Microstructure and type IV cracking behavior of HAZ in P92 steel weldment

The creep rupture behavior and microstructure changes of W strengthened P92 steel weld joints have been investigated at 873 K, 898 K and 923 K. The joints were prepared by submerged arc welding (SAW). The results showed that low ductility type IV fracture took place more easily at higher temperature and lower stress. There would be a critical Larson-Miller parameter (LMP) of 35.5 and a critical applied stress of 120 MPa for type IV fracture. The critical stress was independent of creep temperature. Type IV cracks occurred in the fine grained heat affected zone (FGHAZ), corresponding to the maximum heating temperature just above Ac3, which showed the fine equiaxed microstructure without lath structure and the lowest hardness due to the instability of microstructure. An increased number density of Laves phases precipitated on grain boundaries in FGHAZ compared with other zones of weldment during creep, while the coarsening of M23C6 carbides was not very significant in W strengthened P92 steel. The fracture location in FGHAZ exhibited the most severe creep damage among the various zones of weldment and many cavities formed at the grain boundaries during creep. It was considered that the coarse Laves phase at the grain boundaries acted as the preferential cavity nucleation sites. We believe that the degradation of lath substructure and fast formation of Laves phase may be the main metallurgical factors for the type IV cracking.