Numerical Simulaiton On The Welding Residual Stress And Life Analysis Of Large Scale Rotating Components

With the development of generation III nuclear power and ultra-supercritical fossil turbine towards large scale and high-parameter (high temperature and pressure), welding has become an important means to manufacture the rotating components like rotor. However, the residual stress and microdefect (pores, inclusions, et al) introduced in welding process will affect strength and safe reliability of the structure. On the other hand, fatigue and creep damage will be produced in the rotating component due to complex load history caused by start-up/shut-down operation and high temperature environment. It is still a very important subject in engineering that how the coupling of welding residual stress and working stress effect on service life.In this work, the generation and distribution of welding residual stress in welding steam turbine rotor were investigated systematically by finite element method. It was discussed emphatically about the role of fatigue and creep on residual stress relaxation and redistribution in service load history. The influence of residual stress on life design of rotating component was studied according to damage and fracture theory. The main research contents and conclusions are listed as follows:(1) Thermal elastic-plastic finite element method was used to analyze the welding residual stress in multipass narrow gap welded joint and it’s influence factors. It shows that the simulated results coincide with measured residual stress when solid-state phase transition is considered in welding simulation. The increase of welding pass numbers reduces width of HAZ, but does not alter the distribution trend of residual stresses. With the increase of cooling time after each welding pass, the peak value of residual stresses increases slightly, but the affected zone of which reduces obviously.(2) Based on fatigue and fracture theory, numerical method was employed to calculate the distribution of high/low cycle fatigue life and critical crack size in welded rotation components. Results indicate that the predicted fatigue lives have little difference for using Von Mises stress and maximum principal stress as equivalent stress respectively. Internal fillets near weld are the weakest zone because of the lowest fatigue life. When traditional strain-based method is applied to predicted fatigue life for rotor under thermal-mechanical load, the result is non-conservative (greater than actual life). So stress-based method was proposed to solve this problem.(3) The effect of welding residual stresses on fatigue life was investigated for welded joint of rotor. It turn out that the high/low cycle fatigue life decreases at HAZ and base metal zone by reason of residual stresses. But the fatigue life increases significantly in weld zone due to the existence of residual compressive stress. In addition, Low cycle fatigue life was affacted more apparently by residual stress than high cycle fatigue life.(4) The relaxation and redistribution of residual stress were studied in welded rotor under service condition (fatigue stress during turbine starts and stops, high temperature creep) using thermo-elasto-viscoplastic constitutive model. Results show that welding residual stresses will relief (tensile stress reduces) and redistribute during start-up and shut-down operation, on account of stress superposition and yield strength decrease under high temperature. Creep mechanism causes more relaxation of residual stress than fatigue mechanism under high temperature environment. And the higher temperature leads to more relaxation of residual stress when the weld position close to steam inlet. A large extent of residual stress relaxation occurs in short residence time under creep mechanism. However, residual stress will not always decrease with the residence time prolonged further.(5) The stress/strain concentration around welding micro-defects was studied quantitatively by finite element simulation including varying elasticity modulus, position and spacing of inclusions under different loading conditions. It was found that the driving force for fatigue crack growth can not be characterized by the stress concentration factor Kt around welding micro-defects in case of considering elastic plastic behavior of material. And then a parameter Kp was proposed to characterize coupling effect of stress/strain concentration. Based on the parameter Kp, it is interpreted why cracks are prone to initiate at surface of specimen for low cycle fatigue, and produce inside the specimen for high cycle fatigue. Further studies indicate that the greater is the difference of elastic modulus between inclusion and matrix, the greater is the local stress concentration. Fatigue cracks are easier to be initiated at pores than incluisons in weld. The enhancement of stress field caused by multiple defects interaction depends on both array of defects and loading direction. The cyclic softing leads to an decrease of stress concentration factor around the pore, while which shows an insignificant effect on that around hard inclusions.