Effect of shielding gas on the properties and microstructure of melted steel surface using a TIG torch

Abstract

A surface engineering technique based on a tungsten inert gas (TIG) torch was used to melt single tracks on the surface of a microalloyed steel with a hardness of 150 HV. The influence of three shielding gases, argon, helium and nitrogen, on the microstructure and hardness of the re-solidified surfaces was analysed. In all melting techniques, the heat generated by the source is normally conducted to the substrate ahead of the torch and has been described as ‘preheat’. This leads to a gradually higher substrate temperature, from the start to the finish of a melted surface track. The aim of this research was to analyse any inhomogeneities in the microstructure, due to ‘preheat’, which is rarely considered in the published literature. Three thermocouples were located along the melted track in order to record the temperature at three different points. An energy input of ~840 J/mm was used in each experiment, and the results show that the maximum temperature recorded by the last thermocouple, No three (subjected to the preheat), for argon, helium and nitrogen gas was 590, 1120 and 740 °C, respectively, where a difference of 150 and 200 °C was registered between the first and third thermocouples when using helium and nitrogen, respectively. The corresponding hardness values were 170, 162 and 225 HV, and the corresponding surface roughness values were 6, 12 and 25 μm. A decrease by almost 60% in the roughness value was observed between the initial and last stage of the melted track, when using argon as shielding gas.