Structural formation of high-speed steels during crystallization

Abstract

The L+δ→ψ peritectic transformation in tungsten-molybdenum high-speed steels goes to completion prior to the start of the eutectic reaction (L→ψ+K), which may precede segregation of austenite from the liquid (with a high carbon content). In this case, the eutectic is crystallized on the base of the metastable M2C carbide. Conditions for the formation of thermodynamically more stable MC and M6C carbides are created during slow cooling characteristic for the central zones and upper horizons of an ingot. Eutectics based on these carbides begin to crystallize at comparatively high temperatures, when the peritectic transformation has yet to go to completion. In a certain temperature interval, these transformations occur simultaneously; in this case, they can be suppressed by the eutectic crystallization owing to the characteristic features of the peritectic transformation. As a result, δ-ferrite (the products of its decomposition) is present in the steel. The mechanism responsible for the peritectic transformation in tungsten-molybdenum highspeed steels is characterized by the penetration of austenite into the depths of the δ-crystals during steady contact between the liquid and both the austenite, and the δ-phase, and by the absence of solid-phase pericrystallization. Diffusion processes are carried out via a network of channels connecting the thin liquid interlayer at the δ→ψ-transformation front with the interdendrite melt. This dictates the high rate of the peritectic transformation, and also the possibility of the suppression of its eutectic crystallization of the liquid in the channels. Decreases in the macro-, micro-, and submicroheterogeneity and increases in the technological plasticity can be attained with this effect on crystallization, which prevents overlapping of the regions of the peritectic and eutectic transformations. The amount of residual δ-ferrite and skeletal eutectic, and the chemical heterogeneity of the austenite should be reduced in this case.