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

Munoz De Escalona, Patricia and Mridha, Shahjahan and Baker, neville (2014) Effect of shielding gas on the properties and microstructure of melted steel surface using a TIG torch. In: International Conference on Advances in Materials and Processing Technologies, AMPT 2014, 2014-11-16 - 2014-11-20, Atlantis the Palm.

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A surface engineering technique based on a Tungsten Inert Gas (TIG) torch was used to melt single tracks on the surface of a micro-alloyed steel with a hardness of 150 HV. The influenceof three shielding gases, argon, helium and nitrogen, on the microstructure and hardness of the re-solidified surfaces was analyzed. 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 analyze 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 ~ 840J/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 ºC, 1120º C and 740ºC respectively, where a difference of 150ºC and 200 ºC was registered between the first and thirdthermocouples when using helium and nitrogen respectively. The corresponding hardness values were 170 HV, 162 HV and 225HV, and the corresponding surface roughness values were 6 μm, 12μm and 25μm. A decrease of almost 60% in the roughness value was observed between the initial and last stage of the melted track, when using argon as shielding gas.