DISPERSION OF STRUCTURE

UDC 669.14.018

DISPERSION OF THE STRUCTURE OF STEEL 35Kh UP TO NANOLEVEL

WITH THE AIM OF CREATING A MATERIAL FOR PRESSURE VESSELS

Yu. N. Simonov,1 A. P. Nishta,1 S. S. Yugay,1 and A. S. Pertsev1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 7 12, November, 2010.

A comparative study of the structure and mechanical properties of sheet and tube preforms from steel 35Kh af-

ter heat refining, cold rolling, and wobble forming is performed. It is shown that wobble forming produces a

positive effect on formation of an -phase with homogeneous and dispersed cellular substructure in the whole

of the cross section of tube preforms, which ensures a favorable combination of strength, ductility, and impact

toughness. It is recommended to subject the tube preforms to subsequent annealing in order to raise the para-

meters of ductility and impact toughness without lowering the strength.

Key words: wobble forming, dispersion of structure, mechanical properties of tube preforms.

INTRODUCTION

Development and installation of advanced technologies

for increasing the output, reducing the consumption of mate-

rials, and simultaneously raising the service reliability of ma-

chine parts and structures remains an important task of me-

chanical engineering and materials science.

One such technology is deformation of a material by a

wobbling tool (we will call it wobble forming). According to

the idea of this method of pressure treatment, one of the tools

(the female die or the male die) performs a wobbling (preces-

sional) motion created by a special drive, while the other tool

performs translational motion created by a conventional hy-

draulic drive (Fig. 1). This method of treatment is especially

effective for making tube preforms and articles like pressure

vessels and hydraulic cylinders. Today articles of this type

are primarily produced with the help of mechanical treatment

(drilling) or by bending and welding of initial sheet preforms

followed by hardening heat treatment.

Wobble forming (WF) makes it possible to lower the

consumption of metal and to simplify the process of manu-

facturing of tube preforms on the whole. In addition, results

of studies performed in the last decades show that forming of

this type raises the entire set of characteristics of mechanical

properties and thus the reliability of the articles. Another

positive feature of this kind of deformation is its high loca-

lity, which makes it possible to use conventional equipment

for deforming materials with elevated and high strength

level; the same microvolume of the metal can be deformed

repeatedly, which produces a homogeneous structure and

substructure in the cross section of the metallic preform.

It cannot be excluded that due to the locality and the re-

peated nature of the deformation in wobble forming (or any

other treatment of the kind) the actual strain is substantially

Metal Science and Heat Treatment, Vol. 52, Nos. 11 12, March, 2011 (Russian Original Nos. 11 12, November December, 2010)

517

0026-0673/11/1112-0517 2011 Springer Science + Business Media, Inc.

1Perm State Engineering University, Perm, Russia (e-mail:

mto@pstu.ru).

P

3

2

1

Fig. 1. Scheme of deformation of a tube preform in a through wob-

bling facility: 1 ) precessional tool; 2 ) deformed tube preform;

3 ) arbor.

higher than the design one. Thus, the method of wobble

forming is an example of commercial process that imple-

ments to this or that degree the mode of severe plastic defor-

mation.

The aim of the given work was a comparative study of

the fine structure and properties of sheet and tube preforms

from steel 35Kh obtained by rolling and wobble forming and

development of a process for manufacturing a reliable mate-

rial for pressure vessels.

METHODS OF STUDY

We studied sheet and tube preforms from steel 35Kh.

The chemical composition of the preforms is presented in

Table 1.

The sheet and tube preforms were heat treated in a mode

consisting of hardening from 860C (40 min) in water and

subsequent tempering at 570C for 1 h with water cooling.

After the heat refinement the preforms were subjected to

cold plastic deformation. The sheet preforms were deformed

by rolling with 30 and 60% shrinkage; the tube preforms

were wobble formed with 42 and 63% shrinkage. Then the

cold-deformed preforms were annealed in a temperature

range of 300 650C with 1-h and 2-h holds.

The sheets and the tubes was then cut into preforms with

a thickness of 0.4 0.5 mm in an electrospark discharge ma-

chine and subjected to an electron microscopic study. Then

the preforms were thinned mechanically against sand papers

to a thickness of 0.05 0.10 mm and polished electrically

using anode dissolution in a phosphorus-chromium electro-

lyte (860 ml orthophosphoric acid + 100 g chromic anhy-

dride). The foils obtained were studied under an EM-12M

electron microscope at an accelerating voltage of 100 kV and

a JEM-200CX microscope at 160 kV. One structural state

was studied for at least 5 foils; the number of studied fields

for each foil was at least 10.

The images were computer processed and the longitudi-

nal and transverse sizes of fragments of the -phase (laths,

cells, or subgrains depending on the structural state) and of

the cementite carbides were measured. The number of mea-

surements in every case was no less than 500. We used for-

mulas of mathematical statistics for processing the metallo-

graphic images [1] and computed the mean longitudinal and

transverse sizes of the structural components, the variance,

the standard deviation, the confidence interval, and form fac-

tor of each component.

The characteristics of strength and ductility were deter-

mined in accordance with the GOST 149773 Standard for

short cylindrical specimens with initial diameter of 5 mm in

a 1231U-10A universal testing machine at a deformation rate

of 2 mmmin. The recorded stress-strain diagram was used

to compute the ultimate rupture strength r, the yield strength

0.2, and the elongation . The contraction was evaluated

in therms of the change in the diameter of the specimen. No

less than 4 5 specimens were tested after each variant of

treatment.

The impact toughness was computed using the results of

the tests in an MK-30 impact machine in accordance with the

requirements of GOST 945478 for specimens of types 3 and

17. The tests were performed at a temperature ranging from

+ 20 to 60C. Negative temperatures were obtained by

mixing liquid nitrogen with ethyl alcohol. The temperature

was controlled by a preliminarily calibrated copper-constan-

tan thermocouple connected to a digital millivoltmeter.

The static crack resistance was evaluated in terms of the

crack resistance limit Ic. Preparation of the specimens, con-

duction of the tests, and processing of the results were per-

formed according to GOST 25.50685 for flat specimens

with a thickness of 5 mm, a side notch, and a crack in the

mode of three-point bending using an INSTRON 5882 ma-

chine at a deformation rate of 0.5 mmmin. The relative

length of a crack determined in accordance with GOST

25.50685 as the ratio of the total length l of the notch and of

the fatigue crack grown from it to the width b of the speci-

men; in our tests = 0.45 0.55.

The test error in determination of the mechanical charac-

teristics did not exceed 5% in any case.

RESULTS AND DISCUSSION

Properties of Steel 35Kh in the Initial State

The structure obtained in the preforms after heat refine-

ment may be classified as disperse tempered sorbite repre-

sented by elongated subgrains (or cells) of ferrite inherited

from the martensitic structure and cementite carbides segre-

gated primarily over boundaries of ferrite subgrains. The dis-

location density inside the subgrains is not high.

In addition to the disperse tempered sorbite the structure

bears microvolumes that have experienced secondary recry-

stallization. These microvolumes have a size of about 5 m

518 Yu. N. Simonov et al.

TABLE 1. Chemical Composition of Steel 35Kh

Preform

Content of elements, wt.%

C Cr Mn Si Ni Cu S P

12-mm-thick sheet 0.34 0.96 0.64 0.23 0.03 0.05 0.009 0.019

Tube with wall thickness of 12 mm 0.35 0.92 0.63 0.27 0.04 0.07 0.014 0.022

and their volume fraction does not exceed 10 15%. Cemen-

tite carbides are encountered inside such ferrite polyhedrons.

Statistical analysis of the structure parameters of the tube

preforms (Table 2) shows that the sizes of ferrite subgrains

and cementite carbides in similarly oriented layers over the

thickness of a tube preform remain virtually invariable. At

the same time, comparison of structure parameters in an in-

ternal horizontal layer and in an internal vertical layer shows

that the transverse size of ferrite subgrains in the tangential

direction is somewhat larger than in the radial one (layers 3

and 6 in Table 2).

The results of the tensile and impact toughness tests

show that the tube preforms in such condition have the fol-

lowing mechanical properties: rupture strength r= 860 MPa,

yield strength 0.2

= 550 MPa, elongation = 12%, contrac-

tion = 65%, impact toughness KCU + 20 = 1.65 MJm2, and

KCU 60 = 1.28 MJm2. The sheet preforms in the initial

state have close parameters of structure and mechanical cha-

racteristics.

Structure and Properties of Preforms from Steel 35Kh

after Rolling and Wobble Forming

The structure of disperse tempered sorbite is preserved

on the whole both after rolling and after wobble forming.

The most substantial difference of this structure from the ini-

tial (high-tempered) one is the absence of microvolumes

with features of secondary recrystallization and the presence

of a developed cellular substructure in the -phase with ele-

vated dislocation density inside the cells.

The results of statistical analysis show that when the re-

duction ratio is increased, the sizes of the cells change incon-

siderably (Table 3). The shape factor of the cells remains vir-

tually constant, i.e., K1

= 2.5 2.8, which reflects consider-

able elongation of the cells.

In contrast to the cell substructure formed by rolling, the

cells formed due to wobble forming contain subgrains of a

polygonal type, which divide a cell additionally into several

segments (Fig. 2a ).

Statistical analysis shows that the sizes of the cell sub-

structure after wobble forming are noticeably smaller than

after rolling (Table 3) and the cells themselves are more

equiaxed; K1

= 1.6 1.9. In addition, the given cellular sub-

structure is very homogeneous both over the section of the

tube preform and in various directions; the mean transverse

size of the cells measured in various layers d = 160

170 nm, and the mean longitudinal size l = 270 310 nm

(wobble forming with = 42%, layers 1, 3, 6 in Table 3).

The sizes of the carbide particles after rolling and after

wobble forming differ inconsiderably. However, we should

note that the form factor of the particles after rolling

K2

= 3.0 3.7, whereas after wobble forming K2

1.85 2.0.

In other words, on the whole, wobble forming yields a more

dispersed and equiaxed substructure than rolling.

Mechanical properties of sheets and tube preforms after

rolling and after wobble forming are presented in Table 4. It

Dispersion of the Structure of Steel 35Kh with the Aim of Creating a Material for Pressure Vessels 519

TABLE 2. Parameters of Structure of a Tube Preform in the Initial State

Layer in the cross section

of the tube*d, nm S

d, nm l, nm S

l, nm a, nm S

a, nm c S

c, nm k

1, nm k

2, nm

1 (external surface) 500 30 300 2800 30 950 100 20 60 320 70 200 5.6 3.2

3 (middle layer over the

wall thickness)

580 40 300 2950 30 1030 100 7 50 200 10 80 5.1 2.0

6 (internal surface) 820 60 400 2700 40 1200 100 20 60 400 80 300 3.4 4.0

*The parameters of structure of layers 2, 4, 5 in the longitudinal section are not given in Table 2.

Notations: d and l are the mean transverse and longitudinal sizes of ferrite subgrains, respectively; a and c are the mean transverse and longitu-

dinal sizes of the carbides, respectively; d, l, a, and c are given with a confidence interval at confidence probability 0.95; k1

= ld is the shapefactor of ferrite subgrains; k

2= ca is the shape factor of carbides; S is the standard deviation of the structure parameters.

200 nm

200 nm

b

Fig. 2. Fine structure of steel 35Kh after wobble forming with 63%

reduction (a) and subsequent annealing at 570C for 1 h (b ).

can be seen that both rolling and wobble forming raise sub-

stantially the strength parameters of the preforms, whereas

the ductility decreases somewhat. It is important that the im-

pact toughness of the specimens tested at room and mini-

mum climatic temperatures both with a soft and a sharp con-

centrator is considerably higher after wobble forming than

after rolling.

The static crack resistance at room temperature after roll-

ing and after wobble forming changes by a dependence with

a maximum due to deformation with 30 40% reduction.

The ultimate crack resistance after wobble forming with

mean reduction = 42% is substantially higher than after

rolling with = 30%, i.e., Ic

20= 101.2 and 88.3 MPa m12,

respectively. After testing at minimum climatic temperature

( 60C) the change in the ultimate crack resistance after

rolling is the same as at room temperature. After wobble

forming with reduction of 42% the ultimate crack resistance

increases with respect to the initial value; when the degree of

reduction is increased to 63%, it remains invariable at a level

of 95 MPa m12 in contrast to rolled preforms.

Thus, wobble forming with high enough reduction does

not cause substantial decrease in the characteristics of impact

toughness and static crack resistance in contrast to rolling.

These differences are especially obvious in dynamic tests.

Structure and Properties of Deformed Preforms

after Recrystallization Annealing

Electron microscopic analysis shows that post-deforma-

tion annealing yields a subgrain structure in the preforms

both after rolling and after wobble forming (Fig. 2b ). The

boundaries of the subgrains are thin and well manifested.

The dislocation density inside subgrains is not high. Quite

frequently, subgrain boundaries exhibit a characteristic stri-

ated contrast, which is a feature of high-angle off-orientation

of these subgrains. After annealing, the sizes of the subgrains

are somewhat larger than after the corresponding mode of

520 Yu. N. Simonov et al.

TABLE 3. Parameters of the Structure of Preforms from Steel 35Kh after Rolling and Wobble Forming after Subsequent Annealing

Treatment mode Layer* d, nm l, nm a, nm c, nm k1, nm k

2, nm

Rolling, = 60% External 271 8 660 190 43 4 160 48 2.50 3.70

Rolling, = 60% + annealing at 570C, 1 h External 289 40 531 85 59 3 140 8 1.84 2.37

WF, = 42% 1 160 10 310 30 55 4 170 16 1.90 3.10

WF, = 42% 3 170 10 270 20 70 6 140 7 1.60 2.00

WF, = 42% 6 160 10 290 20 100 10 200 20 1.80 2.00

WF, = 63% 3 160 10 260 20 65 8 120 14 1.60 1.85

WF, = 63% + annealing at 570C, 1 h 3 210 20 350 30 86 4 160 10 1.63 1.86

WF, = 63% + annealing at 570C, 2 h 3 377 56 614 107 120 7 180 9 1.70 1.50

*In cross section of a tube (layers 1, 3, 6 ) or a sheet (external layer).

Notations: WF) wobble forming; the structural parameters are denoted as in Table 1.

TABLE 4. Characteristics of Mechanical Properties of Preforms from Steel 35Kh after Various Treatment Variants

Treatment r , MPa 0.2 , MPa , % , %KCU + 20,

MJm2KCT + 20,

MJm2KCU 60,

MJm2KCT 60,

MJm2Ic + 20,

MPa m12Ic 60,

MPa m12

Rolling

Initial state 785 560 15 62 1.45 0.48 0.98 0.37 83.2 79.2

= 30% 820 760 12 56 0.78 0.32 0.62 0.24 88.3 99.9

= 30% + annealing at 570C 630 540 17 56 0.96 0.38 0.71 95.1 88.0

= 60% 965 830 10 56 0.53 0.20 0.43 0.18 85.5 79.3

= 60% + annealing at 570C 670 585 19 53 0.85 0.33 0.77 0.28 85.0 80.7

Wobble forming

Initial state 800 550 15 65 1.65 0.67 1.28 0.48 80.4 77.2

= 42% 977 900 12 65 1.60 0.60 1.20 0.45 101.2 98.7

= 42% + annealing at 570C 825 700 18 68 2.0 0.57 1.77 106.4 98.7

= 63% 1003 952 11 60 1.48 0.50 1.15 0.40 88.4 95.4

= 63% + annealing at 570C 840 757 17 67 1.80 0.53 1.65 0.44 99.3 90.6

pressure treatment, which is a sign of migration of sub-

boundaries in the annealing process.

The main mass of the cementite-type carbides after an-

nealing, just like after cold plastic deformation, is arranged

over the boundaries of the components of the substructure.

The mean sizes of the carbides after annealing are somewhat

larger than directly after deformation. However, it should be

noted that after annealing very fine carbide segregations are

observable inside subgrains, which are most probably a con-

sequence of the occurrence of processes of strain aging. The

possibility of strain aging is confirmed indirectly by the re-

sults of statistical analysis of the sizes of carbide particles.

Comparison of the data presented in Tables 2 and 3 shows

that cold plastic deformation lowers the sizes of carbide par-

ticles, especially the longitudinal ones. This should be con-

nected primarily with mechanical separation of carbide parti-

cles in the process of cold deformation, which does not, how-

ever, exclude dissolution of individual fragments of particles

due to the action of the deformation.

As an example, we present in Fig. 3a dependences of the

characteristics of strength and ductility of specimens of tube

preforms from steel 35Kh on the temperatures of annealing

performed after wobble forming with 63% reduction.

According to the results of the tests, the behavior of the

strength and of the ductility does not depend on the degree of

reduction. For example, after annealing at 300C the strength

characteristics increase and the elongation decreases some-

what. It should only be noted that after the deformation with

= 63% the growth in the strength characteristics is higher

than due to reduction = 42%; the yield strength increases

by 70 80 MPa and 40 50 MPa, respectively. This fact is

another confirmation of the occurrence of strain aging pro-

cesses. When the annealing temperature is increased to

600C, the strength characteristics decrease and the ductility

characteristics increase; the strength of the specimens wob-

ble formed with reduction of 63% is preserved at a higher

level.

The dependence of the impact toughness of a tube pre-

form from steel 35Kh after wobble forming with reduction of

63% and subsequent annealing is presented in Fig. 3b. It can

be seen that independently of the test temperature the impact

toughness increases upon growth in the temperature from

300 to 600C. Even at the temperature of maximum manifes-

tation of the effect of strain aging (300C) its level is quite

high, i.e., KCU = 1.6 and 1.5 MJm2 at test temperatures of

+ 20 and 60C, respectively.

Comparative analysis of the characteristics of mechani-

cal properties of steel 35Kh (Table 5) shows that wobble

forming ensures a higher strength than rolling, as well as a

higher ductility, static crack resistance and, what is the most

Dispersion of the Structure of Steel 35Kh with the Aim of Creating a Material for Pressure Vessels 521

TABLE 5. Comparative Analysis of the Requirements of OST

3-168680 and the Properties of Rube Preforms after Various Treat-

ment Variants

Requirements of OST 3-168680 for tube-type articles

Strength category

0.2, MPa,

at least

, %,

at least

, %,

at least

KCU 20,

MJm2,at least

KS55 550 12 45 0.66

KS70 700 9 45 0.60

KS100 1000 5 40 0.44

Experimental properties of tube preforms

Treatment mode 0.2 , MPa , % , %KCU 20,

MJm2

Initial state 550 15 65 1.65

WF, = 42% 900 12 65 1.60

WF, = 63% 952 11 60 1.48

WF, = 42% + annealing

at 570C, 2 h 700 18 68 2.00

WF, = 63% + annealing

at 570C, 2 h 757 17 67 1.80

Note. Initial state: heat refining (hardening from 860C in water +

2-h tempering at 570C).

1000

900

800

700

600

200 300 400 500 600 700

0 200 400 600

40

30

20

80

70

60

, %

, %

r 0,2; , P

r

0.2

b

tan , C

tan , C

+ 20C

60C

1.9

1.7

1.5

1.3

1.1

KCU, J m 2

Fig. 3. Dependences of the characteristics of strength, ductility (a),

and impact toughness (b ) of tube preforms from steel 35Kh on the

temperature of post-deformation annealing ( = 1 h). Initial state:

wobble forming with 63% reduction: a) test temperature is 20C;

b ) 20 and 60C (the numbers at the curves).

substantial, a higher impact toughness including the case of

minimum climatic temperature.

Comparison of the requirements of the OST 3-168680

specification for tube articles and the characteristics of the

mechanical properties of tube preforms after various variants

of treatment (Table 5) shows that the tube preform subjected

to heat refining meets only the KS55 strength category. The

tube subjected to wobble forming with 43% reduction and

subsequent annealing at 300C has a strength level meeting

the KS100 category. The tube wobble formed with 63% re-

duction and then annealed at 570C has a strength level

meeting category KS700 and possesses an almost doubled

ductility margin and tripled margin of the impact toughness.

CONCLUSIONS

1. Wobble forming yields a more dispersed and equiaxed

substructure than rolling. The size of ferrite grains after roll-

ing is 270 290 nm; that after wobble forming is 160

170 nm.

2. Post-deformation annealing at 570C results in forma-

tion of a homogeneous subgrain structure with primarily

high-angle off-orientations. The size of the subgrains after

wobble forming with 63% reduction and subsequent 1-h an-

nealing at 570C amounts to 210 nm.

3. Wobble forming produces a higher level of strength

than rolling at a higher ductility, static crack resistance,

and, what is the most important, at a substantially higher im-

pact toughness including that at the minimum climatic

temperature.

4. Comparison of the requirements of the OST 3-168680

specification for tube-type articles with the mechanical pro-

perties of tube preforms after various treatment variants has

shown that wobble forming yields a reliable material for fab-

ricating pressure vessels.

REFERENCES

1. S. A. Saltykov, Stereometric Metallography [in Russian], Metal-

lurgiya, Moscow (1976).

522 Yu. N. Simonov et al.

AbstractKey wordsINTRODUCTIONMETHODS OF STUDYRESULTS AND DISCUSSIONProperties of Steel 35Kh in the Initial StateStructure and Properties of Preforms from Steel 35Kh after Rolling and Wobble FormingStructure and Properties of Deformed Preforms after Recrystallization Annealing

CONCLUSIONSREFERENCES

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 150 /GrayImageDepth -1 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputCondition () /PDFXRegistryName (http://www.color.org?) /PDFXTrapped /False

/SyntheticBoldness 1.000000 /Description >>> setdistillerparams> setpagedevice