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

Abstract

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 M _7 C _3 (M is mainly Cr element) and NbC, which transformed into M _23 C _6 type carbides (M is mainly Cr element) and G phase(Ni _16 Nb _6 Si _7 ) after service, respectively. The G phase and M _23 C _6 carbides mix together and grow up with each other during service. After carburization, the crystal structure of M _23 C _6 type carbide changes again and transforms into M _7 C _3 type carbide. The mechanism of M _23 C _6 → M _7 C _3 carbide reaction is an in situ transformation; the M _7 C _3 type carbide preferentially nucleates at the interface of M _23 C _6 / γ and grows toward the M _23 C _6 type carbide interior. With the diffusion of free carbon atoms into the M _23 C _6 type carbides, the M _23 C _6 type carbides are gradually surrounded by the M _7 C _3 type carbides until they are completely changed into M _7 C _3 type carbides.

Keywords

• carbide transformation
• carburization
• Cr35Ni45Nb type alloy
• M7C3
• M23C6