UDC 621.762:669.152428-194

CONCENTRATION-INHOMOGENEOUS NICKEL-MOLYBDENUM

TRIP-STEELS

A. A. Shatsov1 and I. V. Ryaposov1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 47 50, November, 2010.

The structure and properties of powder steels with 2 4% Ni and 0.5% Mo widely used for making machine

parts and equipment are studied. Heat treated articles from these steels have high operating properties under

complex loading conditions, i.e., under friction (including the case of presence of abrasive particles) and under

impact and alternating loads.

Key words: structure, metastable austenite, martensite, deformation, phase transformation, TRIP-

steel, mechanical properties.

INTRODUCTION

A metastable structure can be obtained at operating tem-

peratures in steels with high content of austenite at nickel

concentration of about 2% in the case of the use of dispersed

powders, for example, mechanically alloyed ones [1]. In al-

loys based on technical iron powders the concentration of

nickel should be less than 4% [2].

The possibility of obtaining low-alloy nickel-molybde-

num powder steels with a low fraction of metastable austen-

ite was first patented in [3] and then reported by S. Takajo

[4]. The mechanical properties of the steels have been raised

by increasing the proportion of the metastable component by

means of thermochemical treatment, additional compaction,

and use of partially alloyed powders [5 7]. Other methods

for fabricating powder metastable austenitic steels are con-

nected with implementing a specified distribution of alloying

elements [8], for example, by sintering in the presence of a

liquid phase [9]. The interest in low-alloy trip-steels is ex-

plainable by their high structural strength at a relatively low

content of alloying additives. Nickel-molybdenum powder

steels are used for the production of highly loaded parts, such

as hammers, anvils, and links. Other applications of such

steels are steel copper pseudoalloys and composite mate-

rials based on them [12, 13]. Articles from powder meta-

stable austenitic steels can compete with parts from cast iron

with metastable austenite and parts produced with the use of

ausforming (gears, links, etc.).

The aim of the present work was to study the structure

and properties of low-alloy metastable austenitic steel

PK70N4M (with 4% Ni and 0.5% Mo) obtained with the use

of double pressing and sintering.

METHODS OF STUDY

The structure of steel PK70N4M was studied by metallo-

graphic, microdurometric, microscopic x-ray spectrum, and

x-ray diffraction analyses. The strength and the ductility was

determined according to GOST 1822785, the crack resis-

tance was determined according to GOST 25.50685 on spe-

cimens of type IV with preliminarily deposited crack, and the

impact toughness was determined for specimens with cross

section of 6 6 mm (at 40-mm distance between the rests).

The initial material was iron powder of grade

PZhR3.200.28. The alloying additives were PNK-OT4 nickel

carbonyl powder, molybdenum powder prepared by TU

49-19-10.573, and colloidal graphite preparation of grade

S-1. In order to improve the compressibility, the blend was

enriched with 0.8 wt.% zinc stearate.

The blend was mixed for 8 h in a mixer with shifted rota-

tion axis. The first pressing was performed at 600 MPa. The

preforms were annealed for 2 h at 850C in an atmosphere of

dissociated ammonia and then pressed again at 800 MPa.

The final sintering was performed in a walking beam furnace

at 1160C for 4 h in an atmosphere of dissociated ammonia.

RESULTS AND DISCUSSION

After the fist pressing the specimens had a porosity of

6.60 gcm3 (Po = 15%); after preliminary sintering and addi-

tional compaction their porosity was 7.15 gcm3 (Po = 9%).

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

558

0026-0673/11/1112-0558 2011 Springer Science + Business Media, Inc.

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

shatsov@pstu.ru).

After sintering, the structure of the specimens is prima-

rily represented by sorbite and troostite (Fig. 1a ). Austenite

spots with inclusions of martensite are noticeable near

pores. The porosity after the sintering is about 9%; the spe-

cific surface of the pores is 0.030 m 1, the distance be-

tween the pores is 125 m, the pore diameter is 12 m. The

content of carbon in the sintered specimens is 0.80 0.65%.

Under the conditions mentioned the density and the po-

rosity change inconsiderably in the sintering process. The

mechanical properties of the steel obtained are presented in

Table 1.

Homogenization in the sintering stage is determined by

the chemical and granulometric compositions and by the

structure of the powders. In order to give an exhaustive sta-

tistical description of the distribution of an alloying element

we should know the distribution law and the coefficient V of

variation of the concentration. As a rule, the law of distribu-

tion of elements in a steel is asymptotically lognormal [9]. In

our work we checked the distribution law in terms of the

n2 test. For a lognormal law to be obeyed a specific value

of the criterion should be lower than the tabulated value at

significance level P (the probability with which the hypothe-

sis may be rejected). For P = 0.2 the tabulated value of

n2 = 0.24 [14]. The coefficients of variation of the concen-

tration are 0.31 for nickel and 1.0 for molybdenum; the val-

ues of n2 are 0.17 for nickel and 0.44 for molybdenum.

Consequently, the distribution of nickel does obey a

lognormal law. The distribution of molybdenum does not

obey a lognormal law, which confirms indirectly the high

value of the coefficient of variation, but this has a low effect

on the properties because the concentration of molybdenum

is not high.

The final heat treatment includes hardening from 850C

and 2-h tempering at 180C. Such heat treatment improves

markedly the mechanical properties of the steel (see Table 1).

In the heat treated state steel PK70N4M contains two

main structural components, i.e., massive martensite and re-

tained austenite (Fig. 1b ) located primarily near pores.

The microhardness of the dark regions of the lap is

670 750 HV (martensite with a low fraction of troostite);

that of the light regions is 350 420 HV (a mixture of aus-

tenite and martensite). This high microhardness of the aus-

tenite-martensite structure seems to be a result of partial de-

composition of austenite upon impression of the indenter [12].

In concentration-inhomogeneous powder steels the trans-

formation of metastable austenite into strain martensite un-

der loading promotes growth in the hardness (due to the

growth in the stresses required for opening of typical defects)

and in the crack resistance (due to the additionally spent en-

ergy required for structural transformations) [15]. In the steel

studied retained austenite transforms completely into strain

martensite under the action of applied stresses (Fig. 2). The

diffractograms taken from the laps give a somewhat distorted

proportion of the - and -phases, because the austenite de-

composes partially in the process of preparation of the laps.

At the same time, an -phase is virtually absent on the frac-

ture surface, which leaves no doubt about realization of the

trip-effect.

Another proof of a strain-induced phase transformation

in the fracture zone was obtained in the metallographic study

of fracture surfaces (Fig. 3). Traditional etching methods

were used to determine the fracture zone. The depth of the

most etched zone, where the main processes preceding frac-

ture develop [8, 15], fluctuated from several tens to several

hundreds microns depending on the loading rate.

Concentration-Inhomogeneous Nickel-Molybdenum Trip-Steels 559

b 50 m

Fig. 1. Microstructure of steel PK70N4M after sintering (a, 320)

and after heat treatment (b ).

TABLE 1. Mechanical Properties of Steel PK70N4M after Sintering and after Heat Treatment

Treatment Hardness r, MPa , % KC, kJm2 K

Ic, MNm32 , gcm3 Po, %

Sintering 88 HRB 590 2.0 200 35 7.15 9.0

Hardening + tempering 50 HRC 950 0.5 120 38 7.15 9.0

The contribution of the austenite-to-martensite transfor-

mation on the fracture surface into the growth in the crack re-

sistance KIc

can be evaluated analytically from the equation [8]:

KIc

= ( )

KW f Eh

cI

tr2

21

,

where Wtr

is the specific work of the phase transformation

[for low-alloy steels Wtr

1.7 kJ(g-atom)], E is Youngs

modulus, is Poissons coefficient, f is the volume fraction

of the phase transformation, h is the thickness of the layer,

and KcI

is the value of KIc

in the absence of phase transfor-

mations.

Evaluation with the help of this equation shows that the

crack resistance of low-alloy steels can be increased by more

than a factor of 1.5 only due to the trip-effect.

We know of models explaining the growth of the

strength parameters of powder trip-steels by increase in the

failure stress in bodies with defects upon implementation of

strain-induced transformation [15]. An equivalent ap-

proach [16] associates the growth in the strength with in-

crease in the compressive stresses at the tip of a defect during

formation of martensite.

Loading causes redistribution of the alloying elements in

the fracture zone [8, 15]. In the climatic temperature range

only two mechanisms of accelerated diffusion in a stress

field are possible, namely, pushing-out of the element with

higher atomic volume and displacement of atoms of the al-

loying elements in the dislocation center [17]. The elevated

concentration of impurity atoms in steels based on the pow-

der of grade PZhR 3.200.28 results in localization of stresses,

which decreases, in its turn, the length of the strained region

and decelerates the motion of dislocations. This seems to be

one of the causes of lower mechanical properties as com-

pared to bidisperse power trip-steels [8, 15]. Another cause is

the worse concentration homogeneity. It manifests itself in

the fact that the strain-induced transformation re-

quires higher deformation (due to the presence of regions

with elevated concentration of alloying element), and there-

fore the thickness of the zone with phase transformation de-

creases as compared to steels with high content of additives

of carbonyl powders.

CONCLUSIONS

1. An approach is suggested for fabricating parts from

powder low-alloy metastable austenitic steels.

2. Elevation of mechanical properties of concentration-

inhomogeneous trip-steels as compared to commercial pow-

der steels is a result of phase transformations on the fracture

surface upon loading.

3. The mechanical properties of concentration-inhomo-

geneous metastable austenitic steels based on domestic pow-

der iron are not inferior to those of traditional low-alloy

steels, which makes these steels suitable for the production

of loaded parts.

REFERENCES

1. V. N. Antsiferov, S. N. Bobrova, and A. A. Shatsov, Structure

and properties of mechanically alloyed steels PK50N2M,

Poroshk. Metall., No. 34, 31 35 (1998).

2. A. A. Shatsov and M. G. Latypov, The role of nickel and car-

bon in concentration-inhomogeneous trip-steels, Metalloved.

Term. Obrab. Met., No. 6, 34 36 (2001).

3. Jr. Jandeska and F. William, Iron Powder Article Having Im-

proved Toughness, US Pat. No. 4618473, Publ. 1989.

4. S. Takajo, Obtaining high strength steel powder, MRP, No. 78,33 (1991).

5. H. Masazum, M. Naok, and H. Tadatoshi, Development of

ultrahigh strength sintered steel, in: Adv. Powder Met. and

Part. Mater, Proc. Power Met. World Congr. (21 26 June,

560 A. A. Shatsov and I. V. Ryaposov

49

49

55

55

2 , deg

2 , deg

I

I

b

Fig. 2. Diffractograms of steel PK70N4M (I is the intensity of the

radiation): a) lap; b ) fracture.

Fig. 3. Microstructure of steel PK70N4M near a fractured zone,

320.

1992, San Francisco, Calf), New York: Princeton (1992), Vol. 5.

pp. 215 226.

6. L. Caroline, E. Uif, and E. Per, Sintered high strength materi-

als, in: Adv. Proc. Powder Met. and Part. Mater., Proc. Power

Met. World Congr. (21 26 June, 1992, San Francisco, Calif.),

New York: Princeton (1992), Vol. 5. pp. 107 114.

7. O. Furukimi, K. Yano, and S. Takajo, Ultrahigh strength fer-

rous sintered components, Int. J. Powder Met., 27(4), 331 337

(1991).

8. M. G. Latypov and A. A. Shatsov, Special features of trip-ef-

fect in powder concentration-inhomogeneous steels with low

nickel content, Metalloved. Term. Obrab. Met., No. 8, 15 19

(1997).

9. V. N. Antsiferov, N. N. Maslennikov, A. A. Shatsov, and

T. V. Smyshlyaeva, Powder steel with structure of metastable

austenite, Poroshk. Metall., No. 34, 42 47 (1994).10. A. A. Shatsov, Special features of structure of steel-copper

metastable pseudoalloys, Metalloved. Term. Obrab. Met.,

No. 6, 21 24 (2007).

11. M. G. Latypov, E. V. Cherepakhin, and A. A. Shatsov, Struc-

ture and properties of steel-copper metastable pseudoalloys,

Perspekt. Mater., No. 2, 63 67 (2008).

12. M. G. Latypov, S. M. Katanov, E. V. Cherepakhin, and A. A.

Shatsov, Interaction between metastable copper-steel pseudo-

alloy and hard alloy, Metalloved. Term. Obrab. Met., No. 7,

22 27 (2009).

13. S. M. Katanov and A. A. Shatsov, Metastable steel copper

pseudoalloys with hard-alloy strengthening, Perspekt. Mater,

Special Issue 6, Functional nanomaterials and high-purity

substances, Part 2, 308 311 (2008).

14. S. L. Akhnazarova and V. V. Kafarov, Optimization of Experi-

ment in Chemistry and Chemical Technology [in Russian],

Vysshaya Shkola, Moscow (1978), 319 p.

15. V. N. Antsiferov, M. G. Litvinov, and A. A. Shatsov, High-

strength crack-resistant concentration-inhomogeneous powder

nickel steels, Metalloved. Term. Obrab. Met., No. 11, 28 32

(1999).

16. M. A. Filippov, V. S. Litvinov, and Yu. R. Nemirovskii, Steels

with Metastable Austenite [in Russian], Metallurgiya, Moscow

(1988).

17. S. N. Peshcherenko, Mutual Diffusion in Structurally Inhomo-

geneous Materials Obtained by the Method of Powder Metal-

lurgy, Authors Abstract of Doctorals Thesis [in Russian], PGU,

Perm (1998).

Concentration-Inhomogeneous Nickel-Molybdenum Trip-Steels 561

AbstractKey wordsINTRODUCTIONMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES

/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