SS41鋼の延性および脆性破壊靱性特性に及ぼすミクロ組織因子の影響

抄録

Static fracture toughness test at room temperature and -196°C was carried out on JIS SS41 steel specimens which had different notch root radii and microstructures, then, effect of microstructural factor on ductile and brittle fracture toughnesses were examined.
At room temperature, it was observed that the local model of ductile fracture initiation criterion was valid. Characteristic distance in ductile fracture has nearly corresponded to void initiation distance ahead of the crack tip and the spacing of inclusion, therefore, it was presumed that a microstructural factor associated with the ductile fracture was mainly inclusions. Ductile crack initiation toughness and crack propagation toughness were measured. It was observed that the crack propagation toughness rather than the crack initiation toughness was susceptible to variation of microstructure.
At very low temperature, it has been shown that the variation of the fracture toughness with notch root radius follows the theoretical formula presented by Williams. Characteristic distance in brittle fracture has corresponded to a multiple of martensite packet size or spacing of precipitated carbide, therefore, it was presumed that a microstructural factor associated with the brittle fracture was mainly carbides. Brittle fracture initiation model taking the process zone at the crack tip and distribution of carbides into consideration was presented.