Mechanism of M23C6 → M7C3 carbides reaction of Cr35Ni45Nb type alloy during carburization

The carbide transformation of Cr35Ni45Nb type alloy during service has been investigated in the present work. The primary carbide of the as-cast Cr35Ni45Nb type alloy is M7C3(M is mainly Cr element) and NbC, which transformed into M23C6 type carbides (M is mainly Cr element) and G phase(Ni16Nb6Si7) after service, respectively. The G phase and M23C6 carbides mix together and grow up with each other during service. After carburization, the crystal structure of M23C6 type carbide changes again and transforms into M7C3 type carbide. The mechanism of M23C6 → M7C3 carbide reaction is an in situ transformation; the M7C3 type carbide preferentially nucleates at the interface of M23C6/γ and grows toward the M23C6 type carbide interior. With the diffusion of free carbon atoms into the M23C6 type carbides, the M23C6 type carbides are gradually surrounded by the M7C3 type carbides until they are completely changed into M7C3 type carbides.