Kinetical behaviour of anomalous austenite formation in turbine blade steel

Moessbauer spectroscopy and X-ray diffractometry were used to study the kinetical behaviour of anomalous austenite formation observed previously in turbine blade steels aged isothermally between 450 and 600deg C. A monotonous increase in austenite content (between 2 and 9%) was found with increasing aging time during isothermal aging of the steel samples. The aging time dependence of the anomalous austenite formation corresponds to the Awrami-kinetics with an exponent of 0.3. This reflects that the stress assisted transformation is more probable as a mechanism of formation of anomalous austenite than normal diffusion. At the same time, the Cr concentration of the ferrite phase is also determined as a function of the aging time with the help of hyperfine field distribution analysis based upon the alloying effect. The Cr content of the ferrite phase decreases significantly after a relative short aging period. The changes in the Cr concentration can promote the austenite formation. Our results also suggest to revise the technological standard of production of turbine blade steel since the prescribed final heat treatment can result in nonfavourable austenite formation. (orig.)