Study on Creep Damage Behavior of 2.25Cr-1.6W Steel Weld Joint

Author:Xu Meng Jia

Supervisor:lu zuo


Degree Year:2016





2.25Cr-1.6W steel(HCM2S,T23/P23) is usually used to manufacture water wall,superheater and reheater pipes,and main steam pipes in the Ultra Super Critical(USC) Power Plants with the benefit of enhancing thermal efficiency.Components employed in power plants are continually exposed to both high temperatures and high steam pressures.The safety and performance of a thermal power plant highly relied on the integrity of its welded joints.The welding residual stresses would be superimposed on any applied loads,such as high steam pressures,and generate a complex stress state on in-service components.They may accelerate creep damage and cracking initiation at high temperatures on these components.Therefore,it is necessary to clarify the combined effects of microstructure change,welding residual stress and primary load on the creep damage and crack initiation.The present thesis presents a newly developed global optimisation method for the finite element simulation of welding process considering bainite transformation.In this method,the pattern search algorithm was applied to determine kinetic parameters in Johnson-Mehl-Avrami-Kolmogorov(J-M-A-K) equation during a continuous cooling process.Meanwhile,the J-M-A-K equation was modified into an explicit form as a function of welding temperature field to improve calculation efficiency in the optimisation process.This methodology improves the accuracy as calculating the temperature dependent volume fraction of bainite transformation in finite element simulation.Based on the modified J-M-A-K equation,the effects of weld metal strength and transformation temperature on the residual stress in 2.25Cr-1.6W steel welded joints was studied by FEM simulation.The higher spatial resolution measurements of the stresses located at regions of different microstructures are applied in order to adequately capture the positions of the maximum stresses across the boundary of the HAZ and base metal.We found that the stress levels in the weld and the peak residual stress position in the base metal can be controlled by the appropriate selection of filler metal.Low transformation temperature weld metals are capable of inducing compressive residual stresses in the weld bead Meanwhile,the high strength weld metal pushes the peak tensile stress outward to the base metal,keeping off the region of weak creep strength.A reasonable agreement is obtained between the numerical modelling considering phase transformation and the local stresses measurement associated to weld microstructure regionsEffect of residual stresses on creep behavior was investigated using a newly designed compact tension creep specimen in 2.25Cr-1.6W steel before and after heat treatment.Residual stresses were generated at the neighbor of crack root by loading in compression beyond yield and then unloading.The heat treatment was carried out to obtain partially austenitized microstructure and to cause zigzag configuration of grain boundaries.Based on damage-coupled creep constitutive model,a three-dimensional finite element model was developed to predict the evolution of creep damage and residual stress within the specimens during creep time.The extent of creep cracking has also been assessed through destructive examination,providing validation for the creep damage prediction model.Fractal analysis method was applied to quantitative determine the correlation between the grain boundary morphology and creep behaviorCreep crack tests were carried out on the tensile creep specimens,in which the residual stresses were generated by local remelting and cooling.Residual stresses in the specimens were measured using Synchrotron X-ray diffraction techniques.The fracture surface of the creep specimen was analyzed using statistical methods and fractal analysis The relation between fractal dimension of the fracture surface and fracture mode of the creep specimen was discussed.Due to different fracture mechanisms,the probability density functions of the height coordinates vary with the intergranular crack percentage Good fitting was found between Gaussian distribution and the probability function graph of height coordinates of the high percentage intergranular crack surfaceFinally,based on the thermo-mechanical coupled model and the modified J-M-A-K equation,FE simulation for the welding process of butt welded 2.25Cr-1.6W steel pipes was conducted.On the basis of simulated results of welding residual stress,we introduce damage-coupled creep constitutive model to calculate the creep damage and stress release of butt welded 2.25Cr-1.6W steel pipes exposed to both high temperatures and high steam pressures.