Characterisation of deformation process and fracture mechanisms of P91 steel at 600 °C in small punch tensile testing

This paper investigates the microstructural evolution and fracture mechanism of the P91 steel during small punch tensile tests. Disc specimens, 8 mm in diameter and 0.5 mm in thickness, were tested in a small punch test rig at 600 °C using a constant displacement rate of 2 μm/s. Interrupted small punch tensile tests were performed to investigate the microstructural evolution in different deformation regimes. Deformed specimens were characterised by scanning electron microscopy and electron backscatter diffraction. Microstructure characterisation showed that block boundary alignment occurred in the plastic deformation direction, before achieving the maximum punch load. It was revealed that the martensitic structure was recovered when the tests progressed. The elongation of recovered grains occurred after achieving the maximum punch load. In addition, it was shown that the voids tended to nucleate along the recovered grain boundaries and became elongated to align with the plastic flow within the specimen.