Microstructure of tool steel upon combined semi-solid processing and thermomechanical treatment

摘要

Semi-solid processing can lead to microstructures with a high proportion of metastable austeriite embedded in carbide-austenite network even in ordinary steels. This type of microstructure is a result of non-uniform distribution of alloying elements in a partially melted material. The paper deals with semi-solid processing and heat treatment of the ledeburitic steel with 1.8% carbon and 11% chromium. Its initial microstructure typically consists of ferrite matrix with primary chromium carbides and globular cementite particles. Upon semi-solid processing, the material comprised 95% metastable austenite, pre dominantly in the form of polyhedral grains with the size of 19 μm. Previous research has demonstrated the grain refinement effect of plastic deformation applied immediately after solidification. In follow-up experiments, the influence of thermomechanical treatment upon subsequent evolution of microstructure was studied. In this experiment, particular attention was devoted to the decomposition of metastable austenite and its contribution to microstructure refinement. Upon the transition through the semi-solid state, the material was cooled down below its solidus temperature and subjected to deformation. This was followed by several cooling schedules. Some of the schedules included a dwell at 600 °C to allow metastable austenite to decompose. Cooling schedules were immediately followed by heating to austen-itizing temperature and cooling to room temperature at an equal rate. These procedures produced various types of microstructures. The treatment with the dwell produced mixed-type microstructures with an exceptional form of martensite and partially decomposed carbide network. Processing without the dwell led to the development of microstructures with polyhedral retained austenite grains embedded in the original carbide network. Their hardness ranged from 445 to 850 HV30.