Retained Austenite and Tempered Martensite Embrittlement in Medium Carbon Steels
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I.
INTRODUCTION
IT has
already been shown that TME in experimental 0.3 pct C and commercial structural steels (such as AISI 4340) 1'2'3 is associated with decomposition of retained austenite into carbides. However, the correlation of microstructural and morphological changes with mechanical properties, especially fracture, still requires some elucidation. It is the objective of the present work to examine these details using characterization techniques including electron microscopy, diffraction, microanalysis, X-rays, and Auger spectroscopy. The steels chosen are based on alloys designed to combine attractive combinations of strength and toughness. 4 They are primarily low alloy steels of approximately 0.3 pct C, so as to provide dislocated lath martensite and interlath retained austenite upon quenching from the austenite range. The basis for the design of these steels is described elsewhere. ~'~ The mechanical properties, especially fracture, of these steels depend strongly upon composition and temperature for tempering above approximately 250 ~ Sudden troughs in the toughness (measured as Charpy V-notch impact energy or plane strain fracture toughness6) vs tempering temperature curves are observed either in the vicinity of 350 ~ tempering1-5. 7-17or 500 ~ tempering, or both, depending on the alloying additions (e.g., Cr, Mn, Mo, Ni, Si, etc.), extent of impurity content (e.g., S, P, Sb, Sn, As), and the history of the heat treatments, especially austenitizing temperature. This embrittlement 1'8'1ssubsequently causes an increase in the ductile-to-brittle transition temperature (DBTT) of the steels. The embrittlement which occurs with tempering in the vicinity of 500 ~ is called "500 ~ embrittlement" or more M. SARIKAYA, formerly Graduate Student, is now Assistant Research Engineer; A. K. JHINGAN, formerly Graduate Student, is now at Memorex Corporation, Santa Clara, CA; and G. THOMAS, Professor, is with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. 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
generally "temper embrittlement" (TE) which is usually attributed to the segregation of impurity elements to the prior austenite grain boundaries causing predominantly intergranular fracture (with respect to preaustenite grain boundariesS'9'lt'12'18). This embrittlement commonly occurs in low purity commercial steels, and the extent and the temperature of the embrittlement depends on the impurity level in the bulk alloys. The embrittlement on tempering near 350 ~ is termed "tempered martensite embrittlement" (TME).1-3'8'9'11'14'23 Since this embrittlement can be found in high purity vacuum-melted alloys,l'2 it is not considered to be controlled by impurities. In the case of TME the fracture path is quasi
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