Factors influencing the secondary hardening of vanadium steels.

Research over the last twenty years has conclusively shown that the secondary hardening of high speed steels is due to precipitation hardening by carbides formed on tempering. Vanadium carbide offers the major contribution to this phenomenon, and primarily for this reason, vanadium is present in all grades of high speed steel. The object of this research was to attempt to improve the secondary hardening response of high speed steels, due to V[4] C[3] precipitation. There were two important aspects, these being the effects of varying elements on the nucleation rate of this carbide on tempering and to attempt to improve the dissolution rate of V[4] C[3] during austenitisation. A considerable amount of research has been carried out on the orientation relationship between precipitating V[4] C[3] particles in a ferrite matrix, but little work has been undertaken on the effect of other elements on this precipitation mechanism. Simple ternary and quaternary alloys were melted, containing in each instance, iron, carbon and vanadium and in the latter case an additional alloying element. A number of commercial purity steels were also employed, these being carburised to give a range of carbon contents in the cases. True secondary hardening did not occur in the high purity steels which only contained vanadium as an alloying addition, but this did take place in the high purity steels in which a second alloying addition was present. All the commercial purity steels showed secondary hardening peaks, over a wide range of carbon contents, with the exception of those with very low vanadium levels (0.05 and 0.06 wt. %). Optimum secondary hardening occurred at or close to the stoichio-metric vanadium: carbon ratios for V[4] C[3] formation in these steels. The nucleation of V[4] C[3] took place on dislocations surrounded by solute atoms. This meant that the greater distortion and higher free energy associated with the presence of solute atoms in the matrix, the higher the nucleation rate of V[4] C[3] lattice. Silicon and manganese had the greatest effects in increasing the nucleation rate of V[4] C[3] as the lattice parameters of these elements differed quite appreciably with that of ferrite. Non-carbide forming elements, with the exception of silicon, increased the dissolution rate of during austenitisation. These elements decreased the interatomic bond strength between vanadium and carbon atoms in the V[4] C[3] lattice. Silicon had the effect of retarding the diffusion of vanadium atoms away from the carbide and into the austenite lattice. A very surprising aspect of this research was that some of the high purity steels graphitised during long term tempering at 560°C. This was due to V[4] C[3] decomposing into graphite, but the addition of molybdenum, manganese, chromium or tungsten prevented this phenomenon. The results of this work allowed a suggestion to be made of the composition of a low alloy tool steel. This steel was a compromise composition between optimum secondary hardening response due to V[4] C[3] precipitation and maximum dissolution rate of primary V[4] C[3] on austenitisation. Such a steel would have a much lower cost than high speed steels.