| Repair welding of ASTM A-470 class 8 high pressure steam turbine rotor steel has been studied. The welds coupon plates were machined from a rotor that has been retired after 20 years of service. Three different types of welding processes were studied i.e., multipass submerged arc welding (SAW), tungsten inert gas welding (TIG), and metal inert gas welding (MIG). The welds were analyzed by microhardness test, stress rupture test, optical and electron microscope investigation and X-ray diffraction.;Samples from welds made with the various welding processes were subject to stress rupture test at 593;The type and morphology of the carbides were examined by transmission electron microscope using aluminum extraction replica. The retired rotor steel had three different types of carbides, which were identified as molybdenum rich M;Many creep cavities were observed along the rupture surface. It was observed that most of the cavities lead to the final fracture during stress rupture test. The creep cavity density as well as the carbide coarseness and population were correlated with the welding heat input. As the heat input was increased, the carbides coarsened to a higher degree, which caused the low density of cavities resulted in a longer stress rupture life.;Microhardness transverse was conducted across the weldment from unaffected base metal (BM) to weld metal. A drop in hardness was observed in the heat affected zone (HAZ) close to unaffected BM. This region of hardness drop, called as softening zone, had a width of 0.5-0.6mm for SAW weld. |
