Interpretation of the Effects of High Austenitizing Temperature on Toughness Behavior in a Low Alloy, High Strength Stee
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I.
INTRODUCTION
O V E R ten years ago it was discovered 1 that austenitizing low alloy, high strength steels at a temperature several hundred degrees higher than conventionally used can result in a significant increase in fracture toughness, Kjc, in the asquenched or low tempered conditions. Since then numerous studies have been made, which revealed for a variety of low alloy, high strength steels that a high temperature austenitizing treatment (h.t.a.) may enhance fracture toughness by as much as a factor of two without a reduction in strength. 2-~9 However, it was also observed in most steels that, simultaneously with this improvement in K/c, the Charpy impact energy or the static bending strength of blunt-notched bar decreased. 2'4'5'7-9'~>15'16'~'1~ Besides, the ductilities of h.t.a, steels were almost invariably re-
duced.Z,4,5.v,9,11,15A9
Two explanations were put forward to clarify the effects of h.t.a, upon the toughness behavior of low alloy, high strength steels. One attributed the improvement in K~c to the changes in microstructure which resulted from h.t.a., including: (a) variations in the amount, distribution, and stability of retained austenite (RA), particularly the occurrence of an interlath austenite f i l m ; 203'2~ (b) elimination of twinned martensite plates; 2'H'2~ (c) improvement in chemical homogeneity by a resolution of residual elements, reduction in elemental segregation effects, and suppression of impurity segregation; 3"11A2'lg"2s'26 (d) dissolution of pro-eutectoid ferrite and upper bainite; 3
WEI-DI CAO, Lecturer, and XIAO-PING LU, Associate Professor, are with the Department of Mechanical Engineering, Tsinghua Umversity, Beijmg, China. Manuscript submitted December 16, 1985 METALLURGICAL TRANSACTIONS A
(e) changes in morphology of carbides precipitated during tempering;8.10,11 (f) dissolution of coarse alloy carbides; 1~ and (g) dissolution of internal inclusions.17'3~ The second interpretation4's attributed the improvement in Klc to an effect of the notch root radius; i.e., the better fracture toughness in h.t.a, steel was the result of an increase in the limiting root radius or the characteristic distance. Because a decreasing blunt-notch toughness was observed as the austenitizing temperature was raised, it was believed that critical fracture stress or strain of a steel was actually reduced by h.t.a. ; i.e., h.t.a, had no beneficial effects on the microstructure of the steel except for increasing the limiting root radius, which was assumed to be equal to the austenite grain size or the inclusion particle spacing. Because h.t.a. effects are of considerable practical and theoretical importance in the modification of industrial heat treatement procedures and in the design of new alloys, further studies are desirable to arrive at a better understanding. Particularly, since the effects of a microstructural constituent on toughness behavior are markedly different in different fracture modes, any studies undertaken should take into account the interplay among microstructure, toughn
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