Microstructural Analysis of Isothermally Heat-Treated Spheroidal Graphite Cast Iron Using Microscopy and X-ray Diffraction Methods

Abstract

  Optical microscopy, scanning electron microscopy, and X-ray diffraction analysis were used to examine spheroidal graphite cast irons. The irons were austenitized, isothermally heat-treated at 673 K or 573 K for various holding times, then water-quenched. The characteristics of the irons―specifically, the diffusion of carbon atoms and the irons’ microstructures during the isothermal transformation process―were investigated. Microstructure observations and X-ray diffraction analysis revealed that the isothermally heat-treated irons exhibited composite microstructures consisting of ferrite, martensite, untransformed austenite, and graphite. The precipitation of carbides, such as cementite, in the irons was suppressed by high silicon concentrations. Classic lenticular martensites, which were connected like lightning to each other, were formed in other regions of graphite and ferrite because numerous carbon atoms diffused into the untransformed austenite during the isothermal treatment. The carbon content in the untransformed austenite was estimated through X-ray diffraction analysis to be between 1.57 mass% C and 1.81 mass% C. Because of this high carbon concentration, the untransformed austenite, which is a stable phase with a martensitic transformation start temperature much lower than room temperature, remained in the isothermally heat-treated irons.