What is Austempering?

08 September 2017

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.

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