Fracture toughness of calcium-modified ultrahigh-strength 4340 steel
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INTRODUCTION
T H E use of commercial ultrahigh-strength, low-alloy steels, i.e., AISI 4340 and 300-M, for critical structural applications in aircraft and aerospace vehicles has increased. However, these steels may often develop fatigue and stress-corrosion cracks during service; consequently, catastrophic fracture may occur and lead to serious consequences which can cause financial loss, environmental contamination, or loss of life. To minimize the risk of failure, during the last decade, considerable effort has been directed toward improving plane-strain fracture toughness (Ktc), which is the critical value of the stress-intensity factor and is used as a fracture criterion in crack growth of the ultrahigh-strength, low-alloy steels. So far, most of these studies have been focused on improving the Klc of the steels with various heat-treating techniques. Among these, for example, are high-temperature austenitizing treatments t~-61 and modified heat treatments, tT-l~ There has recently been increased demand, however, for ultrahigh-strength, low-alloy steel with superior mechanical properties and for large-sized ultrahigh-strength applications. Especially, the isotropy regarding these properties has been required. Although the modified heat treatments are beneficial methods of dramatically developing the mechanical properties, the treatments are not very promising for large-sized applications and for modifying the isotropy of the mechanical properties. In such situations, the author gives one potential solution to the problem by developing the KI~ of the ultrahigh-strength steels through decreased hot-rolling reduction treatment. [1~] The effectiveness of the treatment in improving the Kx~ is attributed to the following facts: (1) sulfide inclusions modified by decreasing hot-
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 June 15, 1989. METALLURGICALTRANSACTIONSA
rolling reduction improved the K~c in the longitudinal (L) orientation by blunting and arresting crack propagation across the specimen; (2) the modified sulfide inclusions also developed the Ktc in the transverse (T) orientation by considerably suppressing the lamellate fracture which occurs in a brittle manner along the interfaces of the inclusions/matrix at the crack tip. As noted, the role of the inclusions has been found to be to blunt and arrest the propagating cracks and suppress the lamellate fracture. Thus, other treatments should cause the same effects, provided they modify the morphology (size, shape, and distribution) of the inclusions. The obvious alternative treatment is modification of the inclusions by chemical means. Among these chemical means are the use of alkaline earth metals, e.g., calcium and magnesium, or rare earth metals, e.g., cerium and lanthanum, which are fed into molten steel in the ladle, t12'131 It has been shown previously that, in principle, the calcium or
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