INFLUENCE OF TIG PROCESS PARAMETERS ON HARDNESS, TENSILE STRENGTH AND MICROSTRUCTURES OF AISI430 FERRITIC STAINLESS STEEL WELDS

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Department of Engineering

ABSTRACT
The influence of tungsten inert gas (TIG) welding process parameters on the microstructural features and mechanical properties (hardness and tensile strength) of AISI430 ferritic stainless steel (FSS) welds based on the central composite design (CCD) approach, was investigated. The process parameters considered in this study include; the welding current (I), welding speed (S) and argon flow rate (AFR). The stainless steel AISI 430 FSS plates having chromium content of 17% Cr and 1.5mm thickness were butt-welded using autogenous tungsten inert gas welding process. The microstructures of the FSS welds were analyzed using optical microscope. The hardness and tensile strength were determined using Vickers’s microhardness testing machine and Hounsfield tensile testing machine respectively. The results showed that all input variables considered have direct influence on the properties as well as the microstructure of the weldments. Microstructural result reveals ferrite structures aligned in the cold rolling direction for the base metal. The microstructure of the weld metal at the optimized condition shows the presence of equiaxed ferrite grains with some grain boundary marten site structures within the fusion and heat affected zones of the weldment. It was observed that welding current shows the greatest effect on the tensile strengthwith34% contribution, followed by the speed and argon flow rate with 15% and 2.54% respectively. For the hardness, the speed plays the major role. Empirical models were generated from the analysis to predict the weld quality. An optimized tensile strength and hardness of 431.014MPa and 307.10 HV respectively, were predicted at the welding current of 22A, the welding speed of 5mm/s and the argon flow rate of 10L/min. In order to validate the results, confirmatory experiments were performed based at the optimized condition and the test results were found to be in good agreement with the predicted values with percentage errors of 1.23% and 1.81% for the tensile strength and hardness respectively. Based on this finding, it can be concluded that the optimization of TIG input parameters using response surface methodology approach produced weld joint with good tensile strength and improved hardness that is very close to that of the base metal.