Abstract: In order to increase the service temperature of mild steel, the 9Cr-1Mo coating was prepared on Q235 base material using cold metal transition welding (CMT) technique. Significant residual stresses are formed in the fusion zone and heat-affected zone of the substrate after cladding due to thermal effects of welding. The residual stress distribution was measured by blind hole drilling method, and a finite element model was developed by SYSWELD software to simulate the temperature and stress field distribution of CMT cladding 9Cr-1Mo coating. The effect of martensitic phase transformation on the residual stress distribution was analyzed. The results showed that the coating was mainly martensitic, and the heat affected zone (HAZ) was mainly consisted of ferrite and pearlite. Multiple thermal cycles in multi-pass welding had a significant effect on the evolution of residual stress in the coating. Welding thermal cycles in the later passes can redistribute the stress generated during the fusion of the first passes and simultaneously change the direction and magnitude of the residual stress. Compressive residual stress formed in the cladding layer due to martensitic transformation, while the tensile stress mainly distributed on the HAZ and adjacent base metal. Among them, the maximum transverse residual tensile stress located in the HAZ adjacent to the fusion line, with a peak value of 300 MPa, and the compressive stress were concentrated on the fusion cladding layer near the fusion depth with a peak value of -200 MPa. The highest longitudinal residual tensile stress appeared in the heat affected zone between the weld passes, and the peak value reaches 550 MPa, and the peak of the compressive stress occurred at the bottom of the passes with a peak value of -400 MPa. The distribution of residual stress in the surface layer was caused by martensitic transformation and thermal cycling in the pass after welding. The simulation results agreed well with the experimental results.

Key words: CMT arc cladding, martensitic transformation, 9Cr-1Mo, numerical simulation, residual stress

摘要： 为了提高低碳钢的使用温度，采用冷金属过渡(Cold Metal Transition, CMT)焊接技术在Q235基板上制备了9Cr-1Mo涂层。由于焊接热效应，熔覆后涂层和基板热影响区会形成显著的残余应力。使用盲孔法测量了试样表面残余应力分布，同时使用SYSWELD软件开发了考虑材料马氏体固态相变的热-冶金-力学耦合计算的有限元方法，模拟了CMT熔覆9Cr-1Mo涂层温度场与应力场分布，并分析了马氏体相变对残余应力分布的影响。结果表明，涂层部分主要为马氏体组织，热影响区主要为铁素体与珠光体组织。多道焊中的多次热循环对涂层残余应力演化影响明显，后道焊道焊接热循环可以使先焊焊道熔覆时产生的应力重新分布，并同时改变残余应力的方向和大小。马氏体相变使熔覆层大部分区域呈现为压应力，而基板热影响区主要为拉应力。其中，最高横向残余拉应力出现在熔合线附近的热影响区，峰值为300 MPa，压应力集中在熔深附近的熔覆层上，峰值达到-200 MPa。最高纵向残余拉应力出现在焊道之间的热影响区中，峰值达到550 MPa，压应力峰值出现在焊道底部，峰值达-400 MPa。在马氏体相变和后焊焊道热循环的作用下使表层残余应力呈现拉压相间分布状态。模拟结果与实验结果吻合良好。

关键词: CMT电弧熔覆, 马氏体相变, 9Cr-1Mo, 数值模拟, 残余应力