Bluing of Steel by Heat Treatment: Why and How it Happens?

28 September 2017

Let’s navigate our way through a passivation process that earns its name by producing a blue-black protective layer around the subjected steel part. The heat treatment technique essentially forms a protective finish around the metal, which is why screws, cutting blades, and even hand weapons can procure that signature “bluing of steel” look. But, as with all heat treatment processes, there’s more to this technique than cosmetic appeal.

A Rust Resistant Finish

Not long after dipping into the whys and wherefores of steel bluing, we come across this primary benefit. Simply put, the passivation process makes the steel rust resistant. That’s because the heat treatment equipment produces magnetite, which is a Fe3O4 black iron oxide compound, not the Fe2O3 red oxide that’s commonly known as ‘Rust.’ As every owner of a ferrous-heavy product knows, rust is a corrosive oxide, one that flakes away and breaks down the metal. The black oxide variant holds firm, so it acts as a protective barrier, plus it delivers big in terms of a visually alluring metal finish.

The Bluing of Steel: How is it Accomplished?

In order to convert a virgin steel exterior surface into a bluish protective finish, we need to send it into a specially equipped heat treatment station. In here, the steel part is suspended in a furnace for a predetermined amount of time. But this isn’t an ordinary furnace. No, there’s a molten salt bath in here, and it’s a dip in that nitrate-rich compound, plus the thermal energy, that gives the steel its deep blue lustre. In effect, this is a controlled rusting procedure, but it’s not the nasty orange-red (Fe2O3) that breaks down ferrous metals. Rather, this is a controlled black oxide finish, a Fe3O4 membrane that uses passivation technology to form a shield material around a steel component, be it a knife blade or a screw, a gun or a hacksaw blade.

The molten salt is maintained at 300°C to 400°C, which is a high enough temperature to trigger the passivation process. Lowered into that bath, the steel part turns blue. Deeper shades of blue are reached if the component is held in the hot salt bath for longer periods. Alternatively, an air circulating furnace method can be employed. freed of messy salt compounds, this latter method uses an air circulating furnace and a predetermined quantity of steam to gain the same protective finish, one that again travels through different shades as long as the workpiece is suspended in that hot, steamy furnace interior. Importantly, the final product is rust resistant but not rustproof.

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