島状マルテンサイトの金属学的特性に関する検討

80及び100キロ級高張力鋼大入熱HAZ部の靱性劣化とその改善に関する研究(第2報)

抄録

The internal microstructure, chemical composition, and hardness of M-A (Martensite-Austenite constituent) formed in weld CGHAZ (Coarse Grained Heat Affected Zone) of 780-980 MPa class HSLA steels have been investigated in order to reveal metallographic characteristics of the M-A constituent. The martensite was classified into lath and twin type. The massive M-A constituent had higher C contents, and included more twin type martensite and retained austenite than the elongated M-A. The cementite was found to be classified into a coarse rod type and a dendritic or fine needle type. The coarse rod type is considered to be precipitated directly from the austenite. The dendritic or fine needle type is probably precipitated by the self tempering of martensite.
The C content of the M-A constituent was increased with Δ_t8/5 (cooling time from 1073 to 773 K) and reached 1.3-2.2% for Δ_t8/5=100-1000s.
The hardness of the M-A constituent was increased with C content. The hardness of the massive M-A constituent (Hv-950) was generally higher than that of the elongated M-A constituent (Hv-700). The hardness and C content of the M-A constituent can be reduced significantly by a post weld heat treatment at temperatures from 623 to 773 K.