Evaluation of the effect of TIG surface quenching process of S45C steel on the hardness of cylindrical surface layers by Taguchi method

Abstract

This study investigates the influence of current intensity, relative travel speed of the TIG torch on the cylindrical S45C steel surface, and axial displacement speed on hardness distribution in deep layers after arc quenching. The process forms three distinct zones: quenched, heat-affected, and base metal. The quenched zone transforms from the original ferrite–pearlite structure into martensite, residual austenite, and bainite. The heat-affected zone contains bainite, pearlite, and ferrite. These phase variations result from rapid heating and cooling. Hardness evaluation across 25 cases shows the 0.4–0.6 mm range provides the most stable and highest hardness. Taguchi analysis reveals that axial travel speed mainly affects arc width and has little influence on hardness by depth. In contrast, current intensity strongly impacts heat input: higher current increases heating from the TIG tip, while higher relative travel speed reduces heat input. The highest hardness values were identified at different depths: 37.7 HRC at 0.2 mm (case 17), 38.2 HRC at 0.4 mm (case 21), and the maximum for 0.6 mm occurred in case 2 with 42.3 HRC. At deeper layers, hardness increased significantly, with 43.8 HRC at 0.8 mm (case 1) and 41.7 HRC at 1 mm (case 1). The results of the study confirm that variations in current intensity and relative travel speed play decisive roles in determining the hardness distribution of S45C steel subjected to TIG arc quenching.