Type IV Damage Evolution during Creep of 316LN SS-P91 Steel Dissimilar Weld Joint

Damage evolution in a creep tested 316LN stainless steel-P91 steel dissimilar weld joint fabricated by shielded metal arc welding has been investigated. Failure in this joint which was tested at 873 K and stress level of 80 MPa occurred by Type IV cracking in the P91 steel's heat-affected zone. Microstructural investigation on the failed specimen revealed presence of creep cavities adjacent to the fractured region. To get an insight of failure mechanism, orientation imaging microscopic analysis was carried out on the fractured and two interrupted creep tested specimens-one tested up to onset of apparent secondary regime and another up to the onset of apparent tertiary regime. The analysis established that the damage associated with creep cavitation was more prevalent in the fine-grained heat-affected zone (FGHAZ) and that nucleated cavities in this region were resolvable after onset of the apparent tertiary regime. Though cavities were observed in the inter-critical heat-affected zone after exposure up to onset of tertiary, their growth was not significant. The weakening of the FGHAZ's microstructure is attributed to extensive precipitation and coarsening of M23C6 in this region. Kernel Average Misorientation studies revealed that the dislocation substructure did not alter considerably in both the regions until the onset of the apparent secondary regime. However, after subsequent exposure damage evolution was quite different in both these regions. Based on the microstructural findings a new failure mechanism has been proposed describing the preferential damage in the investigated dissimilar weld joint during creep.