Evolution of Precipitated Phase and Dendritic Structure Around Nitrogen Pore in 30Cr15Mo1N Ingot.

In this paper, the influence mechanisms of nitrogen pore formation on the evolution of precipitated phase and dendritic structure during solidification process in 30Cr15Mo1N ingot have been investigated by microstructure observation, component analysis, thermodynamic calculation, and secondary dendrite arm spacing statistics. There are ferrite phase, austenite phase, and M₂₃(C,N)₆ at the edge of nitrogen pore. In the columnar region, the M₂₃(C,N)₆ is precipitated in the position where the nitrogen bubble stops growing. In the equiaxed region, M₂₃(C,N)₆ is precipitated not only in the position where the nitrogen bubble stops growing, but also in other positions of the edge of nitrogen pore. In addition, the growth direction of columnar dendrite on both sides in front of the nitrogen pore cap is deflected. It is caused by the larger solute undercooling and the larger temperature gradient between the front and both sides of the nitrogen pore cap. The presence of nitrogen bubble reduces the heat transfer because of the larger thermal resistance of nitrogen gas. It leads to the smaller cooling rate in front of the nitrogen pore cap. As a result, the secondary dendrite arm spacing in front of the nitrogen pore cap is larger than that in front inferior of the nitrogen pore cap. [ABSTRACT FROM AUTHOR]