Effect of microstructure on plane-strain fracture toughness of aisi 4340 steel
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INTRODUCTION
COMMERCIAL ultrahigh strength, low alloy steels used in high performance aerospace systems, i.e., AISI 4340 and 300-M, may develop fatigue and stress-corrosion cracks during service and lead to catastrophic failure which can cause serious financial loss or loss of life. Therefore, during the last decade, increased emphasis has been devoted to fracture mechanics characteristics of ultrahigh strength steels. While a high ductile-to-brittle transition temperature is a requirement which has been a demand for design against crack initiation, an even more critical fracture parameter for design against crack propagation of the ultrahigh strength steels is sharp-notch plane-strain fracture toughness (Ktc). Therefore, the design and quality assurance procedures so as to have higher Kic should be carefully studied and controlled. In such a situation, materials specialists may be required to conduct an investigation to determine mechanisms of fracture, based on the microstructure of the steels. Thus, in the author's laboratory, a fundamental program has been initiated to understand the correlation between the microstructure and Kjc of the ultrahigh strength steels. Recent investigators have studied the effect of microstructural parameters controlling the mechanical property. Among these are the effect of differences in retained austenite levels, t~'2'31amount of twinned martensite] 4j iron and alloy carbide distributions, tS]inclusions spacing distance, 16'71 and grain size. f8"9]However, it is not clear from the information exactly how the microstructure influences Ktc. For example, the use of high austenitizing temperatures in AISI 4340 steel can lead to improved Ktc in the as-quenched and lightly tempered condition, t6'71 Such treatments not only result in more complete dissolution of alloy carbides, but also lead to the formation of films of retained austenite at the martensite lath boundaries. However, the improvement in the fracture
YOSHIYUK/TOMITA, Associate Professor, is with the Department of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, 4-804 Mozu-Umemachi, Sakai, Osaka 591, Japan. Manuscript submitted July 17, 1987. METALLURGICALTRANSACTIONSA
toughness may not be attributed solely to the beneficial effect of retained austenite, tl'2,3J Ritchie and coworkers t6'71 have demonstrated that retained austenite had no effect on the fracture toughness and the increase in strength is associated with an increase in the characteristic distance, e.g., prior austenitic grain size, for ductile fracture, resulting from dissolution of void-initiating particles at high austenitizing temperature. In this regard, Kim et al. tlo] have also shown in the study of AISI M2 high-speed steel and MA matrix tool steel that a high content (about 16 vol pct) of retained austenite appears to raise the fracture toughness, while the prior austenite grain size seems to be the controlling factor if the retained austenite content decreases. On the other hand, Lee et al. is] have studied the eff
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