Powder metallurgy (PM) components typically have a finer and more homogeneous microstructure than conventionally manufactured components, which results in isotropic, improved mechanical properties. The combination of hot isostatic pressing (HIP), and near-net-shape manufacturing enables the production of nearly defect-free PM parts. However, previous HIP systems did not allow for precise temperature control during cooling, due to their inability to remove heat from the unit fast enough. As a result, a subsequent, energy-intensive heat treatment was typically required after the HIP process to adjust the final properties of the material. Advances during the last 20 years now allow heat treatments to be carried out in situ, integrated into the HIP process. This reduces additional processing steps, transportation, energy consumption, and costs.

The pressure applied during in-situ heat treatments can influence phase stability and thus the evolution of phase transformations Due to the lack of research on the influence of pressure on the phase transformation during HIP heat treatment, this work introduces improvements to a measurement method capable of accurately determining the phase transformation.

For the in-situ measurement of phase transformations, the electrical conductivity of the material is measured as a function of temperature. This provides valuable information, as phase changes result in detectable change in the material’s electrical resistivity. To that end, the four-wire measurement is applied to a meander-shaped sheet specimen. The sample geometry is chosen so that the conductor is as long as possible to measure the largest possible total change in electrical resistance. In the four-wire technique, a known electrical current is passed through the sample via two outer connections. The voltage drop over the length of the sample is measured at two inner connections. The resistance of the conductor can then be calculated according to Ohm's law. To obtain geometry-independent values, the specific electrical resistance is calculated from the electrical resistance using the known sample cross-section and the known sample length. 

We validated the specimen and method on measurements of the martensite-start temperature of the high-speed steel PM HS 3-3-4. This descriptor is not only an indicator of phase transformation. It is also a key parameter in materials technology because it is directly related to the resulting mechanical properties.