Papers by Author: S. Chenna Krishna

Abstract: Among the copper alloys, the Cu-3Ag-0.5Zr alloy is one of the potential candidates for combustion chamber of liquid rocket engine because of its optimum combination of high strength with thermal conductivity. The present study is a detailed characterization of microstructure, strength, and electrical conductivity during the aging treatment. The aging cycle for Cu-3Ag-0.5Zr alloy after the solution treatment (ST) was optimized to obtain higher hardness without compromising on electrical conductivity. The precipitates responsible for strengthening in aged samples are identified as nanocrystalline Ag precipitates with an average diameter of 9.0±2.0 nm.

Abstract: Cu-Cr-Zr-Ti alloys are widely used for fabrication of thrust chamber in liquid rocket engines, because of their high thermal conductivity and adequate strength. The alloy should be used in peak-aged condition to achieve the best combination of strength and conductivity. However, realization of final component involves forming and brazing operations which limit the usage of the alloy in peak aged condition. Therefore, an attempt has been made to study the effect of the simulated brazing treatment on the microstructure and mechanical properties using optical microscopy, electron microscopy, mechanical testing and hardness measurement. The high temperature brazing treatments resulted in tremendous grain growth. The measured hardness is observed to be in line with the grain size of the specimens. The hardness and tensile strength decreased in samples subjected to simulated brazing cycle, implying requirement for a post braze treatment. After the post brazing treatment the optimum mechanical properties have been achieved at room temperature. Low ductility was observed at 600°C in the samples subjected to simulated brazing cycle.

Abstract: Inconel 718 is widely used superalloy in the Indian space program for high temperature application. Some of the newer applications envisage use of this alloy in very critical high pressure oxygen carrying vessels. The alloy is frequently used in welded condition which requires extensive characterization of various types of welds viz Electron beam welding (EBW) and Gas tungsten arc welding (GTAW). In many cases the weldability of Inconel 718 is found to be limited by microfissuring phenomenon in the weld heat affected zone. Microfissures are fine intergranular cracks and their severity strongly depends on pre-weld solution treatment temperature (grain size), weld heat input and concentration of impurities (B, P and S) in the base metal. In the present work, a study was undertaken to compare the microfissuring tendency in EBW and GTAW processes using two pre-weld solution treatment temperatures. The samples were solution treated at 970°C and 1050°C to generate different grain sizes. Amount of heat input and cooling rate were calculated since they are known to affect the microfissuring and an effort was made to understand their role on the microfissuring. It was observed that microfissuring susceptibility is more at coarser grain size. Severity is more in EBW. The reasons for this phenomenon have been discussed in this paper correlating microfissuring with microstructures and other factors. Procedures to achieve minimal microfissuring during welding have also been brought out.