Microalloyed Forging Steels-A State of the Art Review 890801
Microalloyed (MA) medium carbon forging steels are gaining acceptance as a replacement for the traditional quenched and tempered grades. Substantial cost reduction provides the driving force for the change. The ferrite-pearlite microstructure is strengthened by precipitation of MA compounds during cooling from the forging temperature. Because of high solubility of vanadium in medium-carbon steels, this element is the preferred MA additive. Forging is generally performed by conventional (high-temperature) practice. The rate of cooling, however, must be controlled. In many instances, air cooling on a conveyer gives satisfactory results. For applications requiring improved notch toughness, small additions of titanium (0.01%) were found to be effective. A dispersion of fine titanium nitride particles contributes to grain refinement by pinning the austenite grain boundaries. The fine grained steel exhibits impact properties comparable to quenched and tempered carbon steel. Improved machinability of MA steels compared to heat-treated steel grades enhances further their cost advantages. To assure a safe and reliable performance of MA steels, the substitution is usually based on the results of component testing, subjected to simulated service conditions. This approach requires a coordinated co-operative effort, involving metallurgists and engineers of the user, steel producer, and forging personnel.