Effect of hot-rolling reduction on shape of sulfide inclusions and fracture toughness of AISI 4340 ultrahigh strength st
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INTRODUCTION
RECENTLY, use of ultrahigh strength low alloy steels, i.e., AISI 4340 and 300M, for critical structural application in aircraft and aerospace vehicles has increased. Ultrahigh strength steels may develop fatigue and stress corrosion cracks during service, and consequently, catastrophic fracture may occur. Therefore, improving plane-strain fracture toughness (Ktc) of the ultrahigh strength steels has been a topic of interest during the last decade attracting the attention of many workers. Considerable research effort has been directed toward improving the mechanical properties of the ultrahigh strength steels. So far, most of these studies have been focused on improving the Klc of the steels with various heat treating techniques. For example, high temperature austenitizing treatments and modified heat treatments have been reported, tl-61 However, a marked improvement in the Klc with high austenitizing treatments is often not paralleled with a corresponding increase in strength, ductility, and Charpy impact energy. There has recently been an increased demand for ultrahigh strength, low alloy steels with superior mechanical properties and for large-sized structural ultrahigh strength applications. Although the modified heat treatment is a beneficial method to develop dramatically the mechanical properties at lower temperature levels, the modified heat-treated steels are not very promising to large-sized structural ultrahigh strength applications because the isothermal transformation of lower bainite (after conventional austenitization is utilized for the improvement. One of the potential solutions to the problem, therefore, is to develop the Ktc property of the ultrahigh strength steels with modification of nonmetallic inclusions. In such a case, the better approach taken to improve the property of the steels is either to reduce the levels of inclusions as much as possible or to modify the shape of the inclusions
YOSHIYUKI 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 August 31, 1987. METALLURGICALTRANSACTIONS A
at a given level of cleanliness. However, the practice employed to produce the reduced levels of the inclusions is generally quite expensive. Thus, in our laboratory, a fundamental program has been initiated to improve the Ktc of the ultrahigh strength steels with modification of the shape of the inclusions at a given level of cleanliness, which is more economical in commercial practice. In recent years, emphasis has been placed on the effect of nonmetallic inclusion on ductility, toughness, and fatigue strength, and the role of nonmetallic inclusions in the microvoid coalescence mechanism of fracture. Leslie tT] has reviewed the state of knowledge of the effect of nonmetallic inclusions on the mechanical properties of steels including fracture. For example, technical importance of the effect of inclusions on ductile fracture, anisotropy
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