Measurement of kinetics of Nb(C,N) precipitation using hot torsion testing

Precipitation of Nb(C,N) in austenite of microalloyed steels is a well studied subject. Strain induced precipitates play a critical part in retarding recrystallization of austenite in the finishing passes of hot rolling. Hence, knowledge of how precipitation takes place is of major industrial significance. Measurements of fraction precipitated as a function of time at constant temperature (PTT curves) have been obtained through several experimental methods. One of them uses the principle of stress relaxation in axial compression for measurements of kinetics of TiC precipitation in austenite of Ti microalloyed steels. However, the total true deformation that can be applied in compression is usually limited to about 0.8. On the other hand, deformation of austenite in industrial processes is executed in several passes under continuous cooling before precipitation takes place. Successive cycles of deformation followed by recrystallization leads to substantial grain refinement which in turn may effect the process of precipitation. Therefore, the capability of axial compression to reproduce a more realistic hot rolling scenario seems to be very limited as far as austenite conditioning is concerned. A viable alternative may be torsion testing. Hot torsion can be used, for instance, to apply several passes with strains, strain rates, delay times and temperatures similar to those found in an industrial process. It would be then desirable to combine the ability of torsion to simulate industrial schedules with the stress relaxation technique to obtain PTT curves. But before building up PTT curves from simulated 'as hot rolled' austenite, it is necessary to investigate if torsion experiments are able to detect precipitation. This paper describes an application of torsion testing and the stress relaxation techniques to measure kinetics of precipitation