Influence of liquid nitrogen on residual circumferential distortion, weld penetration and delta-ferrite distribution in clamped tungsten inert gas welded butt joint of 316 stainless steel

Abstract

AISI 316 stainless steel (SS316) is an attractive material for industrial applications. Welding of this alloy can lead to severe distortion that often results in dimensional inaccuracies. With the help of specially designed fixture and clamping mandrels, tungsten inert gas (TIG) welding on SS316 pipes is employed to investigate the effect of liquid nitrogen on the circumferential distortion, weld penetration and delta-ferrite distribution in different welding zones. The experimental results reveal that the presence of trailing heat sink in TIG welded butt joint has almost zero distortion at a distance of 30 mm from the weld centerline and 64% decrement in residual circumferential distortion at the center of the weld bead. A metallurgical microscope is used to find the weld penetration, (fusion zone) FZ and (heat affected zone) HAZ. The results illustrate that increase in current and the presence of intensive cooling media yield deeper penetration in TIG welding. Liquid nitrogen (LN2) also constricts the FZ and HAZ. Instead of evaluating delta-ferrite contents using the ferrite number technique, a MATLAB® code for image processing is developed to evaluate the delta-ferrite distribution in the different regions of a single-pass weldment. Delta-ferrite contents are maximum in the presence of intensive cooling media near HAZ and increase with an increase in the current value. Hence, TIG welding with LN2 as trailing heat sink is the most suitable scheme to weld industrial pipes owing to its higher weld penetration, higher delta-ferrite contents and minimum circumferential distortion.