Correlation of microstructure and fracture toughness in two 4340 steels
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INTRODUCTION
CORRELATING microstructure and fracture toughness has become one of the primary aims of modern research in mechanical metallurgy. In fact, the ability to document and understand the micromechanics of fracture processes, especially as it relates to macroscopic failure mechanisms, along with the influence of chemistry, microstructure and processing, now underlies a good deal of alloy design methodology. ~'2'3 Recent advances in fracture mechanics, in particular for ductile alloys, has provided new means of studying the interaction of cracks moving under quasi-static and dynamic conditions in various microstructures, and this in turn has led to a considerable range of studies aimed directly'at explaining, and even predicting, toughness variations in terms of microstructural variations. 4'5'6 A particularly interesting set of recent studies of this type has focused on the role of microstructures, produced by simple heat treatments in quenched and tempered ultrahigh strength steels, in controlling plane strain fracture toughness. 7-1° It has been shown, for example, that in AISI 4340 steels austenitizing at higher than the conventional 870 °C generally leads to higher K~c values at 20 °C, whereas other measures of toughness, such as Charpy impact energy and tensile ductility, undergo expected degradations. Up to twofold increases in Kzc have been reported in as-quenched or quenched and tempered specimens when the austenitizing temperatures are raised from 870 °C to 1200 °C. Attempts to explain this phenomenology have focused on the possible effects of differences in retained austenite levels, 18 amounts of twinned vs dislocated martensites, 9 grain size, 6'7 and iron and alloy carbide distributions 1° as key metallurgical variables. In addition, the discussion of Ritchie, Francis, and Server6 on this problem has called specific attention to the need for analyzing the fracture pro-
S. LEE, Graduate Student, and R. J. ASARO, Professor of Engineering, are with the Division of Engineering, Brown University, Providence, RI 02912. L. MAJNO, formerly Graduate Student at Brown University, is now Product Specialist, Instron Corporation, 100 Royall Street, Canton, MA 02021. Manuscript submitted July 12, 1984. METALLURGICAL TRANSACTIONS A
cesses within the respective fields of the initially "sharp" cracks characteristic of plane strain fracture toughness specimens and the relatively "blunt" notches of Charpy impact specimens. The observed behavior, then, poses some very interesting and fundamental questions regarding the role of microstructure in controlling toughness in an important class of ultrahigh strength materials. The phenomenology also poses an interesting study of the use of micromechanical models to predict realistically, or at least describe, observed microstructural effects. Aside from a desire to understand how microstructure affects Klc values (which pertain to the initiation of crack growth), there is also a vital need to understand the micromechanics of continuing fracture at the crack tip during
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