Microstructural Characterisation of Copper-Stainless Steel Brazed Joint with Copper Based Filler Metal

Abstract

Copper to stainless steel brazed components are used in the thrust chamber of cryogenic propulsion system of Indian Space Programme. Temperature and time play a major role in the optimization of brazing cycle. An effort has been made to study the influence of the above parameters and correlate with the microstructure and shear strength achieved. In the present work, brazing was performed using Cu based filler metal. Temperature selected was within the range of 1010-1045 °C for two different time durations (20 minutes and 30 minutes) under high vacuum (10-5 Torr). Extensive scanning electron microscopy with energy dispersive spectrometry was carried out to understand the kinetics of elemental diffusion and phase formation at and near the brazed joint. Microhardness was evaluated across the joints to facilitate further understanding of the joint chemistry. The optimized shear strength of the joints was as high as 155 MPa for the joint brazed at 1030 °C for 30 minutes against the required strength of 150 MPa. The joint could retain strength at cryo temperature and meet the minimum expected value of 320 MPa. At the same time, it exhibited good mechanical properties at elevated temperatures. This paper brings out details of the investigation carried out.