Splat cooling of iron-molybdenum-carbon alloys

Abstract

Two molybdenum alloy steels, which normally undergo the austenite → martensite phase transformation during solid state quenching, have been rapidly cooled from the melt in a controlled atmosphere “gun” splat cooling device. The matrix phases produced wereδ-ferrite, martensite, and austenite; the carbide Mo₂C was also present in the as-quenched condition in the higher alloy composition studied. The amount of austenite retained to room temperature was found to be inversely related to the cooling rate. The morphology of the martensite in the splat-cooled alloys exhibited a marked change compared with its characteristic appearance in the conventionally solid-state quenched material. This was attributed to the dual effect of increased cooling rate on carbon segregation in the parent austenite and of decreased section thickness in which the martensite forms. The degree of solute segregation observed in the microstructures of the matrix phases was shown to depend on the extent of the equilibrium liquidus-solidus temperature range. The precipitation of Mo₂C during ageing in the range 600 to 700° C paralleled the behaviour of conventionally quenched and tempered alloys, although local inhomogeneities did produce precipitation phenomena not encountered in solid-state quenched material.