Download - Chapter7 alloy steels 2 - Teaching & Learning Site · PDF fileSME 1613 : Materials Science 2007 Heat Treatment • Heated to austenite temperature prior to quenching • Tempered to

Alloy SteelsAlloy Steels

77

SME 1613 : Materials Science 2007

3

Low Alloy High Alloy

low carbon <0.25wt%C

med carbon 0.25-0.6wt%C

high carbon 0.6-1.4wt%C

Uses auto struc. sheet

bridges towers press. vessels

crank shafts bolts hammers blades

pistons gears wear applic.

wear applic.

drills saws dies

high T applic. turbines furnaces V. corros. resistant

Example 1010 4310 1040 4340 1095 4190 304

Additions noneCr,V

Ni, Monone

Cr, Ni

Monone

Cr, V,

Mo, WCr, Ni, Mo

plain HSLA plainheat

treatableplain tool

austentitic

stainlessName

Hardenability 0 + + ++ ++ +++ 0

TS - 0 + ++ + ++ 0EL + + 0 - - -- ++

increasing strength, cost, decreasing ductility

CLASSIFICATION OF STEELS


Introduction

Alloy steels differ from carbon steels in that they have compositions that extend beyond the limits set for carbon steels.

As a general guide, alloy steel will have:-

Manganese content >1.65%

Silicon content >0.5%

Copper content >0.6%

•Type of alloy steels:-

1. Tool steels

2. High speed steels

3. High strength low alloy steels (HSLA)

4. Maraging steels

• Used in manufacturing processes as well as for machining metals.

• Able to withstand high specific loads

• Stable at elevated temperatures (200oC)

• Composition:

high carbon: 0.6-1.4 %wt C

and others elements : chromium (Cr) ,vanadium (V), tungsten (W), and

molybdenum (Mo) to form carbide

•Six types of tool steels

1. High speed,

2. Hot work,

3. Cold work,

4. Shock resisting,

5. Special purpose and

6. Water hardening

Tool Steels


Effects of Alloying Elements on Tool Steel Properties

• Carbon (C): increases hardness slightly and wear resistance.

• Manganese (Mn): small amounts reduce brittleness and improve forgeability. Larger amounts of manganese improve hardenability, permit oil quenching, and reduce quenching deformation.

• Silicon (Si): Improves strength, toughness, and shock resistance.

• Tungsten (W): Improves hardness - used in high-speed tool steel.

• Vanadium (V): Refines carbide structure and improves forgeability, also improving hardness and wear resistance.

• Molybdenum (Mo): Improves deep hardening, toughness, and in larger amounts, "hot hardness". Used in high speed tool steel because it's cheaper than tungsten.

• Chromium (Cr): Improves hardenability, wear resistance and toughness.

• Nickel (Ni): Improves toughness and wear resistance to a lesser degree.


Heat Treatment

• Heated to austenite temperature prior to quenching

• Tempered to form martensite

• Never allow cooling to room temperature after quenching as this will lead to cracking.

• The tool steel must be transferred to the tempering furnace whilst still warm - about 50-80°C. During multiple tempering operations tool steels may be allowed to cool to room temperature between tempers.

• Residual stresses need to be removed prior to heat treatment and it is recommended that a stress relief heat treatment at 500-550°C be carried out allowing the tooling to cool to room temperature after stress relief


• The normalising: cycle involves heating slowly and carefully to the

normalising temperature for that particular steel, holding at

temperature sufficient to allow homogenisation to occur and then air

cooling to room temperature.

• The full annealing: process involves heating the steel slowly and

uniformly to a temperature above the upper critical transformation

point and holding until complete austenitisation and homogenisation

occurs. Cooling after heating is carefully controlled at a particular

rate as recommended by the steel manufacturer for the grade of tool

steel involved.

• Cooling at this specified cooling rate is continued down to 550°C

when the steel may be removed from the furnace and air cooled to

room temperature.

Normalizing & Annealing


Properties of tool steels & Applications

• wear resistant

• capable of holding sharp cutting edge

• very hard

Applications

• wood working tools

• concrete drills

• cutting tools

• drawing dies

• embossing dies and etc


TYPICAL APPLICATIONS

Woodworking tools1.1C, 0.15Cr, 0.2Ni, 0.1Mo. 015W and 0.1 V

W1

Cutlery and drawing dies

1.5C, 12Cr, 0.3Ni, 0.95Mo and 1.1 V (No tungsten)

D2

Punches and dies1.0C, 5.5Cr, 0.3Ni, 1.15Mo and 0.35 V (No tungsten)

A2

Drills, saws, lathe, and planer tools

0.85C, 3.75Cr, 0.3Ni, 8.7Mo 1.75W and 1.2 V

M1

ApplicationsCompositions AISI

NUMBER


High Speed Steel (HSS)

• Capable of cutting metal at a much higher rate (high speed) than

carbon tool steel

• Used to cut other metals/alloys

• continues to cut and retain its hardness even when the point of the tool

is heated to a low red temperature.

Composition

• Tungsten is the major alloying element

• Also combined with molybdenum, vanadium and cobalt in varying

amounts.



Mechanical Properties

1. High attainable hardness

• Minimum attainable hardness of High-Rupturing Capacity (HRC)

• Typical cutting tools may be HRC 64/68, depending on grade & application.

• High carbon, along with elements to promote strong secondary hardness (W, Mo), are common to all high speed steels for this purpose

2. High hardness at elevated temperatures (600oC)

• This involves both red hardness (the ability to stay hard at elevated temperature during cutting) and temper resistance (the ability to resist permanent softening over time due to high temperature exposure).

• The W and/or Mo contents of high speed steels promote these properties, and cobalt enhances it further when needed.



3. High wear resistance

• To promote edge retention during cutting.

• Constant abrasion wears away tool surfaces.

• The high volumes of wear-resistant carbides in high speed steel

microstructures aids in resisting this abrasion.

4. Sufficient impact toughness

• To handle interrupted cutting applications, to avoid chipping during

cutting, and to avoid breakage in fragile tools.

• High speed steels are notably tougher than carbide or ceramic

materials.

• CPM (Crucible Powder Metallurgy)-produced high speed steels offer

the ultimate in impact resistance for cutting tools.


Influence of alloying elements on the properties


Microstructure of HSS

Fine grain structure at 1000x showing uniform dispersal of

carbides. Sample was austenitized at 2240°F and triple

tempered at 1025°F to a hardness of HRC 70.


Turning, planning tools of all types, broaches and hobs,

taps, twist drills, economical high speed steel grade,

drills, reamers, milling tools, wood working tools, cold

work tools, tool bits

High Speed Steel: Applications