Powder metallurgy (PM) component producer GKN Sinter Metals, a division of GKN plc, recently started local production of stainless steel and other PM components for the local and international markets.
PM is a forming and fabrication technique that offers a potential alternative to processes, such as forging, stamping, casting and machining. This high-volume process is characterised by energy efficiency, high material use and low scrap rates, which result in lower costs. Although secondary processing is sometimes necessary, components with complex geometries are routinely produced to close dimensional tolerances and good surface finish without secondary operations.
While PM is a relatively new technique in South Africa, it, in itself, is not new. Cemented carbides and self-lubricating bearings have been produced using this method since the 1920s, and the development of iron and steel structural parts, followed shortly after. PM stainless steel has been around since the early 1940s: however, its use by industry gained momentum in the last 20 to 30 years, owing to greatly improved corrosion resistance.
A large range of ferrous and nonferrous alloys is available to cater for the wide selection of applications where PM is used. Stainless steels make up part of this range of alloys, with the most common, austenitic, ferritic and martensitic grades, all readily available.
The ferritic grades account for the highest volume of PM stainless steels produced. GKN reports that these alloys perform better in thermal cycling environments, which have made them good options for automotive exhaust components. These alloys are cheaper than the austenitic grades and can be used in applications where moderate corrosion resistance is required.
Austenitic PM grades offer corrosion resistance and elevated temperature properties.
The martensitic grades of PM stainless steels are particularly suited to applications requiring good wear resistance and strength, combined with fair corrosion resistance.
The ferritic and martensitic grades are ferromagnetic and are used in applications where this is a desired property. Current applications for PM stainless steels include aerospace, automotive, general industrial and appliance components.
The conventional PM process entails the consolidation of metal powder in a closed die, under high pressures, and subsequent sintering at high temperatures, resulting in net or near-net shape components. Certain components may require finishing operations to add functionality, additional complexity, or to achieve improved tolerances.