A study on fractography in the low-temperature brittle fracture of an 18Cr-18Mn-0.7N austenitic steel
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INTRODUCTION
BECAUSE of a favorable combination of strength and toughness at room temperature and excellent stress corrosion cracking resistance, 18Cr-18Mn-N type austenitic stainless steels have recently been used widely as nonmagnetic retaining ring materials. Moreover, the applicability of this type of steels for cryogenic structural materials has also been investigated. At low temperatures, however, the yield and tensile strength increased markedly and the steels underwent a ductile-brittle transition, which is contrary to usual expectations for face-centered-cubic materials.[1–6] Brittle fracture modes for 18Cr-18Mn-N type and other CrMn-N and Cr-Ni-Mn-N austenitic steels have been studied extensively. Defilippi et al.[7] examined the fracture surface of an 18Cr-15Mn-0.5N steel at 77 K and observed a planar facet formed by transgranular cracking along {111} planes and twin boundary cracking. In their study, however, only optical microscopy was used to observe the fracture path, and no detailed fractographic investigation was conducted. The brittle transgranular fracture at cryogenic temperatures in high N containing Cr-Mn austenitic steels was scarcely discussed after Defilippi et al. Tobler and Meyn[8] presented a highly reflective facet on the fracture surface of a fracture toughness specimen in an 18Cr-3Ni-13Mn-0.37N austenitic stainless steel tested at 4 K. They attributed the facet to slip-band cracking and called it transgranular cleavagelike SHI CHENG LIU, Visiting Researcher, and T. HASHIDA, Associate Professor, are with the Research Institute for Fracture Technology, Faculty of Engineering, Tohoku University, Sendai 980, Japan. H. TAKAHASHI, formerly Professor, Research Institute for Fracture Technology, Faculty of Engineering, Tohoku University, is deceased. H. KUWANO, Professor, is with Muroran Institute of Technology, Muroran 050, Japan. Y. HAMAGUCHI, formerly Professor, Muroran Institute of Technology, is Non-Regular Employee, Advanced Science Research Center, Japan Atomic Energy Research Institute, Ibaraki-ken 319-11, Japan. Manuscript submitted January 22, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
fracture facet, because it satisfies the definition of cleavage except the presence of river patterns such as observed on classical cleavage facets in ferritic steels. Tomota and Endo[4] examined the fracture mechanism of an 18Cr18Mn-0.5N austenitic steel that ruptured brittlely at 77 K. They observed that transgranular fracture occurred along {111} planes, but they could not determine whether the mechanism was pure cleavage or cleavagelike fracture because no river pattern was observed. Ishizaka et al.[5] reported flat facets and an obtuse angle between adjacent flat facets in 18Cr-18Mn-(0.55–0.84)N austenitic steels, and reached a similar conclusion that the ductile-brittle transition in these steels was caused by transgranular cleavagelike cracking. Similar flat fracture surfaces have also been observed in low-temperature fracture of other austenitic steels with high Mn content, and man
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