Influence of cooling rate on the microstructural features of a remelted white-solidified cast iron surface and its effects on nitriding behaviour

Abstract

Electron beam remelting (EBR) of cast irons is an established procedure for generating hard and wear-resistant functional surfaces. Due to process-specific rapid cooling after the surface melting of cast iron, solidification occurs according to the metastable system, and a ledeburitic surface-layer microstructure is generated (eutectic carbide + austenite + pearlite). When a subsequent nitriding process is added, carbide morphology and distribution have a decisive influence on the formation of nitride layers.In this analysis, investigations on the influence of different EB process parameters are presented and discussed, such as beam current and acceleration voltage, as well as the effect of focus position on the ledeburite morphology, the layer thickness and surface deformation after the EBR process. Cast iron with globular graphite was used as the base material for the present investigations.The classification of the ledeburite was carried out by examining polished metallographic cross-sections for the determination of the EBR layer thickness, the layer hardness and the secondary dendrite arm spacing. The latter factor also allowed conclusions to be reached regarding the rates of cooling. Furthermore, the local distribution of C and Si in the microstructural constituents of the ledeburitic surface layer was essential in understanding the diffusion processes and phase transformations during the subsequent nitriding process. For this reason, EBR layers with defined coarse and fine ledeburitic carbides were generated and characterized in detail using EDX surface scans.The influence of the local microstructure, the carbide morphology and the element distributions in the EBR layer on the formation and properties of the nitride layers are discussed in light of initial nitriding experiments. Based on these findings, further optimization of the surface layer is possible with respect to the selection of parameters for both the EBR and nitriding processes.