Abstract:

After heat treatment of metal active gas arc welding (MAG weld) joints of 316LN austenitic stainless steel for fusion reactor at different temperatures, Charpy impact test at liquid nitrogen temperature was carried out. The effects of heat treatment temperature on microstructure, fracture morphology and precipitates were studied with optical microscope, SEM and EDS analysis. Results indicated that the impact toughness of weld metal could be significantly improved by 873K heat treatment. However, with the rising of heat treatment temperature, the precipitates were generated gradually along the grain boundary and a gradual decrease of toughness occurred as well. All fracture was ductile. The dimple became shallow and less with the increase of temperature. Spherical and irregular precipitates were found at the bottom of the dimple. The spherical precipitates generated during welding were free from the influence of the heat treatment, and the irregular precipitates increased with the rising of temperature. High content of Mo in filler materials led to a great segregation of Mo at the grain boundary, which would promote the precipitation of σ phase. Impact toughness of weld metal decreased significantly when continuous distribution of σ phase formed at grain boundary.

Key words: MAG welding, 316LN, Heat treatment, Impact toughness; &sigma, phase

摘要：

对聚变堆用316LN奥氏体不锈钢熔化极活性气体保护电弧焊(MAG焊)接接头进行不同温度的热处理，并在液氮温度下进行夏比冲击试验。利用光学显微镜、扫描电镜、EDS分析等研究了热处理温度对接头微观组织、断口形貌及析出物的影响。结果表明，873K热处理可以显著提高焊缝金属冲击韧性，但随着热处理温度的上升，焊缝金属逐渐出现沿着晶界分布的析出物，韧性逐渐下降。断口均为延性断裂，但随着热处理温度的升高，韧窝变浅、数量变少。韧窝底部存在球状析出和不规则状析出，球状析出在焊接过程中产生，不因热处理温度而变化，不规则析出随着热处理温度的升高逐渐增多。焊材中的Mo含量过高导致焊缝金属中Mo在晶界大量偏聚，促进了σ相的析出，当σ相在晶界形成连续分布后，焊缝金属冲击韧性显著下降。

关键词: 熔化极活性气体保护电弧焊；316LN；热处理；冲击韧性；&sigma, 相