Microstructure of 10% Cr martensitic heat-resistant steel welded joints and type IV cracking behavior during creep rupture at 650 °C

The creep rupture behavior and microstructure changes of a new type of 10% Cr martensitic heat-resistant steel (MHRS) welded joints during short-term creep at 130-250 MPa and 650 °C were investigated. The results showed that type IV cracking took place in the fine-grained heat affected zone (FGHAZ). Creep cavities mainly formed at the intersection points of prior austenite grain (PAG) boundaries. M23C6 carbides coarsened significantly, coarse Laves phases formed next to M23C6 along the PAG boundaries and acted as the nucleation sites for creep cavities. It was considered that the high-density of PAG intersection points of FGHAZ in per unit area, combined with the coarsening of M23C6 precipitates and the formation of Laves phases on PAG boundaries caused the formation of type IV cracking.