Duplex treatment of high-strength steels
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I.
INTRODUCTION
THE heat
treatment technology improving the strength and especially the toughness of high-strength steels has been actively investigated for a long time. So far, several new austenitization methods have been advanced, including high-temperature austenitization (HTA), double austenitization (DA), and rapid cyclic austenitization. Many investigations tl-4j have shown that HTA could increase fracture toughness by as much as a factor of two in some quenched or low tempered high-strength steels but, unfortunately, decrease the ductility and notch toughness of steels in most cases. Some researchers t3,4] believed that the increase in fracture toughness in HTA steels would be due to the increase in characteristic distance or limiting root radius over which the fracture process occurs, whereas the microstructure of steels might be damaged rather than improved by HTA, as shown by the decrease in ductility or notch toughness. Thus, the industrial application of HTA cannot be justified except for some special applications. To overcome the technical difficulties (for example, quenching cracking) encountered when using HTA, a method called double austenitization treatment (DA) which involved two cycles of austenitization, first at high temperature and then at conventional temperature, was developed, rS'6J It was reported t~-8~ that DA can avoid the disadvantage of HTA; not only high fracture toughness but also high ductility and notch toughness could be obtained in high-strength steels such as AISI 4340 steel and some experimental alloys. Some explorative works on the application of DA have been made in AISI 52100 steel, I9~ high-speed steel, f~~ and A533-B steel, cllj However, as a careful examination of available information revealed that DA did not always give an optimum combination of strength and toughness, a further improvement is still possible. Cyclic rapid austenitization treatment tl2~ was developed with the aim of obtaining steels with ultrafine austenite grain size to enhance their strength and toughness. A number of investigations, however, did not prove WEI-DI CAO and XIAO-PING LU, formerly with the Department of Mechanical Engineering, Tsinghua University, Beijing, China, are Visiting Scholars with the Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 276957907. Manuscript submitted June 22, 1988. METALLURGICAL TRANSACTIONS A
its beneficial effect on fracture toughness of AISI 4340 C131 and 300M, t~41 although the strength of steels was increased by ultrafine austenite grain. More important is that cyclic austenitization involves repeatedly rapid heating and cooling from austenitization temperature to ambient temperature and could easily lead to the change in shape and cracking of treated parts. Thus, the industrial application of cyclic austenitization treatment is severely confined. Besides, it is also very difficult to attain a high and uniform heating rate through the cross-section of the treated part. Based on the results of a research program o
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