Vanadium Fine-Grain Hardening
Vanadium fine-grain hardening increases yield strength through submicroscopic vanadium carbides in the ferritic structure, boosting mechanical load capacity.
Physical-Chemical Working Mechanism
Vanadium reacts with carbon in the molten pool to form submicroscopic vanadium carbides. These fine particles refine the grain in the ferritic structure and hinder dislocation movement of atoms.
Metallurgical Relevance & Welding Risks
The fine-grain hardening raises the yield strength and improves the steel's mechanical load capacity. However, too high a vanadium content (> 0.1 %) can cause fusion issues, such as excessive carbon loss and high residual stress in the structure, leading to cracks and hot cracks.
Workshop Practice & Action Items
To achieve vanadium fine-grain hardening, the steel should be welded in a protected molten pool with vanadium treatment. The optimal vanadium amount is typically 0.05 to 0.1 %. The cooling time after welding should be slow (t8/5 > 25 s) to keep the grain large and increase mechanical load capacity. If cooling is too fast (> t8/5 < 5 s), glass-hard martensite forms, leading to cracks.