CHARACTERIZATION OF WELD HEAT INPUT EFFECTS ON MECHANICAL PROPERTIES OF Cr-Mo STEEL BAR USING TIG WELDING PROCESS

Abstract

Welding as an energy-consuming process is inevitable as-welded joint can be subject to various loads without failure. Therefore, this paper presents the welding thermal cycle of Cr-Mo steel bar (ASTM A304) of dimensions 100 by 50 and various thicknesses of 5, 10- and 15-mm. Pure tungsten with 2% thoriated TIG electrodes sizes 1.6 mm X 175 mm, 2.4 mm X 175 mm, and 3.2 mm X 175 were used without filler materials for the welding process. A double V-groove weld joint, with a moving heat source, was employed to determine the temperature fields and transformation in the single-pass butt-welded joint. A calibration process was attached at each point of interest using a datalogger type K-type thermocouple 3-channel–LU-MTM-380SD during the welding process on the Cr-Mo steel bar. The results showed that the welding temperature became higher at the welding centreline and decreased towards the edges of the bar. An indication that a weld thermal cycle is a veritable tool, a function of heat input to access likely consequence of the welding process at both welded and parent metal portions of steel bar. Design of Experiment using Taguchi L9 orthogonal array matrix L9(3^4), Factors:4 and Runs:18 in Minitab 17 Taguchi Design Method that suited the experimental method used. Taguchi’s L9 orthogonal array to restrict the number of experimental runs was used for the design of the experiment (DOE). Mechanical and Microstructure tests were carried out on the samples to investigate the effect of weld heat input. The hardness test result showed that samples C15 and D15 have the highest hardness values 165.0HV and 164.0HV respectively at Base metal (BM) 20mm away from weld centreline and it was also observed that samples C15 and D15 have the highest impact values 48.53J and 48.7J respectively. The microstructure of the C15 at the weld zone WZ, which consists majorly of pearlite and less ferrite, BM shows the appearance of alpha ferrite and pearlite and heat-affected zone HAZ consists majorly of pearlite and a very small proportion of ferrite resulted in increased hardness and impact values at the HAZ.