Recently, Powder Metallurgy has become a chosen method of production. The difference in the processing of a powdered metal allows for steel chemistries not possible with traditional steelmaking practices. Conventional steelmaking begins by melting the steel in a large electric arc furnace. Although the steel is very homogeneous in the molten state, as it slowly solidifies in the molds, the alloying elements segregate resulting in a non-uniform as-cast microstructure. In high speed steels and high carbon tool steels, carbides precipitate from the melt and grow to form a coarse intergranular network. Subsequent mill processing is required to break up and refine the microstructure, but the segregation effects are never fully eliminated. The higher the alloy content and the higher the carbon content, the more detrimental are the effects of the segregation on the resultant mechanical properties of the finished steel product. Powder metallurgy was developed to eliminate segregation and make alloys with elemental concentrations significantly higher than previously possible in traditional processes.
The process starts out the same as wrought steels – alloying elements are added and dissolved into molten iron. Then comes the main difference. The molten steel is atomized (misted into microscopic droplets) into liquid nitrogen where the steel is instantly frozen, leaving no time for diffusion to take place. The chemistry of the resulting powder is identical to that in the molten vat. Additionally, there are no inclusions or large carbides that form. The austenite grain size is the size of the powder at the very largest, which is small. The powder is then cleaned and sorted by size and then the remaining ideal powder is sintered in a hot isostatic press to solidify the steel. Sintering is heating the steel to a temperature just below its melting point, and then pressing it together at high pressures to solidify or remove the voids between powder spheres. This allows for drastic changes in the steel chemistry namely in carbon and vanadium. A larger volume of the highly wear resistant vanadium carbides form upon heat-treating. Since vanadium has a greater propensity to interact with carbon and form carbides than it does with chromium, most of the excess carbon is utilized in the formation of vanadium carbides. These leave the chromium free to help keep the steel corrosion resistant. The result is a premium steel product with properties of exceptional wear-resistance and good corrosion-resistance. (See: Part 4, Heat Treatment and our Steel Glossary)