Effect of solidification path and volume shrinkage on cracking susceptibility at different cooling rates

Understanding the solidification characteristics and microsegregation under varying cooling rates is essential to comprehend the formation of center cracks in large section round billets. P91 high-alloy steel was taken as the research object. The peritectic solidification process, steel solidification shrinkage and microsegregation of solute elements at different cooling rates were studied and revealed by high-temperature confocal scanning laser microscopy, Thermo-Calc thermodynamic software, hybrid laser microscopy and electron probe microanalysis. The results showed that as the cooling rate increased from 10 to 100 °C/min, the percentage of δ-Fe involved in peritectic reaction decreased from 98.6% to 36.4%, the surface roughness of the sample decreased from 8.59 to 5.14 μm, and the volume shrinkage decreased from 5.92% to 2.18%. Moreover, the solidification path enters the crack sensitivity area at lower cooling rates (10 and 50 °C/min), while the solidification path is far from the crack susceptibility area at higher cooling rate (100 °C/min). With the increase in cooling rate, the segregation deviation parameters of the elements V, C, Mo and Cr were decreased by 9.52%, 22.2%, 29.4% and 70.5%, respectively. Solidification path changed and microsegregation weakened by adjusting cooling mode might be a way to improve central crack.