PHASE TRANSFORMATIONS DURING TEMPERING

OF QUENCHED Fe- -N ALLOYS

M. V. Be lous , Yu . N. Moska lenko , and V. G. Permyakov

UDC 620.181:669.1'786

The addition of nitrogen to steels is of practical interest due to the deve lopment of surface hardening procedures and also f rom the standpoint of replacing several scarce alloying e lements by alloying with nitrogen. Nitrogen is s imilar to carbon, and therefore s imi lar changes in the strueture and propert ies can be expected after quench ing of austenite and temper Ing of martens i te with nitrogen. These changes are of considerable theoretical and practical interest.

Iron--nitrogen alloys with 0.3-2.05% N were prepared by nitriding samples obtained by press ing and sintering iron powder (0.02% C, 0 .014% S, 0 .016% P) in hydrogen at l000~ Nitriding was conducted at 550 ~ in ammonia . The quantity of nitrogen in the samples was control led by the process ing t ime and the degree of dissociation of ammonia . The phase compos i t ion of the alloys was determined by x - ray and magnet ic analysis. It was found that the alloys consist of ferrite with nitrogen and the 7' nitride, Fe4N. The e phase fo rmed in some cases was el iminated by remova l of a surface layer. The alloys were quenched f rom tem- peratures ensur ing homogeneous austenite. The martens i te contained 0.5-2.05% N.

The quenched samples consisting of martensi te with nitrogen and retained austenite were tempered at 100-600 ~ (50 ~ Intervals) for 1 h and cooled in air. The tempered samples were subjected to x - ray analysis. Some of the samples were tempered at constant temperature and with heating in a di latometer and differen- tial magnetometer in saturat ing fields. According to the x - ray analysis, tempering at 150 ~ results in the formation of high-nitrogen phase with a tetragonal lattice, with c /a larger than for the original martensite. Temper martensite with a low degree of tetragonality occurred along with this phase after 10w-temperature tempering of the Fe--N alloys. It is probable that the formation of high-nitrogen phase during low-tem- perature temper ing is due to order ing in the arrangement of nitrogen ions in the pores of the bcc lattice of iron. Similar resul ts were obtained in [1, 2] for tempered martensi te with carbon in several h igh-manga- nese and high-nickel steels. The increase in the tetragonality of the martensi te with carbon at room tem- perature as compared with the quenched condition at negative temperatures is associated with ordering of interst i t ia l atoms.

Ordering occurs at al l temperatures In the f i rs t stage of tempering (20-180 ~ of the quenched Fe--N alloys. It acce lerates with increasing tempering temperatures . The coercive force increases at the same time (~40% as compared withthe quenched condition), the intensity of magnetization decreases (by 20% for the al loy with 2% N), and the electr ical res ist iv i ty decreases ~30% (see Fig. 1). The results of the mag- netometr ic analysis indicate that the reduction of the magnetization can be explained if the composition of the nitride phase formed during low-temperature tempering matches formula Fel6N2 and the specific magnetization is equal to 110-130 G-cm3/g [3]. No e phase is formed in low-temperature tempering, which is conf irmed by the magnetization graphs, on which no Curie temperature was observed for e nitr ides.

After tempering at 200-300 ~ (second stage) the res ist iv i ty is somewhat lower than after low-tempera- ture tempering, the magnetization increases sharply, and the coercive force decreases. The di latometric analysis showed a reduction in the volume of the sample (0.15% for the alloy with 1.5% N), accompanied by a considerable thermal effect (3 ca l /g for the alloy with 1.9% N). According to the structural analysis, the volume effect must be considered outside the rebuilding of the crysta l lattice of the lowFtemperature nitride

VAev Polytechnical Institute. Translated f rom Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 28-30, March, 1974.

9 Consultants Bureau, a division of Plenum Publishing Corporation, 227 g'est 17th Street, New York, N. Y. I0011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the gr~blisher. ,4 copy of this article is available from the publisher for $15.00.

225

Hc, Oe Hc, Oe 1

C

t.o

E

?

/ /

cm O,6O

r~

o, Jo

IUO 200 JUg ZllO JUU 'tuO '~C '~ Tempering temperature a b

Fig. i. Change in physical propert ies with temper ing of quenched Fe + 1% N. a) First stage of temper ing; b) second stage, i) Coerc ive force; 2) magnet i za - tion intensity; 3) electrical resistivity; 4) di latometric curve dur ing temper ing; 5) calor imetr ic curve during tempering. I, 3) I sothermal temper ing for I h; 2, 4, 5) continuous heating at rate of 3-4~

phase Fe I6N 2 into the T' Fe4N phase. This is conf i rmed by the quantitative analysis of the vo lume effect, taking account of the specific vo lume of the phase participating in the transformation. F rom the results reported in [3], the increase in magnet izat ion in the second stage of temper ing is due to the superposit ion of two effects -- decompos i t ion of retained austenite with nitrogen and the Fe I6N 2 -~ Fe4N transformation.

In the third stage of temper ing (300-550 ~ the t ransformat ions associated with a change in the phase compos i t ion of the Fe - -N alloys do not occur. The increase in the temper ing temperature leads to a reduc- tion in the width of interference lInes of the a and T' phases and to changes in the coercive force and hard- ness, s imilar to those in carbon steels.

CONCLUSIONS

It was found that temper ing processes are similar in quenched steel and nitrided iron -- in the first stage of temper ing (20-180 ~ the martensi te with nitrogen t ransforms, with format ion of metastable F I6N 2 and temper martensite; in the second stage (180-300 ~ the retained austenite decomposes and the Fe I6N 2 --" Fe4N transformat ion occurs; in the third stage (300-550 ~ the number of lattice defects decreases and the Fe4N particles coalesce. After quenching and temper ing at 550-600 ~ the alloy consists of a ferrite--nitride mixture of the type of temper sorbite in carbon steel.

I.

2.

3.

L ITERATURE C ITED

L. I. Lysak and Ya. N. Vovk, "Format ion of phase with a cubic lattice dur ing quench ing of steels, " Fiz. Metal. Metal loved., i_99, 5, 669-705 (1965). Yu. L. Al 'shevskii and G. V. Kurdyumov, "The crystal lattice of martensi te in manganese steels, " Fiz. Metal. Metal loved., 25, i, 172-174 (1968). M. V. Belous, Yu. N. Moska lenko, and V. G. Permyakov , "Phase t ransformat ions dur ing temper ing of quenched alloys in the Fe - -N system, " Fiz. Metal. Metal loved., 28, 5, 894-902 (1969).

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