INVESTIGATIONS ON FORMATION MECHANISMS OF BRAZING CRACKS AT THE AUSTENITIC STAINLESS STEEL/FILLER METAL BRAZING JOINT INTERFACES

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 thelow 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 interlace arc 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.