Repair Process for Turbine Casting

Alloy706 is used as exhaust casing material in gas turbines of cryogenic rocket engines in view of its excellent properties. The defective zones in super-alloy castings usually are repair welded in solution treated condition to obtain good room temperature and high temperature properties. However a need came-up to carry out in-situ repair welding in one of the gas turbine castings made of aged Alloy 706 in cryogenic rocket engine with a limitation that the component cannot be subjected to any heat treatment cycle subsequent to the repair welding. Hence it became necessary to develop process and process parameters with reduced strength mismatch and HAZ free from micro-fissures in order to obtain required mechanical properties and further high temperature service associated with high pressure hydrogen environment. This has been approached with process control using reduced heat input and alloying control.In order to reduce the heat input, two processes such as GTA welding and GTA braze welding have been adopted with multi pass technique.