Influence Mechanism of Pressure on Nitrogen Bubble Formation During Solidification Process in 30Cr15Mo1N Ingot

In this paper, the influence mechanism of pressure on nitrogen bubble formation during solidification process in 30Cr15Mo1N ingot was investigated by experimental and calculation methods. The effects of pressure on the critical nucleation radius and the growth rate of nitrogen bubbles were investigated. The growth of nitrogen bubbles was closely associated with the direction of solidification. In addition, nitrogen bubbles were generally egg-shaped and spherical in the columnar and equiaxed regions, respectively. With increasing pressure from 0.3 to 0.5 MPa, an increment existed in the critical nucleation radius and a decrement in the growth rate of nitrogen bubbles, mainly caused by the increment of the Gibbs free energy change of the system for nucleation and the internal pressure in the nitrogen bubbles, respectively. It led to decrements in number and mean area of nitrogen bubbles in 30Cr15Mo1N ingots with increasing pressure from 0.3 to 0.5 MPa. Based on the effect of pressure on formation of nitrogen bubbles, a special fitting formula was proposed to obtain the critical pressure for the 30Cr15Mo1N cylindrical ingot without nitrogen bubbles: P_c = max{0.8867[pct N]_0 + 0.098,0.053e^[pct N]_0 /0.212 + 0.0415}, which is applicable under low pressure (≤ 2 MPa).