Hardenability, transformation and precipitation effects in vanadium steels.
PLATT, Geoffrey K. (1988). Hardenability, transformation and precipitation effects in vanadium steels. Doctoral, Sheffield Hallam University (United Kingdom)..
Recent work has highlighted unusual effects of vanadium when used in conjunction with other microalloying additions on the hardenability of steels. Positive and negative synergistic effects have been observed, but studies into the mechanisms have been limited. To investigate the effects, vanadium interactions with aluminium, molybdenum, niobium and titanium were studied in low (0.1%) and medium (0.4%) carbon steels, containing normal (0.008%) and enhanced (0.020%) nitrogen. Utilising standard jominy test conditions of 950°C for one hour resulted in classical hardenability responses being obtained, where increasing quantities of microalloying additions in solution increase the hardenability.However, when the jominy test conditions were varied unexpected effects were observed. Extending the austenitising time to eight hours showed that the hardenability was dependent upon kinetic effects such as the rate of solution of the alloy carbides/nitrides and the rate at which the microalloying elements in solution segregated to the austenite grain boundaries. It was also observed that if the austenitising temperature was increased to 1200°C a decrease in hardenability could be obtained by increasing the quantity of vanadium, niobium or titanium. These effects were attributed to a combination of thermal dispersion of microalloying clusters from the austenite grain boundaries, preferrential transformation on large alloy carbides/nitrides and migration of the austenite grain boundaries. Therefore it was considered inadequate to explain hardenablity solely in terms of the carbon concentration, austenite grain size and amount of other alloying elements present. Additional factors such as cluster formation, grain boundary pinning etc., were identified and applied to the results to successfully explain the effects of the alloy interactions on hardenability.Recent studies on vanadium alloyed pearlitic steels showed significant increases in strength could be obtained by precipitation within the pearlitic ferrite. Mechanical property investigations of two steels indicated that a maximum precipitation effect was obtained at an isothermal transformation temperature of 600°C.