Structure and crystallography of martensite-austenite constituent in the intercritically reheated coarse-grained heat affected zone of a high strength pipeline steel

The structure and crystallography of martensite-austenite (M-A) constituent in the intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of X100 (690 MPa) pipeline steel weld joint was studied via multi scale characterization. The results suggested that majority of the necklace-type M-A constituent in the ICCGHAZ preferred to form lamellar lath structure, which primarily consisted of lath martensite (87%). Only small fraction of retained austenite (9%) was found between martensite laths. The retained austenite had K-S orientation relationship with its neighboring martensite: (10-1)(M)// (11-1)(gamma) and [111](M) // [011](gamma). Adjacent martensite laths within M-A constituent had large misorientation but no fixed crystallographic orientation relationship which implied that martensite laths may nucleate independently and encounter with each other during growth. M-A constituent and matrix microstructure belonged to different prior austenite grains. The martensite laths within the M-A constituent did not inherit the crystallographic orientation of the parent matrix during the intercritical reheating and subsequent cooling process of the second pass welding.