Microplastic deformation during stress relaxation

MICROPLASTIC DEFORMATION DURING STRESS

I. L. Mirkin, Z. N. Petropavlovskaya, and S. A. II'inykh

R E L A X A T I O N *

UDC 539.371

The relaxation of mechanical s t r e s se s is one of the most important phenomena in sol id-s ta te physics . The study of the s t r e s s relaxation p roces s in metals is difficult, since the total plast ic deformation is small (not over 0.2%). We studied plast ic deformation in different steels during s t ress relaxation by the optical in terference method [1]. This method makes it possible to determine the microplas t ic deformation with a prec is ion of up to 0.03 /z, which is no more than 3% of the value measured.

The s t r e s s relaxation experiments were made with flat samples at different t empera tures in vaccum, using the "Relaxometer" apparatus, which makes it possible to plot s t r e ss relaxation f rom the beginning of the application of the load [2]. The mater ia l s investigated were: a - i r o n ; steel with 127o Cr, 1% W, and 0.5% Mo hardened by rela t ively coarse M23C 6 carbides with an average size of 0.8/z; and steel with 12% Cr, 8% W, and 0.5% Mo, hardened with f iner par t ic les of Fe2W with an average size of 0.4 #.

F igure 1 shows the s t r e s s relaxation curves for the three mater ia ls at 580~ It can be seen that in relat ion to the s t ruc tura l condition of the metal the cumulative plast ic deformation is not the same and leads to different ra tes of s t r e s s relaxation. Thus, for a - i r o n a reduction of the elast ic deformation f rom 0.12 to 0.025% and its conversion into plast ic deformation af ter 1 h lowers the relaxation res i s tance by 80%, while for the steels tes ted under the same conditions the elast ic deformation decreases f rom 0.15 to 0.07- 0.04% and the s t r e s s drops by only 50-60%. The relaxation p rocess in the samples of u - i r o n ceases com- pletely af ter 2 h at 580~ while in either of the two steels tested there are sections where the p rocess still continues after 4 h, the s t r e s s remaining at 30-40% of the original value.

The in ter ference pat terns of the surface of (~-iron (Fig. 2) indicate that some of the grains turn during s t r e s s relaxation - the in ter ference bands change in cross ing the grain boundaries. The difference in the value of local deformation amounts to ~0.2 #. Thus, there is a difference in the small p las t ic deformation

* Presen ted at the VII Conference on Prob lems Concerning the Scattering of Energy during Vibrations of Mechanical Systems, Kiev, Institute of Problems of Strength of the Academy of Sciences of the Ukrainian SSR, 1968.

Sel, % ~74~= 27kg/mm~

o 7 2 h

Fig. 1 Fig. 2 Fig. 1. Stress relaxation curves at 580~ for u - i ron (1) and steels with 12% Cr hardened by carbides (2) and intermetal l ic compounds (3).

Fig. 2. In ter ference patterns of the surface of a - i r o n before and after s t ress relaxation tests at 580~ for 1 h (• a) Before the test; b) after the test.

Central Scient i f ic -Research Institute of Technology and Mechanical Engineering. Trans la ted f rom lKetallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 62-64, March, 1970.

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0,2O 8,18 i o,76 o,1~ % 0.12 2

o,,o ' ~ . 0,08 o, o6 < o, o4 ~02

Lq 1 2 3 h

Fig. 4. Stress relaxation curves at 750~ for steels hardened by carbides (1) and by intermetal l ic com- pounds (2).

~ <~ i �84 <~i~% : _ i ~ , ~ , ~ , ~ ! < ~ , ~ ~i �84 ~ - ~

Fig. 3. Interference patterns of steels hardened by carbides (I) and intermetall ic compounds (II)

before and after s t r e ss relaxation tests (x500). a) Micros t ruc ture of the original; b) before the test; c) after testing 4 h at 580~ d) after testing 4 h at 750~

in different grains of s-iron during stress relaxation~ The interference patterns for samples of the steel hardened by M23C G carbides before and after relaxation tests are shown in Fig. 3. After 4 h at 580~ the inter- ference patterns show small undulations near the car- bide particles, pointing to inhibition of microplastic deformation at these points. The interference pattern for the other steel, hardened by intermetallic com- pounds, shows hardly any distortion after 4 h at 580~ In this case the difference in the value of local plastic deformation was less than 0.03 p, and could not be detected by this method.

A notable difference in the course of m ic ro - plast ic deformation was observed when the testing tempera ture for the two steels was ra i sed to 750~ The s t r e s s relaxation curves at 750~ are shown in Fig. 4. For the steel hardened by intermetal l ic com- pounds the conversion of elast ic into plast ic deforma-

tion occurs at a s lower rate than for the steel hardened by carbides. The cumulative plastic deforma- tion after test ing 2 h was 0.13% in the f i rs t case, while in the lat ter case the elast ic deformation was com- pletely t r ans fo rmed into plast ic deformation. No plast ic deformation in the grain boundaries was noted in ei ther steel. As can be seen f rom the mic ros t ruc tu re in Fig. 3, the microplas t ic deformation at 580 as well as at 750~ occurs mainly inside the grains close to the par t ic les . Evidently the precipi ta tes of second phase are b a r r i e r s to plast ic deformation and have a decisive influence on the inhibition of the relaxation p rocess . Both the type of precipi ta te and the dispers i ty a re important. For example, the distortion in the in terference pat tern of the steel hardened by carbides af ter 1 h at 580~ and par t icu lar ly at 750~ is con- s iderably g rea te r than for the steel hardened by intermetal l ic compounds.

C O N C L U S I O N S

1. The in terference method can be used to demonstrate that the microplas t ic deformation differs during s t r ess relaxation of steels at high tempera tures .

2. In a - i r o n microplas t ic deformation is mainly in tergranular and distinctly local in character .

3. With d ispersed hardening phases (carbides and intermetal l ic compounds) in chromium steels the microplas t ic deformation is mainly intragranular . There are local differences in the magnitude of m ic ro - plas t ic deformation in the vicinity of the hardening par t ic les .

1 o

2.

L I T E R A T U R E C I T E D

A. V. Velikanov and Ya. P. Rauzin, Fiz. Metal. i Metalloved., 1_.99 (1965). Z. N. Petropavlovskaya, Proceedings of the Central Scient i f ic-Research Institute of Technology and Mechanical Engineering [in Russian], Book 38 (1963).

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