Type IV cracking behaviour of modified 9Cr-1Mo steel weld joints

Abstract

Creep deformation and rupture behaviour of modified 9Cr–1Mo steel weld joints fabricated by single-pass activated TIG (A-TIG) and shielded metal arc welding (SMAW) processes have been investigated at 923 K over a stress range of 50–110 MPa after post-weld heat treatment. The formation of coarse M23C6 precipitates and recovery of martensite lath structure into subgrain in the ICHAZ of the weld joints resulted in hardness trough. The weld joints exhibited significantly lower creep rupture lives than the base metal at lower applied stresses. Creep rupture location of the weld joints was found to occur in the ICHAZ. The reduced ductile mode of failure was observed in the low stress level in comparison with high stress level. An extensive localised creep deformation, coarsening of M23C6 precipitates in the ICHAZ with creep exposure led to the premature Type IV failure of the joints. The coarsening of M23C6 precipitates was extensive in the mid-section of the ICHAZ than the sub-surface of the joints, and was more predominant in the SMAW joint. While A-TIG weld joint exhibited reduced creep cavitation and coarsening of M23C6 precipitates due to lower deformation constraints by adjacent regions in the ICHAZ. Hence, A-TIG weld joint exhibited higher creep rupture life than the SMAW joint.