Brazing of Martensitic Stainless Steel to Copper Using Electroplated Copper and Silver Coatings
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Brazing of Martensitic Stainless Steel to Copper Using Electroplated Copper and Silver Coatings T. Venkateswaran, Vincent Xavier, D. Sivakumar, Bhanu Pant, P.K. Jayan, and G.D. Janaki Ram (Submitted May 7, 2018; in revised form September 15, 2018) Brazing of 3 mm thick sheets of low-carbon martensitic stainless steel and Cu-Cr-Zr-Ti alloy was attempted using electroplated coatings of Cu and Ag. Ni coating of 5 lm thick was provided on the MSS base metal to enhance the wettability. Brazing experiments were carried out at 985 °C for a holding of 15 min. Detailed microstructural studies, hardness, lap-shear, tensile, and pressure tests were carried out on the brazed joints. The reaction between the Cu and Ag coatings lead to the formation of in situ braze metal. The microstructure of the braze metal was found to consist of Cu-rich primary alpha (aP) and Ag-rich eutectic mixture (b + aE). In lap-shear tests, the brazed joints produced with an overlap of 9 and 6 mm were found to fail in copper base metal, while the joints produced with an overlap of 3 mm failed in the braze metal exhibiting a lap-shear strength of 147 MPa. In tensile tests, the butt brazed joints were found to fail in copper base metal as well with an average tensile strength of 213 MPa. The braze metal hardness was found to be higher than the hardness of the copper base metal. In pressure tests, the brazed panels were found to survive a pressure of 750 bar without undergoing any damage. These results show that electroplated coatings of Cu and Ag can be advantageously utilized for brazing of large, complex-shaped parts such as thrust chambers in space industry. Keywords
Ag coating, brazing, Cu coating, martensitic stainless steel
1. Introduction Liquid propulsion engines are widely used in space launch vehicles. Thrust chamber, a regeneratively cooled structure, is an important sub-system of these engines (Ref 1). The thrust chamber consists of an outer shell and an inner shell with several ribs to form the cooling channels. One of the liquid propellants is circulated as coolant through these channels, which reduces heat accumulation (due to combustion of propellants) in the thrust chamber (Ref 1-4). The inner shell is usually made of a copper alloy with high thermal conductivity (to efficiently transfer heat to the coolant) and sufficient strength (to meet the pressure requirements) (Ref 5). The outer shell is usually made of a low-carbon martensitic stainless steel to provide adequate structural stability and corrosion resistance. The thrust chamber is typically produced by brazing the copper inner shell to the stainless steel outer shell. Brazing is the only viable process for the above-stated application, and joining by any other processes is not feasible due to complex geometry of the parts, dissimilar materials, and thickness. T. Venkateswaran, Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Trivandrum, India; and Department of Metallurgical and Materials Engineering, Indian Institute of Technolo
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