S. Panneer Selvi

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

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.

Mechanistic approach for prediction of creep deformation, damage and rupture life of different Cr–Mo ferritic steels

Abstract A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.

Effect of Normalization Heat Treatment on Creep and Tensile Properties of Modified 9Cr–1Mo Steel

AbstractCreep and tensile properties have been investigated on modified 9Cr–1Mo steel subjected to single and double normalization heat treatments. Optical, scanning and transmission electron microscopic investigations revealed the presence of finer prior austenite grain size and coarsening of MX precipitates in the double normalized steel as compared to the steel subjected to single normalization heat treatment. Increased minimum creep rate and significant reduction in creep rupture life were observed in the double normalized steel. Increased tensile ductility coupled with marginal decrease in tensile strength was observed with double normalized steel compared to single normalized steel. The double normalized steel exhibited lower creep strength and higher tensile ductility than the single normalized steel.