对316LN奥氏体不锈钢的钎焊界面裂纹形成机制进行了系统研究.首先以Cu-Si和Ag-Cu-Sn两种含Cu钎料对316LN不锈钢管进行了氩弧钎焊,根据焊后工件的打压泄漏情况, 使用OM和SEM观察了钎焊界面的微观组织和裂纹形貌,并用EDS分析了裂纹内表面成分. 结果显示, 钎焊界面裂纹在母材/钎料界面萌生,沿母材晶界扩展, 裂纹内可检测到钎料中的元素, 且裂纹开始形成于钎料凝固之前.为进一步确定钎焊裂纹产生机制, 进行了316LN不锈钢的Cu-Si钎料浸渍实验,Cu-Si钎料真空钎焊实验, 以及Ag-Sn, Ag-Al钎料和Ni焊丝氩弧钎焊实验,证明了钎料中的低熔点元素沿母材晶界扩散导致的晶界弱化,以及钎焊过程中母材内温度梯度导致的内应力是钎焊界面裂纹发生的必要条件.最后定性描述了钎焊界面裂纹的形成过程, 并讨论了抑制该类裂纹的钎焊工艺.

The austenitic stainless steels are widely used as structural materials of the workpieces served in severe environments, while the interfacial cracking in brazing joints of the austenitic stainless steels is a drawback limiting their application. Thus far the reports have not comprehensively revealed the formation mechanisms of the brazing cracks. To help solving this problem, formation mechanisms of the interfacial cracks at the 316LN stainless steel/filler metal brazing joints were comprehensively investigated in this study. The 316LN cooling pipes were firstly arc brazed with Cu-Si and Ag-Cu-Sn filler metals. According to the leakage test results of these pipes, microstructures of the brazing joints and the interfacial cracks were observed by OM and SEM, and compositions around the cracks were analyzed by EDS. The results show that the interfacial cracks initiate at the 316LN/filler metals interface and propagate along the grain boundaries of the stainless steel, elements of the filler metals were detected in the cracks, and it is confirmed that the cracks formed before solidification of the filler metals. To further reveal the crack formation mechanisms, verification tests including dipping (1100℃, 30 s) and vacuum-brazing (1100℃, 10 min) of 316LN with Cu-Si filler metal, arc brazing of 316LN with Ag-Al, Ag-Sn and Ni filler metals were conducted. The cracking was not observed at the vacuum-brazed 316LN/Cu-Si joint interface and the arc-brazed 316LN/Ni interface, but the other three brazing joints show similar cracking behaviors with the 316LN/Cu-Si joint. Base on the results, it was predicated that weakening of the grain boundaries in the 316LN induced by GB diffusion of the low melting point elements, and the brazing stress result from the temperature gradient in the 316LN substrate material during the brazing process are necessary formation conditions of the brazing cracks. Cracking at the brazing joint interface are affected by composition of filler metals, heating rate, thermal input, and heat treatment conditions of substrate materials. Brazing techniques were optimized according to the findings, and it was found that occurrence of the cracks can be restrained through decreasing the temperature gradient or avoid to used the low melting point elements-contained filler metals.