Influence of direct current (DC) on mild steel coating stainless steel rods via TIG welding: A model for the rehabilitation of steel components

Abstract

In this paper, the influence of varying DC current on microstructures of mild steel substrates coated with stainless steel using TIG welding technique was investigated. To achieve the desired results, mild steel substrates of sizes 40 mm × 10 mm × 1.5 mm were used. The mild steel substrates were ground using Silicon Carbide (SiC) papers to achieve smooth surfaces and to remove oxides that might be present on their surfaces. Then the substrates were coated using stainless-steel rods on TIG 2200i AC/DC machine as a welding power source. The process was undertaken under six varying currents of 40 A, 50 A, 55 A, 60 A, 65 A and 70 A. For purpose of characterization, microstructures of the coated samples were studied using optical microscopy and quantitative image analyses. The depth of penetration of the melt pool below the substrates and the coating thickness were measured for all the samples. Based on the optical microscopy, four zones were identified on the TIG coated samples; the heat-affected zone (HAZ), fusion, coating and a skirt (the region that formed between the substrates and the coatings). Minute porosities were also observed in the coated samples. It was observed that the maximum coating thickness was 5.345 mm and the highest average coating thickness was 3.522 mm, which both occurred at 40 A. The maximum length of penetration was 3.864 mm and was observed when 60 A DC was used. Similarly, the highest average depth of penetration was 2.422 mm and occurred at 65 A. The results of this study showed that for direct current, as the current increased from 40 A to 50 A, the porosities in the coated samples increased. However, at 55 A, the number of porosities decreased, but continued to increase from 60 A to 65 A, but the porosities decreased again at 70 A.