Development of wear and corrosion resistant cold-work tool steels produced by diffusion alloying

The atomization of tool steel powders containing high amounts of niobium carbide does not work due to an early solidification of primary NbC from the melt in the tundish. A new powder-metallurgical production route called ‘diffusion alloying’ was introduced in previous papers: a melt containing a low amount of interstitial elements like carbon or nitrogen is gas-atomized. Afterwards the powder is blended with graphite. This technique offers the possibility to produce PM-steels by hot isostatic pressing with high amounts of niobium carbides which are known to increase the wear resistance significantly. In those MC-type carbides hardly any chromium is bound which hence remains in the metal matrix. This makes niobium an excellent substitution for vanadium in martensitic stainless steels, which are resistant to both wear and aqueous corrosion. Based on this new production technique, several new alloys were designed using the thermodynamic simulation tool “Thermo-Calc”. Due to the fact that interstitials are added subsequently to atomization, alloys with different C-contents can be produced from the same initial powder. All produced metal powders contain more than 12% chromium in a martensitic hardenable metal matrix and a fine dispersion of NbC. Some contain additional chromium-rich carbides of the type M7C3and/or image. The new materials were investigated thoroughly with regard to their hardening capacity as well as their tempering behavior. Subsequently differently treated steels were tested with a focus on their resistance to abrasive wear using pin-on-grinding paper tests. Scanning electron microscope (SEM) was used to characterize the micro-mechanisms of wear as well as the influence of the microstructure on the wear behavior.