Metallographic investigation of M‐A constituent Deterioration and improvement of HAZ toughness in 780 and 980 MPa class HSLA steels welded with high heat inputs (2nd report)

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Abstract
This paper describes an investigation of the internal microstructure, chemical composition, and hardness of the M‐A (martensite‐austenite) constituent formed in one type of simulated HAZ ‐ the CGHAZ (coarsegrained HAZ for high heat input welding process) ‐ in 780 and 980 MPa class HSLA steels to determine the metallographic characteristics of the M‐A constituent. The M‐A constituent is classified into two morphological types: massive M‐A and elongated M‐A. The massive M‐A constituent has a higher C content and includes more twin type martensite and retained austenite than the elongated M‐A. The cementite is classified into a coarse rod type and dendritic or acicular type. The coarse rod type is considered to be precipitated directly from the austenite. The dendritic or acicular type is probably precipitated by self‐tempering of the martensite. The C content of the M‐A constituent increases with ?t[8/5 (cooling time from 1073 to 773 K), reaching 1.3–2.2% when ?t8/5 = 100–1000 sec. The hardness of the M‐A constituent increases with the C content. The hardness of the massive M‐A constituent (Hv950) is generally higher than that of the elongated M‐A constituent (Hv700). The hardness and C content of the M‐A constituent can be significantly reduced by post weld heat treatment at temperatures from 623 to 773 K.