562-567

THERMOCHEMICAL TREATMENT

UDC 669.14.018.298:621.785.062

STRUCTURE AND PROPERTIES OF CARBURIZED COATINGS

WITH REVERTED AUSTENITE ON LOW-CARBON MARTENSITIC STEELS

A. S. Ivanov,1 S. A. Kokovyakina,1 and A. S. Pertsev1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 51 56, November, 2010.

The process of creation and subsequent hardening of a gradient carburized layer in low-carbon martensitic

steel 17Kh2G2NMFTB is studied. It is shown that the structure and properties of the carburized layer can be

optimized due to formation of reverted austenite hardened by quenching from the intercritical temperature

range.

Key words: gradient layer, reverted austenite, lath martensite, low-carbon martensitic steels

(LCMS).

INTRODUCTION

Low-carbon martensitic steels (LCMS) possess a number

of unique features distinguishing them from other structural

steels traditionally used in machine building [1 5]. Rational

alloying and low content of carbon in LCMS promote eleva-

tion of the stability of supercooled austenite and yield a mar-

tensite structure in large cross sections after comparatively

slow cooling in air.

In order to widen the range of application of LCMS to

operation under conditions of contact friction it is necessary

to develop processes of their surface hardening. LCMS of

grades 07Kh3GNMYuA, 10Kh3GNM, and 08Kh2G2F ope-

rating under contact friction (gears, turbodrill parts) are sub-

jected to surface hardening by carburizing and nitriding.

However, grades 12Kh2G2NMFT, 15Kh2G2NMFB, and

17Kh2G2NMFTB having the highest strength are not used in

a surface-hardened state [7 10].

In the present work we had an aim to study the formation

of a surface layer in the process of carburizing of low-carbon

martensitic steel 17Kh2G2NMFTB, to determine the struc-

ture and properties of the layer, and to estimate the possibi-

lity of its hardening by optimizing the modes of subsequent

heat treatment at preserved structure and properties of the

matrix of the metal.

METHODS OF STUDY

We studied low-carbon martensitic steel 17Kh2G2NMFTB

and (for comparison) 12Kh2G2NMFT. The chemical com-

positions, the hardness, and the structures of the steels in the

initial state are presented in Tables 1 and 2.

Gas carburizing of the steels was performed in a Ts-75

shaft furnace at 900 910C for 4 h at benzene feeding rate

of 80 100 dropsmin. Subsequent heat treatment was per-

formed in a SNOL-type laboratory furnace. The carburized

specimens were placed into a container with waste carburizer

in order to avoid decarburization.

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

562

0026-0673/11/1112-0562 2011 Springer Science + Business Media, Inc.

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

[email protected]).

TABLE 1. Chemical Compositions of Steels

Steels

Content of elements, wt.%

C Si Mn Cr Ni Mo Nb V Ti S, at most P, at most

17Kh2G2NMFTB 0.17 0.37 2.07 2.42 1.50 0.39 0.17 0.12 0.22 0.011 0.015

12Kh2G2NMFT 0.13 0.24 2.24 2.39 1.38 0.45 0.10 0.04 0.119 0.004

The microstructure of the steels was studied using an

OLYMPUS GX-51 microscope at magnification of 100

1000 and a Video Test-Master. Structure 4-0 software for

image analysis. The hardness was measured by the Rockwell

method in accordance with GOST 901359; the microhard-

ness was measured in accordance with GOST 945076 using

a PMT-3 microhardness meter at a load of 0.5 and 0.2 N.

X-ray diffraction analysis was performed with the help

of a DRON-3 x-ray diffractometer in iron K

radiation; the

results were recorded using a DRON System for Automa-

tion of x-ray Diffractometers.

The content of retained austenite (in %) was determined

using the ratio of the integral intensities of line (311) of aus-

tenite and line (211) of martensite, i.e.,

Aret

= 100[1.273(I(211)

I(311)

) + 1], (1)

where I(311)

and I(311)

are the intensities of the lines of re-

tained austenite and martensite, respectively.

The microstresses and the sizes of subgrains in austenite

and martensite were computed by the method of approxima-

tion from x-ray lines (110) (211) of martensite and (111)

(311) of austenite. The approximating functions were [6]

y = 1(1 + x 2 ) and y = 1(1 + x 2 )2. (2)

The standards were powders of technically pure iron and

copper.

The process of decomposition of retained austenite was

studied and the critical points of the carburized layers were

determined using a Chevenard dilatometer on specimens

3.5 mm in diameter subjected to through carburizing. The

standard was a piros alloy. Layer-by-layer chemical analy-

sis was performed using an ARL-31.100 emission quantum

meter.

RESULTS AND DISCUSSION

The microstructure of the carburized layer of steels

17Kh2G2NMFTB and 12Kh2G2NMFT is presented in

Fig. 1. The microhardness and the content of retained auste-

nite are presented in Fig. 2a and b.

The structure of the carburized layer on steel

17Kh2G2NMFTB may be divided conventionally into three

zones. In the first (surface) layer martensite and retained aus-

tenite are accompanied at a distance of up to h = 0.10 mm by

a high amount of carbides. In the second (austenite) zone the

content of austenite at a distance h 0.20 0.30 mm from

the surface attains 77% and the microhardness decreases

abruptly to 460 HV (Fig. 2a and b ). At h 0.40 0.45 mm

the content of retained austenite decreases to 40% due to the

decrease in the carbon concentration in the layer. This causes

formation of a third (martensite) zone, where coarse acicular

martensite gradually transforms into lath martensite of the

matrix. The microhardness in this zone increases to 620 HV

in steel 17Kh2G2NMFTB and to 800 HV in steel

12Kh2G2NMFT. As the distance from the surface increases

and the carbon concentration decreases, the microhardness

decreases uniformly and attains 400 HV, which corresponds

to the microhardness of the matrix of the steel. The hardness

of the carburized layer is 58 HRC.

A distinctive feature of the microstructure of the carbu-

rized layer of steel 12Kh2G2NMFT is a high content of re-

tained austenite (up to 85%). The distribution of the micro-

hardness and of the amount of retained austenite after carbu-

rizing of steels 17Kh2G2NMFTB and 12Kh2G2NMFT has

the same nature (Fig. 2a and b ). The hardness of the carbu-

rized layer of steel 12Kh2G2NMFT is only 40 HRC due to

the higher content of retained austenite in it as compared to

steel 17Kh2G2NMFTB.

In order to decrease the content of retained austenite and

to optimize the structure and properties of the carburized

Structure and Properties of Carburized Coatings with Reverted Austenite 563

TABLE 2. Structure and Hardness of the Steels in the Initial Condi-

tion

Steel Initial condition StructureHRC

hardness

17Kh2G2NMFTB Hot-rolled bar Lath martensite 37

12Kh2G2NMFT Hot-rolled sheet,

6 mm

Lath martensite 39

100 m b

Fig. 1. Microstructure of carburized layer on steels 17Kh2G2NMFTB

(a) and 12Kh2G2NMFT (b ).

layer, the carburizing was followed by hardening of the steels

from temperatures of the austenitic and two-phase ( + )

ranges with oil and air cooling.

Steels 17Kh2G2NMFTB and 12Kh2G2NMFT were

hardened from 810C (0.5 h) in oil and in air by the regime

worked out at the Inkar Company for steel 12Kh2N4A (the

hold time is corrected with allowance for the charge).

After oil cooling of steel 12Kh2G2NMFT the surface

zone of the carburized layer acquires high-carbon martensite,

preserves a carbide zone, and the content of the austenite de-

creases. Formation of a martensite structure and decrease in

the content of retained austenite cause growth in the hardness

of the carburized layer to 62 HRC. In the case of air cooling

of the steel a carbide zone and a zone of fragmented austenite

are preserved in the structure in addition to the formed

martensite.

Oil cooling and air cooling of steel 17Kh2G2NMFTB

lower considerably the content of retained austenite as com-

pared to the initial carburized state (to 49 and 47%, respec-

tively, Fig. 2c ).

The distribution of the microhardness over the thickness

of the layer for steels 17Kh2G2NMFTB and 12Kh2G2NMFT

after oil and air hardening is the same (Fig. 2d ) and does not

change in the cause of the hardening. The fall in the micro-

hardness in the austenite zone is preserved but becomes

lower. The highest microhardness is observed over the whole

of the thickness of the layer after oil cooling.

It should be noted that the content of austenite in the

austenitic zone of the carburized layer on steel 12Kh2G2NMFT

is considerably higher and the microhardness is lower than in

steel 17Kh2G2NMFTB.

In has been shown in [11, 12] that steel 12Kh2G2NMFT

promises much after hardening from the intercritical tempe-

rature range (ITR). Hardening from a temperature exceeding

Ac1

but lower than Ac3

yields a homogeneous structure of

lath martensite. The close levels of mechanical properties af-

ter complete and incomplete hardening make it possible to

lower the temperature of heating for hardening, which should

reduce the level of internal stresses in the metal.

We estimated the possibility to use incomplete hardening

for steel 17Kh2G2NMFTB in a carburized state. According

to the data of dilatometric analysis of the carburized layer on

steels 12Kh2G2NMFT and 17Kh2G2NMFTB (we studied

specimens subjected to through carburizing), A1

= 710

720C and A3

= 790 800C. After hardening of carburized

specimens of steel 12Kh2G2NMFT from 740 and 770C

both in air and in oil, the structure and the properties of the

carburized layer remained virtually unchanged. Therefore,

we chose the temperature of 750C for hardening the studied

steels from the ITR.

The structure of the carburized layer on steel

17Kh2G2NMFTB after hardening from the ITR is repre-

sented by carbide-martensite, austenite-martensite, and mar-

564 A. S. Ivanov et al.

100

90

80

70

60

50

40

30

20

10

80

70

60

50

40

30

20

10

00 0.05 0.15 0.25 0.35 0.45 0.55 0.05 0.15 0.25 0.35 0.45Matrix Matrix

17Kh2G2NMFTB 17Kh2G2NMFTB

17Kh2G2NMFTB17Kh2G2NMFTB

12Kh2G2NMFT 17Kh2G2NMFTB

17Kh2G2NMFTB

12Kh2G2NMFT

12Kh2G2NMFT

12Kh2G2NMFT

c

b d

h, mm h, mm

h, mm h, mm

1200

1000

800

600

400

200

1200

1000

800

600

400

200

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

HV, kgf mm 2 HV, kgf mm 2

Aret , % Aret , %

Fig. 2. Distribution of retained austenite and of microhardness over the thickness of surface layer (h is the dis-

tance from the surface) in steels 17Kh2G2NMFTB and 12Kh2G2NMFT: a, b ) after carburizing; c, d ) after car-

burizing and hardening from 810C in oil (, ) and in air ().

tensite zones arranged from the surface to the core of the

specimen, respectively.

After oil cooling of steel 17Kh2G2NMFTB the hardness

of the carburized layer is 61 HRC, which is comparable with

the hardness due to hardening from 810C; the core has a

hardness of 32 HRC. After air cooling the hardness of the

layer is somewhat lower, i.e., 54 HRC; that of the core is

29 HRC. After oil and air cooling from 750C the content of

the retained austenite is lower than after hardening from

810C (compare Figs. 3 and 2c). Its maximum content

(35 36%) is detected at h 0.15 0.25 mm.

The distribution of the microhardness after hardening

from 750 and 810C differs (compare Figs. 3 and 2d ). After

hardening from 750C the dip of the microhardness in the

zone of retained austenite disappears; the microhardness de-

creases uniformly from about 1100 HV on the surface of the

layer to 300 HV in the matrix. After air cooling the micro-

hardness in the zone of retained austenite at the place of the

dip increases to about 90 HV. It should be noted that in steel

12Kh2G2NMFT similar properties are obtained after treat-

ment in more complex mode, namely, (1 ) carburizing, high

tempering at 620C. hardening from 740 and 770C with oil

or air cooling, respectively, and (2 ) carburizing, hardening

from 740C with 2-h heating at 620C, and oil or air cooling.

The variation of the parameters of the subgrain struc-

ture of the austenite in the carburized layer of steel

17Kh2G2NMFTB after hardening from the ITR (750C)

presents special interest. The sizes of the blocks and the

microstresses in the austenite differ substantially after air

cooling and after oil cooling (Fig. 4).

After air cooling the block sizes are much smaller and

the level of the microstresses is higher than after oil cooling.

This may be connected with the fact that the reverted austen-

ite (the retained austenite not decomposed and newly formed

during heating to the ITR temperature) undergoes precipita-

tion hardening in air, i.e., segregation of dispersed carbides

that harden the austenite and increase in the stresses.

Figure 5 presents the curves of variation of microstresses

in austenite over the thickness of the layer after hardening

from 750 and 810C in oil and in air. After hardening from

750C in air the level of the microstresses is considerably


40

35

30

25

20

15

10

5

0

0.06 0.16 0.26 0.36 0.46 Matrix

b

h, mm

h, mm

1200

1000

800

600

400

200

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2

HV, kgf mm 2

Aret , %

1

1

2

2

Fig. 3. Distribution of retained austenite (a) and microhardness (b )

over the thickness of surface layer (h is the distance from the sur-

face) in steel 17Kh2G2NMFTB after carburizing and hardening

from 750C in air (1 ) and in oil (2 ).

b

h, mm

h, mm

0 0.1 0.2 0.3 0.4 0.5

0 0.1 0.2 0.3 0.4 0.5

1

1

2

2

Q, m

m , P

0,25

0,20

0,15

0,10

0,05

400

350

300

250

200

150

100

50

Fig. 4. Sizes of blocks (Q ) (a) and microstresses (m

) (b ) in the

austenitic zone of surface layer on steel 17Kh2G2NMFTB after car-

burizing and hardening from 750C in air (1 ) and in oil (2 ).

h, mm

0 0.1 0.2 0.3 0.4 0.5

1

4

2

3

m , P

500

400

300

200

100

Fig. 5. Microstresses in austenite in steel 17Kh2G2NMFTB after

carburizing and hardening by different regimes: 1 ) from 750C in oil;

2 ) from 819C in oil; 3 ) from 750C in air; 4 ) from 810C in air.

higher than after hardening in oil; in the austenite region

(h 0.20 mm) it even exceeds the level of the microstresses

attained after hardening from 810C. The high compressive

stresses in the surface layer promote increase in the fatigue

resistance and stabilize the austenite.

Precipitation nature of the hardening of reverted auste-

nite is mentioned in [13 15]. Target measurements of the

microhardness of austenite have shown that its hardness is

higher than the integral hardness of the austenite-martensite

structure in the same region of the layer (Fig. 6).

Thus, austenite hardens upon comparatively slow cool-

ing in air and is fragmented forming blocks of a smaller size.

Summarizing the facts mentioned above we may suggest

the following scheme of formation of reverted austenite in

the carburized layer of steel 17Kh2G2NMFTB due to heat-

ing in the ITR and subsequent cooling at different rates.

At 750C the carburized layer preserves (acquires) re-

verted austenite, the behavior of which in the processes of

cooling in air and in oil differs substantially. It could be ex-

pected that in oil cooling the stresses in the layer should be

higher than in comparatively slow air cooling. However,

such behavior is observed in martensite, but in austenite the

microstresses after air cooling are considerably higher than

after oil cooling.

It seems that this is explainable by the fact that carbon is

redistributed during air cooling in reverted austenite and

ultrafine carbides of a cementite type are segregated there.

This causes substantial precipitation hardening of the re-

verted austenite. Simultaneously, there occurs redistribution

of dislocations and formation of nanosize subgrains.

In the case of oil cooling these processes are suppressed,

and the level of the microstresses in the austenite is consider-

ably lower and the block sizes are higher than in air cooling.

This is conformed by the results of a comparison of the

microhardness of the layer after hardening from 750C in air

and in oil (see Fig. 3). In the surface carbide-martensite zone

(h 100 m), where the content of austenite is compara-

tively low, the hardness of the layer hardened in oil is higher

than that after cooling in air. In the austenitic zone

(h = 100 200 m), where we have detected a dip in the

hardness after carburizing and after carburizing followed by

hardening from 810C, the hardness after air hardening from

750C increases abruptly and remains higher than after air

cooling over the entire layer up to h = 550 m.

Precipitation hardening of austenite accompanied by re-

arrangement of its dislocation structure is similar to the pro-

cess of cell formation. As a result, the blocks are refined

markedly (see Fig. 4). Locking of dislocations in walls

causes growth in the strength and hardness of the austenite. It

can be assumed that fixation of the dislocation structure of

the austenite and increase in its stability are promoted by

segregation of dispersed carbides over boundaries of

subgrains described in works [16, 17].

Variation of the parameters of the crystal structure of

austenite over the thickness of carburized layer after carbu-

rizing and after carburizing followed by hardening from 810

and 750C in oil and in air is presented in Fig. 7. In all the

cases the decrease in the content of carbon in the layer results

in decrease in the lattice parameter of the austenite. The

value of the lattice parameter differs somewhat depending on

the heating temperature and on the rate of the hardening

cooling. In the process of hardening from 810C retained

austenite with the same lattice parameter is preserved upon

cooling in oil and in air. After cooling from 750C, the lattice

parameter of the austenite is less after cooling in air than af-

ter cooling in oil. It is obvious that this is a consequence of

the earlier described process of segregation of dispersed car-

bides and carbon depletion of the reverted austenite due to

the comparatively slow air cooling.

Thus, the reverted austenite differs from the retained aus-

tenite in carburized layer of steel 17Kh2G2NMFTB by the

566 A. S. Ivanov et al.

h, mm

1200

1000

800

6000 0.05 0.10 0.15 0.20 0.25 0.30 0.35

HV, kgf mm 2

1

2

Fig. 6. Distribution of microhardness over the thickness of surface

layer (h is the distance from the surface) of steel 17Kh2G2NMFTB

after carburizing and hardening from 650C in air: 1 ) hardness of

austenite (target measurement); 2 ) hardness of austenite and mar-

tensite.

h,

h,

0 0.1 0.2 0.3 0.4 0.5 0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

1

1

4

2

5

3

a, nm

a, nm

0.362

0.361

0.360

0.359

0.362

0.361

0.360

0.359

b

Fig. 7. Variation of the lattice parameter of austenite over the thick-

ness of hardened layer (h is the distance from the surface) of steel

17Kh2G2NMFTB after carburizing and hardening from 750C (a)

and from 810C (b ): 1 ) after carburizing; 2, 3 ) after carburizing

and hardening from 750C in oil and in air respectively; 4, 5 ) after

carburizing and hardening from 810C in oil and in air respectively.

fact that its precipitation hardening and stabilization occur as

a result of segregation of ultrafine carbides and formation of

a cellular structure with nanometric (25 30 nm) sizes of

subgrains and an elevated level of microstresses.

CONCLUSIONS

1. The complex alloying system of steel 17Kh2G2NMFTB

promotes formation of stable retained austenite in carburized

layer.

2. Retained austenite in carburized layer is distributed by

a law with an extremum; its maximum content (80 85%)

occurs at a distance h = 0.2 0.3 mm from the surface.

3. It is possible to form reverted austenite with a high

carbon content and unusually high hardness in carburized

layer of steel 17Kh2G2NMFTB by hardening from the

intercritical temperature range with cooling in air.

4. The most probable cause of the hardening of the aus-

tenite is segregation of ultrafine carbides of a cementite type.

5. Strengthening and stabilization of the reverted auste-

nite is also promoted by rearrangement of its dislocation

structure, formation of nanosize subgrains, and elevation of

the level of microstresses.

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AbstractKey wordsINTRODUCTIONMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES

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