Martensitic transformations in metastable austenitic steel with coarse-grained and ultrafine-grained structure during torsion

The X -ray method was used to study martensitic transformations in different areas on the fracture surface of the samples made of coarse-grained (CG) and ultrafine-grained (UFG) Fe – 0.02C – 18Cr – 8Ni steel after torsion testing. The fine structure of UFG steel was analyzed on JEM-2100 transmission electron microscope (TEM). The authors carried out the steel hardness tests on TN 300 hardness tester. Static tension of cylindrical samples with a diameter of 3 mm was performed at a temperature of 20 °C on N50KT universal testing machine. Torsion testing of cylindrical samples with a working part diameter of 10 mm and a length of 100 mm was carried out at a temperature of 20 °C using MK-50 unit. The equal-channel angular pressing (ECAP), forming UFG structure, improves the mechanical properties of steel under tension and torsion, and also helps to stabilize the austenitic structure of Fe – 0.02C – 18Cr – 8Ni steel under torsion. 100 % of α-martensite is formed on the fracture surface of CG steel samples, regardless of the X -ray diffraction area. On the fracture surface of UFG steel samples, the maximum amount of α-martensite (30 %) is formed in the peripheral area of ​​the fracture; the minimum amount of α-martensite (15 %) – in the fracture central part. The authors made a comparative analysis of the martensitic phases distribution on the samples fracture surface after torsion testing with the martensitic phases distribution in the samples of the same steel after severe plastic deformation by torsion (SPDT), when both ε- and α-martensite are formed. The absence of ε-martensite on the fracture surface of the samples made of CG and UFG Fe – 0.02C – 18Cr – 8Ni steel during torsion is associated with an insignificant pressure for this type of loading, less than in the SPDT process.