Effect of Heat Treatment on Microstructure of Steam Turbine Shaft Overlaid by TIG Welding

Welding of power plant shafts have many problems like، martensite formation، remained Austenite، sensitivity to cold crack and residual stress. Accurate selection of welding process and variable، determination of suitable filler metals and appropriate post weld heat treatment will decrease this kind of problems. Because of existence of carbon and alloying elements in steel shaft 26 NiCrMoV 11-5، low hydrogen Cr-Mo fillers for overlaying welding on steel shaft surface was selected. Normalizing and Annealing heat treatments were selected and compared with as-welded specimen. The purpose of this work was specifying desirable post weld heat treatment and evolution of microstructure in weld and HAZ zones with micro hardness، LOM، SEM and XRD. According to metallographic and XRD results it was concluded that in welded specimen with Cr-Mo filler rod and normalized heat treatment، the amount of carbide phases were decreased، and hardness variation in weld zone and base metal was low when compare with other process. As a result، welded specimen with Cr-Mo filler rod and normalized heat treatment had appropriate microstructure and uniform hardness distribution.