Microstructural aspects of the causes of type IV cracking in Cr-Mo steel weld joints

Fusion welded joints of Cr-Mo steels fail prematurely under creep condition at the heat affected zone (HAZ) close to the base metal, termed as type IV cracking. Optical metallography and hardness testing across the joint establish that the type IV cracking occurs in the soft intercritical HAZ. Based on detailed microstructural studies carried out to understand the evolution of the microstructure and its role in determining the tendency for type IV cracking, the factors that lead to deterioration of creep strength in intercritical HAZ in weld joint of Cr-Mo steels are:

1. fine grained structure
2. coarse M₂₃C₆ carbides at grain and sub-grain boundaries
3. dissolution of M₂X and MX types of intragranular precipitates.

In the case of low Cr steels, the dissolution of intragranular Mo₂C is an important factor among others in determining the tendency to type IV cracking in the weld joint. On the other hand, in higher Cr alloys, M₂₃C₆, which plays a dominant role in determining substructure strengthening by stabilizing the substructures, is found to be the main cause of type IV cracking in the weld joint. The dissolution of finer M₂₃C₆ and the accompanying coarsening of the large particles leads to the modification of lath-like substructure having high dislocation density into fine polygonal ferrite having low dislocation density, which in turn reduces the creep strength profoundly. The preferential Z-phase formation accompanied with the dissolution of intragranular (Nb,V)(C,N) in the intercritical HAZ is also considered as a factor for the type IV cracking on longer creep exposure. The paper would highlight and discuss in detail some of our results on these lines.