Role of microstructure influencing on the methane bubble formation in the heat-affected zone of 3Cr-1Mo-0.25V steel

Methane bubble formation in a 3Cr-1Mo-0.25V steel was investigated in association with carbides formation and prior austenite grain size. The reaction between hydrogen and carbon arises the formation of methane bubbles affected by carbides as well as the prior austenite grain size. The formation of stable carbides through heat treatments led to the lowering carbon activity, and thereby giving rise to improved resistance to hydrogen attack. When the steel has larger prior austenite grain size and the larger occupancy of carbides on grain boundary, the severe hydrogen attack occurred due to the reduction of total grain boundary areas relative to the amount of bubbles.