Evolution of Eutectic Carbide during M<sub>7</sub>C<sub>3</sub>/M<sub>23</sub>C<sub>6</sub> in situ Transformation in Martensitic Stainless Steel
Jie Zhang, Jing Li, Jing Li, Chengbin Shi, Jihui Li, Jihui Li
Abstract
The M 7 C 3 /M 23 C 6 in situ transformation in high‐carbon martensitic stainless steel is investigated. Eutectic Cr‐rich M 7 C 3 carbide is generated during solidification; it is metastable and can almost completely transform into other phases when the temperature is <1000 °C. The eutectic M 7 C 3 carbide transforms into an M 7 C 3 –M 23 C 6 core–shell structure after heat treatment at 1000 °C for 1 h and then completely transforms into M 23 C 6 carbides after heat treatment at 1000 °C for 7 h. The eutectic M 7 C 3 carbide with the same orientation in the as‐cast state transforms into dozens of M 23 C 6 carbide grains with varying orientations during in situ transformation, and the transformed M 23 C 6 carbides have no constant orientation relationship with the adjacent matrix and eutectic M 7 C 3 carbide core. The formation of the M 23 C 6 shell is the result of not only the in situ transformation but also the coarsening of M 23 C 6 carbides. The diffusion of carbon atoms in the M 23 C 6 shell is the restrictive factor of M 7 C 3 /M 23 C 6 in situ transformation and maintains transformation. The diffusion of chromium atoms in the matrix is the restrictive factor in the coarsening of the M 23 C 6 shell, which continues coarsening and results in a significant increase in the final area fraction of carbides.