Structural Characterization of Gas Tungsten Arc Welded Stainless Steel at High Heat Input

Abstract

Performance integrity of weldment differs considerably from the base metal due to microstructural changes, resulting from welding heat input effect. In this study, effects of high gas tungsten arc welding heat inputs on microstructure of 304L austenitic stainless steel (ASS) weldments were investigated. The weldments were produced based on ASTM A778/778M at GTAW speed, current and voltage of  1.7 mm/s, 200 A and 40 V respectively. After which, they were characterised, using X-ray diffraction (XRD) and optical microscopy. The experimental samples, both control and weldments were prepared for the XRD and microstructural analysis following ASTM E 975-03 and ASTM E3 – 11 respectively. Results showed that control sample and weldments were characterized by varying amounts of major and minor compounds. Crystal structure of the control sample is comprised of a mixture of face centred cubic (FCC) and body centred cubic (BCC) with major peak (111). While those of the 1.7 mm/s, 200 A and 40 V produced weldments were comprised majorly of FCC with major peak (511), BCC with major peak (211) and BCC with major peak (110) respectively. Microstructure of the control sample is homogenous, comprising of equiaxed – grained austenite (γ) matrix with precipitates of varying sizes (dark spots), which are distributed non-uniformly within the γ matrix and small amounts of δ-ferrite (dark lines) along the γ matrix grain boundaries. While microstructures of the weldments are heterogeneous, comprising of austenite (γ) matrix and ferrite (α) grains, which are dispersed within the γ matrix. Generally, grains of the fusion zones (FZs) microstructures are fine relative the heat affected zones (HAZs) microstructures. The FZs and HAZs microstructures are characterised by precipitates, δ-ferrite, inclusions and dendrites of varied number and size.