ue_techbook

FORGED HARDENED STEEL ROLLS

SERVICE PROBLEMS

CAUSES AND PREVENTION

UNION ELECTRIC STEEL CORPORATION

Union Electric Steel Corporation was founded in 1923 as a manufacturer of forged steel products.Since 1930, Union Electric Steel has made forged steel rolls our sole product. This specialization hasmade Union Electric Steel the recognized world leader in the forged steel roll market.

Leadership in the highly technical field of roll manufacturing is achieved only through a focused planof continuous improvement. The basis of this plan is firmly rooted in an extensive research anddevelopment program.

Union Electric Steel is continually developing new chemistry and heat treatment combinations to provide new products with increased value for the ferrous and non-ferrous industries.

Our global customer base is served by a highly trained staff of Sales Engineers. Practical and innovative problem solving is the core of our customer service program. Training seminars providean ongoing education of current developments in both roll technology and roll maintenance.

Union Electric Steel provides the roll user with not only the highest level of technology, but also anunparalleled level of after-sales service.

Copyright ©1999 Union Electric Steel Corporation

TABLE OF CONTENTSPAGE NUMBERS

I. INTRODUCTION 5

II. ROLL SURFACE INDICATIONS

A. INCLUSION 7 - 8

B. ORANGE PEEL 9 - 10

C. ROLL MARK 11 - 14

D. BRUISE 15 - 20

E. FIRE CRACKS

1. HOT MILL WORK ROLL 21

2. COLD MILL WORK ROLL 22 - 29

III. SPALLING

A. SURFACE INITIATION 31 - 49

B. SUB-SURFACE INITIATION

1. MATERIAL QUALITY 50 - 52

2. CONTACT STRESS

a. GENERAL MECHANISM 53 - 54

b. COLD MILL WORK ROLLS 55 - 64

c. HOT MILL WORK ROLLS 65 - 67

d. BACK-UP ROLLS 68 - 73

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PAGE NUMBERS

IV. NECK BREAKAGE

A. FATIGUE

1. SURFACE INITIATION 75 - 81

a. NECK STRESS CALCULATION 77 - 79

2. SUB-SURFACE INITIATION

a. ROLL DESIGN or MATERIAL QUALITY 80 - 85

3. NECK REPAIR 86 - 89

B. INSTANTANEOUS


2. MILL OVERLOAD 92 - 94

V. BODY BREAKAGE

A. FATIGUE 95 - 97

B. INSTANTANEOUS



VI. ROLL INSPECTION 103

A. EDDY CURRENT INSPECTION 105 - 106

B. SURFACE WAVE ULTRASONIC INSPECTION 107 - 112

C. DYE PENETRANT INSPECTION 113 - 115

D. ETCH TESTING 116 - 118

E. MAGNAFLUX TESTING 120 - 121

F. HARDNESS TESTING 122 - 126

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VII. ROLL HANDLING AND STORAGE 127

VIII. ROLL NOMENCLATURE 129

IX. REFERENCES 131

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INTRODUCTION

The performance characteristics of rolls in service are critical to mill productivity and to the quality and acceptance of the rolled products. Rolls also represent a significant investment and input to a value analysis of cost per ton rolled. The purpose of this investigation is to present a summary of service problems encountered with forged hardened steel rolls and provide the following analysis:

- Type of service problem- Characteristics- Examples (photographs, illustrations)- Mechanism- Prevention

Nondestructive testing (NDT) of forged rolls is important to both the roll manufacturer and the roll user. NDT is employed by the roll manufacturer to verify that both the surface and the interior of the roll are acceptable prior to hardening and subsequent to the finish machining operations. End-user roll shops utilize NDT to ensure that grinding removals are adequate for restoration of the roll surface prior to further usage. Common NDT methods and their application are also included as a guide for optimizing roll maintenance procedures.

Roll handling and storage is also a factor that can impact premature roll problems. Guidelines forproper movement and placement of rolls are listed.

Roll nomenclature is described as an aid in communication of roll problems and their location.

In the event of a roll problem, it is recommended that the following applicable steps be taken by the roll user:

Roll Spalls/Breakage - Collect all pieces of the fracture and protect them from oxidation.

Roll Records - Review current and past history for abnormal conditions (mill incidents, grinding removals, length of mill campaign).

Surface Indications - Document (photograph) prior to removal.

NDT - Verify calibration and proper testing procedure.

Communication - Notify the roll manufacturer for assistance and review of the rollmanufacturing history.

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ROLL SURFACE INDICATIONS

II. ROLL SURFACE INDICATIONS

A. INCLUSION 7 - 8

B. ORANGE PEEL 9 - 10

C. ROLL MARK 11 - 14

D. BRUISE 15 - 20

E. FIRE CRACKS

1. HOT MILL WORK ROLL 21

2. COLD MILL WORK ROLL 22 - 29

CATEGORY: ROLL SURFACE INDICATIONS

TYPE: INCLUSION

CHARACTERISTICS

Typical inclusions that are visible on the roll surface are irregular in shape with the major axis in the longitudinal direction. Their length can be in the range of 0.05 mm (0.002”) to 5 mm (0.020”). The “hole” left in the roll surface after removal of the included material is rough in appearance.

EXAMPLES

EXAMPLE 1Exogenous inclusion on the roll surface (50X).

EXAMPLE 2Close-up of the exogenous inclusion shown in Example 1 (200X).

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MECHANISM

Inclusions that are visually detectable are normally exogenous in nature. The source material can range from refractory, slag or other external materials entrapped during ingot solidification. Indigenous inclusions require the aid of a microscope for detection. This type of inclusion is inherent in the steel making process and can be classified according to its composition (sulfide,aluminate, silicate or oxide).

PREVENTION

Prevention of inclusions is the responsibility of the roll manufacturer. Inclusions are inherent in all steels, however, their size and multitude can be reduced through identification and control of critical melting variables. The probability for an irregularity to exist in the material after solidification can be reduced by changing from electric arc furnace vacuum degassed material to ESR (Electroslag Remelt) material.

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TYPE: ORANGE PEEL(WOOD GRAIN, INGOTISM)

CHARACTERISTICS

Orange peel can be characterized as a textured “wood grain” pattern or “differential roughness”that develops on the roll body during a campaign. The surface finish highlights the dendritic pattern developed during solidification. This pattern is located over the entire rolling surface and usually appears after a significant amount of stock removal.

EXAMPLES

EXAMPLE 1Roll exhibiting an orange peel pattern on the rolling surface.

EXAMPLE 2Roll exhibiting an orange peel pattern on the rolling surface.

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MECHANISM

This degree of “ingotism” or “dendritic pattern” can be associated with either minimal forge reduction or an excessive amount of stock removal from the original roll diameter (cut down rolls). The dendritic macrostructure of the roll material can be described as a “tree-like” structure of almost pure iron. The iron rich dendrites are softer (lower in hardness) than the alloy rich areas between them. The friction induced on the roll surface during rolling will wear down the softer dendrites faster than the alloy rich zones forming the textured surface. Insufficient lubrication during mill service can accelerate the development of orange peel.

PREVENTION

The development of orange peel can be delayed through the use of sufficient lubrication of the roll surface during rolling. Orange peel can be further prevented through proper forge reduction duringnew roll processing and scheduling appropriate cut down candidates.

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TYPE: ROLL MARK(PIN-HEADS, PITS, BUTTONS AND HOLES)

CHARACTERISTICS

Roll marks are characterized as random localized indentations, typically “round”, with a maximumdiameter of approximately 3 mm (0.125”) and a depth of 0.08 mm (0.003”). The surface finish ortexture of the roll is typically retained within the indentation.

EXAMPLES

EXAMPLE 1 (ref.1)

Three dimensional optical interferometer plot of a roll mark on the surface of a roll.

EXAMPLE 2 (ref. 1)

Two dimensional optical interferometer plot of a roll mark on the surface of a roll.Note the retention of the surface finish within the indentation.

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EXAMPLE 3 (ref. 1)

Three dimensional optical interferometer plot of a roll mark imprinted on the surface of the strip.

EXAMPLE 4Arrows highlight typical roll marks on the surface of a roll.

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EXAMPLE 5Circle highlights a typical roll mark on the surface of a roll.

EXAMPLE 6Close-up view of the roll mark shown in Example 5 revealing the retention of the surface roughness within the impression (50X). Arrows highlight the edges of the impression on the surface.

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MECHANISM

Roll marks are generated as debris enters either the roll bite or the work roll/back-up roll (intermediate roll - 6 hi mills) contact zone. The size of the particles required to mark the work rolls within the roll bite would have to be greater than the gauge of the product being rolled. As particles become trapped within the bite or the work roll/back-up roll contact zone, stresses can develop in excess of 100,000 psi, causing the particle to indent the work roll or both the work roll and the back-up roll. Potential sources of the debris include grit, weld spatter, scale and the incoming strip quality. With continued mill service, roll marks can initiate surface cracks that propagate radially and circumferentially until spalling occurs (see Spalling - Surface Initiation)

PREVENTION

Roll marks can be prevented by the following:

• Identification and elimination of the mill debris source.

• Increased work roll hardness/depth of hardness.

• Increased hardness difference between the work roll and the back-up roll (intermediate roll - 6 hi mills).

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TYPE: BRUISE(SOFT SPOT, PINCH MARK)

CHARACTERISTICS

An area on the roll surface that exhibits a softened condition (lower hardness) than the remainder of the roll surface. Detection methods include eddy current inspection, hardness testing, acid etching and shot blasting. These areas typically have fire cracks within the bruised area (see Roll Surface Indications - Fire Cracks). In extreme cases, the bruise may also contain a hardened condition and a tempered color (blue/brown).

EXAMPLES

EXAMPLE 1Grinding wheel “bump” mark after etching and Rockwell Hardness testing. The “light” area is 7 HRc points harder than the base material (re-hardened). The “dark” area is 11 HRc points softer than the base material (re-tempered). The surface was etched with a 20% Nital reagent.

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MECHANISM

A bruise occurs in mill service when the local temperature exceeds the tempering temperature the roll was subjected to during processing. The tempering temperature is set to establish the roll hardness. In general, the higher the tempering temperature, the lower the roll hardness. Therefore, a bruise is created when the local temperature exceeds the tempering temperature of the roll, which reduces the hardness within the bruised (re-tempered) area. A change in specific volume also occurs when martensitic steel is hardened (BCC > BCT). During tempering and re-tempering, contraction occurs with increasing temperature (see Figure 1 below). This contraction can cause fire cracking within the bruise (see Roll Surface Indications - Fire Cracks). With continued mill service, the fire cracks can propagate inward and spalling is inevitable (see Spalling - Surface Initiation).

FIGURE 1Dimensional changes that occur during heat treatment of the roll body.

PREVENTION

Some of the potential “heat” sources for manifestation of a bruise are: grinding wheel “bump”marks, strip breaks, skids, mill wrecks (metal wrap or metal weld), non-uniform spray cooling,variation in gauge of product being rolled, coolant temperature and mill speed. Avoiding any one of these potential causes will reduce the occurrence of bruises being formed on the roll surface. Use of “softer” rolls (higher tempering temperatures during processing) is a consideration for mills with a high incidence rate for the above problems.

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FURTHER EXAMPLES OF BRUISES

EXAMPLE 2Evidence of bruising (localized over tempering) resulting from heat induced during a mill wreck where a metal wrap was involved. The “dark” areas are softer than the surrounding base metal. The surface was etched with a 20% Nital reagent.

EXAMPLE 3Evidence of bruising resulting from friction heat induced on the roll surface during a skid. The “dark” area is softer than the surrounding base metal. The surface was etched with a 20% Nital reagent.

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EXAMPLE 4Arrows highlight the location of bruising resulting from friction heat induced on the roll surface during a skid. The surface was etched with a 20% Nital reagent.

EXAMPLE 5Evidence of bruising resulting from friction heat induced on the roll surface by foreign particles being “dragged” across the roll surface. The surface was etched with a 20% Nital reagent.

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EXAMPLE 6Evidence of bruising resulting from friction heat induced on the roll surface by a foreign particle being “dragged” across the roll surface. The surface was etched with a 20% Nital reagent.

EXAMPLE 7Close-up of the bruise shown in Example 6.

Note the presence of longitudinal fire cracks within the bruise.

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EXAMPLE 8Arrow highlights the location of a small bruise manifested on the roll surface during an aggressive grinding operation. The surface was etched with a 20% Nital reagent.

EXAMPLE 9Evidence of bruising resulting from heat induced on the roll surface during a mill wreck where metal wrap was involved. The numbers written on the surface indicate the hardness difference between the bruised areas and the parent metal (measured in HLd). The surface was etched with a 20% Nital reagent.

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TYPE: FIRE CRACKS (STRESS CRACKS)

CHARACTERISTICS

Fire cracks range in appearance from small (1 mm, 0.040”), tight longitudinal cracks to various patterns (“crazing”, “dry river bed”) and densities depending on the mill operation and cause (hot mill/cold mill). Typically, fire cracks are also associated with areas of bruising (see Roll Surface Indications - Bruise).

EXAMPLES

EXAMPLE 1Evidence of fire cracking on an aluminum hot mill work roll.

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EXAMPLE 2Evidence of fire cracking on the surface of a spall from a cold mill work roll.

MECHANISM

A bruised area (see Roll Surface Indications - Bruise) is the first stage for the generation of a fire crack. Non-uniform stress within the bruise is created as the re-tempered martensite contracts from the surrounding roll material. A fire crack (stress crack) is initiated as the bruise undergoes stress relief. With continued mill service, fire cracks can propagate radially and circumferentially into the roll until spalling occurs (see Spalling - Surface Initiation).

Thermal shock (spalling resulting from rapid heating and cooling of the roll body surface), a moresevere form of fire cracking, occurs in an instantaneous manner. Thermal shock is usually associ-ated with a mill induced metal wrap (metal weld) where the heat generated on the roll surface islarge enough to instantly result in cracking and spalling.

PREVENTION

Avoiding the conditions that create a bruise or result in thermal shock will reduce the possibility of fire cracks being formed on the roll surface.

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FURTHER EXAMPLES OF FIRE CRACKS

EXAMPLE 3Evidence of fire cracking on a cold mill work roll.

EXAMPLE 4Arrows highlight a small fire crack that manifested within a small grinder burn (50X).

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EXAMPLE 8Fire cracks highlighted during dye penetrant inspection of a cold mill work roll.

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EXAMPLE 9Fire cracks highlighted during dye penetrant inspection of a cold mill work roll.

EXAMPLE 10Fire cracks highlighted during dye penetrant inspection of a cold mill work roll. Notice the cracks to be circumferentially in line with a spall that occurred.

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EXAMPLE 11Evidence of spalling resulting from thermal shock (a severe form of fire cracking) that manifested during a mill wreck where metal wrap was involved.

EXAMPLE 12Evidence of spalling resulting from thermal shock that manifested during a mill wreck where metal wrap was involved.

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EXAMPLE 13Evidence of spalling resulting from thermal shock that manifested during a mill wreck where metal wrap was involved.

EXAMPLE 14Close-up view of the thermal shock shown in above in Example 13 showing a typical non-distinct fracture face.

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EXAMPLE 15Evidence of spalling on an aluminum hot mill work roll that manifested from thermal shock that occured during a cobble.

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SPALLING

III. SPALLING

A. SURFACE INITIATION 31 - 49

B. SUB-SURFACE INITIATION


2. CONTACT STRESS

a. GENERAL MECHANISM 53 - 54

b. COLD MILL WORK ROLLS 55 - 64

c. HOT MILL WORK ROLLS 65 - 67

d. BACK-UP ROLLS 68 - 73

CATEGORY: SPALLING

TYPE: SURFACE INITIATION(FATIGUE PATH, WRECK PATH, WAGON TRACKS)

CHARACTERISTICS

Surface initiated spalling can be identified by the presence of a fatigue “wreck” path on the fracture face. The fatigue path is marked as a circumferential path of fracture that can in some cases be visually traced back to the surface. The distinguishing characteristics of a fatigue path are the typical fatigue arrest marks (beach marks) and the “fan” shaped fracture flow lines on the fatigue fracture face. A fatigue path can range in length from a few inches to several complete laps around the roll circumference and sometimes has a shiny (“rubbed”) or dark (oxidized) appearance. The surface initiation point is typically associated with a thermally initiated crack (fire crack - see Roll Surface Indications - Fire Cracks) or a crack from a mechanically initiated indication (roll mark - see Roll Surface Indications - Roll Mark). The direction of fatigue “wreck” path propagation is opposite to the direction of roll rotation. Radial crack propagation is characteristic of a reversing mill application.

EXAMPLES

EXAMPLE 1The above figure diagrams the steps involved in surface initiated spalling. It should be pointed out though that initiation does not necessarily need to come from a bruised and fire cracked area. Fatigue cracks can initiate at any point on the surface where stress is concentrated. For example:roll marks, inherent non-bruise related surface cracks, gouges etc.

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EXAMPLE 2Fire cracks on a roll surface (stages 1-3).

EXAMPLE 3Transverse view of a surface initiated spall (stage 4). Cracking can be seen propagating radially and circumferentially from the rolling surface (rolling surface highlighted with arrows). The directionof crack propagation is opposite to the direction of roll rotation.

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EXAMPLE 4Fatigue path evident after spalling occurred (stages 5 and 6).

EXAMPLE 5Fatigue path evident on a portion of a spall fracture face. Large arrow highlights typical fatigue arrest mark (beach mark). Small arrow highlights direction of fatigue propagation.

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EXAMPLE 6Close-up of a fatigue path fracture face. Note fracture flow lines in a “fan” type pattern. In this case the fatigue arrest marks are present but not as evident due to the speed of fatigue propagation. Arrow highlights the direction of fatigue propagation.

MECHANISM

Surface initiated spalling occurs in several distinct stages (see example 1).

Stages 1 to 3 - A crack is induced on the roll surface. This crack can be the result of a bruise (as discussed in Roll Surface Indications - Bruise and illustrated in Example 1) or a non-bruise related surface crack. Roll marks, gouges etc. can create surface cracks by acting as stress concentration factors. With each roll revolution, the entire roll surface cycles between states of high tensile stress to states of high compressive stress. Any concentration of stress at a single point could result in the development of a surface crack.

Stage 4 - With each revolution, the crack propagates radially and circumferentially via a fatigue mode through the transition zone (depth of hardness). Stage 4 is noted by the presence of a radial and circumferential fatigue path showing distinct arrest marks and “fan” type fracture flow lines.

Stage 5 - The crack continues to propagate circumferentially in the sub-surface transition zone. Stage 5 is noted by the continuation of the circumferential fatigue path showing distinct arrest marks and “fan” type fracture flow lines within the fatigue path.

Stage 6 - The yield strength of the surrounding material is reduced to such a degree thatspalling occurs. Stage 6 can occur any time between stage 4 and 5 depending on thematerial strength of the roll and the induced rolling stresses. This final stage of fracture is instantaneous and brittle in nature and can be noted by fibrous fracture flowlines originating from the fatigue path on the fracture face.

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PREVENTION

Surface initiated spalling can be prevented by the following:

• Avoid mill related surface damage such as bruises, cracks, roll marks, gouges or any surface indication that may act as a stress concentration factor.

• If the induced rolling stresses are high compared to the material strength of the roll, it is possible for surface crack initiation, propagation and spalling to occur all within a single campaign. Shorter campaign times along with reduced rolling pressures could prevent surface indications, that are induced during a single campaign, from initiating into fatigue paths before they are removed during the grinding operation.

• Sufficient stock removal during the grinding operations to insure that any surface damage induced during the last campaign is eliminated.

• Consistent use of eddy current and ultrasonic inspection techniques on every roll after completion of the grinding operation. This will insure that every roll that is returned to the mill for service is free of any surface damage that may initiate a fatigue path.

FURTHER EXAMPLES OF FATIGUE PATHS

EXAMPLE 7Arrow highlights a fatigue path evident on the fracture face after spalling occurred.

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EXAMPLE 8Arrow highlights the location of a fatigue path evident after spalling occurred.

EXAMPLE 9Close-up view of the initiation site for the fatigue path shown above in Example 8. Arrow highlights a portion of the surface crack that initiated the fatigue path.

EXAMPLE 10Side view of the fatigue path shown in Example 8. Arrows highlight the radial and circumferential path of propagation from the surface into the rolls interior.

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EXAMPLE 11Transverse view of a surface initiated spall from a reversing cold mill work roll. Cracking can be seen propagating radially and circumferentially (in both directions) from the rolling surface (rolling surface highlighted with arrows).

EXAMPLE 12Arrow highlights a fatigue path evident on the fracture face after spalling occurred.

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EXAMPLE 13In some cases, the fatigue path cannot be initially seen after spalling occurs, but is hidden by a “bridge” surrounded longitudinally by one or two cone shaped-spalls.

EXAMPLE 14Fatigue path evident on the fracture face after removal of the “bridge” shown above in Example 13.

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EXAMPLE 15Fatigue path evident on the fracture face after spalling occurred. Arrow highlights the direction of fatigue propagation.


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EXAMPLE 17Fatigue path evident on the fracture face after spalling occurred.


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EXAMPLE 19Fatigue path evident on the fracture face after spalling occurred. Arrows highlight the direction of fatigue propagation.

EXAMPLE 20Fatigue path evident on the fracture face after spalling occurred. Small arrows highlight the direction of fatigue path propagation. Large arrows highlight general fatiguing away from the fatigue path.

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EXAMPLE 22Small fatigue path evident on the fracture face after spalling occurred.

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EXAMPLE 23Large arrows highlight dual fatigue paths evident on the fracture face after spalling occurred. Small arrows highlight the direction of fatigue propagation.

EXAMPLE 24Dual fatigue paths evident on the fracture face after spalling occurred. Arrows highlight the direction of fatigue propagation.

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EXAMPLE 25Fatigue path evident on the fracture face after spalling occurred. Small arrow highlights the direction of fatigue propagation. Large arrow highlights the indication on the surface where fatiguing initiated.

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EXAMPLE 26Fatigue path evident on the fracture face after spalling occurred. Large arrow highlights the location where radial fatiguing from the surface ended. Small arrows highlight the direction of longitudinal fatiguing away from the radial fatigue path.


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EXAMPLE 29Fatigue path evident on the fracture face after spalling occurred. Large arrow highlights the location where radial fatiguing from the surface ended. Small arrows highlight the direction of longitudinal and circumferential fatiguing away from the radial fatigue path.

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EXAMPLE 30Fatigue path evident on the fracture face after spalling occurred. Arrows highlight the direction of fatigue propagation.

EXAMPLE 31Large arrow highlights a fatigue path evident on the fracture face after spalling occurred. Small arrow highlights the direction of fatigue propagation.

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EXAMPLE 32Fatigue paths evident on the fracture face after spalling occurred. Arrows highlight the direction of fatigue propagation.

EXAMPLE 33Fatigue path evident on the fracture face after spalling occurred.

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CATEGORY: SPALLING

TYPE: SUB-SURFACE INITIATION - MATERIAL QUALITY RELATED(DEEP SEATED DEFECT “DSD”, FISH EYE)

CHARACTERISTICS

Sub-surface initiated spalling resulting from poor material quality can be identified by the presence of a concentric fatigue pattern (fish eye) on the fracture face. The fatiguing can be seen initiating from a single point with fatigue arrest marks (beach marks) emanating in an oval pattern away from the initiation site. The fatigue arrest lines are only associated with the deep seated material indication (“DSD”) and not with any other sources (eg. “wreck” paths). Thisfatigue pattern should not be confused with surface initiated fatiguing (see Spalling - SurfaceInitiation) which appears as a fatigue “wreck” path.

EXAMPLES

EXAMPLE 1Typical fracture face of a subsurface fatigue spall initiating from a material defect. Small arrow highlights the initiation site as a small material defect. Large arrow highlights a fatigue arrest mark (stage 1). Instantaneous brittle fracture flow lines can also be seen emanating from the outerfatigue arrest mark (stage 2).

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MECHANISM

During solidification, it is possible for an irregularity to become “entrapped” within the ingot. This irregularity can be anything from refractory, slag, localized segregation, porosity etc. These irregularities can then act as stress concentration factors when the roll is put into service.

Spalling resulting from a sub-surface material defect occurs in two distinct stages:

Stage 1 - When the localized stress induced during rolling at these irregularities exceeds the fatigue strength (but not the tensile strength) of the material, fatigue cracks initiate and propagate away from the initiation site. The fatigue arrest marks propagate in all directions away from the initiation site usually within a single plane of propagation.

Stage 2 - The strength of the surrounding material is reduced to such a degree that spalling occurs. This final stage of fracture is instantaneous and brittle in nature and can be noted by fibrous fracture flow lines originating from the outer fatiguemark on the fracture face.

PREVENTION

Spalling resulting from a sub-surface defect can be prevented by the following:

• Identification and control of critical melting variables by the roll manufacturer to reduce the possibility of an irregularity existing in the ingot after solidification.

• Changing from electric arc furnace vacuum degassed material to ESR (Electroslag Remelt) material for high productivity mill applications. The ESR process reduces the probability for an irregularity to exist in the material after solidification.

• Ultrasonic inspection techniques using a straight beam transducer on every roll after completion of the grinding operation. If accurate inspection records on every roll are maintained, rolls with sub-surface indications which are initiating fatigue can be identified and removed from service before they spall.

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FURTHER EXAMPLES OF “DSD” SPALLS

EXAMPLE 2Fracture face of a fatigue spall that originated from a subsurface material defect. Arrow highlights the location of the fatigue initiation site.

EXAMPLE 3Close-up view of a typical fracture face of a fatigue spall that originated from a subsurface material defect. Arrow highlights the fatigue initiation site.

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CATEGORY: SPALLING

TYPE: SUB-SURFACE INITIATION - CONTACT STRESS(CRUSHING)

GENERAL MECHANISM

Due to the applied mill load and the localized flattening of the rolls at their contact point, the maximum resultant shear stress (commonly referred to as “Hertzian Stress”) is located at a short distance below the roll surface (Figures 1 and 2 illustrate the stress condition). Cracks at multiple locations can initiate and propagate at that sub-surface location when the Hertzian Stress exceeds the compressive strength of the roll. This can occur via two modes:

(1) Instantaneously; by a sudden increase in the Hertzian Stress. This occurs when work rolls wrap, skid or stop suddenly. The Hertzian Stress increases dramatically and can easily exceed the compressive strength of the roll. Sub-surface cracks can then form instantaneously and with further cycling of rolling stresses, propagation via a fatigue mode can occur and spalling is probable. In extreme cases of excessive contact stress, subsurface cracks can both initiate and propagate to spalling instantaneously.

(2) High Cycle Fatigue Fracture; this mode of sub-surface crack initiation occurs more commonly in back-up rolls and usually occurs without mill incidents. Typically, this type of fatigue fracture is described as a “crumbly” type spall that manifests from cracks that initiate beneath the roll surface over time. This is readily explainable by a typical S-N fatigue graph where the number of cycles to failure is on the order of 1 million. Repeated application of stress lower than the inherent material strength of the roll can lead to crack initiation if the number of stress applications is sufficient. High cycle contact stress fatigue spalls initiate in many locations within the stressed area as very small cracks oriented parallel to the tangent of the roll surface. Repeated cycling of stress then propagates these cracks toward the surface until spalling occurs. In some cases, the contact stress fatigue cracks can begin to propagate radially and circumferentially, forming a fatigue “wreck” path and spalling is inevitable (See Spalling - Surface Initiation). A number of factors contribute to produce contact fatigue spalls including:length of time in the mill, stock removal, rolling pressures and work roll to back-up roll diameter differential.

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FIGURE 1 (ref. 2)

FIGURE 2 (ref. 2)

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CONTACT STRESS SPALLS CONTINUED

COLD MILL WORK ROLLS

CHARACTERISTICS

Contact stress spalling can occur on cold mill work rolls via both modes described previously.

Instantaneous - This mode can be characterized as an area that appears “crushed” and does not show any distinct initiation site.

High Cycle Contact Stress Fatigue Fracture - This mode has different characteristics depending on whether the stress was located on the body or the body edge.

Roll Body - High cycle fatigue fracture on the work roll body is usually not directly responsible for the spalling that occurs. In this case, the contact stress fatigue cracks that form below the surface do not propagate towards the surface but rather propagate radially and circumferentially forming a fatigue “wreck” path. It is this mode of crack propagation that eventually causes spalling to occur (see Spalling - Surface Initiation). High cycle contact stress fatigue is difficult to diagnose when it occurs on a work roll body because the cracks that initiated the fatigue “wreck” path are not readily seen on the fracture face but are hidden in the sub-surface.

Roll Body Edge - High cycle contact stress fatigue fracture on the work roll body edge can be present as a small “crumbly” type fatigue spall. The “crumbly” fatigue spall is usually located at the contact point between the work roll and back-up roll body edge,strip edge or at the work roll body edge. The “crumbly” spall is usually small in area and no greater than 0.150” deep. In many cases, a fatigue “wreck” path originating from the area of contact stress fatigue is also present (see Spalling - Surface Initiation).

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EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLSINSTANTANEOUS

EXAMPLE 1Arrows highlight the fracture face of a work roll body edge that spalled instantaneously. In this case, the contact stresses applied to the body edge were large enough to exceed the compressive strength of the material and subsurface crack initiation and propagation occurred instantaneously.

EXAMPLE 2Fracture face of a work roll body edge that spalled instantaneously. In this case, the contact stresses applied to the body edge were large enough to exceed the compressive strength of the material and subsurface crack initiation and propagation occurred instantaneously.

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EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLSFATIGUE

EXAMPLE 1Fracture face from a portion of spall that occurred on the body of a cold mill work roll. Arrow highlights the initiation site associated with high cycle contact stress fatigue. Note the presence of several distinct fatigue “wreck” paths originating from the area of contact stress fatigue.

EXAMPLE 2Transverse view of the spall shown in Example 1. Arrow highlights the high cycle contact stress fatigue cracks that were formed below the surface.

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EXAMPLE 3Network of high cycle contact stress fatigue cracks present in the sub-surface of the spall shown in Example 1.

EXAMPLE 4Cold mill work roll exhibiting a body edge spall.

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EXAMPLE 5Fracture face of the body edge spall shown in Example 4. Note the distinct fatigue “wreck” path responsible for the major portion of spalling. Arrow highlights the direction of fatigue propagation.

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EXAMPLE 6Close up of the fatigue “wreck” path initiation point that resulted in the spall shown in Example 4. Arrows highlight small “crumbly” spalls associated with high cycle contact stress fatigue. The fatigue path can be seen initiating from the area of high cycle contact stress fatigue spalling.

EXAMPLE 7Cold mill work roll exhibiting a body edge spall initiating from the edge of the bevel.

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EXAMPLE 8Cold mill work roll exhibiting “crumbly” contact stress fatigue spalling on the body edge.

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EXAMPLE 9Cold mill work roll exhibiting plastic deformation resulting from contact stress that exceeded the compressive yield strength.

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MECHANISM

Instantaneous Contact Stress Spalling:

As was described in the General Mechanism section of Contact Stress Spalling,the maximum resultant shear stress is located a short distance below the surface. A sudden mill stop, or a skid in extreme circumstances, can cause the maximum resultant shear stress to exceed the compressive strength of the roll. This sudden rise in shear stress can cause sub-surface cracks to form instantaneously. In extreme cases, the contact stress is severe enough to cause the subsurface cracks formed to also propagate to spalling instantaneously.

High Cycle Contact Stress Fatigue:

Body - Due to the high hardness, high cycle contact stress fatigue on a work roll body is usually the result of a point load being induced on the roll. This point load can be a small piece of grit stuck in the roll bite, a strip weld being passed through the roll bite or anything that would act to concentrate stresses at a single point on the roll body. If the stress is greater than the compressive strength of the material small cracks can initiate in the sub-surface which over time propagate via a fatigue “wreck” path mode and spalling is inevitable.

Body Edge - Areas on the work roll body such as the strip edge, contact zone between the work roll and the back-up roll edge or the body edge act as stress concentration factors when the roll is put into service. With each revolution of the roll, if the maximum resultant shear stress , located just below the roll surface (see General Mechanism section of Contact Stress Fatigue), exceeds the compressive fatigue strength of the material, cracks will form at that location. Further stress cycles will propagate the cracks toward the surface where small “crumbly”spalling then occurs. In many cases, fatigue “wreck” paths will initiate at locationswhere sub-surface contact stress fatigue cracks have formed. The fatigue “wreck”path can then propagate radially and circumferentially until the strength of the surrounding material is reduced to such a degree that large spalling occurs.

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PREVENTION

Instantaneous Contact Stress Spalls

Avoid mill incidents such as skids, mill wrecks, metal wrap etc.

High Cycle Contact Stress Fatigue -

Several steps can be taken to prevent the occurrence of high cycle contact stress fatigue on cold mill work rolls:

• Avoid increases in maximum resultant shear stress above the compressive fatigue strength of the roll by the passing of grit or welds etc. through the roll bite.

• Ultrasonic inspection techniques using a dual probe (“pitch/catch”) and surface wavetransducer on every roll after completion of the grinding operation. This will insure thatevery roll that is returned to the mill for service is free of both surface and sub-surface cracks.

• Sufficient stock removal during the grinding operation to either assure the removal of any cracks formed in the sub-surface or to relocate those cracks further away from the zone of maximum resultant shear stress. Relocating the cracks will subject them to a lower stress state where they will be less likely to propagate.

• Shorten campaign times to decrease the number of stress cycles the roll is subjected to. Repeated application of stress above the compressive fatigue strength of the roll will lead to sub-surface crack initiation if the number of stress cycles is sufficient enough.

• Reduce the rolling pressures to reduce the maximum resultant shear stress.

• Develop a surface profile (work roll/back-up roll crowning practice) to insure a uniform contact stress pattern along the entire work roll/back-up roll (intermediate roll) contact zone.

• Change the roll design from a body bevel to a body radius to reduce the stress concentration on the roll body edge.

• Change the back-up roll body edge design to a body radius and proper relief (approximately 0.5°) to reduce the stress concentration on the work roll body at the point of contact between the work roll and the back-up roll body edge.

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HOT MILL WORK ROLLS(THERMAL PITTING)

CHARACTERISTICS

Contact stress fatigue spalling on hot mill work rolls can be characterized as small “pits” that may be random or clustered on the roll body at areas of stress concentration. Individual pits can exhibit evidence of fatigue arrest marks (beach marks) and erosion on the rolling surface of the spall.

EXAMPLES

EXAMPLE 1Hot mill work roll exhibiting contact stress fatigue in the form of thermal pitting on the roll body surface.

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EXAMPLE 2Close-up view of the thermal pitting condition exhibited by the hot mill work roll shown in Example 1.

MECHANISM

In 4-high mills, thermal pitting typically initiates at a sub-surface location defined by the maximum shear stress (see General Mechanism section of Contact Stress Fatigue). During rolling, any one point on the hot mill work roll surface will cycle between high temperatures (when in contact with the hot strip surface) and low temperatures (when cooled with the sprays). As the material cycles in temperature, it undergoes dimensional changes. Cracks formed at the sub-surface location of maximum shear stress can then propagate, via fatigue resulting from the thermally induced dimensional changes, along shear planes both toward the surface and deeper into the roll until the strength of the surrounding material is reduced to such a degree that spalling occurs. Surface initiated thermal pitting can also occur in 2 high mills where cracks form at the roll surface and propagate via fatigue into the interior until spalling occurs. Foreign material (oxide) “sticking” to the roll surface can accelerate the pitting process by a localized increase in the contact stress.

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PREVENTION

Thermal pitting of hot mill work rolls can be prevented by the following:

• Proper selection of roll material (alloy), heat treatment and hardness to optimize the thermal fatigue strength of the roll material.

• Use of “scratch brushes” to clean the roll surface and prevent debris from entering the work roll/back-up roll contact zone where a localized increase in the maximum shear stress can occur.

• Use of spray cooling to insure proper coverage of the roll body. Nonuniform and/or inadequate spray cooling can lead to excessive heating of the roll body during service and increase the probability of thermal fatigue spalling in the affected area.

• Chrome plating the roll body to improve the lubricity and resistance to “oxide pick-up”

• Ultrasonic inspection techniques using a dual probe (“pitch/catch”) and surface wavetransducer on every roll after completion of the grinding operation. This will insure thatevery roll that is returned to the mill for service is free of both surface and sub-surface cracks.


• Shorten campaign times to decrease the number of stress cycles the roll is subjected to. Repeated application of stress above the compressive and thermal fatigue strength of the roll will lead to sub-surface crack initiation if the number of stress cycles is sufficient enough.


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BACK-UP ROLLS

CHARACTERISTICS

Instantaneous contact stress spalling does not normally occur on back-up rolls but high cycle contact stress fatigue is quite common. The high cycle contact stress fatigue spalling can occur at any location of the roll body and usually has a “crumbly” spall appearance. Due to the lower hardness of the back-up roll, the “crumbly” spalls are usually larger and their appearance is usually more enhanced compared to work roll high cycle contact fatigue spalls. The subsurfacecontact stress fatigue cracks can go unnoticed for quite sometime before being exposed to the surface (spall). The only way to detect subsurface contact stress fatigue cracks prior to spalling is through the use of straight beam ultrasonic inspection using a dual probe (“pitch/catch”) transducer. Fatigue “wreck” paths can, in extreme cases, be seen originating from areas of high cycle contact stress fatigue (see Spalling - Surface Initiation).

EXAMPLES

EXAMPLE 1Back-up roll exhibiting a “crumbly” high cycle contact fatigue spall.

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EXAMPLE 2Close-up of the “crumbly” high cycle contact fatigue spall shown in Example 1. The bottom of the “pits” is the location beneath the roll surface where cracks initiated and then propagated toward the roll surface.

EXAMPLE 3Close-up view of a high cycle contact fatigue spall that was located in the center of a back-up roll body. Arrow highlights area of “crumbly” high cycle contact fatigue spalling. Note the fatigue path initiating from the sub-surface cracks.

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MECHANISM

The mechanism for high cycle contact stress fatigue in back-up rolls is as described in the General Mechanism section of Contact Stress Fatigue. Spalling occurs wherever the maximum resultant shear stress, located below the roll surface, exceeds the compressive fatigue stress of the material. The most common locations for high cycle contact stress fatigue on back-up rolls are the crown in the center of the body, the roll body edge and the contact point between the back up roll and the work roll body edge. These areas can act as stress concentration factors during rolling and can significantly increase the maximum resultant shear stress below the surface.

PREVENTION

High cycle contact stress fatigue in back-up rolls can be prevented by the following:

• Ultrasonic inspection techniques using a straight beam dual probe (“pitch/catch”) and surface wave transducer on every roll after completion of the grinding operation. This will insure that every roll that is returned to the mill for service is free of both surface and sub-surface cracks.


• Shorten campaign times to decrease the number of stress cycles the roll is subjected to. Repeated application of stress above the compressive fatigue strength of the roll will lead to sub-surface crack initiation if the number of stress cycles is sufficient enough.


• Increase the hardness of the back-up roll to increase the fatigue strength.

• If contact stress fatigue spalling is occurring later in the rolls life, an increase in the depthof hardness by proper selection of grade and heat treatment will increase the fatigue strength at reduced diameters.

• Change from a standard bevel to a body radius or “relief” taper design to reduce the stress concentration at the contact zone between the back-up roll and the work roll body edge. This design would also reduce the stress concentration at the back-up roll body edge.

• Reduce the amount of body crown to reduce the maximum resultant shear stress at center of the body.

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FURTHER EXAMPLES OF CONTACT STRESS FATIGUE SPALLING OF BACK UP ROLLS

EXAMPLE 4Back-up roll exhibiting an extreme case of “crumbly” high cycle contact fatigue spalling.

EXAMPLE 5Back-up roll exhibiting “crumbly” high cycle contact fatigue spalling at the location of contact between the back-up roll and the edge of the work roll.

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EXAMPLE 6Back-up roll exhibiting “crumbly” high cycle contact stress fatigue spalling at an improperly shaped edge relief. The sharp edge ground into the roll profile near the body edge acted to concentrate stress at that location. In this case, the spalling condition could be eliminated by grinding a tapered relief instead of a sharp change in diameter.

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EXAMPLE 7Back-up roll exhibiting “crumbly” high cycle contact stress fatigue spalling located at the body bevel.

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NECK BREAKAGE

IV. NECK BREAKAGE

A. FATIGUE

1. SURFACE INITIATION 75 - 81

a. NECK STRESS CALCULATION 77 - 79

2. SUB-SURFACE INITIATION

a. ROLL DESIGN or MATERIAL QUALITY 80 - 85

3. NECK REPAIR 86 - 89

B. INSTANTANEOUS



CATEGORY: NECK BREAKAGE

TYPE: FATIGUE - SURFACE INITIATION(ROTATING BENDING FATIGUE, RATCHET MARKS)

CHARACTERISTICS

The fracture face of rotating bending fatigue is characterized as exhibiting multiple fatigue “ratchet marks” initiating from the surface, oriented perpendicular to the neck surface tangent. The areas between the ratchet marks sometimes have visible fatigue arrest marks (beach marks) propagating from the surface toward the center of the neck. Ratchet marks are the result of multiple fatigue initiation points. In some cases where fatiguing initiates at a single point, fatiguearrest marks will be visible on the fracture face in a widened “tear drop” pattern with the pointbeing at the neck surface.

EXAMPLES

EXAMPLE 1Fracture face of a neck that broke due to surface initiated rotating bending fatigue. Large arrows highlight some of the fatigue ratchet marks. Small arrows highlight the fatigue arrest marks (beach marks) and their direction of propagation.

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MECHANISM

Rotating bending fatigue is the result of the applied rolling stresses exceeding the material strength of the neck during roll service. During rolling, force is applied to the roll necks to create the rolling forces necessary to reduce the incoming strip. Therefore, every point on the roll cycles between tensile and compressive states of stress. The surface points of the neck, being located the farther from the centerline are subjected to the highest tensile and compressive states of stress. This stress can concentrate at different locations along the roll neck surface at changes in diameter or other surface indication is present. This is usually located in the form area where the rotating bending stresses are higher than any other location along the roll neck. If the rolling pressure applied to the necks exceeds the material’s tensile fatigue strength at the point of stress concentration, circumferential surface crack(s) will form. These crack(s) will then propagate radially in the transverse plane via a fatigue mode until the strength of the surrounding material is reduced to such a degree that neck fracture occurs. Fatigue ratchet marks are created when multiple fatigue initiation sites exist. Fatiguing propagates radially in the transverse plane until the edges of the propagating cracks meet. The minute longitudinal difference in location is then compromised and a ratchet mark is formed.

PREVENTION

Neck breakage resulting from rotating bending fatigue can be prevented by the following:

• Increase the rolls material strength to resist crack initiation and propagation. The material strength can be increased by several means. If the cracks are formed near the body end face, special heat treating practices can be employed to raise the hardness of the neck past the body end face. The material strength can also be increased through alteration of the pre-hardening heat treatment of the roll.

• Review the roll design to avoid small fillets and corners adjacent to the body which will increase stress concentration in the zone of maximum bending stress. A separate or integral ring design should also be considered.

• Reduce the rolling pressures to reduce the applied bending stress.

• The rotating bending stress along the neck is influenced by the location of stress application. Apply the rolling pressure closer to the roll body to reduce bending stresses along the roll neck.

• The neck design and material strength, in conjunction with the applied mill loads should be considered to avoid crack initiation and propagation. The text on the following pages covers the calculation of bending stresses and the maximum resultant shear stresses induced on the roll neck during mill service (ref. 3 and ref. 4)

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ROLL NECK STRESS CALCULATION

Shearing Stresses In A Roll Neck - Ss:

When a torque is applied to one end of a roll neck, a shear stress (Ss) exists at all points on that roll neck. In addition to these stresses, elongation of the outer fibers due to twisting causes longitudinal tension over the outer half of the transverse cross section and longitudinal compression over the inner half of the transverse cross section. The maximum shear stress that can be sustained by the roll material before rupture is given by:

Ss = 16 T Kt/(πd3)

Where:T = Torque (inch pounds)Kt = Stress Concentration Factor (defined below)d = Roll Neck Diameter (inches)

Bending Stresses In A Roll Neck- Sb:

When a radial force is applied to a roll neck, as in the case of back-up rolls (work rolls in a two-high mill) or by the application of roll bending forces to achieve control of flatness of the rolled product, cyclical bending stresses are introduced into the roll neck. The maximum bending stress in the outer fiber of the roll neck cross section at the location of maximum resultant bending stress (usually located in the form/fillet area) is given by:

Sb = 10.19 P L Kb/d3

Where:

P = Applied Load (pounds)L = Lever Arm (inches)Kb = Form/Fillet Stress Concentration Factor (defined below)d = Roll Neck Diameter (inches)

Maximum Resultant Shear Stress - SMax:

The maximum resultant shear stress due to bending and torsion is given by:

Smax = (0.25 Sb2 + Ss2)1/2

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Stress Concentration Factor - Kt and Kb:

The stress concentration factor is a function of the following parameters:

d = Roll Neck Diameter

h = body diameter - neck diameter2

r = Form/Fillet Radius

Figures 1 and 2 can be used to determine the stress concentration factors Kt and Kb.

FIGURE 1

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FIGURE 2

Safety Factor - Sf:

The safety factor is a ratio of the yield strength of the roll material to the maximum resultant shear stress. The minimum desired value for Sf is 2.0.

Yield Strength - Y.S.:

The yield strength is an indication of the maximum stress that can be developed in a material without causing plastic (permanent) deformation.

Note: The yield strength of a material in a state of shear is approximately half the yield strength in a state of tension

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FURTHER EXAMPLES OF FATIGUE NECK BREAKAGE

EXAMPLE 2Fracture face of a neck that broke due to surface initiated rotating bending fatigue. Arrows highlight some of the fatigue ratchet marks.

EXAMPLE 3Close-up view of a neck that fractured due to rotating bending fatigue. Large arrows highlight some of the fatigue ratchet marks. Small arrows highlight the fatigue arrest marks (beach marks) and their direction of propagation.

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FURTHER EXAMPLES OF FATIGUE NECK BREAKAGE

EXAMPLE 4Fracture face of a neck that broke due to single point rotating bending fatigue. Large arrow highlights the initiation point on the roll surface. Small arrow highlights a typical fatigue arrest mark.

EXAMPLE 5Fracture face of a neck that broke due to single point rotating bending fatigue. Arrow highlights the initiation point on the roll surface.

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TYPE: FATIGUE - SUB-SURFACE INITIATIONROLL DESIGN OR MATERIAL QUALITY(DEEP SEATED DEFECT “DSD”, FISH EYE)

CHARACTERISTICS

Sub-surface fatigue can initiate from either a point of poor material quality (deep seated defect) or from a portion of the roll design (blow off hole), and is identified by the presence of an oval fatigue pattern (fish eye) on the sub surface portion of fracture face. The fatiguing can be seen initiating from a single point with fatigue arrest marks (beach marks) emanating in an oval pattern away from the initiation site. This fatigue pattern should not be confused with surface initiated fatiguing (see Neck breakage - Surface Initiation) which appears as either fatigue “ratchet marks” or a fatigue “tear drop” with the point at the surface.

EXAMPLES

EXAMPLE 1Neck surface exhibiting a circumferential crack running through the groove for a blow off hole.

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EXAMPLE 2Fracture face of the neck shown in Example 1. Fatigue arrest marks can be seen initiating in the sub-surface from the longitudinal portion of the blow off hole. Arrow highlights typical fatigue arrest mark.

EXAMPLE 3Fracture face of a neck that broke as a result of sub-surface fatigue initiation from a deep seated defect. Arrow highlights the area of fatiguing that occurred prior to fracture.

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EXAMPLE 4Close-up of the fracture shown in Example 3. Arrow highlights the area of poor material quality (deep seated defect). Typical fatigue arrest marks can be seen initiating from the deep seated defect.

MECHANISM

During solidification, it is possible for an irregularity to become “entrapped” within the ingot. This irregularity can be anything from refractory, slag, localized segregation, porosity etc. These irregularities can then act as stress concentration factors when the roll is put into service. Specific sub-surface roll design features, such as blow off holes, can also act as stress concentration factors

Neck breakage resulting from a sub-surface stress concentration factor occurs in two distinct stages:

Stage 1 - When the localized stress induced during rolling at sub-surface stress concentration sites exceeds the fatigue strength of the material, fatigue cracks initiate and propagate away from the initiation site. The fatigue arrest marks propagate in all directions away from the initiation site and usually within a single plane of propagation.

Stage 2 - The strength of the surrounding material is reduced to such a degree that instantaneous fracture occurs.

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PREVENTION

Neck breakage resulting from a sub-surface stress concentration factors can be prevented by the following:


• Increase the material strength of the neck through a change in the pre-hardening heat treatment.

• Change the roll design to move design-induced sub-surface stress concentration factors away from areas of high neck bending stress such as the form area.

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TYPE: FATIGUE - NECK REPAIR

CHARACTERISTICS

The characteristics of a fracture resulting from a neck repair are very similar to those described under the section for rotating bending fatigue (see Neck Breakage - Fatigue - Surface Initiation). The fracture face is characterized by multiple fatigue “ratchet marks” initiating from the surface,oriented perpendicular to the neck surface tangent. The areas between the ratchet marks sometimes have visible fatigue arrest marks propagating from the surface toward the center of the neck. Ratchet marks are the result of multiple fatigue initiation points. In some cases where fatiguing initiates at a single point, fatigue arrest marks will be visible on the fracture face in a widened “tear drop” pattern with the point being at the neck surface. The fatigue arrest marks can be viewed initiating from the point of a previous repair.

EXAMPLES

EXAMPLE 1Arrow highlights a circumferential crack located at the bottom end of the form. This location wasthe site of a previous repair where the journals were machined and welded up to the bottom end ofthe form.

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EXAMPLE 2Fracture face of the neck shown in Example 1. Arrows highlight numerous fatigue ratchet marks initiating from the surface.

EXAMPLE 3Longitudinal view of a neck that fractured at a previous weld repair. Large arrow highlights the location of fracture within the heat affected zone of the weld repair. Small arrow highlights a typical weld bead highlighted on the surface when etched with a 20% Nital reagent.

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EXAMPLE 4Longitudinal view of a neck that cracked at a previous weld repair. Large arrows highlight the location of the crack close to the transition zone of the weld repair. Small arrow highlights a typical weld bead highlighted on the surface when etched with a 20% Nital reagent.

MECHANISM

Rotating bending fatigue is the result of the applied rolling stresses exceeding the material strength of the neck during roll service. During rolling, force is applied to the roll necks to create the rolling forces necessary to reduce the incoming strip. Therefore, every point on the roll cycles between tensile and compressive states of stress. The surface points of the neck, being located the furthest from the centerline of the neck, are subjected to the highest tensile and compressive states of stress. This stress can concentrate at different locations along the roll neck surface at changes in diameter, previous repair or other surface indication is present. This is usually located in the form area where the rotating bending stresses are higher than any other location along the roll neck. If the rolling pressure applied to the necks exceeds the material’s tensile fatigue strength at the point of stress concentration, circumferential surface crack(s) will form. These crack(s) will then propagate radially in the transverse plane via fatigue mode until the strength of the surrounding material is reduced to such a degree that neck fracture occurs. Fatigue ratchet marks are created when multiple fatigue initiation sites exist. Fatiguing propagates radially in the transverse plane until the edges of the propagating cracks meet. The minute longitudinal difference in location is then compromised and a ratchet mark is formed.

Areas of previous repair are especially prone to rotating bending fatigue. Common repairs include welding, and under-cutting to remove mill damage. If areas like weld/parent metal interfaces and under-cuts are too close to highly stressed portions of the neck such as the form,the tensile fatigue strength of the material can easily be exceeded at these locations.

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PREVENTION

Neck breakage resulting from rotating bending fatigue can be prevented by the following:

• Increase the rolls material strength to resist crack initiation and propagation. The material strength can be increased by several means. If the cracks are formed near the body end face, special heat treating practices can be employed to raise the hardness of the neck past the body end face. The material strength can also be increased through alteration of the pre-hardening heat treatment of the roll.

• If possible, locate repairs such as weld interfaces and under-cuts away from the form or other highly stressed portion of the neck.

• Reduce the rolling pressures to reduce the applied bending stress.

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TYPE: INSTANTANEOUS - MATERIAL QUALITY(DEEP SEATED DEFECT “DSD”)

CHARACTERISTICS

Instantaneous neck breakage resulting from a deep seated defect typically occurs along the transverse plane. The fracture face can be characterized by fracture flow lines originating from a single interior point. Close examination of the fracture origin will not reveal any evidence of fatigue (fatigue arrest marks), indicating that cracking did not occur over a period of time, but occurred instantaneously.

EXAMPLES

EXAMPLE 1Fracture face of a neck that broke instantaneously as a result of a deep seated defect. Arrow highlights the fracture initiation site.

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MECHANISM

During solidification, it is possible for an irregularity to become “entrapped” within the ingot. This irregularity can be anything from refractory, slag, localized segregation, porosity etc. These irregularities can then act as stress concentration factors when the roll is put into service. When the localized stress induced during rolling at subsurface stress concentration sites exceeds the tensile strength of the material, crack initiation and propagation will occur instantaneously. The loads required to exceed the tensile strength of the material are large compared to those required to exceed the fatigue strength. Therefore, a mill related incident where the typical rolling pressure applied to the neck is exceeded is usually required for fracture to occur in an instantaneous manner from a deep seated defect. It is more likely that under typical rolling conditions, fatigue fracture would occur before instantaneous fracture.

PREVENTION

Instantaneous neck breakage resulting from a sub-surface stress concentration factor can be prevented by the following:


• Avoid mill related incidents that result in a large increase in the pressure applied to the roll necks.

• Increase the material strength of the neck through a change in the pre-hardening heat treatment.

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TYPE: INSTANTANEOUS - MILL OVERLOAD

CHARACTERISTICS

Instantaneous neck fracture resulting from a mill overload typically occurs along the transverse shear (45 degree) plane. The fracture can be characterized as initiating at a single surface point with flow lines running from the initiation site across the entire fracture face. The fracture face ofthe interior in typically ductile in appearance. Close examination of the fracture origin will notreveal any evidence of fatigue (fatigue arrest marks), indicating that the fracture did not occur overa period of time, but occurred instantaneously.

EXAMPLES

EXAMPLE 1Side view of an instantaneous neck fracture that occurred due to a mill overload.

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EXAMPLE 2Fracture face of the breakage shown in Example 1. Arrow highlights the surface initiation site. Note the ductile appearance of the fracture face.

EXAMPLE 3Close-up view of the initiation site for the instantaneous neck fracture that shown in Example 1. Arrow highlights the surface initiation site.

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MECHANISM

Instantaneous neck fracture typically occurs when a mill related incident dramatically increases the bending stresses applied to the roll necks. If the applied bending stress exceeds the material strength of the neck, instantaneous fracture will occur. Fracture initiates at a point on the roll neck surface where the bending stress is the highest (typically the form) and flows radially and longitudinally in the transverse shear plane.

PREVENTION:

Instantaneous neck breakage resulting from a mill overload can be prevented by avoiding mill incidents where the applied bending stress is dramatically increased.

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BODY BREAKAGE

V. BODY BREAKAGE

A. FATIGUE 95 - 97

B. INSTANTANEOUS



CATEGORY: BODY BREAKAGE

TYPE: FATIGUE(DEEP SEATED DEFECT “DSD”, FISH EYE)

CHARACTERISTICS

Sub-surface fatigue initiated body fracture can be identified by the presence of an oval fatigue pattern (fish eye) on the fracture face. The fatiguing can be seen initiating from a single point with fatigue arrest marks (beach marks) emanating in an oval pattern away from the initiation site. Body fracture associated with deep seated defects is catastrophic in nature. The roll may in some severe cases completely split longitudinally and/or break up into several large pieces.

EXAMPLES

EXAMPLE 1Fracture face of a roll body exhibiting fatigue initiation from a deep seated defect. Arrow highlights the fatigue initiation site.

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EXAMPLE 2Close-up of the fatigue initiation point shown in Example 1. Large arrow highlights the initiation point. Small arrow highlights typical fatigue arrest mark.

EXAMPLE 3Fracture face of a roll body exhibiting fatigue initiation from a deep seated defect. Arrow highlights the fatigue initiation site.

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MECHANISM

During solidification, it is possible for an irregularity to become “entrapped” within the ingot. This irregularity can be anything from refractory, slag, localized segregation, porosity etc. These irregularities can then act as stress concentration factors when the roll is put into service.

Body fracture resulting from sub-surface fatiguing occurs in two distinct stages:

Stage 1 - When the localized stress induced during rolling at sub-surface stress concentration factors (DSD’s) exceeds the fatigue strength but not the tensile strength of the material, fatigue cracks initiate and propagate away from the initiation site. The fatigue arrest marks propagate in all directions away from the irregularity and usually within a single plane of propagation.

Stage 2 - The strength of the surrounding material is reduced to such a degree that fracture occurs. This final stage of fracture is instantaneous in nature and can be noted by fibrous flow lines originating from the outer fatigue mark on the fracture face.

PREVENTION

Body fracture resulting from sub-surface fatiguing can be prevented by the following:


• Change from electric arc furnace vacuum degassed material to ESR (Electroslag Remelt) material especially for high productivity mills. The ESR process reduces the possibility for an irregularity to exist in the material after solidification.

• Ultrasonic inspection techniques using a straight beam transducer on every roll after completion of the grinding operation. If accurate inspection records on every roll are maintained, rolls with sub-surface indications which are initiating fatigue can be identified and can be removed from service before they fracture.

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TYPE: INSTANTANEOUS - MILL OVERLOAD

CHARACTERISTICS

Instantaneous roll body fracture resulting from a mill overload typically occurs along the transverse shear (45 degree) plane. The fracture can be characterized as initiating at a single surface point. The fracture face of the interior is typically ductile in appearance. Close examination of the fracture origin will not reveal any evidence of fatigue (fatigue arrest marks),indicating that the fracture did not occur over a period of time, but occurred instantaneously.

EXAMPLES

EXAMPLE 1Side view of a roll that fractured instantaneously. Note the transverse shear (45 degree) plane of fracture. Arrow highlights the surface initiation site.

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EXAMPLE 2Fracture face of the roll shown in Example 1.

MECHANISM

Instantaneous body breakage typically occurs when a mill related incident dramatically increases the stress applied to the roll body. If the applied stress exceeds the material strength of the roll body, instantaneous fracture will occur. Fracture initiates at a point on the roll surface where the stress is the highest and flows radially and longitudinally in the transverse shear plane.

PREVENTION

Instantaneous body breakage resulting from a mill overload can be prevented by controlling mill loads and operating conditions to avoid abnormal stress concentration an the roll body.

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TYPE: INSTANTANEOUS - MATERIAL QUALITY(DEEP SEATED DEFECT “DSD”)

CHARACTERISTICS

Instantaneous fracture of a roll body resulting from a deep seated defect typically occurs along the 180 degree longitudinal plane. The fracture face can be characterized by fracture flow lines originating from a single interior point. Close examination of the fracture origin will not reveal any evidence of fatigue (fatigue arrest marks), indicating that cracking did not occur over a period of time, but occurred instantaneously.

EXAMPLES

EXAMPLE 1Close-up view of the initiation point for an instantaneous body fracture that resulted from a deep seated defect. Arrow highlights the initiation site associated with a deep seated defect.

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MECHANISM

During solidification, it is possible for an irregularity to become “entrapped” within the ingot. This irregularity can be anything from refractory, slag, localized segregation, porosity etc. These irregularities can then act as stress concentration factors when the roll is put into service. When the localized stress induced during rolling at subsurface stress concentration sites exceeds the tensile strength of the material, crack initiation and propagation will occur instantaneously. The loads required to exceed the tensile strength of the material are large compared to those required to exceed the fatigue strength. Therefore, a mill related incident where the typical rolling pressure applied to the body is exceeded is usually required for fracture to occur in an instantaneous manner from a deep seated defect. It is more likely that under typical rolling conditions, fatigue fracture would occur before instantaneous fracture.

PREVENTION

Instantaneous body breakage resulting from a sub-surface stress concentration factor can be prevented by the following:

• Avoid mill related incidents that result in a large increase in the pressure applied to the roll bodies.


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ROLL INSPECTION

VI. ROLL INSPECTION 103

A. EDDY CURRENT INSPECTION 105 - 106

B. SURFACE WAVE ULTRASONIC INSPECTION 107 - 112

C. DYE PENETRANT INSPECTION 113 - 115

D. ETCH TESTING 117 - 119

E. MAGNAFLUX TESTING 120 - 121

F. HARDNESS TESTING 122 - 126

ROLL INSPECTION

Advances in nondestructive testing have improved the methods used by both the roll manufacturer and the roll shop to evaluate the quality of the roll material. The objective of a proper roll maintenance program is to detect the earliest stage of a roll problem and to prevent that roll from being returned to mill service unless corrective action is made. The general inspection methods that can be employed by a roll shop are eddy current, surface wave ultrasonic, dye penetrant, etch, magnaflux and hardness testing. With all of the inspection techniques available, the quickest, most accurate and reliable are the eddy current and ultrasonic inspection. When used together, all surface conditions that could be detrimental to the roll or the rolled product during mill service can be detected with 100% accuracy (Table 1). Dye penetrant,and etch testing have the benefit of being an inexpensive method for inspection, however, they are time consuming and not always 100% reliable, therefore, they should be used in conjunction with eddy current and ultrasonic inspection.

TABLE 1

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Inspection Method

Eddy UltrasonicCurrent Surface (2) Pitch Catch Straight Beam

Dual Probe

EffectiveRadial Depth (1) 0-.003" 0-.050" 0-6" .5" -Bore

Bruise/Soft Spot X

Surface Micro Cracks X(<.006")

Surface Macro Cracks X X(>.006")

Sub-Surface Indications X X

Residual Magnetism X

Work Hardening X

(1)Function of equipment parameters (Frequency crystal, type crystal, probe design)

(2) Circumferential Direction (3 scans required)Longitudinal Direction (2 scans required)

EDDY CURRENT INSPECTION

Eddy current testing is an inspection method for locating indications such as soft areas (bruises),wide cracks and magnetism and is performed after completion of the grinding operation. (Figure 1). While the roll is still in the grinder, a dual wire differential probe is brought close to theroll surface on one end of the roll body. With the roll rotating at a set speed, the probe is then slow-ly traversed across the entire length of the roll body. Synchronization of the traverse rate and rollRPM is designed to insure that every point on the roll surface is passed between the dual wires. Asthe probe traverses the roll body, eddy currents are induced on the roll surface between the wiresby the application of an alternating current. Instantaneous changes in either the electrical conductivity or the path length between the wires can be detected and are displayed on two separate channels named the Pinch/Bruise channel and the Crack/Spall channel respectively. The specific procedures for performing eddy current inspection are dependent on the eddy current equipment used and is provided by the manufacturer.

Changes in electrical conductivity are detected on the Pinch/Bruise channel and are the result of changes in hardness and microstructure between adjacent points on the roll surface. Areas of magnetism will also result in a continuous change in electrical conductivity between all adjacent points within the magnetized area. A gauss gauge (magnetic field indicator) can be used to confirm the presence of residual magnetism (>30 Gauss). Typical roll conditions that can yield changes in the electrical conductivity of the roll surface include but is not limited to: localized over tempering (bruise), localized work hardening, a roughly ground roll surface and inclusions where exogenous material is entrapped in the roll surface. These conditions are displayed on the Pinch/Bruise channel hard copy print out as individual spikes above the residual noise level or as large areas of grass that also exceed the residual noise level (Figure 2).

Changes in path length are detected on the Crack/Spall channel and are the result of surface cracks. As the probe passes over a crack, the generated current must travel down the crack wall and back up the other side in order to reach the opposite wire. This difference in path length is then displayed on Crack/Spall channel as individual spikes (Figure 2). Eddy current inspection is unable to detect cracks less than 0.006” wide and is only semi-accurate at detecting cracks wider than 0.006”. For accurate detection of all surface cracks, ultrasonic inspection should be used.

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106

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EDDY CURRENT INSPECTION

FIGURE 1Eddy current inspection being performed on a roll after completion of the grinding operation.

FIGURE 2Example of a typical eddy current inspection chart record.

Large arrows highlight typical spikes from a bruise on the roll surface.Small arrow highlights residual noise on the Pinch/Bruise channel.

Medium size arrows highlight cracks displayed on the Crack/Spall channel.

Threshold

BRUISE CHANNEL

Threshold

CRACK CHANNEL

SURFACE WAVE ULTRASONIC INSPECTION

Surface wave ultrasonic inspection is performed using a transducer attached to a 90° surface wedge and is the most accurate method for detection of surface cracks. Surface wave ultrasonic inspection transmits high frequency sound waves around the circumference to detect conditions that reflect or absorb the sound waves. All interfaces will reflect or scatter the waves to somedegree. This includes cracks, inclusions, grain boundaries and other discontinuities. Metal-to-airinterfaces (cracks) reflect most of the sound wave while metal-to-solid interfaces (inclusions) partially reflect the sound waves. The reflected sound wave is then returned to the transducer andis displayed on the testing screen as a spike. The following is a basic procedure for performing the surface ultrasonic test:

1.0 Ultrasonic Inspection Technique

Utilize the ultrasonic contact method based on the back reflection technique referenced in ASTM A388.

1.1 Ultrasonic Equipment

1.1.1 An ultrasonic pulse-echo type instrument shall be used to generate and display theultrasonic signals.

1.1.2 A contact search unit utilizing a piezoelectric material shall be used to transmit and receive the ultrasonic signals.

1.1.3 A coaxial cable with appropriate adapters shall be used to connect the ultrasonic instrument and the search unit.

1.1.4 A couplant shall be used to effectively transmit and receive the ultrasonic signals to and from the forged steel roll, typically oil, glycerin or water.

1.2 Ultrasonic Tests

1.2.1 Circumferential Surface Test

This test is designed to detect longitudinal surface and shallow sub-surface indications.

1.2.2 Longitudinal Surface Test

This test is designed to detect transverse surface and shallow subsurface indications.

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1.3 Circumferential Surface Test

1.3.1 Search Unit

Utilize a 2.25 MHz, 0.5” x 1.0” contact search unit with a 90° lucite wedge to generate the surface wave.

1.3.2 Prepare Roll

Place the roll on machine centers, stands or “V” blocks with the supports located away from the roll body. Clean the body with a solvent if necessary and dry with a cloth or rag.

1.3.3 Apply Couplant

Apply a thin, light band (≈ 1”) of couplant on top of the body along the full body length.

1.3.4 Setup, Display and Sensitivity

Setup on an area that has no relevant indications. Set the sweep line to represent a distance greater than 1/2 the circumference of the body with the initial pulse at the extreme left side of the screen. Place the search unit on the couplant and obtain a circumferential back reflection. Adjust the gain (dB) to obtain 1 full back reflection (100% amplitude). Adjust the sweep line to position the back reflection at the extreme right-hand side of the screen (Figure 1).

1.3.5 Conduct Test - First 180° Of The Rolls Circumference

Place the search unit at one end of the body and direct the surface wave circumferentially. Traverse the length of the body along the couplant at less than 8” per second while maintaining 1 full back reflection (Figures 2 and 3). Monitor the screen for 100% back reflection amplitude. Locate and mark the indications for further evaluation and report documentation.

1.3.6 Conduct Test - Second 180° Of The Rolls Circumference

Reverse the circumferential position of the search unit and repeat step 1.3.5 (Figure 2).

1.3.7 Conduct Test - Overlap Dead Zone

Wipe the couplant from the roll body and reapply couplant approximately 90°from the original location. Repeat steps 1.3.5 and 1.3.6.

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1.4 Longitudinal Surface Test - Roll Body

1.4.1 Search Unit

Same as 1.3.1.

1.4.2 Prepare Roll

Same as 1.3.2.

1.4.3 Apply Couplant

Apply a thin band (≈ 1”) of couplant around the circumference (360°) on the drive end of the roll body. Note: If the roll rotating mechanism is not available, apply on 1/2 of the circumference (180°), reposition the roll and repeat.

1.4.4 Setup, Display and Sensitivity

Same as 1.3.4 except set the sweep line to represent a distance greater than the body length. Place the search unit on the couplant and obtain a longitudinal back reflection (Figure 1).

1.4.5 Conduct Test - Drive End

Place the search unit at the drive end of the body and direct the surface wave longitudinally. Traverse the circumference along the couplant at less than 8” per second while maintaining 1 full back reflection. Monitor the screen for 100% back reflection amplitude and any indications greater than 10% amplitude. Locate and mark the indications for further evaluation and report documentation (Figure 4).

1.4.6 Conduct Test - Operator End

Wipe the couplant from the roll body and repeat steps 1.4.3, 1.4.4 and 1.4.5 on the operator end.

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FIGURE 1Ultrasonic testing unit screen display.

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INITIALPULSE

BACKREFLECTION

SWEEP LINE

AMPLITUDE


FIGURE 2Circumferential surface test

FIGURE 3Circumferential surface test being performed on a roll body.

111

1ST 180°

2ND 180°

SURFACE WAVE

SURFACE WAVE

REVERSESEARCH

UNIT

TRAVERSE BODY LENGTH

TRAVERSE BODY LENGTH

COUPLANT

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FIGURE 4Longitudinal surface test

112

COUPLANT

TRAVERSEBODY

DIAMETER

TRAVERSEBODY

DIAMETER

OPERATOR ENDSURFACE WAVE

DRIVE ENDSURFACE WAVE

COUPLANT

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DYE PENETRANT INSPECTION

Dye penetrant inspection can be performed at any time (after the grinding operation is most common) and is used to highlight cracks on the roll surface. A red colored dye penetrant is applied to the roll surface and the dye enters crack interfaces through capillary action. After a specific amount of time, the roll is wiped dry with clean, dry cloths. The dye will seep back out ofthe cracks through reverse capillary action. Developer is applied to the surface and the cracks arehighlighted as red lines on the white background. Dye penetrant testing can also be done using aflorescent dye. The cracks are then highlighted using a florescent black light instead of developer. Dye penetrant inspection is accurate at highlighting large, wide cracks, however, if thecrack is too narrow, the penetrant cannot seep into the crack and will not be highlighted whendeveloped. It is generally preferred to perform the test after locating surface indications with eddycurrent and ultrasonic inspection. Only the general area of the indications can then be tested ratherthan the entire roll body. The following is a simplified procedure for performing a dye penetranttest using the red colored dye technique.

• Identify the defect area using Ultrasonic and Eddy Current techniques, approximately within one quadrant. Position this area where it is convenient to work on.

• Wipe the quadrant clean with a rag to remove excess grease, dirt, etc.

• Take a rag and spray the Magnaflux cleaner/remover onto the rag (spray heavily), Use this damp rag to wipe the area again to remove residue and allow a minute to dry. Don’t spray Cleaner directly on the roll; it will yield substandard results!

• Spray the area to be tested with penetrant (red dye), holding the can about 8” away from the roll surface (Figure 1). It is OK to overlap and to spray heavily, but too muchprovides no benefit.

• Allow the penetrant to soak into the roll for at least 10 minutes (longer is better).

• Wipe the roll down with a clean dry rag to remove most of the penetrant. Change to a clean portion of the rag periodically to avoid smearing the dye (Figure 2).

• After the area is visibly clean, spray the cleaner/remover onto another clean rag. Use the moistened rag to remove any residual red dye. Periodically change to a clean portion of the rag as before.

• Spray the developer onto the area to be tested, holding the can about 8” to 10” away from the roll surface (Figure 3). Use a side to side motion to apply the developer evenly.

• As the developer begins to dry, red lines will appear to highlight any macro cracks that exist (Figure 4). If no lines appear, then no macro surface cracks exist in the tested area.

• Evaluate the size of the cracks and determine the appropriate salvage method (grinding, turning in a lathe, hand buffing, etc.).

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DYE PENETRATE INSPECTION

FIGURE 1Penetrate (red dye) being applied to the suspected area on the roll body.

FIGURE 2Removal of the Penetrate after a few minutes wait.

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DYE PENETRATE INSPECTION

FIGURE 3Developer being applied to the suspected area on the roll body.

FIGURE 4Close-up view of the results of a dye penetrant test where macro cracks were found.

Red lines highlights the location of the cracks.

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117

ETCH TESTING

Etch testing can be performed at any time (after the grinding operation is most common) and is used to highlight conditions on the roll surface where cracks or a change in hardness exists. When acid is applied to the roll surface, softer areas will “burn” or “darken” at a faster rate than harder areas leaving a differential etch appearance. Cracks are also highlighted during an etch test when reagent enters the crack interface through capillary action. Once the acid is cleaned from the surface, the residual acid within the crack seeps out and burns the area surrounding it. Etch testing is accurate at highlighting bruises and large, wide cracks, however, if the crack is too narrow, the acid cannot enter into the crack and it will not be highlighted. It is generally preferred to perform the test after locating surface indications with eddy current and ultrasonic inspection. Only the general area of the indications will then need to be tested rather than the entire roll body.The following is a simplified procedure for performing an etch test:

• Clean and dry roll surface to be etched by grinding off a few thousandths to insure the surface is free of any plating (chrome) and any superficial mill conditions.

• Swab the area to be etched with 20% Nital reagent (nitric acid and methanol) using cotton for at least 1.5 minutes (Figure 1). Do not let the area dry.

• Thoroughly rinse the Nital from the roll surface with methanol while at the same time using a different cotton swab to clean off the residue (Figure 2).

• Dry the etched surface using compressed air (Figure 3).

• Soft or hard areas will be highlighted as a difference in shade between the affected area and the surrounding metal. Soft areas will appear darker and hards areas will appear lighter (Figure 4).

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ETCH TESTING

FIGURE 120% Nital reagent being applied to an area on the roll body.

FIGURE 2Etched area being cleaned with cotton and methanol.

119

ETCH TESTING

FIGURE 3Etched area being dried using moisture free compressed air.

FIGURE 4Results of and etch test where a soft spot (bruise) was found.The bruise appears as the darker area on the etched surface.

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MAGNAFLUX TESTING

Magnaflux testing can be performed at any time and is mostly used to inspect for cracks on the rollnecks. Cracks in the neck portion of the roll are typically circumferential and are usually locatedin the form area. Magnaflux testing as described here cannot be performed on the roll body surface because the strength of the magnetic field used would induce magnetism into the surface which can be detrimental to the quality of the rolled product. The test is performed by inducing a magnetic field in the longitudinal direction with the area to be tested in the middle. A fine magnetic powder is then lightly blown over the area in-between the two magnetic poles. Any interface (crack) between the two poles acts to distort the magnetic field which attracts the fine magnetic particles, highlighting the crack. Magnaflux is accurate at highlighting large, wide cracks, however, if the crack is too narrow, fewer magnetic particles will be attracted to the interface and the crack and will not be highlighted. The following is a simplified procedure formagnaflux testing.

• Clean the portion of the roll neck to be tested using solvent to remove any oil, grease,dirt or build-up.

• Place both poles of the electromagnetic yokes in the longitudinal direction over the area of the neck to be tested (Figure 1).

• Activate the electromagnetic yokes to induce the magnetic field across the area to be tested.

• While the yokes are energized, take the blower bulb in one hand, and carefully and evenly, squeeze the bulb to apply the magnetic particles to the area under the yokes(Figure 1).

• Move the yokes circumferentially around the area of the neck being tested and continue to apply the magnetic powder.

• As the magnetic powder adheres to interfaces, the cracks are then highlighted as thin lines of magnetic particles.

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MAGNAFLUX TESTING

FIGURE 1Magnaflux test being performed on a shoulder radius.

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HARDNESS TESTING

Hardness testing should be performed before and after the grinding operation. It is used to determine the overall hardness of the roll as well as verifying the existence of localized hardnessdifferentials (bruises, work hardening, etc.). The typical methods of hardness testing include indentation (Rockwell - HRc and Vickers - HV) and rebound testing (Equotip - HLd and HLe,Shore - HFRSc and HSd) - see Figures 1 - 8. Rockwell testing is not generally used in the roll shopdue to the special surface requirement needed for accurate testing and the time involved to preparea test spot. The Equotip and Shore tests are the most commonly used hardness testing methods.The rebound tests indicate the hardness by dropping a small impact device or “hammer” onto theroll surface and measure the height (Shore) or speed (Equotip) of the rebound. The test can be performed on any clean surface and should be performed multiple times to obtain the average hardness of the area being tested.

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123

HARDNESS TESTING

FIGURE 1Rockwell test (HRc) being performed on a roll body.

FIGURE 2Vickers test (HV) being performed on a roll.

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124

HARDNESS TESTING

FIGURE 3Equotip hardness testing equipment including an Equotip Unit,

HLd hammer and HLe hammer.

FIGURE 4Equotip hardness test being performed on a roll body.

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125

HARDNESS TESTING

FIGURE 5Shore HSd hardness tester.

FIGURE 6Shore HSd hardness test being performed on a roll body.

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ROLL HANDLING AND STORAGE

VII. ROLL HANDLING AND STORAGE 127

126

HARDNESS TESTING

FIGURE 7Shore HFRSc hardness tester.

FIGURE 8Shore HFRSc hardness test being performed on a roll body.

ROLL INSPECTION ROLL

INSPE

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ROLL HANDLING AND STORAGE

Special care and attention must be used in the handling and storage of forged hardened steel rolls to avoid premature roll problems and to maintain a safe environment for employees. The following recommendations are given for this purpose:

A. Roll Handling

1. Avoid roll on roll contact by moving only one roll at a time.

2. Proper slings should be used in the appropriate lift area of the roll neck.

3. Roll bodies should not come into contact with any “hard” surface during transportation.

4. Welding of “attachments” to broken rolls to facilitate handling should never be attempted.

5. Rolls should never be handled with an electromagnet.

B. Roll Storage

1. Avoid roll contact by using a type of separator between the roll bodies (wood wedge, rubber strapping).

2. Roll journals, seal areas and body should be protected from corrosion (rust).

3. Avoid sudden temperature changes in the roll body by storing in a proper environment.

4. Insure that the storage area, racks and equipment are free of any residual magnetism.

5. Damaged rolls (adhering spalls) should be either enclosed or covered with aprotective blanket. As a minimum, the damaged area should be turned towards the floor. Acoustic emission testing can also be performed to help determinewhether or not the damaged roll has attained a state of equilibrium (no significantacoustic activity).

6. Rolls from the mill should not be ground until the roll body temperature approaches the ambient temperature in the roll shop. Sudden cooling to achievethis condition is not advisable.

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ROLL

HAND

LING A

ND ST

ORAG

E

ROLL NOMENCLATURE

VIII. ROLL NOMENCLATURE 129 - 130

ROLL NOMENCLATURE

129

ROLL

NOME

NCLAT

URE

130

ROLL

NOME

NCLAT

URE

REFERENCES

1. F. J. Barchfeld, “Investigation of Roll Marking Relationships,” Rolls 2000, 1996.

2. W. L. Roberts, “The Rolling Process Mechanical Engineering Explanation,”Rolls For The Metalworking Industry, ISS, 1990, pp. 40 - 41.

3. W. L. Roberts, Flat Processing of Steel, Marcel Dekker, 1988.

4. F. A. K’Isa, H. Erzurum and J. Gross, “Stress Concentration Curves For VariousNeck Configurations of Steel Mill Rolls,” AISE Yearbook, 1969, pp. 637 - 641.

SUGGESTED REFERENCES

1. J. Pinkowski, M. Andjelich, “Tandem Roll Mark Source Cause Determination andResulting Corrective Action,” AISE, 1995.

2. K. Tiitto, “Measuring Stress in Rolls By Magnetoelastic Method,”27th Mechanical Working and Steel Processing, 1985, pp. 273 - 279.

3. W. Tait, “An Automatic NDT Inspection System for Forged Steel Work Rolls,”22nd Mechanical Working and Steel Processing, 1980, pp. 375 - 387.

4. J. F. Morris, M. J. Sorby, “The Development and Application of the Sarclad RollscanRoll Inspection System,” 25th Mechanical Working and Steel Processing,1983,pp. 469 - 488.

5. K. A. Ridal, M. J. Sorby, “Further Developments in the Use of Sarclad RollscanTechnology for the Inspection of Rolling Mill Rolls,” 27th Mechanical Working and Steel Processing, 1985, pp. 243 - 247.

6. G. A. Ott, “The Application, Metallurgy, and Maintenance of High Hardness, Ultra DeepHardened Forged Steel Work Rolls,” 38th Mechanical Working and Steel Processing,1996, pp. 91 - 105.

7. D. D. Lang, D. P. Geier, “ New Mill Roll Inspection System For Firecrack Detection,”38th Mechanical Working and Steel Processing, 1996, pp. 15 - 27.

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FORGED HARDENED STEEL ROLLS

SERVICE PROBLEMS

CAUSES AND PREVENTION

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