Welding Residual Stress Analysis And Optimization Of Post Weld Heat Treatment Of Narrow Gap Submerged Arc Welding

The steam turbine rotor is one key component of nuclear turbine set and its reliability and stability have a decisive effect on the safety and service life of nuclear turbine set.Now,the LP welded rotor has been used in many nuclear turbine sets,so it is important and necessary for the design of low pressure rotor to capture and observe the welding residual stress evolution during welding and its distribution after welding.The method of combining experimental measurement and finite element analysis has become an effective way to investigate welding residual stress distribution of a thick-wall weldment,but there is still little research of welding residual stress for theses weldments with wall thickness larger than 150mm.On the other hand,post weld heat treatment(PWHT)plays an important role in decreasing or eliminating welding residual stress in engineering project,but the settings of PWHT parameters mostly depend on the engineering experience.In order to accomplish the numerical simulation of welding,a 2D axisymmetric finite element model for simulated component(welded pipe)of low pressure welded rotor was established using commercial package ABAQUS,while the subroutines of FILM,DFLUX and UEXPAN were compiled to realize heat exchange,welding heat source model and SSPT,respectively.In addition,the main PWHT parameters(PWHT target temperature,holding time and heating rate)were considered to investigate the stress relaxation behavior during PWHT.The main contents and related results are as following:(1)Based on sequential coupling method,the numerical simulation of welding process was conducted to investigate the welding residual stress evolution during welding and its distribution after welding.The results show that there is large welding residual stress in the welding zone and its vicinity,and the welding residual stress decreases gradually with the distance from weld centerline increasing.Solid-state phase transformation(SSPT)has a significant influence on the distributions of welding residual stress at the area of welding zone and its vicinity,which does not only influence the amount but also even the direction of welding residual stress.The distribution of welding residual stress on the weld centerline shows relatively uniform state with a high stress.About the evolution of welding stress,when the subsequent weld height reaches some level during welding process,the welding stress state ether on the weld centerline or the path parallel to weld outer surface almost keep steady.(2)Based on transient thermal analysis,Norton-Bailey constitutive equation was employed to make it clear that which stress relaxation mechanism is the dominant role and investigate the evolution of welding residual stress in PWHT.The results show that the stress relaxation mechanism of creep plays the dominant role in decreasing or eliminating the welding residual stress in PWHT,not the material strength reduction in high temperature.The welding residual stress relieves very remarkably before the holding stage and the holding stage slightly affects the final welding residual stress after PWHT.About the evolution of welding residual stress in PWHT,when the temperature within the simulated component is higher than the creep temperature,the welding residual stress on the weld centerline shows a quick decrease from the surface to the inner.(3)Based on control variable method,the influence of PWHT parameters on stress relaxation was discussed.The results show that PWHT temperature is the key factor for the sufficient amount of stress relaxation,so it is necessary to make sure that the temperature within the whole weldment can reach the target temperature during the hold time period.The holding time has little influence on the amount of stress relaxation,so when only considering the stress relaxation,it is suggested that the holding time could be reduced appropriately.By discussing the effect of different ramp up time on temperature curve of weldment with different thickness,the maximum heating rate and minimum hold time can be determined.