Exploring residual stress analysis in the machining of hypoeutectic high chromium white cast iron alloys through the hole-drilling method

Abstract

High chromium white cast irons (HCWCIs), ASTM A352, Type A and Class III, i.e., 25%Cr iron in as-cast condition consists of proeutectic austenite (γ-Fe), transformed martensite (α-Fe) and discontinuous Cr-rich, i.e., M7C3/(Cr,Fe)7C3 type of carbides, which are hard and brittle in nature. Fully annealed thermal treatment was performed to improve iron’s machinability leading to fully pearlitic matrix with minor retained γ-Fe content. Eutectic (Cr,Fe)7C3 type of carbides are not affected by heat treatment processes. Resulting from corresponding manufacturing process, the magnitude and distribution of residual stresses (RSs) in as-cast and after machining were measured using hole-drilling method (HDM), as they are known to be harmful to corrosion and fatigue resistance. Furthermore, general metallurgical material characterisation was performed in as-cast and heat-treated conditions. As a result, this study revealed hardness variation, 547 and 555BHN in as-cast as compared to 327BHN in heat-treated condition. Furnace and actual cast component chemical analysis revealed a slight variation, especially between carbon (C) and chromium (Cr). Furthermore, eutectic type of carbides and precipitated secondary, i.e., (Cr,Fe)23C6 type of carbides within fully pearlitic matrix with minor amounts of retained γ-Fe were detected within the dominant matrix, i.e., pearlitic matrix in as-annealed condition. Detected magnitude and distributions of RSs on heat-treated sample resulted in higher tensile stresses in the surface and compressive in the interior as compared to sample in as-cast condition. Thus, this study was successfully in measuring RSs in as-cast and upon machining of hypoeutectic irons of HCWCI alloys using HDM.