Flow Stresses during Hot Deformation of Three Different Steels (C–Mn, Nb–Ti, Nb) and Ni Base Superalloys

Using the Gleeble 1500, incremental and continuous hot compression straining tests were performed on C–Mn, Nb–Ti and Nb steels with test temperatures varying between 875 and 1100°C; strain rates between 0.5 and 20 s−1. Maximum stresses obtained varied from 150 MPa for Nb–Ti at 1100°C to 330 MPa for C–Mn steel at 875°C. For comparison with hot compression tests, actual rolling was carried out on Hille reversible rolling mill on Ni80Cr20 and Ni78Cr16Fe6 superalloys. The rolling reduction per pass was 10 and 30% at the predetermined rolling temperatures. The results indicate non-linearity relationship between rolling stresses and rolling temperatures, with rolling stresses increasing more sharply with decreasing rolling temperatures below 900°C. The highest rolling stress of 696 MPa was obtained for Ni80Cr20 at a temperature of 395°C for 10% reduction per pass. The relationship was observed to be linear for steels.Existing and proposed stress peak models were applied to the experimental results in order to predict multistage flow stress curves from experimental single stage flow stress curves and vice versa. The results indicate fair accuracy of both the existing and proposed models. The stress restoration index k obtained from proposed stress peak model was found to be constant and unique to particular sets of experimental conditions.Because of the peculiar differences in the cooling properties of steels and superalloys during rolling, the existing and proposed models were also applied to the experimental results obtained from rolling superalloys. The proposed stress peak model indicates fair accuracy in predicting the single stage curve from the multistage curve and vice versa. The existing models showed less accuracy as the rolling temperature dropped below 900°C. Stress strain equation constants have been obtained for single stage deformation of nickel-based superalloys.