Tempering Kinetics of Alloy Steels as a Function of Quench Rate and M s Temperature
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I.
INTRODUCTION
THEREhas been a series
of recent studies of the effects of quench rate during the austenite to martensite transformation in steel on the transformation kinetics, ~'2 the morphology, 3,4,5the Ms temperature, 6-9 and the as-quenched hardness. 7'sA~ The hardness studies8'l~ have shown that the as-quenched hardness decreases with increasing quench rate, while after aging at room temperature, the same hardness is reached for all samples of an alloy regardless of the quench rate. The results have been interpreted in terms of the effect of cooling rate during quenching on the degree of carbon segregation either to defects in the austenite prior to the transformation or due to autotempering after the transformation and, therefore, in both cases implicitly to the carbide or carbon segregate particle size in the as-quenched martensite. There has not been, however, any direct measurement of the carbide particle size as a function of quench rate in either the as-quenched condition or during the aging process. Paradoxically, for a series of steels in the fully aged condition (where one would expect identical hardnesses regardless of the prior quench rate) a TEM studys of the carbide particle size as a function of quench rate reported that the carbide size decreased with increasing quench rate, the resolution limit of the study being 100~ due to the presence of dense substructure. The purpose of this investigation was to examine this apparent paradox: first, to reexamine the effect of quench rate on both the as-quenched hardness and subsequent room temperature aging behavior of a set of ferrous alloys identical to those studied previously. 8'~ Second, using the high resolution small-angle X-ray scattering technique, to deterDANIEL G. HENNESSY, formerly Research Associate, Department of Materials Engineering, Rensselaer Polytechnic Institute, is now with the Timken Company, Canton, OH 44706. VIJAY SHARMA, formerly Research Associate, Department of Materials Engineering, Rensselaer Polytechnic Institute, is now with USHA Alloys and Steels, Ltd., Jamshedpur, Birhar, India. G.S. ANSELL is Dean of Engineering, Rensselaer Polytechnic Institute, Troy, NY 12181. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees. METALLURGICALTRANSACTIONS A
mine the effect of quench rate on the carbon segregate or carbide particle size and volume fraction in the martensite for these same alloy steels both as-quenched and as a function of subsequent room temperature aging.
II.
EXPERIMENTAL PROCEDURE
Alloy Selection and Specimen Preparation. The alloys studied in this investigation were ternaries selected such that a comparison and use of previous morphology, hardness, and Ms temperature data would be possible. 8'~1 The alloy chemistry and measured ~1conventional quench rate Ms temperatures are shown in Table
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