Characterization of the solidification structures within the dendritic core of M2 high speed steel

Abstract

The phases which are present within the dendritic core region during the solidification of AISI type M2 high speed steel (6 W, 5 Mo, 4 Cr, 2 V) were studied and characterized by metallography and quantitative microprobe analysis. A series of samples were quenched from various temperatures during solidification, “freezing in” the different solidification phases. The first phase observed to solidify in M2 is ferrite which contains very little carbon. As the solidification process continues, most of the liquid surrounding the ferrite transforms to austenite by virtue of a peritectic reaction which initiates at 1330°C,(L + F →A). The ferritic cores also transform at around 1330°C into an austenite plus car-bide aggregate. By the time the ingot cools to 1255°C, the carbides at the center of the dendrites dissolve completely, leaving an austenitic phase of uniform carbon and alloy content. At temperatures below the solidus, very fine carbides precipitate from the aus-tenite. No eutectoid decomposition by products such as those commonly observed in Tl tool steel were observed in these specimens nor in samples from a commercial ingot.