The behaviour of the carbides of ledeburitic AISI D2 tool steel during multiple hot deformation cycles

In order to improve the hot workability of AISI D2 tool-steel ingots during several heats hot-deformation process, laboratory hot-compression tests as well as industrial investigations of the carbides' behaviour were carried out. The conditions that led to the occurrence of undesired, oversized carbides in the matrix were estimated and explained. The area fraction of carbides with respect to their size, their number per μm2 and their sphericity after each hot-deformation cycle were determined. It was found that too high soaking temperature results in an increased size of carbides which decreases hot workability. The results of industrial investigations show that area fraction of carbides after the end of each deformation cycle remains almost constant, but their mean size more than double during deformations in several heats which implies that the final microstructure is not dependent primarily on the last soaking-deformation cycle but depends on entire processing history, i.e. hot workability over several hot-deformation cycles can change considerably from cycle to cycle.