BRIE F COMMUNICATIONS

EFFECT OF ROLLING ON THE LOW-CYCLE ENDURANCE OF STEEL IN WORKING MEDIA G. V. Karpenko, I. P. Pistun, A. B. Kuslitskii, and K. B. Katsov

UDC 669.14:539.434

The rolling of metal during its hot or cold plastic deformation from the cast ingot to the finished section usually has a beneficial effect on the strength characteristics of steels. Thus, the fatigue limit of steel increases monotonically with the degree of rolling reduction [i, 2]. No information is available about the effect of rolling on the low-cycle endurance of steels, although we know that surface strengthening may raise by several times the life of metals in low-cycle fatigue [3]. In view of the importance of this question, we have investigated the effect of rolling to various degrees of reduction on the low-cycle life of steel St. 3, from which more than 80% of all rolled products are made. The metal was rolled on a laboratory mill in such a way that the strip was reduced by I0, 20, 40, and 60%, the final thickness in all cases being 2.5 mm. Specimens were cut from the strips across the rolling direction and subjected to low-cycle fatigue testing in reverse bending at three different relative deformation levels in an IP-2 machine [4] in air, in a corrosive medium (3% NaCI solution), and in a hydrogenating medium (the same solution with cathodic polarization at a current density of I0 A/dm2). The experimental results were treated by mathematical statistical methods, the results of five or six tests being used for each point. The microhardness of the metal of the rolled specimens was measured on a PMT-3 machine, and the deformation force acting on the specimens and the total deformation work were determined by means of strain gauges [5]. The electrode potentials of the surface of the specimens were also measured during the cyclic elastoplastic deformation [6]. Analysis of the test results (Fig. i) reveals the indeterminate nature of the relationship between the low-cycle endurance of the steel and the rolling reduction. A significant factor is the optimum degree of reduction that produces the maximum life of the steel. ~i~e greater the amplitude of the cyclic deformation, the more is the life displaced in the direction of the maximum degrees of reduction. Consequently, whereas at comparatively low levels of cyclic stressing (high-cycle fatigue) rolling results in an increase in endurance [7], at higher stresses it is found only at small degrees of reduction, and a sharp reduction in the low-cycle endurance of the steel occurs at very high levels of cyclic deformation. Rolling leads to a rise in the hardness and strength of the steel (work hardening), so reducing the ductility index. Thus, with a reduction of 20%, the microhardness increases by 40%. At greater reductions, the metal softens or disintegrates instead of becoming stronger; for example an increase in the reduction to 60% causes the microhardness of the steel to fall to half the maximum value. Hence, the cyclic elastoplastic deformation of steel takes place under conditions in which the metal to some extent exhausts its reserve of ductility owing to the work hardening caused by rolling; consequently, its resistance to low-cycle fatigue will depend, firstly, on the extent of this reserve (i.e., on the degree of reduction) and, secondly, on the amplitude of the cyclic deformation. At comparatively small reductions and low amplitudes of cyclic deformation the steel is further strengthened in the first stage of fatigue on account Physicomechanical Institute, Academy of Sciences of the Ukrainian SSR, L'vov. Trans- lated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. ii, No. i, pp. 96-97, JanuaryFebruary, 1975. Original article submitted December 15, 1973.

9 1976 Plenum Publishing Corporation, 227 West 17th Street, N e w York, N.Y. 10011. No part o f 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 o f the publisher. A copy o f this article is available from the publisher for $15.00.

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5 4 J

5 q

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Reduction, % Fig. Fig. 3 in ing and

1

Fig.

2

1. The e f f e c t of the degree of reduction on t h e l o w - c y c l e e n d u r a n c e o f s t e e l St. reverse bending tests in air (1), in a corrosive medium (2), and in a hydrogenatmedium (3) with relative deformations of the extreme fibers o f 0.38% ( a ) , 0 . 6 3 % ( b ) , 1.12% ( c ) .

F i g . 2. rolling

Microstructures (• reduction o f 60% ( b ) .

of steel

St.

3 in

its

original

state

(a)

and after

a

of the available reserve of ductility, and as a result the low-cycle endurance of the steel increases. If, however, the steel has undergone large reductions during rolling or the tests are carried out at high amplitudes of cyclic deformation, the metal will lose strength literally from the first cycles of repeated stressing, and this leads to a sharp fall in the low-cycle endurance. These relationships are confirmed by measurements of the force acting during deformation of the specimens and of the total work of deformation. Thus, in tests on specimens made from steel that had been reduced by 40%, the curve of the deformation force began to fall even at the second cycle, so indicating a loss of strength in the metal. The total work of deformation also reveals this tendency; the area of the hysteresis loop for steel with a large reduction is only one half to one quarter that of the original steel. As can be seen from Fig. i, the aggressive media -- the corrosive and hydrogenating media -- not only reduce the low-cycle endurance of steel, but they also change somewhat the character of the effect of the degree of reduction on this endurance. Rolling steel considerably alters the microelectrode potentials, which are an index of the thermodynamic state of the metal, so that the difference in electrode potential at the surface increases monotonically with rise in the degree of reduction. Thus, even at e = 0.38% in the presence of these media, the optimum reduction is not 20%, as it is in tests carried out in air, but 10%. Consequently, we may take it as proved that the corrosion and hydrogenating media have an additional embrittling effect on rolled steel, and in this way lead to a substantial r e d u c t i o n in its low-cycle endurance.

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It is undoubtedly of interest to determine more accurately the effect of the rolling direction. With this object in view we have carried out tests on specimens cut in the rolling direction; on the whole, the specimens showed the same qualitative features as those characterizing the transverse specimens. The quantitative indices of the effect of the degree of reduction on the life of the longitudinal specimens were different, however. In particular, the maximum increase in the life of these specimens as a result of rolling (at a minimum deformation E = 0.38%) was 123% (compared with 84% for transverse specimens). It should be noted that the increase in the degree of reduction leads to a considerable rise in the stability of the endurance indices, particularly in the corrosive and hydrogenating media, so that the embrittling effect of these two media is very marked in the case of the transverse specimens which have a comparatively low ductility. To judge from the photomicrographs shown in Fig. 2, the rolling direction forms a texture, which is also responsible for the marked difference in the endurance under conditions of low-cycle fatigue. LITERATURE CITED i. 2. 3. 4. 5. 6. 7.

D. R0dzinak, Metalloberfl~che, 25, No. 3, 84; No. 7, 226 (1971). M. Ogirima, Trans. Jap. Inst. Metals, iO, No. 3~ 182 (1969). S . I . Kishkina, in: Surface Work Hardening of High-Strength Materials [in Russian], ONTI, Moscow (1971), pp. 16-36. V . I . Tkachev and Yu. I. Babei, Fiz.-Khim. Mekh. Mater., No. 2 (1966). V . I . Tkachev and A. B. Kuslitskii, Fiz. Met. Metalloved., 26, No. 4 (1968). G . V . Karpenko et al. ; Fiz.-Khim. Mekh. Mater., No. 3 (1969). G . V . Karpenko, The Effect of Mechanical Working on the Strength and Endurance of Steel [in Russian], Mashgiz (1959) .

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