Solidification and Segregation Behaviors during Directional Solidification of High Chromium Martensitic Steels

Directional solidification experiments were carried out on modified 9Cr1Mo steel and 10Cr1MoW steel to investigate their solidification and segregation characteristics. Under a given temperature gradient, both steels were solidified at the same growth rates of 1, 5, 50 and 100 mu m/s, respectively. A detailed study of the solidification sequence for both steels revealed that delta ferrite was the leading phase and gamma austenite nucleated from the solid solidified earlier. A quick phase transformation of delta into gamma was made at growth rates faster than 50 mu m/s. Therefore, almost all delta dendrites were identified as gamma phases to be transformed. On the other hand, compositional analyses performed on the solid/liquid interface confirmed that alloying elements in both steels were hardly segregated ahead of the solid/liquid interface. This appears to be responsible for a very narrow freezing range which suppresses elemental segregation.