Determination of heated band width criterion of local postweld heat treatment for Cr-Mo steel pipe

Optimum heating conditions for local post weld heat treatment(PWHT) of Cr-Mo steel pipe were discussed from the point of view of residual stress using thermal-visco-elastic-plastic finite element method(FEM).The whole history of the stress relaxation during local PWHT could be clarified through FEM analysis.The investigation showed that the transient and residual stresses induced by local PWHT were significantly affected by creep behavior and temperature dependency of Young's modulus.The stresses decreased quickly in the heating stage,with decreased yield strergth.They continued to decrease slowly according to the creep law in the holding stage and increased slightly due to recovery of Young's modulus when the cooling stage started.The residual stress after PWHT decreased with increase of heated band width.When the heated band width was large enough,the residual stress after local PWHT become close to that of the uniform PWHT.Compared with the existing codes,it was found that the heated band width of 5√Rt on center of the weld seems reasonable from the residual stress point of view.A new phase transformation model was proposed considering the low temperature expansion during the cooling stage of welding of Cr-Mo steel pipe.Experiment results of welding residual stress and the stress after local post weld heat treatment are compared with numerical simulation.