Tanaka Limited of Osaka, Japan, has developed a plasma carburisation treatment that hardens the surface of austenitic stainless steel nuts and bolts. Carburisation of austenitic stainless steel is typically a challenge: at high temperatures of 550 to 850°C, chromium carbides are generated in the metal, and these are then precipitated at grain boundaries where they can lead to intergranular corrosion. However, because the process developed by Tanaka operates at a much lower temperature of less than 500°C, the carbon provides solid solution strengthening without the formation of chromium carbides. “For austenite-stable materials, such as S31600 and S31700, the process has a beneficial effect,” says Tsunenari Murakami, general manager of Tanaka. The new carburisation process improves the wear resistance, galling and seizing characteristics of stainless steel in nut-and-bolt applications. Harder nuts and bolts produced by this process can be used in food and beverage processing equipment, in which lubricating oils are not allowed, and in medical equipment applications, where hygiene is essential, says Murakami. Carburised stainless steel has been used commercially by a major manufacturer for a threaded lid on a beverage-processing vessel. The hardened lid prevents galling and seizing, which would result in wastage of the product being processed in the vessel.
