The effect of small additions of vanadium on the yielding, impact and carbide percipitation characteristics of mild steel

• Main Author: Towers, T. A.
• Published: University of Leicester 1967
• Subjects: 620
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.737075

A study was made of a series of steels containing varying amounts of carbon and vanadium (Table l). The yield and impact characteristics were compared using the relationships between grain size and yield strength and transition temperature as established by PETCH. The precipitation characteristics were studied by micro-hardness testing the ferrite formed under both isothermal and continuous cooling, and the effect of controlling the finishing temperature on rolling on the mechanical properties of one of the steels was also investigated. The results showed an increase in ?i, which increases the yield strength by 2 tons in-2 for the addition of 0.2% V when sufficient carbon is present (0.25%). This also gave a rise in transition temperature of 9°C, the majority of the increase being caused by the initial additions (0.02%). The relationships obtained were all non-linear, with the raising of the austenitising temperature giving a continuous increase in ?i. Grain refinement occurred in the 0.25% C steels, giving a reduction in transition temperature sufficient to counteract the rise due to the vanadium additions together with an increase in yield of 4 tons in-2. When insufficient carbon was present, grain refinement did not occur and the yield strength was lowered by 0.2% V showing that lowering of ?i due to the loss of carbon from solid solution is greater than the increase due to VC. The impact transition temperature showed a similar rise to the 0.25%C steeds. Nitrogen has a greater effect on ?i giving a larger increase in yield strength and transition temperature. A very fine grain size was obtained, but the reduction in transition temperature did not completely offset the rise due to ?i. Vanadium carbide precipitation followed the mode of known theory, secondary hardening being present with 0.25%C and 0.2% V; overageing, however was rapid. Nitrogen gave secondary hardening at lower vanadium and carbon levels, overageing being much slower. All high carbon steels gave hardening peaks on continuous cooling; rapid overageing again occurred. The final constant hardness increased with increasing cooling rate and it is probable that the highest strength occurs between 2000 and 7000°C/hr. Controlled rolling is beneficial if rolling is continued after precipitation occurs resulting in high yield strength but low transition temperature.