experimental investigation on the effect of post weld heat treatment parameters on sensitization recovery of auto tig welded austenitic stainless steels tube

Abstract

newline quotAustenitic stainless steel plays a crucial role in the final phases of superheater/reheater tubing used in supercritical boilers. Its notable resistance to intergranular corrosion is essential, contributing to increase overall efficiency and the diminished emission of NOx in power plants. If these steels are subjected to post weld heat treatments in the temperature regimes of 400 900°C due to the requirement of essential dissimilar joints between austenitic stainless steel and Cr-Mo-V steel, chromium carbides are formed along the grain boundaries leading to chromium depletion in the vicinity of these carbides. This is known as sensitization and makes the microstructure sensitive to intergranular corrosion. Regaining the chromium percentage in the vicinity of the Cr depleted zone near the grain boundary is referred to as recovery of sensitization (desensitization) which may occur owing to several reasons. The sensitization and desensitization behavior are greatly influenced by several heat treatment factors such as soaking temperature, soaking time and cooling rate. In order to study the effect of above-mentioned individual factors, Auto TIG welding using optimized welding parameters has been performed on Nb-stabilized TP347H and TP304 grade austenitic stainless steel tubes followed by suitable post weld heat treatments (PWHT). newline newlineThe study examines and discusses the impact of PWHT parameters on microhardness, microstructural (Cr23C6, Nb carbides and twin boundaries) and intergranular corrosion properties of grade 347 and 304 weldment. In grade TP347H weldment, significant decrease in microhardness from 280 to 180 VHN has been observed with an increase in PWHT soaking time up to 5 hours. However, an additional increase in soaking time (10 and 15 hours) leads to minor increase in microhardness from 180 to 200 VHN in base, weld, and heat affected zone due to formation of annealing twins in base metal and precipitation of Nb carbides within weld and heat affected zone validated by optical microscopy an