Study on microstructure and properties of CGHAZ(Coarse Grain Heat Affected Zone) in X100 pipeline steel

The CGHAZ(Coarse Grain Heat Affected Zone) is the weak part of welded joint. In the paper, the change of microstructure and properties of CGHAZ of X100 pipeline steel was studied by welding thermal simulation method, microstructure observation, mechanics performance testing. The relationship between cooling time, microstructure and properties was discussed as well. The results indicated that the CGHAZ was mainly composed of granular bainite, with the increase of cooling time, grain size and bainite ferrite coarse, the volume fraction of M-A component increased, the shape changed from strip to chunk at the same time, M-A component on the grain boundary also coarsened, all of the phenomenon aforementioned lead to the decrease of impact toughness and crack resistance. The proportion of brittle fracture in impact fracture morphologywas becoming increasingly larger and the hardness, yield strength and tensile strength descended.