Microstructure and Properties of the Fusion Zone in Steel-Cast Iron Composite Castings

Abstract

Casting is an established way to manufacture metal components with complex geometry and high wall thickness at low cost. However, the possibilities for locally modifying material properties are limited in conventional casting processes. These limitations can be remedied by utilizing composite casting. Within this study, a lamellar graphite cast iron melt was poured into low carbon steel shells comprised the steels S235JR and 25CrMo4, differing in their mechanical properties and their thermal conductivity, with the aim to form a material bond. Three different temperatures for the steel shells at the time of pouring were investigated: 293, 383 and 583 K. After preheating, the shells were embedded in unheated sand moulds, which were then filled with an EN-GJL-250 melt. The resulting fusion zone was characterized using optical microscopy, and the mechanical properties were assessed by Brinell hardness tests. The properties and microstructural morphology around the fusion zone strongly depend on the initial temperature of the shell at the time of casting. For shell temperatures of 293 K, no proper bonding was achieved between shell and core. A pearlite layer with a thickness of up to 500 µm was formed at a temperature of 583 K, whilst the adjacent region of the cast iron became decarburized. The hardness of the fusion zone reached a maximum of 275 HBW for a shell temperature of 383 K with an overall span from 235 to 275 HBW.