Shucai Zhang

Relationship between Microstructure and Corrosion Behavior of Martensitic High Nitrogen Stainless Steel 30Cr15Mo1N at Different Austenitizing Temperatures

The relationship between microstructure and corrosion behavior of martensitic high nitrogen stainless steel 30Cr15Mo1N at different austenitizing temperatures was investigated by microscopy observation, electrochemical measurement, X-ray photoelectron spectroscopy analysis and immersion testing. The results indicated that finer Cr-rich M2N dispersed more homogeneously than coarse M23C6, and the fractions of M23C6 and M2N both decreased with increasing austenitizing temperature. The Cr-depleted zone around M23C6 was wider and its minimum Cr concentration was lower than M2N. The metastable pits initiated preferentially around coarse M23C6 which induced severer Cr-depletion, and the pit growth followed the power law. The increasing of austenitizing temperature induced fewer metastable pit initiation sites, more uniform element distribution and higher contents of Cr, Mo and N in the matrix. In addition, the passive film thickened and Cr2O3, Cr3+ and CrN enriched with increasing austenitizing temperature, which enhanced the stability of the passive film and repassivation ability of pits. Therefore, as austenitizing temperature increased, the metastable and stable pitting potentials increased and pit growth rate decreased, revealing less susceptible metastable pit initiation, larger repassivation tendency and higher corrosion resistance. The determining factor of pitting potentials could be divided into three stages: dissolution of M23C6 (below 1000 °C), dissolution of M2N (from 1000 to 1050 °C) and existence of a few undissolved precipitates and non-metallic inclusions (above 1050 °C).

Effects of Nitrogen Segregation and Solubility on the Formation of Nitrogen Gas Pores in 21.5Cr-1.5Ni Duplex Stainless Steel

The nitrogen gas pore-formation mechanism was discussed with regard to the solidification of 21.5Cr-1.5Ni duplex stainless steels (DSSs) by considering nitrogen segregation and solubility. The segregation behavior of nitrogen was investigated with phase transformation using experimental detection methods and Thermo-Calc software calculations. The process associated with the formation of gas pores was illustrated clearly. The factors that influenced the formation of gas pores, including shrinkage, nitrogen content, solidification pressure, and alloying elements (Mn and Cr), were discussed in detail. The formation of nitrogen-rich phases [austenite phase (FCC), AlN, and hexagonal close packed] is beneficial to eliminate nitrogen segregation and suppressing gas pore formation. The nitrogen-depleted phase (ferrite phase (BCC)) exhibits an opposite effect. Regular gas pores are initially formed in locations consisting of the austenite phase. As the gas pores lengthen, ferrite and austenite phases alternately form around the gas pores. Solidification shrinkage can promote the formation of irregular gas pores at the centerline of the ingots. Increasing the nitrogen content is favorable to the formation of gas pores. Increasing solidification pressure is effective with regard to suppressing the formation of gas pore defects in DSSs. Increasing the Mn content can reduce the likelihood of gas pore formation; this can be attributed to the increased nitrogen solubility in the residual liquid surrounding the dendrites and the formation tendency of the nitrogen-rich phase. Increasing the Cr content exhibits a dual effect on gas pore formation, which is caused by the increased nitrogen solubility and segregation in the residual liquid.

Numerical simulation of Mo macrosegregation during ingot casting of high-Mo austenitic stainless steel

Mo macrosegregation was studied through the comparison of numerical simulation of the ingot pouring process and experiment on as-cast 500 kg high-Mo austenitic stainless steel ingot. The simulated results showed the evolution of temperature, melt velocity and the patterns of Mo macrosegregation, and revealed the effects of pouring temperature and cooling rate on macrosegregation. The predicted variation of Mo macrosegregation was compared with measurement values from an industrial ingot along the vertical centreline and horizontal direction. Severe normal and gravity segregation were observed. Although a basic agreement was obtained, the lack of a sufficiently fine numerical grid and the neglect of sedimentation for free equiaxed grains in the prediction brought about the absence of A-segregation and V-segregation. Further investigation would be needed to perform this investigation. The predicted results also confirmed that Mo macrosegregation in the ingot could be effectively diminished by improving cooling rate and decreasing pouring temperature.