ITER热锚TIG钎焊采用铜基钎料致316L不锈钢渗透裂纹的研究

Liquid copper induced cracking of 316L austenitic stainless steel in TIG brazing of ITER's thermal anchor

热锚是安装在ITER磁体支撑上,内部通有液氦,用以减少磁体支撑对超导线圈的热传导,保证超导线圈正常运行的一个关键部件。由于磁体支撑运行在低温,强电磁力冲击等复杂环境下,故要求热锚连接应具有较高的强度、韧性以及热导率。采用铜基钎料的TIG钎焊被选为热锚安装的主要方法,但是工程测试结果显示,由于在焊接过程中铜基钎料渗入316L奥氏体不锈钢晶界并沿着晶界扩展,导致不锈钢管出现裂纹并产生泄漏。通过金相、扫描SEM以及能谱EDS对采用不同钎料不同钎焊方法制作的接头试样进行分析对比,得出液态金属对固态金属的扩散渗透以及焊接部位所受应力都是影响液态金属渗透致裂的重要因素,在二者共同作用下导致固态金属脆化并出现裂纹。

Thermal anchor is one of the key components of ITER magnet supports and serves to maintain them at low temperature for the normal operation of superconducting coils.Thermal anchor pipes which contain liquid helium are connected to reduce the heat transferred from supports to superconductor.High strength,high toughness and thermal conduction are required for the connection.Argon arc brazing with Cu- based filler was used as the main method to install the thermal anchor.The preliminary test result shows that the leakage in some brazed area of the pipe was found.The further metallographic examination shows that the leakage is caused by the cracks on the wall of 316L austenitic stainless steel cooling pipes,originates from the surface of stainless steel and expands along the grain boundary of austenite.Metallographic examination,SEM observation and EDS analysis were used to analyze the joints welded with different filler metals and different brazing methods.The analysis results show that both the penetration of liquid metal to solid metal and stress in the joints are key factors caused the liquid metal induced embrittlement and cracking.