Solidification Characteristics and Eutectic Precipitates in 15Cr-22Ni-1Nb Austenitic Heat-Resistant Stainless Steel

The influence of the cooling rate of liquid steel on solidification characteristics, eutectic NbC and Laves phase in 15Cr-22Ni-1Nb steel was investigated viain situ observation and microstructure characterization. The initiation and finishing solidification temperatures of the steel increase first and then decrease as the cooling rate is increased from 10 to 100 & DEG;C/min, whereas its solidification temperature interval is monotonically enlarged. The rate constant of austenitic nucleation and growth decreases with the increase in cooling rate because of the reduction in the effective activation energy of liquid steel solidification. The nucleation rate of austenite continuously increases with increasing the cooling rate. The effective activation energy of austenitic nucleation and growth is - 88.86 and - 396.76 kJ/mol, respectively. The secondary dendrite arm spacing of the steel is reduced from 69.43 to 27.27 & mu;m as the cooling rate is increased from 10 to 100 & DEG;C/min. Increasing the cooling rate aggravates the microsegregation degree of C, Si, Ni, Cr, Nb and Mo, which enhances the generation of eutectic NbC and Laves phase. The volume fractions and number densities of eutectic NbC and Laves phase increase with increasing the cooling rate. The mass ratio of Nb/C in the residual liquid steel increases as the cooling rate is increased, which is favorable to the precipitation of Laves phase. The starting-precipitation solid fraction and starting-precipitation temperature of eutectic NbC and Laves phase increase initially and then decrease with increasing the cooling rate. The precipitation tendency of eutectic NbC and Laves phase is enhanced as the cooling rate is increased.