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High-temperature diffusion process extends the service life of steel tools and other components by forming a hard (~2000 HV), wear-resistant case consisting of single- or two-phase iron borides.
If the process has been performed properly and the right material and layer thickness have been chosen, boriding can extend the service life of ferrous metal tools and other parts beyond that imparted by traditional surface treatments such as carburizing, nitriding, and hard chromium plating. The diffusion process enhances surface hardness and provides resistance to cold welding, abrasive wear, and certain adds.
Boriding process basics
Boriding agents consist of mixtures of boron carbide and activators, in either powder or paste form. Boriding with a powder agent is performed in a manner similar to pack carburizing. Parts to be treated are packed together with the powder into a heat-resistant steel box. Parts are separated from each other with at least 10 mm (0.4 in.) of boriding agent, and covered with a layer of the material approximately 50 to 100 mm (2 to 4 in.) deep. This ensures uniform boriding and guarantees that both the formation and microstructure of the boride layer will be influenced only by the activity of the boriding agent, the treatment temperature, and the material being treated.
During subsequent furnace heating at 900 to 100 deg C (1650 to 1830F), boron diffuses into the metal and forms the boride layer. After a sufficient time at the boriding temperature, the box is removed from the furnace and allowed to cool at room temperature. The borided surface that results is very hard: ~2000 HV.
Powder agents provide a boride layer of excellent quality and are particularly suitable for treating small tools and other parts. The fine granular powders also are ideal for treating intricately shaped parts. The very small granules ensure excellent contact with the metal surface. Note that powder agents may form a crust-like deposit on the parts during boriding. However, the crust can be easily broken off afterparts have cooled.
These agents may be reused several times in normal boriding operations, which typically take 3 to 5 hours. After each cycle,3 40% fresh powder is mixed in with-the recycled material. Complete replacement of the boriding...