Kinetics of Nitrogen Absorption/Vacuum Denitrigenization and Precipitation Behavior of Nitrogen Bubbles in 42CrMoA Molten Steel

Abstract

Nitrogen absorption and vacuum denitrigenization experiments of 42CrMoA molten steel are carried out in a vacuum suspension furnace. Based on the results, the effects of absorption time on the nitrogen absorption process and denitrigenization time/pressure on denitrigenization process are studied, and the morphology of nitrogen bubbles in denitrigenization samples are analyzed. At temperature of 1777 K, the nitrogen solubility of 42CrMoA steel under pressures of 1 × 105 and 0.2 × 105 Pa are measured as 510 and 230 ppm, which are higher than the value calculated by Satir's nitrogen solubility formula. Nitrogen bubbles are observed in denitrigenization samples at denitrigenization pressure ranging from 0.2 × 105 to 0.8 × 105 Pa, indicating that bubbles can precipitate when the supersaturation degree of nitrogen in molten steel exceeds 38 ppm. The bubble diameters are in the range of 2.7–22.8 μm; it is presumed that the critical‐nucleation radius of nitrogen bubbles is 2.7 μm. The size of nitrogen bubbles measured in this experiment is much smaller than the critical‐nucleation radius calculated by classical‐nucleation theory for both homogeneous and heterogeneous‐nucleation method. Moreover, the pressure inside bubbles are analyzed and found to be much larger than nitrogen precipitation pressure, which is not in accordance with classical‐nucleation theory, highlighting the need for further study.