Precracking structures in a creep-fatigued low-carbon CrMo steel

A grain boundary phenomenon observed, after metallographic polishing and etching, in a 1Cr-0.5Mo steel is shown to be due to the selective etching of the boundaries. The phenomenon is a feature only of material that has been subjected to strain-controlled fatigue cycles incorporating a dwell time at peak tensile stress (creep fatigue) and is largely confined to boundaries whose orientation is approximately normal to the direction of the principal tensile stress. The structural feature responsible has not been identified, although it has been established that it is not a physical discontinuity, such as a void cavity or a decohered carbide particle. The phenomenon develops concurrently with the precipitation of M3C and M7C3 carbides at the boundaries, which also is a feature of creep fatigue, and develops at the interface between these particles and the ferrite matrix as well as at the ferrite grain boundaries themselves. It seems to have been initiated in a very thin layer at these interfaces, perhaps in a layer only of the order of atom layers in thickness.