This study investigated the influence of thermal stability of powdered oxides on the joint penetration of tungsten inert gas (TIG) welding of high-strength low-alloy (HSLA) steels. The microstructure and microhardness of the weldments were evaluated and the results were compared. The results shows a greater improvement in the joint penetration ability (JPA) of AISI 4130 steel TIG welds produced with MoO3 and Cr2O3, as compared to TIC welds produced with Al2O3, MgO, and CaO. TIG welding with CaO resulted in a broadening of the arc contour, which resulted in the shape of the weld becoming relatively shallow and wide compared to TIG welding without powdered oxide. The use of powdered oxide with high thermal stability may generate a heavy slag over the surface of molten metal, acting as a barrier for thermal energy transfer from the electric arc to the molten pool during powdered oxide assisted TIG welding. Moreover, the hard microstructures formed in weld metal (WM) and partial heat-affected zone (HAZ) of AISI 4130 steel weldment exhibited a considerable microhardness value (>370 Hv). (C) 2015 Elsevier B.V. All rights reserved.