Microstructural Development of HP9/4/30 Steel During Partial Remelting

Abstract

Semi‐solid processing, also known as thixoforming, is a forming process that shapes metal components in their semi‐solid state. Prior to forming, the microstructure of the alloy consists preferably of solid metal spheroids in a liquid matrix. This paper describes the microstructural development within the semi‐solid zone of a typically banded high performance HP9/4/30 steel through a direct partial remelting process from as‐received and as‐deformed conditions. Partial remelting was carried out at temperatures between 1430 and 1470°C. Liquation occurred initially at the grain boundaries, then also along the segregation bands. With increasing time and hold temperature, these “columns” broke down into shorter, more equiaxed segments, offering a better chance of being thixoformed. The microstructures revealed distinct polygonal cells at 1430°C that changed to more rounded solid grains with diminishing sharp edges at 1450°C, followed by smaller truncated cell structures due to the liquation of the bands at 1460°C and 1470°C. The partial remelting procedures carried out in this study are from material that is in a recrystallised state. Thixoforming from this recrystallised state is shown to be successful. This indicates a widening of the range of potential routes to thixoformable microstructures.