Metallurgical Characterization of Defect Observed During Ultrasonic Testing (UT) of Copper Alloy Forgings Used in Cryogenic Engines with the Aid of Computed Tomography

The inner lining of the cryogenic steering engine, used for thrust vector control in the upper stage of Indian launch vehicles, is made from shaped forgings of the indigenized Russian copper alloy, Cu-0.5Cr-0.05Ti-0.05Zr, due to its superior thermo-structural properties. This product is realized by 5-axis milling of the forging after being qualified through conventional ultrasonic testing (UT) to class A level of AMS 2630, using both normal and angle beam techniques. Recently, inclusions coupled with cracks opened to the surface at an advanced stage of machining of UT cleared forgings. Due to the defect severity, the other forgings belonging to the same batch were recalled and re-inspected using conventional UT and radiographic testing (RT). Surprisingly, a large defect was detected in one of the recalled forgings. The sample (with defect) that was extracted from the defective forging with the results of UT & RT was then subjected to Computed Tomography (CT) for determining the precise location of the defect. Several rounds of wire EDM and CT were performed to increase the defect width to sample width ratio by removing excess parent material. Subsequently, this sample was pulled open at the defect using a universal tensile testing machine and extensive metallurgical characterization was performed using optical microscope (OM), stereo microscope, and environmental scanning electron microscope (ESEM). The defect was identified as graphite inclusion that was traced to the graphite crucible used for melting of the alloying elements (during realization of input billets). Results of the NDE in conjunction with the material characterization studies provided inputs to improve both the UT capability and the melting process. The detailed analysis that was performed in this study is presented in this paper.