Effects of test temperature on internal fatigue crack generation associated with nonmetallic particles in austenitic ste
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I.
INTRODUCTION
THE generation of subsurface (internal) fatigue cracks in high-strength alloys is usually associated with the presence of nonmetallic inclusions, as illustrated in the case of fish-eye-type fractures.[1,2] While the mechanism of the crack generation and its growth originating from a nonmetallic inclusion near the specimen surface has been discussed,[3,4] it has not been sufficient to understand the internal crack-generation mechanism associated with preexisting defects, such as inclusions. Table I provides a summary of our results[5–10] on the fatigue crack-initiation site and subsurface crack origin at cryogenic temperatures. Subsurface crack initiation is dominant at lower temperatures and in the long-life range, while surface crack initiation occurs in high peak stress tests or in short-life tests. For example, the Ti-6Al-4V alloys and 24Cr-15Ni-4Mn-0.3N steel listed in Table I have the highest strength level among the cryogenic structural alloys, and their subsurface crack initiation apparently occurs due to transgranular or intergranular cracking.[6,9] The dependence of subsurface crackinitiation site size on the peak stress was able to be accounted for by a threshold condition assumption such that microcracks grew until a critical crack size depending on the peak stress. Furthermore, the possibility that, even in an S-N curve, different crack initiation mechanisms can exist has not been focused. For an A2219 alloy, two types of internal crack initiation origins, of which were precipitate and intergranular cracking, were detected at cryogenic temperatures.[10] This means that the microcrack originating from a pre-existing defect may not always provide a critical crack size. In order to determine the defect that is finally responsible for fatigue failure, not only the metallurgical origin but also OSAMU UMEZAWA, Senior Scientist, and KOTOBU NAGAI, Unit Leader, are with the Frontier Research Center for Structural Materials, National Research Institute for Metals, Ibaraki 305-0047, Japan. Manuscript submitted April 29, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
the geometry of the initial cracks have to be discussed. Hence, the metallurgical and micro-fractographical investigations are necessary to clarify the mechanism of internal crack generation from defects such as nonmetallic particles. The internal crack initiation in high-cycle fatigue has been detected in 25Mn-5Cr high-manganese austenitic steel and nitrogen-strengthened 25Cr-13Ni austenitic stainless steel at cryogenic temperatures. For the 25Mn-5Cr steel, in which the important role of inclusions on its fatigue crackgrowth behavior was already reported elsewhere,[11] the internal crack initiation associated with nonmetallic inclusions was dominant at cryogenic temperature.[8] The 25Cr-13Ni steel also contained fine particles.[12] In the present study, the internal crack generation in each steel has been characterized. Special attention is given to present the important role of nonmetallic particles at initiation sites for the mi
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