Type IIIa cracking in 1/2CrMoV steam pipework systems

A mode of cracking termed Type IIIa has been found to occur in the welds of 1/2CrMoV pipework systems in the UK. This cracking mechanism is not as well documented as the more familiar Type IV cracking, which occurs on these and other ferritic steel systems, but is likely to increase in importance as plant ages. This paper summarises the available information on the initiation and development of Type IIIa cracks and brings up to date recent work on carbon diffusion local to welds, which is believed to play a role in the susceptibility to this form of cracking.