Segregation and Solidification Mechanism in Spray-Formed M3 High-Speed Steel

Spray forming of various steels and iron-based alloys has been investigated since the 1960s. The microstructure and properties of spray-formed steels are superior to those of cast material, typically resembling those of equivalent powder metallurgy steels. While the wight of the deposits produced in pilot-scale plants is typically less than 100 kg, in some cases, industrial plants are capable of producing preforms that weigh up to several tons. However, in actual industrial production processes, segregation can easily appear in the product structure, especially in large-scale high-speed steel. In this work, M3 high-speed steels with a 250 mm diameter after forging were prepared through spray forming to study their cross-section segregation morphology. An arc-spark direct reading spectrometer, an original position analyzer for metals, OM, SEM, and EDS were used to analyze the distribution of alloy elements and microstructure characteristics of the different parts of a cross-section area. The results show two segregation morphologies in the cross-section of spray-formed M3 high-speed steel: ingot segregation and ring segregation. Carbon and alloy elements are enriched in the ingot segregation, whereas carbon and molybdenum are mainly enriched in the ring segregation, where the degree of segregation is less than that of the ingot segregation. From edge to center, the morphology of carbide changes from plate to massive. In the ring segregation area, there were two morphologies of carbide: one is M6C-wrapped MC composite carbide of the network distribution and the other M6C and MC both nucleating at the carbide/matrix interface of the composite carbide. In the ingot segregation area, carbides were mainly distributed independently of massive M6C and MC, with severe carbide segregation in the macrostructure. On the basis of the above experimental results, the solidification and microstructural evolution of spray forming were discussed, and the slow cooling rate in the deposition stage was the fundamental reason for the above experimental results. It is believed that spray forming loses rapid solidification characteristics in the deposition stage when preparing large-scale products.