固态相变对P92钢焊接接头残余应力的影响

INFLUENCE OF SOLID-STATE PHASE TRANSFORMATION ON RESIDUAL STRESS IN P92 STEEL WELDED JOINT

采用光学显微镜、显微硬度仪和盲孔法研究了P92钢平板焊接接头的微观组织、显微硬度和表面残余应力分布.同时,基于SYSWELD软件开发了考虑材料固态相变的热-冶金-力学耦合的有限元计算方法,并采用该方法模拟了P92钢的Satoh试验和单道堆焊接头的温度场及应力场分布,探讨了固态相变引起的体积变化、屈服强度变化和相变塑性(TRIP)对焊接残余应力形成过程及最终残余应力分布的影响.实验结果表明,P92钢平板焊接接头焊缝组织为淬火马氏体,其平均显微硬度为440 HV,母材(BM)组织为回火马氏体,其显微硬度为240 HV.Satoh试验的数值模拟表明,固态相变引起的体积变化和屈服强度变化不仅对残余应力的形成过程及最终应力的分布和峰值大小有显著影响,甚至可以改变应力的符号;而TRIP效应则具有减缓因体积膨胀和屈服强度变化所引起应力变化趋势的作用.进一步的计算结果表明,P92钢堆焊接头焊缝和热影响区(HAZ)的纵向残余应力为压应力,而靠近HAZ的BM上存在较大的纵向拉应力,峰值为600 MPa,该值超过了P92钢的室温屈服强度;整个焊接接头的横向残余应力峰值为130 MPa,远小于其纵向残余应力的峰值.数值计算结果与盲孔法测量得到的结果比较吻合,表明了所开发的热-冶金-力学耦合的有限元计算方法有较高的计算精度.

Microstructure and welding residual stresses in ferritic heat-resistant steels such as P92 have been considered as one of the most important factors in the structural integrity and life assessment of power plant weldments. Applying computational tools to predict microstructure and residual stress distribution in practical welded structures is a preferable way to create safer, more reliable and lower cost structures. In this work, the effects of volume change, yield strength variation and transformation induced plasticity（TRIP） on the generation of residual stresses in P92 steel welded joints were investigated experimentally and numerically. Optical microscope and Vickers hardness tester were used to characterize the microstructure and hardness of the weldments. The hole- drilling strain- gage method was employed to determine the residual stress distribution across the weldments. Based on SYSWELD software, a thermal-metallurgical-mechanical finite element method（FEM） was developed to simulate welding temperature field and residual stress distribution in P92 steel joints. Firstly, numerical simulations of Satohtest were carried out to clarify the influence of solid-state phase transformation on the formation of residual stresses. The simulation results show that the volume change and the yield stress variation have a great effect on the magnitude and distribution profiles of residual stresses in the fusion zone（FZ） and heat affected zone（HAZ）, and even alter the sign of the stresses, while TRIP have a relaxation effect on the tendency of stress variation during phase transformation. Secondly, a FEM was established to calculate the welding residual stress distribution in a singlepass bead-on P92 steel joint. In the FEM, three main constituent phases（austenite, untempered martensite and tempered martensite） in P92 steel were taken into account. Finally, the simulation results of welding residual stress were compared with the experiments obtained by hole-drilling method. The numerical simulation results are generally in a good agreement with the measured data.