Kinetics of reverse martensitic-austenitic transformation in duplex steel CF8 under thermal action

Abstract

Using the methods of Mössbauer spectroscopy and X-ray diffraction, the kinetics of martensitic-austenitic transformation under thermal action (300 - 850 °C) in duplex steel CF8, previously subjected to cold plastic deformation with a deformation degree of ε = 55%, was studied. The dynamics of the reverse a'-martensite - у-austenite transformation was not observed at temperatures up to 500 °C. However, there was a noticeable increase in the average hyperfine magnetic field <H_n>, from 241 ± 1 to 269 ± 1 kOe. A further increase in the annealing temperature led to a decrease in the average effective hyperfine field. At the same time, the distribution of the magnetic field p(H_n) showed a gradual increase in the contribution from H_n ≥ 330 kOe, which is associated with a decrease in the number of chromium atoms in the nearest iron environment. The intense a'-у transition occurred in the temperature range of 600-700 °C. At 850 °C, the restoration of the original structure of у-austenite was completed. The process of transformations mainly occurred by means of a shear mechanism. The results of X-ray diffraction analysis confirm the data of Mossbauer spectroscopy: the structure of CF8 steel before annealing is a ferromagnetic phase (a'-martensite), after annealing at 850°C, the X-ray diffraction patterns characteristic of у-austenite are reflected.