In Situ Observation of P91 High‐Alloy Steel Solidification Characteristics at Different Cooling Rates

Abstract

To clarify the effect of different radial cooling intensities on the formation of central cracks in large round bloom continuous casting. It is necessary to study the solidification characteristics and dynamics of P91 high alloy steel at different cooling rates to improve the central defects. In this paper, the solidification characteristics of P91 high‐alloyed steel at different cooling rates were studied by the Thermo‐Calc software, high‐temperature laser confocal microscopy, scanning electron microscopy, and optical microscopy. Meanwhile, the growth kinetics of δ‐Fe and γ‐Fe phases under different cooling rates were determined. The results show that the solidification path of P91 high‐alloyed steel is L→(L+δ‐Fe)→(L+γ‐Fe+δ‐Fe)→(δ‐Fe+γ‐Fe). With lower cooling rates (10°C/min), the grains undergo nucleation during the solidification process, then fusion and growth. With higher cooling rates (50°C/min and 100°C/min), the grains grow as dendrites. The δ‐Fe and γ‐Fe phase precipitation process is divided into two stages. Stage I is the nucleation and rapid growth phase, in which a high undercooling is required. Stage II is the slow growth stage, where the undercooling decreases and remains constant. The initial growth linear velocities of the δ‐Fe phase are 0.51, 2.72 and 2.09 µm/s at cooling rates of 10, 50 and 100 °C/min respectively, while those of the γ‐Fe phase are 0.10, 1.42 and 1.41 µm/s.This article is protected by copyright. All rights reserved.