Coarsening kinetics of multicomponent MC-type carbides in high-strength low-alloy steels

Morphology and coarsening kinetics of MC-type carbide (MC-carbide) precipitating during the tempering process have been investigated in V- and Nb-bearing Cr-Mo martensitic steels. Detailed transmission electron microscopy (TEM) observations show that the addition of V and Nb stabilizes the B1-type MC-carbide instead of L’3-type M2C-carbide. The morphology of the MC-carbide is characterized as disk-like with Baker and Nutting orientation relationships with the matrix. When the specimens are fully solution treated followed by quenching, the MC-carbide precipitates as a multicomponent system with continuous solid solution of VC, NbC, and MoC. The V-, Nb-, and Mo-partitioning control the lattice parameter of MC-carbide and consequently affect the coherency between MC-carbide and the matrix. The coherent MC-carbide grows into an incoherent one with the progress of tempering. The numerical analysis on TEM observations has shown that the coarsening kinetics of MC-carbide is equated to (time)1/5 criteria, while the coarsening kinetics of the coexisting cementite is equated to (time)1/3 criteria. It is thus suggested that the Ostwald ripening of MC-carbide is controlled by pipe diffusion of V, Nb, and Mo along dislocations. It has been confirmed that the coarsening rate of the multicomponent MC-carbide is affected by V, Nb, and Mo content. Applying the thermodynamic solution database, the rate equation for MC-carbide coarsening can be expressed as a function of V, Nb, and Mo content, and the activation energy for pipe diffusion can be estimated as ΔQv: ΔQNb: ΔQMo=1:3.9:0.6.