Analysis of Effect of Chemical Composition on Hot Forming Operations of P91 Steel

The mechanical properties, the creep response and the microstructural features of P91 steel have been extensively investigated in recent years since this steel and its derivatives have been used for the production of pipes and tubes. Nevertheless, in plant experience on rolling facilities, showed that the formability of different heats of similar composition can be dramatically different. The obvious observation that the optimization of the rolling cycle by means of in-plant experiments is very expensive and time-consuming, led to the conclusion that laboratory-scale simulation of the process could give a useful preliminary estimate of the hot-formability of each heat. Torsion testing was thus used to investigate the formability of different 9Cr-1Mo ingots; torsion tests were carried out between 850 and 1250°C, under strain rates ranging from 0.05 to 5 s The constitutive equation relating peak flow stress (σ), temperature (T) and strain rate was calculated for the investigated materials. The equivalent-strain to fracture in torsion was used to identify the regions of poorest formability; optical microscopy was then used to investigated the presence of delta-ferrite in the deformed steels. The large difference in ductility in the high temperature regime was thus explained in term of the dramatic effect of minor differences in chemical composition on ferrite content at the rolling temperature.