Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading

Abstract

Surface modification of medium carbon AISI 4340 structural steel using TIG arcing process under various heat input (Ω) of varying arcing current (I) and arc travel speed (S) has been studied. Weld isotherm and thermal cycle of the fused zone has been analytically established and validated with experimentally measured results. TIG arcing process has been found to change the microstructural characteristics of the fusion modified matrix by transformation of fine niddle shape martensite, which has significantly enhanced its hardness from 256 ± 10 HV to 745 ± 8 HV and thus its mechanical properties. Formation of residual stresses in the surface modified the substrate is examined by hole drill method up to a depth of 1.3 mm. A residual stress of tensile nature is found to exist up to an appreciable depth from the top modified of the substrate, which has adversely affected its static and dynamic properties by enhancement of resolved stress in the matrix during the bend test. Surface modification improves flexural properties of the substrate with a maximum improvement of flexural yield strength at a relatively slow arc travel speed of 6 cm/min, while the Ω lies in the range of 0.6–1.21 kJ/mm. A favourable to improvement in static and dynamic properties of the substrate were found at relatively higher Ω of TIG arcing at slower S. It has primarily happened due to development of relatively slower cooling rate in the matrix introducing comparatively ductile lath martensite in it.