使用有限元法研究了不同层间温度(IPT)时,在9%Cr热强钢管道多层多道焊接头残余应力演化中马氏体相变作用的差异,揭示了层间温度对残余应力作用的机理。结果表明,提高层间温度可以显著降低接头内的残余拉应力,特别是管道中部区域焊缝(WM)内的残余拉应力降低明显。其机理主要有2方面：一是提高层间温度可保留较高含量的奥氏体,屈服强度低的奥氏体在冷却时积累的残余拉应力较低;二是高的层间温度阻止了马氏体相变在每道焊道焊完后立即进行,从而避免了马氏体相变降低拉应力的效果被后焊焊道的焊接热循环所消除和在随后焊道的焊接热循环中重新积累较大的拉应力。层间温度对9%Cr热强钢管道多层多道焊残余应力分布的影响取决于热收缩和马氏体相变的综合作用,当层间温度较低(低于马氏体转变终了温度M_f)时,热收缩占主导作用,此时接头的大部分区域以残余拉应力为主,只在末道焊道焊缝及其热影响区(HAZ)内形成较大的压应力;当层间温度较高(高于马氏体转变开始温度M_s)时,马氏体相变占主导作用,此时接头以残余压应力为主。

9%Cr heat-resistant steels have been abundantly used in boilers of modern thermal plants. The 9%Cr steel components in thermal plant boilers are usually assembled by fusion welding. Many of the degradation mechanisms of welded joints can be aggravated by welding residual stress. Tensile residual stress in particular can exacerbate cold cracking tendency, fatigue crack development and the onset of creep damage in heat-resistant steels. It has been recognized that welding residual stress can be mitigated by low temperature martensitic transformation in 9%Cr heat-resistant steel. Nevertheless, the stress mitigation effect seems to be confined around the final weld pass in multi-layer and multi-pass 9%Cr steel welded pipes. The purpose of this work is to investigate the method to break through this confine. Influence of martensitic transformation on welding stress evolution in multi-layer and multi-pass butt-welded 9%Cr heat-resistant steel pipes for different inter-pass temperatures (IPT) was investigated through finite element method, and the influential mechanism of IPT on welding residual stress was revealed. The results showed that tensile residual stress in weld metal (WM) and heat affected zone (HAZ), especially the noteworthy tensile stress in WM at pipe central, was effectively mitigated with the increasing of IPT. The reasons lie in two aspects, firstly, there is more residual austenite in the case of higher IPT, as a result, lower tensile stress is accumulated during cooling due to the lower yield strength of austenite; secondly, the higher IPT suppresses the martensitic transformation during cooling of each weld pass, thus the tensile stress mitigation due to martensitic transformation was avoided to be eliminated by welding thermal cycles of subsequent weld passes and reaccumulating tensile residual stress. The influence of IPT on welding residual stress relies on the combined contribution of thermal contraction and martensitic transformation. When the IPT is lower than martensite transformation finishing temperature (M_f), thermal contraction plays the dominant role in the formation of welding residual stress, and tensile stress was formed in the majority of weld zone except the final weld pass. While, compressive stress was formed in almost whole weld zone due to martensitic transformation when the IPT is higher than martensite transformation starting temperature (M_s).