Brazing of ferrite stainless steel to austenite stainless steel

摘要

In these days, brazing of ferrite stainless steel (F-SS) is very important technology for fabrication of EGR cooler. However there are a few brazing studies of F-SS by Ni-based brazing filler metal. On the other hand, it should be needed that F-SS is brazed to austenite stainless steel (A-SS) by Ni-based brazing filler metal when EGR will be fabricated for an automobile. On our previous works, it was cleared that dissolution of F-SS was occurred slightly as compared with that of A-SS. Purpose of this study is to clarify an interfacial reaction mechanism during brazing of F-SS with a few brazing filler metals, and to obtain sound brazed joint of F-SS to A-SS. Typical commercial A-SS SUS316L (JIS), F-SS SUS444 (JIS) and F-SS SUS430J1L (JIS) were used as base metals in this study. Ni-based brazing filler, BNi-2 and BNi-5, and Ni-Cr-Si-P filler were employed as brazing filler metal. T-joint specimen was used for estimation of fillet form-ability of brazing filler metal to base metal and investigation of interfacial reaction during brazing. All specimens were clumped by stainless steel jig. Brazing was done by electric heated vacuum furnace. Heating rate was 10 K/min. And brazing temperature was determined for consideration of liquidus temperature of each brazing filler metal. Brazing times were 5, 10 and 30 min. Brazingability was estimated by cross-sectional observation results and EPMA elemental distribution results and mechanical properties after brazing. Following typical results were obtained in this study: (1) Brazed layer at the brazed joint, which was brazed with Ni-Cr-Si-P filler, consisted of remained eutectic, primary crystal and Cr_4Ni_3P_4 IMC. (2) Brazed layer at the brazed joint, which was brazed with BNi-5 filler, consisted of remained eutectic, primary crystal and Ni_3Si IMC.