Dissolving behavior of carbide in Nb-Mo-V-Ti and Nb-Mo-V microalloyed high-strength invar steel

The low thermal expansion properties of invar steel make it widely used in precision instruments, aerospace components and other fields, but the low strength limits its further development. Adding alloying elements to invar steel and inducing the precipitation of carbide second phase through solid solution-aging can significantly improve the mechanical properties of invar steel. Solid solution-aging is a key prerequisite for precipitation, and carbide dissolution behavior in this process is particularly important. The alloying elements Nb, Mo, V and Ti were added to invar steel, and the carbide dissolving behavior was analyzed and the mechanism was investigated by scanning electron microscopy and high-temperature laser confocal microscopy. The results show that there are two types of carbides in Nb-Mo-V-Ti and Nb-Mo-V invar steels. The larger carbide particles with an average size greater than 5 μm, formed during the solidification process, are called as the primary carbides. The smaller carbides with an average size of less than 1 μm, precipitated from the matrix, are called as the secondary carbides. Both types of carbides are composite precipitation phases of(Nb, Mo, V)C or(Nb, Mo, V, Ti)C corresponding to their respective components. The thermodynamic results and HT-CSLM observation indicate that the dissolution temperature of the primary carbides is close to the solidification transition temperature of the invar steel, so that it is difficult to dissolve. The secondary carbides realize the basic dissolution after heating and prolonging the holding time, and from this, Ti has a hindering effect on the dissolution of the carbides, which makes the carbides have a higher thermal stability. In addition, the Johnson-Mehl-Avrami-Kolmogorov equation is used to characterize the dissolution kinetics of the secondary carbides, and the dynamics model is experimentally verified to be able to predict the dissolution fraction of the secondary carbides better.