Studies on the effects of cryogenic cooling on microstructure and mechanical properties of plasma arc welded SS 316

Abstract

The stainless steel of grade 316 has significant use in nuclear engineering, aerospace industry and submarines and has become a material of choice due to its diversified properties. While welding this material, the heat input supplied weakens the fusion zone by grain growth, widening the heat-affected zone (HAZ), reducing yield and tensile strengths, and introducing distortion. Therefore, to address these issues, this study investigated the effect of cryogenic cooling during the welding process of thin sheets of SS 316 to improve the microstructure, mechanical properties, and reducing the distortion of the welded material. The keyhole mode plasma arc welding process was used to weld in a single pass without using filler with three different cooling methods. The microstructures, microhardness, and tensile properties of cryogenically and conventionally cooled weld samples were investigated at room temperature. The microstructural behavior of samples was characterized by metallurgical microscopy and scanning electron microscopy. The SEM Analysis reveals γ austenite and δ ferrite phases in conventionally welded test samples. M23C6 is formed in small amounts from δ ferrite, surrounding the δ ferrite on grain boundaries. In cryogenic cooled samples, delta ferrite is detected on grain boundaries of the austenitic matrix. Additionally, traces of (Cr, Fe)2 N are also revealed in specific cryogenic cooled samples due to liquid nitrogen impingement. In gel cooled samples, M23C6 is shown due to comparatively prolonged duration of cooling, and the results reveal that the liquid nitrogen and thermo gel improved average grain size up to 83.53% and 66.84%, respectively, as compared to an average grain size of conventional plasma weld. The reduction in HAZ is observed to be about 43.38% and 7.92% for liquid nitrogen cooled and thermo-gel samples, respectively, compared to conventional weld. Moreover, the tensile and yield strength of liquid nitrogen-cooled weldments increased up to 22.28% and 28.96%, respectively, while for gel-cooled welded sample, a 10.50% improvement in tensile strength and 3.10% in yield strength was observed. Furthermore, a reduction of 75% in distortion is achieved for welded samples with liquid nitrogen cooling.