Influence of Vanadium on the Heat-Affected-Zone Properties of Mild Steel

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Abstract
To obtain specified mechanical properties in high-strength steel plates, vanadium is often used as a grainrefiner and precipitation-strengthener. As excess microalloying is usually considered to impair the toughness of the heat-affected zone, an investigation with the aid of a weld cycle simulator was made. Eight steels of composition 0·15% C and 1·30% Mn with up to 0·45% V were studied. Submerged arc, electrogas, and hand welding were simulated in test-pieces from which Charpy V-notch specimens were prepared and tested for ductility and hardness. From these tests and metallography, including thin-foil electron microscopy, the following conclusions can be drawn. Vanadium in excess of 0·10% embrittles the heat-affected zone at high heat inputs. The transition, temperature increases with vanadium content in a semi-parabolic manner. At lower weld energies, a V content > 0·25% is detrimental to toughness. The ductility loss can be attributed to hardening by vanadium nitrides, presumably in the form of coherent precipitates. Higher welding energies give higher ductility losses, which would be due to a coarser or more abundant precipitation. Without tempering, the nitrides are, as a rule, too small to be observed even with the aid of electron microscopy. After tempering, three types of precipitates can be observed in the electron microscope—on dislocations, in rows, and homogeneously distributed. Vanadium in the ranges studied does not significantly affect the optical metallographic structure of the weld heat-affected zone.

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