Surface Oxidation of Steel Powder

Powder metallurgical production of fully dense steel such as high‐speed steel, tool steel and stainless steel is of large industrial importance. The process route is as follows: melting – inert gas atomization – encapsulation – hot isostatic pressing and subsequently often also hot forming. Final products comprise near‐net‐shape components, billets and semi‐finished parts e.g. rods. The mechanical and functional properties of the PM steels are often superior to those of conventional manufactured steels over casting + forging with the same chemical composition. The properties of the PM‐steels are highly dependant on their oxide contents. There are three types of oxides in PM‐steels: exogenous and endogenous slag inclusions and oxides originating from surface oxidation of the atomized particles. The negative effect from the inclusions on the properties of the PM‐steels is dependant on their origin. The exogenous inclusions are normally highly detrimental while the endogenous inclusions are harmless. Surface oxides are harmful when present in larger amounts. A special experimental technique was developed to study surface oxidation of powders. Large emphasis was also focused on sampling to enable that the oxygen determination was representative of the bulk powder. The results thus obtained gave most important information on when and how the atomized powder was oxidized during the process. Furthermore, a new methodology was developed based on the experimental results. It is now used world‐wide for quality control of atomized steel powders. This method enables the distinction between exogenous + endogenous inclusions and surface oxides.