550-557

MARAGING AND NICKEL-MOLYBDENUM TRIP-STEELS

UDC 669.15-194.55:539.4.015

ELEVATION OF RELIABILITY CHARACTERISTICS OF MARAGING STEEL

03N18K9M5T BY CREATING A NANOTRIPLEX-TYPE STRUCTURE

V. P. Vylezhnev,1 S. A. Kokovyakina,1 Yu. N. Simonov,1 and A. A. Sukhikh2

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 39 47, November, 2010.

Characteristics of mechanical strength of maraging steel N18K9M5T after various variants of heat treatment

are studied. The treatment mode providing a structure with retained austenite and two types of reverted austen-

ite is determined. An optimum structure ensuring enough strength at high impact toughness and static and cy-

clic crack resistances is chosen.

Key words: maraging steel, retained and reverted austenite, strength, crack resistance.

INTRODUCTION

Maraging steels (MAS) are promising structural materi-

als possessing high resistance to fracture. The high properties

of MAS are primarily a result of an inhomogeneous struc-

ture. The scale of the inhomogeneity may differ from a

microlevel (boundaries of former austenite grains or mar-

tensite lath packets) to a submicrolevel and even to a nano-

level (lath boundaries and particles of intermetallic harden-

ing phases respectively).

The structural strength of MAS can be raised by creating

additional inhomogeneities at the submicro- and nanolevels,

for example, by forming a martensite-austenite structure

[1 3]. It is common practice to form what is known as re-

verted austenite rev

, which is obtained by heating a steel

with initially martensitic structure in the intercritical tempe-

rature range (ITR) [4 6]. Reverted austenite possesses high

stability, which is associated with the elevated concentration

of -stabilizers and the presence of particles of hardening

phases. Formation of 20 40% reverted austenite in the

structure promotes increase in the impact toughness, cold re-

sistance, fatigue resistance, and cyclic crack resistance, but

the strength level is not maximum.

The proportion of rev

in steel 03N18K9M5T can be va-

ried by changing the temperature of heating in the inter-

critical range and the duration of the hold. As a rule, MAS

are heated to 575 625C (the lower half of the intercritical

range) in order to obtain a noticeable amount of rev

. In-

crease in the hold time from 15 to 60 min at 575, 600, and

625C yields 10 35, 15 55, and 40 60% rev

, respec-

tively [5].

In order to form retained austenite ret

in the structure of

MAS the latter has to be cooled from the single-phase -re-

gion. However, hardening with conventional heating and

long-term hold does not yield a noticeable amount of ret

.

Formation of ret

in steel 03Kh18K9M5T is possible only

when the concentration inhomogeneity obtained as a result

of a preliminary thermal operation, for example, hardening

from the intercritical temperature range, is not removed by

subsequent hardening from the austenitic range.

In follows from the results of [5, 7] that in order to obtain

retit is expedient to resort to rapid heating with subsequent

short hold in the austenitic range, the duration of which does

not exceed 10 min. The data presented in [5, 7] show that in

heating to 800C the proportion of ret

decreases from 45 to

20% upon prolonging of the hold from 3 to 10 min. In heat-

ing to 840C the amount of ret

decreases from 35 to 10%; at

840C it falls from 20% to almost 0% upon the same change

in the duration of the hold.

The most interesting result of preliminary study [5] is the

fact that in the presence of ret

in the structure of MAS the

critical temperature A1

decreases substantially and the ap-

pearance of new rev

may be expected starting with

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

550

0026-0673/11/1112-0550 2011 Springer Science + Business Media, Inc.

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

[email protected]).2

Institute for Applied Mechanics of the Ural Branch of the Russian

Academy of Sciences, Izhevsk, Russia.

440 450C. In MAS without ret

the temperature of the ap-

pearance of the first portions of rev

is close to 520 540C.

Therefore, aging at a temperature corresponding to maxi-

mum hardening of MAS (490 500C) simultaneously with

formation of particles of a hardening phase in the martensite

leads to formation of about 15 20% new rev

.

The joint presence of two types of austenite in MAS (in

this case the sizes of the austenite inclusions approach the

nanometer range) should elevate substantially the properties

of the MAS, the reliability in the first turn.

The aim of the present work was to study the effect of dif-

ferent types of austenite on the mechanical properties of steel

03N18K9M5T under static, dynamic, and cyclic loading.

METHODS OF STUDY

We studied maraging steel 03Kh18K9M5T melted in an

induction furnace from pure blend materials. The chemical

composition of the steel was as follows (in wt.%): 0.009 C,

18.3 Ni, 8.9 Co, 5.1 Mo, 0.68 Ti, 0.06 Al, 0.03 Si, 0.03 Mn,

0.004 S, and 0.007 P.

The phase composition of the steel was determined with

the help of x-ray diffraction analysis by comparing the inte-

gral intensities of the x-ray interference lines from the - and

-phases (radiation from the Co anode). Micrographs were

taken from the surfaces of etched laps and fracture surfaces.

The structure was studied by the method of transmission

electron microscopy using MMA-3, M-125, and

JEM-200 CX electron microscopes; the structure of the frac-

tures was studied using a REM-100U scanning electron mi-

croscope.

Uniaxial tensile tests and computation of the strength and

ductility characteristics were performed in accordance with

the requirements of GOST 149784. The tests were carried

out in an R-5 testing machine for five-fold cylindrical speci-

mens 5 mm in diameter and 25 mm long. The impact tough-

ness was determined according to GOST 945478 for speci-

mens with a U-notch (size type 3 ).

The tests for cyclic crack resistance under harmonic

loading were performed in accordance with RD 50-34582

[8] in a rigid-load machine at an almost trivial cycle

(R = 0.05) and a frequency of 14 Hz. The specimens were

prepared as for eccentric tension and had a size of 62.5

60 10 mm. Crack growth was controlled using a binocular

microscope with scale division of the ocular equal to

0.05 mm. The results of the tests were used to plot diagrams

of cyclic crack resistance (DCCR) in the form of a depen-

dence log V = f (log K ), where V is the rate of growth of the

crack and K is the amplitude of the stress intensity factor.

Specimens of type 1 of (GOST 945478) were subjected

to fatigue tests under cyclic impact loading using a KPU-2

impact testing machine at a frequency of 10.4 sec 1 and a

cycle asymmetry factor R = 0. The maximum cyclic stresses

were max

= 1650 and 2600 MPa. Diagrams of fracture of the

specimens were recorded by the method of [9]. We deter-

mined the total number of cycles before failure Ntot

and the

periods of nucleation of a fatigue crack N1

and of its propa-

gation N2

(Ntot

= N1

+ N2

) and evaluated the rate of growth

of the crack. Period N1

was evaluated as the number of cy-

cles from the start of the test to formation of a fatigue crack

with a depth of 0.1 mm. The rate of growth of the fatigue

crack in any studied segment of the fracture diagram of a

specimen was determined as a derivative V = dldV. The un-known dependence l = f (N ), where l is the length of the

crack and N is the current number of cycles in the stage of

crack propagation (N = 0 at l = 0.1 mm), was approximated

by an exponential function l = c exp (bN ), where c and b are

the sought-for factors. Whence V = bc exp (bN ) or V = bl.

RESULTS AND DISCUSSION

Structure of Steel 03N18K9M5T with Different Types

of Austenite

Reverted Austenite. This type of austenite is formed both

inside crystals of the -phase in the form of differently ori-

ented rods (Fig. 1) and over boundaries of martensite laths

and packets in the form of fragments of extended crystals.

After aging at 550C for 1 h the structure of the metal bears

about 23% rev

. The width of its crystals is 30 110 nm and

the length is 110 550 nm. After 1-h aging at 600C the con-

tent of rev

increases to 53%, and the maximum width of its

crystals grows to 150 nm. Dislocations and intermetallic par-

ticles of acicular (Ni3Ti) and equiaxed (Fe2Mo) shapes are

observable in the crystals of rev

. The rev

phase forms in a

crystallographically ordered manner [10 12]. This mecha-

nism of transformation promotes inheriting of the dis-

location structure of the -phase, whereas the enrichment of

the austenite crystals with nickel is one of the causes of its

stabilization.

Heating of the steel preliminarily aged at 600C for 1 h

to 820C with a hold of from 12 to 2 min and subsequent

rapid cooling to room temperature yields from 10 to 60%

ret. The retained austenite is preserved in the former vo-

lumes of reverted austenite. At the same content of -phase

the shape and sizes of the crystals of ret

are close to those of

the crystals of rev

that have emerged during the hold in the

intercritical range (Fig. 2).

After a short high-temperature hold (2 min) the structure

preserves equiaxed intermetallics, most probably of a Fe2Mo

phase. They are located in crystals of both -phase and

-phase. In the crystals of ret

the dislocation structure that

has appeared as a result of the reverse transformation

is preserved partially.

Final aging by a conventional regime (490C, 3 h) gives

rise to new kind of rev

in the ( + ret

)-structure (Fig. 2). It

forms by epitaxial growth on ret

[10, 13] at a temperature

lower than that of the formation of the old rev

. This is ex-

Elevation of Reliability Characteristics of Maraging Steel 03N18K9M5T 551

plainable by the fact that the microvolumes of the -phase

lying close to ret

have an elevated content of nickel due to

incomplete homogenization of the high-temperature -phase.

The epitaxial nature of its nucleation is also a substantial

factor.

Crystals of the newly formed rev

inherit particles of the

hardening phase and, partially, the dislocation structure of

the martensite, which is typical for reverse martensitic trans-

formation [14]. The reverted austenite formed at a tempera-

ture of 490C will be called reverted austenite II to differ-

entiate it from reverted austenite I formed at 550C and

higher temperatures. Thus, we may speak of three types of

austenite, i.e., retained austenite, and two kinds of reverted

austenite (I and II ).

Figure 3 presents a diagram throwing light on formation

of different types of austenite in steel 03N18K9M5T. The

structure formed after the final aging (Fig. 3d ) represents a

nanotriplex.

Strength of Steel 03N18K9M5T with Different Types

of Austenite

We have already mentioned that the content of ret

can be

controlled by varying the hold of the metal in the austenitic

range. The total content of austenite increases in subsequent

aging due to formation of revII

(Fig. 4).

Evaluation of the strength of steel 03N18K9M4T in

hardened condition at 0, 20, and 40% ret

in its structure has

shown that in all the three cases r

1100 MPa and 0.2

1000 MPa. This gives us grounds to assume that the strength

characteristics of retained austenite and of unaged martensite

are close to each other. The characteristics of ret

obtained in

our study are close to those presented in [4].

552 V. P. Vylezhnev et al.

0.3 m 0.3 m

brevII

ret

Fig. 2. Structure of steel 03N18K9M5T against light (a) and dark (b )

backgrounds.

revI

revII

retret

-phase

-phase

with reduced

content of Ni

-phase

with elevated

content of Ni

c

b

d

Fig. 3. Diagrams of formation of different types of austenite in steel

03N18K9M5T: a) reverted austenite I due to hardening from the

intercritical temperature range; b ) austenite formed due to rapid

heating to 820C with a short hold (with reduced and elevated con-

tent of Ni, respectively); c) retained austenite after rapid hardening

from 820C; d ) reverted austenite II in the process of aging at

490C.

0.3 m

0.3 m

0.1 m

b

c

revI

Ni Ti3Fe Mo2

Fig. 1. Reverted austenite in steel 03N18K9M5T after hardening

from a temperature of the lower half of the intercritical range

(600C): light (a) and dark (b ) backgrounds in (200)g

reflex; re-

verted austenite with intermetalides (indicated with arrows) inhe-

rited from martensite (c).

It follows from the data of Fig. 5 that as a result of aging

the strength of the specimens containing ret

is lower than

that of the control specimens (without ret

), which seems to

be natural because aging process in martensite are known to

be connected with formation of nanoparticles of a hardening

Ni3Ti phase.

Starting with the aging temperatures of 430 460C the

total content of austenite in the structure of the steel in-

creases due to formation of rev

, but its strength does not de-

crease. This indicates that the reverted austenite (revII

) pos-

sesses higher strength than ret

. Additional proofs of this can

be obtained if we analyze the dependences of 0.2

and r

on

the total content of austenite for the specimens aged at 490C

(Fig. 6). Using the additivity rule we may estimate the yield

strength of the austenite under the assumption that the yield

strength of martensite in the steel with ( + ) structure is

equal to 0.2

of the steel with purely martensitic structure. As

a result, we will see that the yield strength of the austenite

(ret

+ rev

) decreases upon growth in its content at simulta-

neous decrease in the fraction of rev

. Since the fractions of

retand

revare known, we may use extrapolation and find as-

sessed values of their yield strength, i.e., about 1000 and

2000 MPa, respectively. Undoubtedly, these values are rank

ones, but we may assume that the characteristics of the

strength of rev

are considerably higher than those of ret

and

are close to the strength of aged martensite. Such determina-

tion of the strength of reverted austenite is applicable only to

the case when the aging is carried out at 490C. For the tem-

peratures below 490C such estimation is hard to perform

due to the low amount of the formed rev

. After aging at a

temperature exceeding 500C (in the intercritical tempera-

ture range) this is impossible, because the strength of the

martensite aged at these temperatures is unknown. However,

it is probable that at a temperature exceeding 500C the

strength of rev

is close to that of aged martensite, because the

sizes and the type of the hardening phases are important fac-

tors in this case too.

It can be assumed in this connection that the softening

observed at aging temperatures exceeding 500C is caused

not by the content of austenite but rather by the processes of

coagulation of the hardening phases and decrease in the den-

sity of crystal structure defects.

We may expect that the yield strength of retained austen-

ite is determined by phase hardening, fineness of the crystals

of ret

, and partial preservation of the intermetallics. The

considerable increase in the strength of reverted austenite is

most probably caused by particles of the hardening phase in-

herited by the austenite from aged martensite; such particles

are detectable by an electron microscope study. Particles of

hardening phase are seen especially well when the austenite

forms at 550C and higher temperatures. The arrangement of

the particles of the hardening phase in the - and -phases

differs inconsiderably (see Fig. 2). It should be noted that at


50

40

30

20

10

0400 430 460 490 520 550 580

2

1

revII

revII

rettot

, %

tag ,

tot ret rev= + II

Without

aging

Fig. 4. Dependence of the content of austenite in steel 03N18K9M5T

with an initial martensite (1 ) and martensite-austenite (2 ) structure

on the aging temperature.

2200

2000

1800

1600

1400

1200

1000

800

Without

aging

2

41

3

tag ,

370 400 430 460 490 520 550 580 610

r 0.2; , P

Fig. 5. Effect of aging temperature on the rupture strength (1, 3 )

and yield strength (2, 4 ) of steel 03N18K9M5T with an initial

martensite (1, 2 ) and martensite-austenite (3, 4 ) structure.

2200

2000

1800

1600

1400

r 0.2; , P

r

0.2

0 10 20 30 40 50 60

tot , %

Fig. 6. Rupture strength r

and yield strength 0.2

as a function of

the total content of austenite in steel 03N18K9M5T.

aging temperature of 550C we can detect oval particles,

most probably of a Fe2Mo phase.

Fracture Resistance of Steel 03N18K9M5T

upon Single Loading

The characteristics of impact toughness and static crack

resistance of the steel, as well as the strength characteristics,

are determined not only by the total content of the -phase

but also by the relative fraction of this or that austenite.

In the whole of the studied range of aging temperatures

the impact toughness of the control specimens (without ret

)

is 1.5 2 times lower than that of the specimens containing

24% ret

before aging (Fig. 7). The effect of ret

on the resis-

tance to crack propagation (parameter Js) is still inconsider-

able; at 24% ret

the value of Js

is 3 times higher than in the

specimens without ret

.

The impact toughness of the steel after tempering at

490C increases virtually linearly from 0.5 to 1.0 MJm2

upon growth in the content of retained austenite from 0

to 30%.

It should be noted that after aging at 490C the impact

toughness of the metal with retained austenite does not de-

crease despite the growth in the strength, whereas in the steel

without ret

is decreases considerably. At higher aging tem-

peratures the impact toughness of the metal without ret

does

not increase despite the appearance of reverted austenite revI

in the structure and the considerable softening (see Fig. 5)

and increases only when the softening develops intensely (at

tag

> 550C). For this reason we may state that revI

does not

possess high toughness.

The impact toughness of the steel containing ret

starts to

grow at lower aging temperatures. It follows from Fig. 4 that

at such aging modes reverted austenite revII

forms in the

metal. It cannot be excluded that the appearance of one more

phase with properties differing from those of the phases al-

ready present in the structure increases the energy spent on

the fracture additionally. The revII

phase may also have a

higher toughness (as compared to revI

). The reverted austen-

ite formed at aging temperatures of 490C and below this

value inherits highly dispersed particles of the hardening

Ni3Ti phase responsible for the high strength of martensite in

steel 03N18K9M5T. The reverted austenite formed at aging

temperature of 500C and higher temperatures inherits

coarser particles of Ni3Ti with less dispersity, as well as par-

ticles of Fe2Mo appearing at such aging temperatures. It is

possible that the difference in the natures of these particles

(their dispersity and kind) is responsible for the differences

in the levels of fracture resistance.

The presence of different types of austenite in the struc-

ture of steel 03N18K9M5T affects substantially the tempera-

ture dependence of its impact toughness (Fig. 8a ). The im-

pact toughness of the control specimens (without austenite)

aged at 490C and of the specimens containing about 20%

revIdecreases progressively upon decrease in the test tem-

perature from 40C. This is connected with the fact that

high-strength steels typically fracture by a low-energy duc-

tile mechanism.

The impact toughness of the steel with retained and re-

verted (type II) austenite varies over a curve with a maxi-

mum at ttest

from + 20 to 40C (Fig. 8a ). This should be

connected primarily with the fact that decrease in the tempe-

rature is accompanied by increase in the resistance to plastic

strain; while the fracture remains ductile, the fracture resis-

tance increases.


1

2

3

4

tag ,

370 400 430 460 490 520 550 580 610

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.16

0.14

0.12

0.10

0.08

0.06

0.04

KCU, J m 2 Jc2, J m

Fig. 7. Effect of aging temperature on impact toughness KCU (1, 2 )

and fracture toughness Js

(3, 4 ) of steel 03N18K9M5T with an ini-

tial martensite (1, 3 ) and martensite-austenite (2, 4 ) structure.

1

2

2

3

3

4

4

ttest ,

ttest ,

1.0

0.8

0.6

0.4

0.2

0

KCU, J m 2

b

200 100 0 100 200 300

200 100 0 100 200 300

100

80

60

40

20

0

n

, %

Fig. 8. Effect of test temperature on the impact toughness (a) and

on the degree of transformation of austenite n

(b ) of steel

03N18K9M5T with different structures: 1 ) martensite + revI

(40%);

2 ) reverted austenite (40%); 3 ) martensite + ret

(35%); 4 ) marten-

site + ret

(20%) + revII

(20%).

In order to explain the results obtained we estimated the

degree of the transformation of austenite in cooling up to the

temperature of liquid nitrogen and in the process of testing

for impact toughness. We established that ret

and rev

in steel

03N18K9M5T have high thermal stability, because the con-

tent of the austenite does not decrease in cooling up to the

temperature of liquid nitrogen.

At the same time, the stability of retained and reverted

austenite under the action of plastic deformation differs

(Fig. 8b ).

Reverted austenite does not undergo transformation even

when the tests for impact toughness are performed at the

temperature of liquid nitrogen. Retained austenite begins to

transform already at ttest

= + 100C; at the temperature of li-

quid nitrogen the degree of the transformation of retained

austenite is 100%.

In the metal with both types of austenite (% rev

% ret

)

subjected to impact testing in liquid nitrogen the degree of

the transformation is about 60%. Therefore, we may state

that the strain-induced transformation in steel

03N18K9M5T concerns only the retained austenite, whereas

the reverted austenite fractures without transformation. We

may name the following causes of increase in the impact

toughness of the steel with retained austenite:

load-induced transformation of retained austenite into

unaged austenite with high toughness;

spending of energy for the transformation;

appearance of a phase with high specific volume in the

transformation.

In order to study the micromechanisms of fracture of

steel 03N18K9M5T with different types of austenite we per-

formed an electron analysis of fracture surfaces, some of the

results of which are presented in Fig. 9. We studied the struc-

ture of the central parts of fracture surfaces of impact speci-

mens tested at + 250, + 20, and 196C. On the fracture sur-

faces of specimens with martensite structure fractured at

250C we observed only dimples. The depth of the dimples

was low. This reflects the relatively low energy capacity of

the ductile fracture of such specimens. Lowering of the test

temperature to room values gives rise to quasi-cleavage fac-

ets on the fracture surfaces in addition to the dimples. After

testing in liquid nitrogen quasi-cleavage facets become a

dominant component on the fracture surfaces of specimens

with martensite structure.

On fractures of steel 03N18K9M5T containing retained

austenite, which have formed at ttest

= + 250C, the micro-

scopic relief is represented by dimples. However, we should

note that in addition to small shallow dimples typical for

fracture surfaces of specimens with purely martensite struc-

ture, we observed in this case large deep cone-shaped dim-

ples 10 15 m in size, which determine the high level of

impact toughness of the steel bearing ret

.

When ttest

is decreased to + 20C, the ductile fracture pat-

tern is preserved on the whole. We should mention only the

appearance of individual facets surrounded by high ridges of

microplastic strain. Inside these facets we observe a deve-

loped regular infrastructure.

Testing at the temperature of liquid nitrogen makes the

facets surrounded by high ridges of microplastic strain,

which often form a closed net, the dominant component of

the texture. Judging by the distance between the ridges,

which is equal to 20 40 m, we may assume that they form

over boundaries of former austenite grains. Inside the facets

we observe an exceptionally developed infrastructure repre-

sented by regularly arranged and often almost parallel steps

or ridges (Fig. 9a ). Since in this case the degree of the

strain-induced transformation attains 100%, we may

infer that this infrastructure has formed as a result of a

strain-induced transformation of retained austenite into

martensite (Fig. 8b ).

In the case when the steel contains both types of austen-

ite, a dimple texture dominates on fracture surfaces for any

test temperature. At ttest

= + 20C a microrelief in the form of

protrusions or tongues can be observed on the surfaces of

cone dimples. At the temperature of liquid nitrogen cone

dimples are more rare but there appear cleavage plateaus

with a similar microrelief over slip planes (Fig. 9b ). It can-

not be excluded that this microrelief is nothing else but a

trace of a strain-induced transformation of ret

under the con-

ditions of predominantly ductile fracture.

Fracture Resistance of Steel 03N18K9M5T

under Cyclic Loading

The results of the tests for cyclic crack resistance pre-

sented in Fig. 10 show that the presence of this or that type of

austenite in the structure of the high-strength steel raises the

resistance to fatigue crack growth, which is especially no-

ticeable when the rate of crack growth is determined at a

fixed value of the amplitude of the stress intensity factor K.

This effect is especially noticeable in the near-threshold re-

gion. The highest level of threshold values of K (Kth

) is

observed when the structure of the steel contains reverted

austenite; as the value of K increases, the differences in the


10 m 10 m b

Fig. 9. Fracture surfaces after testing impact specimens of steel

03N18K9M5T with different structures in liquid nitrogen: a) mar-

tensite + 23% ret

; b ) martensite + 23% ret

+ 23% revII

.

cyclic crack resistance of steel 03N18K9M5T with structures

of martensite and martensite with different types of austenite

become progressively lower. Computation of exponent n in

the Paris equation shows that in the steel studied the value of

n changes little and ranges within 2.3 2.7. Therefore, we

may state that within the Paris segment of the diagrams of

cyclic crack resistance one and the same fatigue micro-

mechanism of crack growth is implemented independently of

the structural state of the steel.

Tables 1 and 2 present the results of the determination of

parameters characterizing the fracture resistance under im-

pact cyclic loading of the steel containing austenite and bear-

ing no austenite. In the metal aged for maximum strength the

rate of crack growth is lower than in the metal in an overaged

condition. Reverted austenite lowers the rate of crack growth

under impact cyclic loading but decreases the number of cy-

cles before failure. The best results on the impact crack resis-

tance (the largest number of cycles before failure) and the

lowest rate of crack growth) have been observed after aging

at 430C (the structure of the steel contains only retained

austenite).

CONCLUSIONS

1. The structure of maraging steel 03N18K9M5T can

consist of martensite and retained and reverted (two types)

austenite depending on the heat treatment.

2. Retained austenite has a yield strength close to that of

unaged martensite (0.2

1000 MPa), whereas the yield

strength of reverted austenite is close to that of aged (for

maximum strength) martensite (0.2

2000 MPa). The high

difference in the strength levels of the different types of aus-

tenite should be associated with the fact that the reverted aus-


TABLE 1. Ultimate Rupture Strength, Impact Toughness, and Resistance to Low-Cycle Impact Fatigue of Steel

03N18K9M5T after Aging

Initial

structure*tag

, C r, MPa

KCU,

MJm2

N1

, cycle N2

, cycle Ntot

, cycle N1

, cycle N2

, cycle Ntot

, cycle

max= 1650 MPa

max= 2600 MPa

490 2100 0.45 15,628 7073 22,701 2971 1100 4071

550 1750 0.51 12,916 7590 20,516 2422 1308 3730

+ ret

430 1800 0.93 16,494 7633 24,127 2650 1495 4145

+ ret

490 19,000 0.90 14,283 8280 22,563 2364 1668 4032

*Before aging.

Notations: tag

) aging temperature; N1, N

2) periods of nucleation and propagation of fatigue crack, respectively; N

tot) total

number of cycles before failure, Ntot

= N1

+ N2.

TABLE 2. Empirical Dependences of the Crack Growth Rate in

Steel 03N18K9M5T with Different Structural States under Cyclic

Impact Loading

Initial

structuretag

, CStructure

after aging

Rate of crack growth dldN,mcycle, at a stress

max, MPa

1650 2600

490 Martensite + in-

termetallics

5.68 10 7

l 27.1 10 7

l

550 + 22%

rev5.17 10

7l 23.3 10

7l

+ ret

430 + 24%

ret5.46 10

7l 26.4 10

7l

+ ret

490 + 24%

ret+

15% rev

5.05 10 7

l 23.6 10 7

l

Note. The rate of crack growth was determined at a crack length of

up to l = 3.2 mm.

1

2 3

4

v, m cycle

K, P m1 2

3 4 5 6 7 8 9 10 20 30 40 50 60

108

6

4

2

7

6

4

2

1086

4

2

8

1086

4

2

9

10 10

Fig. 10. Diagrams of cyclic crack resistance (v = dldt is the rate

of crack growth, K is amplitude of the stress intensity factor) of

steel 03N18K9M5T with different structures: 1 ) martensite (without

austenite); 2 ) martensite + 30% ret

; 3 ) martensite + 23% ret

+

23% revII

; 4 ) martensite + 40% revI

.

tenite inherits the dispersed intermetallics formed as a result

of aging of the martensite.

3. The presence of about equal fractions of both types of

austenite (about 20% ret

and about 20% revII

) in steel

03N18K9M5T at a high strength level (0.2

= 1800

1900 MPa) provides high impact toughness (KCU = 1.0

1.2 MJm2 ) and static (Js

= 160 180 MPa m12 ) and cy-

clic crack resistance.

4. The high level of cold resistance of steel 03N18K9M5T,

the structure of which bears both types of austenite, is pro-

vided by a strain-induced transformation of retained austen-

ite, while the reverted austenite remains untransformed.

5. Under cyclic loading the reverted and retained austen-

ite lowers the crack growth rate in the initial stage of crack

development (at low values of the amplitude of the stress in-

tensity factor).

6. The properties of the reverted austenite are largely de-

termined by the temperature of its formation. The reverted

austenite formed at 490C and lower temperatures (revII

) has

a higher set of mechanical properties than the reverted aus-

tenite formed above 500C (revI

).

REFERENCES

1. M. D. Perkas and V. M. Kardonskii, High-Strength Maraging

Steels [in Russian], Metallurgiya, Moscow (1970), 224 p.

2. S. S. Ryzhak, L. N. Belyakov, Ya. M. Potak, et al., Some regu-

lar features of phase transformations in steel 000N18K9M5T,

Metalloved. Term. Obrab. Met., No. 2, 55 60 (1972).

3. S. D. Antolovich, A. Saxena, and G. R. Chanani, Increased

fracture toughness in a 300 grade maraging steel as a result of

thermal cycling, Metall. Trans., 5(3), 623 632 (1974).

4. S. B. Nizhnik, S. P. Doroshenko, and G. I. Usikova, Effect of

hardening temperature on the development of transfor-

mation and mechanical properties of maraging steel, Fiz. Met.

Metalloved., 56(2), 327 33 (1983).

5. V. P. Vylezhnev, A. A. Sukhikh, V. G. Bragin, et al., Formation

of austenite and its structure in maraging steel N18K9M5T, in:

Problems of Mechanics and Physical Metallurgy [in Russian],

IPM UrO RAN, Izhevsk (1994), pp. 118 133.

6. V. M. Shchastlitsev, I. L. Barmina, and I. L. Yakovleva, Forma-

tion and stability of reverted austenite in low-carbon nickel-mo-

lybdenum steels, Fiz. Met. Metalloved., 53(2), 316 322

(1983).

7. V. P. Vylezhnev, Yu. N. Simonov, A. A. Sukhikh, and V. G. Bra-

gin, Retained and reverted austenite in maraging steel

03N18K9M5T, in: Proc. Int. Conf. Ferrous Metallurgy in the

CIS Countries in the XXI Century [in Russian], Moscow

(1994), Vol. 5, pp. 200 201.

8. RD 50-34582 Specification, Strength Computations and Tests.

Methods of Mechanical Testing of Metals. Determination of

Crack Resistance Characteristics (Fracture Toughness) under

Cyclic Loading [in Russian], Izd. Standartov, Moscow (1983),

96 p.

9. I. V. Pestov, V. A. Ostapchenko, M. D. Perkas, et al., Kinetics

of crack growth in maraging and medium-carbon steels under

low-cycle fatigue, Metalloved. Term. Obrab. Met., No. 7,

6 11 (1977).

10. V. I. Zeldovich and N. Yu. Frolova, Effect of heating rate on

the process of formation of austenite and recrystallization of

maraging steel, Fiz. Met. Metalloved., No. 2, 178 185

(1990).

11. K. A. Malyshev, V. V. Sagaradze, I. P. Fokin, et al., Phase

Hardening of Austenitic Iron-Nickel-Base Alloys [in Russia],

Nauka, Moscow (1982), 260 p.

12. V. V. Sagaradze, N. L. Pecherina, T. P. Vasechkina, and I. G. Ka-

banova, Inherited substructure of -phase in austenite and ap-

pearance of new dislocations in bcc fcc transformation, in:

High-Strength Austenitic Steels [in Russian], Nauka, Moscow

(1987), pp. 120 126.

13. V. P. Vylezhnev, A. A. Sukhikh, V. G. Bragin, and S. A. Koko-

vyakina, Mechanical properties of maraging steel N18K9M5T

with retained and reverted austenite, Fiz. Met. Metalloved.,

75(4), 157 165 (1993).

14. A. N. Belikov, N. L. Nikolskaya, and S. S. Ryzhak,

Transformation in maraging steel N18K9M5T, Metalloved.

Term. Obrab. Met., No. 6, 26 32 (1968).


AbstractKey wordsINTRODUCTIONMETHODS OF STUDYRESULTS AND DISCUSSIONStructure of Steel 03N18K9M5T with Different Types of AusteniteStrength of Steel 03N18K9M5T with Different Types of AusteniteFracture Resistance of Steel 03N18K9M5T upon Single LoadingFracture Resistance of Steel 03N18K9M5T under Cyclic Loading

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

Documents

550-557