T. Sakthivel

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.

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.

Creep deformation and rupture behaviour of thermal aged P92 steel

Creep deformation and rupture behaviour of thermal aged P92 steel have been investigated over a stress range of 110–130 MPa at 923 K. Laves phase was observed in the thermal aged steel. Laves phase predominantly coarsens towards the intra-lath region assisted by substructure within the lath region. The stress dependence of minimum creep rate followed Nortons power law of creep with comparable apparent stress exponent value in the thermal aged, and normalised and tempered steels. The creep rate and creep rupture lives of both the steels were relatively comparable under lower stress level. The steel obeyed Monkman–Grant and modified Monkman–Grant ductility relationships. Enhanced precipitation of Laves phase has been observed at the necked region in both the conditions of the steel, which experienced higher plastic flow. Thermal aging of P92 steel has not significantly influenced the creep strength under longer creep duration.

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

ABSTRACT Effect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.

Influence of Thermal Ageing on Microstructure and Tensile Properties of P92 Steel

Abstract Microstructure and tensile properties of P92 steel in the normalized and tempered, and thermal aged at 923 K for 5000 h and 10,000 h conditions have been investigated. Laves phase precipitate was observed in the thermal-aged steels. The size of Laves phase precipitate increased with increase in thermal exposure. This was also confirmed from the observation that the area fraction of Laves phase precipitate was higher in the 5000 h aged condition which decreased with further increase in thermal exposure. On the other hand, the size and area fraction of M23C6 precipitate were found increased in the 5000 h aged steel, further continued to enhanced precipitation of fine M23C6 in the 10,000 h aged steel. This resulted in significant increase in area fraction and comparable size with the steel aged for 5000 h. Hardness of the steel was decreased with increase in the duration of ageing. Thermal-aged steels exhibited lower yield stress, ultimate tensile strength and relatively higher ductility in comparison with steel in the normalized and tempered condition. The increase in lath width and recovery of dislocation structure under thermal-aged condition resulted in lower tensile strength and hardness. An extensive Laves phase formation and coarsening by loss of tungsten in the matrix led to decrease in the tensile strength predominantly in the 5000 h aged steel. The tensile strength of 10,000 h aged steel was comparable with that of 5000 h aged steel due to enhanced precipitation of fine M23C6 in the steel due to enhanced mobility of carbon in the absence of tungsten in the matrix.