Vanadium Carbide Precipitation Research Articles

Strengthening of Ferrite by Vanadium Carbide Precipitation

AbstractWhen iron–vanadium–carbon alloys are transformed from the γ to the α state, a banded dispersion of vanadium carbide is formed as the transformation fronts move through the crystals. The strengthening resulting from this reaction in a series of alloys isothermally transformed in the range 725–850° C is shown to be dependent on the transformation temperature, since this determines the state of the dispersion. The mechanical properties are also sensitive to the volume fraction of carbide precipitate, which has been studied by changing the vanadium and carbon contents of the alloys. The dispersion can be further influenced by addition of alloying elements, e.g. manganese, which alters the kinetics of the reaction.

Influence of vanadium on the strengthening of 0.4% C-4% Cr-18% Mn austenitic steel during aging

1. The optimum content of vanadium in austenitic 0.4 C-4 Cr-18 Mn-V steel, guaranteeing the most desirable combination of strength and ductility characteristics upon aging, is 1.2–1.5%. 2. The primary mechanism of strengthening during the aging of this alloy is the precipitation of vanadium carbide, VC. The optimum aging temperature is 650°C. 3. The maximum level of strength attained upon aging rises with increasing quenching temperature from 1050 to 1175°C. 4. After quenching from 1100°C and after the optimum aging treatment at 650°C, the following properties are typically obtained: ultimate tensile strength, 100–110 kg/mm2; yield strength (0.2% offset), 75–80 kg/mm2; elongation, 35–45%; reduction in area, 40–50%.

Structural transformation in 12Kh1MF steel induced by heat treatment

1. In the case of uninterpreted cooling of 12Kh1MF steel the austenite is transformed into ferrite-pearlite, intermediate phase, or martensite, depending on the cooling rate. Therefore, machine parts of various sizes must be cooled at approximately the same rate, changing only the cooling conditions. 2. Tempering of quenched or normalized 12Kh1MF steel at 600\2-650\dgC induces precipitation of highly dispersed particles of vanadium carbide; this, in turn, increases the ultimate strength, yield strength, and hardness, but decreases the impact strength. Increasing tempering temperatures lead to coagulation of vanadium carbide, which decreases the strength of the alloys but increases the ductility and the impact strength. Tempering of annealed steel does not induce precipitation of vanadium carbides, and the mechanical properties remain unchanged, since the vanadium carbide is completely precipitated during annealing. 3. The low and variable values of the impact strength of heat treated thick-walled pipes result from insufficiently high rates of cooling and the fluctuation of the temperature during tempering in industrial furnaces. 4. High heat resistance and good mechanical properties during short exposures to high temperatures are obtained as the result of heating to 960\2-980\dgC, cooling from this temperature at a rate not less than 250\2-300 deg/min, and tempering at 730\2-750\dgC.

The diffusion of carbon and oxygen in vanadium

Carbon diffuses in vanadium at a slightly more rapid rate than oxygen. From internal friction and elastic after-effect measurements, the diffusion coefficients of these interstitial impurities as functions of temperature were found to be D C− V = 0.0047 exp (−27,300/ RT) cm 2/ sec and D o− V = 0.019 exp (− 29,300/ RT) cm 2/ sec. In contrast to solutions of oxygen in vanadium, those of carbon are unstable, as evidenced by a decline in the height of the carbon- vanadium damping peak with time. This decline is attributed to the precipitation of vanadium carbide from a supersaturated solution. A similar instability was found previously in solutions of carbon in niobium and tantalum.