The effect of the austenitizing heat-treatment variables on the fracture toughness of high-speed steel
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I.
INTRODUCTION
HIGH-speed steel has been used to fabricate various engineering cutting tools, particularly in the form of wheels, in quenched and highly tempered conditions. The materials used for these purposes require very high hardness as well as a combination of toughness and high strength. To meet such requirements, many improvements have been made in the microstructure. The concept of microstructure control as a function of chemical composition and heat-treatment process has been well advanced in recent years, but the safe prediction of mechanical behavior and the determination of the widely applicable relationship between heat-treatment macroparameters and fracture toughness remain unanswered. In such heat-treatment performance, the target is twofold: (1) to increase the hardness of the steel after step quenching from an extremely high soaking temperature of ;1200 7C, and (2) to improve the mechanical properties of the already hardened steel by subsequent tempering. During tempering, the hardness that had been obtained by direct quenching is increased due to transformation of retained austenite to martensite and at the same time the toughness and strength are improved. If the tempering is prolonged, however, then the hardness starts declining as primary martensite is being decomposed. This approach is quantitative in nature, so predicting the mechanical properties imparted to the components by a particular heat treatment presents various uncertainties. In order to obtain components of optimum mechanical properties, a compromise has to be made between hardness and toughness. So far, the best known approach of defining the results is by a process of trial and error. However, a better approach taken to solve the problem of optimum mechanical properties was the association N. SARAFIANOS, Assistant Professor, is with the Physical Metallurgy Laboratory, Department of Mechanical Engineering, Aristotle University, 540 06 Thessaloniki, Macedonia-Greece. Manuscript submitted August 1, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
of hardness, toughness, and strength to fracture toughness KIC and JIC integral in the context of the crack extension mode I. Fracture mechanics adopts the fracture toughness as a critical value (KIC) causing crack propagation at the crack tip. Nevertheless, other parameters such as operating temperature, microstructure, environmental conditions, and mostly the heat treatment[1,2,3] are those determining the actual fracture behavior. Since the main point of interest here is the elastic-plastic behavior of the crack tip, the J-integral provides a means to determine the energy release rate and through this, the effect of microstructure parameters controlling the mechanical behavior. This approach comprises the effect of grain size,[4] carbide distribution, and particularly the retained austenite level.[5] Although there are conflicting views about the effect of retained austenite on the improvement of the fracture toughness[4] of ultra-highstrength steel,[5,6] recent investigation[7] has
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