Influence of hydrostatic pressure on the austenite grain growth and subsequent phase transformation of hot work steel AISI H13 during hot isostatic pressing

Newly developed hot isostatic pressing (HIP) units with integrated rapid cooling systems that are capable of operating at gas pressures up to 200 MPa provide the opportunity to densify and harden the metallic materials in a single step. For the application of these advanced HIP units in producing steels with defined microstructure, a crucial factor is the austenite grain size, which strongly influences the phase transformation during cooling and is effectively affected by the pressure during HIP. To study the effects of pressure, temperature and holding time on the growth of austenite grains and subsequently the final microstructure, a series of interrupted HIP experiments on hot work steel AISI H13 were conducted using such an advanced HIP unit. The austenite grain growth during HIP was analysed by metallographic microstructural investigations after rapid cooling. Thereafter, the effect of the austenite grain size on the phase transformation was studied using quenching dilatometry. Following this, the relationship between austenite grain size and martensitic/bainitic phase transformation temperature was determined, which indirectly indicates the influence of temperature, pressure and holding time on the final microstructure. By implementing this relationship into numerical models, the effect of HIP parameters on the microstructure of large-scale components, particularly those that experience large temperature gradients during the heating and cooling stages, can be predicted in the future.