Cr-Mo钢管子局部焊后热处理加热宽度准则的确定

Determination of heated band width criterion of local postweld heat treatment for Cr-Mo steel pipe

关键词:  局部焊后热处理  低温相变  Cr-Mo钢  数值分析

Key words:  local post weld heat treatment  low temperature phase transformation  Cr-Mo steel  numerical analysis

摘要:

采用粘弹塑性有限元方法从应力释放角度讨论了Cr-Mo钢局部热处理下各种优化加热条件。研究表明,局部热处理过程产生的瞬态和残余应力受蠕变性能和温度关系弹性模量等因素的影响。当加热宽度足够大时,局部热处理后的应力与炉中热处理的应力接近。通过与现有标准的比较,从应力释放角度位于焊缝中心5√Rt的加热宽度较为合理。提出了考虑焊接冷却过程Cr-Mo低温膨胀现象的相变模型。给出了焊接残余应力与局部焊后热处理应力的试验结果与模拟结果的对比。

Abstract:

Optimum heating conditions for local post weld heat treatment(PWHT) of Cr-Mo steel pipe were discussed from the point of view of residual stress using thermal-visco-elastic-plastic finite element method(FEM).The whole history of the stress relaxation during local PWHT could be clarified through FEM analysis.The investigation showed that the transient and residual stresses induced by local PWHT were significantly affected by creep behavior and temperature dependency of Young's modulus.The stresses decreased quickly in the heating stage,with decreased yield strergth.They continued to decrease slowly according to the creep law in the holding stage and increased slightly due to recovery of Young's modulus when the cooling stage started.The residual stress after PWHT decreased with increase of heated band width.When the heated band width was large enough,the residual stress after local PWHT become close to that of the uniform PWHT.Compared with the existing codes,it was found that the heated band width of 5√Rt on center of the weld seems reasonable from the residual stress point of view.A new phase transformation model was proposed considering the low temperature expansion during the cooling stage of welding of Cr-Mo steel pipe.Experiment results of welding residual stress and the stress after local post weld heat treatment are compared with numerical simulation.