In order to investigate phase transformations in a hot isostatic press (HIP) with integrated rapid cooling in situ, the electrical conductivity of PM HS 3-3-4 (1.3377) was measured as a function of temperature in the HIP process. This approach leverages the fact that phase changes result in distinct variations in electrical resistance. A four-wire measurement was conducted on a specifically designed PM HS 3-3-4 sheet metal sample. The experimental setup was integrated into the HIP system to determine the martensite start ( M s ) temperature. HIP cycles were performed at an austenitizing temperature of 1000°C under isostatic pressure of 100 MPa. For comparison, unpressurized heat treatments following an identical temperature profile were conducted using a dilatometer to measure the M s temperature. It was shown that the measurement setup with the newly developed sample geometry can be successfully used to determine phase transformations in the HIP. The comparison of the M s temperatures of the samples heat treated under isostatic pressure and without pressure showed that the austenite phase is stabilized by the pressure during quenching, which leads to a shift of the M s temperature to lower values.
Müller et al. (Tue,) studied this question.