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.
Today’s journey through heat treatment technology looks at Austempering. As always, we’re industriously seeking the best, most cost-effective way to improve the mechanical and physical characteristics of a workpiece. More than this, we’re using specific engineering methods to ensure the workpiece absolutely exhibits the traits that have been set in stone by a client. So what is austempering? Does this isothermal heat treatment procedure support our thermal processing ideal?
Austempering Process
Unlike conventional workpiece tempering, the austempering process does not use a traditional quench phase. That means there are no tanks of oil or water employed in this bath. No, the workpiece is instead bathed in molten salt. Raised to the austenitizing temperature, the carbon steel or cast iron workpiece is lowered into the hot salt. The temperature of the salt treatment varies between 240°C and 400°C, and that temperature range is governed by the type of iron or steel passing through this tempering phase. Basically, the red-hot transformative furnace is unchanged, but the tempering sequence here has undergone significant alterations. What’s the result of the thermally active tempering solution? Well, the austenitized microstructure does not cool rapidly and harden as a martensitic material. Instead of this rapid cooling result, the steel component transforms directly into a microcrystalline Bainite form.
Clarifying the Benefits of Austempering
Martensitic steel and iron parts are incredibly hard. Unfortunately, the traditional tempering path adds stress and distortion to the workpiece because the abrupt cooling method causes the carbon steel to flex. This non-uniform flex cannot be allowed in certain components, especially the parts that rely on a uniform metallic structure. Car springs, for example, are manufactured so that they uniformly deliver material strength and physical elasticity throughout the product. Here, then, is one of a host of applications that benefits from a shock-less tempering phase, as delivered by the molten salt bath used in the austempering process. Engine components, crankshafts, transmission parts, suspension springs and more, all of these stress-heavy parts gain tempered effectiveness when they’re exposed to a high-temp heat treatment stage that toughens carbon steel and cast iron components without adding cooling-incurred mechanical distortion to the final product.
Intended as a shock less workpiece toughening phase, this heat treatment process produces distortion-free springs and other dimensionally critical components. The austempering salt bath holds the treated part for minutes, perhaps even hours, at which point the now austempered workpiece is uniformly hardened, ductile and entirely wear resistant. That Bainite steel part or ausferrite cast iron component is then ready to serve in the most mechanically torturous work conditions.