Deformation Characteristics and Mechanism of Martensitic Stainless Steel during Thixoforming

Abstract

Semi-solid billet of 9Cr18 martensitic stainless steel with globular grains was made by a wavelike sloping plate experimental device, and hot compression tests were carried out in the semi-solid state of 9Cr18 semi-solid billet on Gleeble-1500 thermal simulation testing machine at the temperatures of 1250°C ~1300°C and the strain rates of 0.1 s-1~5.0 s-1to investigate the effects of thixoforming parameters on its deformation characteristics and mechanism. According to the true stress-strain curves obtained from the test, the influence of deformation temperature and strain rate on 9Cr18 semi-solid billet deformation resistance was investigated, and the deformation resistance model of specimen with coexistence of solid and liquid phases was established. In this paper, it was found that deformation mechanism changed because of different deformation temperature and strain rate. Dynamic recrystallization occured at 1250°C in different phases separately. So that big fine recrystallized grains were achieved at the soft primary austenite region while small recrystallized grains were achieved at the hard solidified liquid region. The melted metal would be extruded from the centre of the specimen to the free surface completely when the temperature was higher than 1275°C. And then specimen became FGM (functionally graded materials), with phases and properties graded distribution perpendicular to the stress direction. When thixoforming temperature reached 1300 °C, martensitic transformation occurred after rapid cooling. The mathematics models of the relation between stress and temperatures, fraction of solid, deformation rates and deformation degree of 9Cr18 semi-solid billet were regressed and established based on the dates attained from the compression deformation experiments. The R value was 0.991, and the RMSE value was 3.57.