Investigation of effect of post weld heat treatment on microhardness and microstructure of auto TIG welded stabilized SA213 TP347H weldments

Abstract

Investigation explores the weldability of Niobium stabilized TP347H austenitic stainless steel tubes through the application of optimized welding parameters using Auto TIG welding. This research investigates the influence of post weld heat treatment conditions on microhardness, microstructures, sensitization due to Cr23C6 precipitation, presence of Niobium carbides and formation of twin boundaries in the weldment. Extended post weld heat treatment duration resulted in notable decline in microhardness from 280 to 180 VHN within initial 5-h of soaking. Nevertheless, further prolongation of soaking time (10–15 h) exhibited subsequent rise in microhardness levels to 200 VHN in base, weld and heat affected zone. Increase in hardness is attributed to the emergence of annealing twins in base metal and the precipitation of Niobium carbides within weld and heat affected zone, confirmed by advanced analytical scanning electron microscope. Energy dispersive spectrum line mapping reveals presence of Cr23C6 precipitation following 1-h soak at 720 °C. Subsequent prolongation of soaking time (5-h) demonstrates desensitization in the base metal. Additionally, cooling rate of 300 °C/hr during post weld heat treatment reduces required time for microstructure homogenization, resulting in increased hardness compared to 100 °C/hr rate in the weldment. However, beyond a 5-h soaking time, the cooling rate has a negligible impact on microhardness. Ultimate Tensile Stress of 674.82 and 665.28 N/mm2 for TP347H was achieved against the minimum requirement of 515 N/mm2 as per American Society of Mechanical Engineers Section IX standard.