Austempering

Bainite must have been present in steels long before its acknowledged discovery date, but was not identified because of the limited metallographic techniques available and the mixed microstructures formed by the heat treatment practices of the time.

Through his contacts at the American Steel and Wire Company, Bain was aware of isothermal transformations being used in industry and he began to conceive new experiments [1] Further research into the isothermal transformation of steels was a result of Bain and Davenport's discovery of a new microstructure consisting of an "acicular, dark etching aggregate."

[1][3] Commercial use of bainitic steel thus came about as a result of the development of new heat-treating methods, with those that involve a step in which the workpiece is held at a fixed temperature for a period of time sufficient to allow transformation becoming collectively known as austempering.

These components, which are usually thin, formed parts, do not require expensive alloys and generally possess better elastic properties than their tempered Martensite counterparts.

Currently, austempered steel is also used in bearings, mower blades, transmission gear, wave plate, and turf aeration tines.

The basic steps are the same whether applied to cast iron or steel and are as follows: In order for any transformation to take place, the microstructure of the metal must be austenite structure.

As with conventional quench and tempering the material being heat treated must be cooled from the austenitizing temperature quickly enough to avoid the formation of pearlite.

The specific cooling rate that is necessary to avoid the formation of pearlite is a product of the chemistry of the austenite phase and thus the alloy being processed.

In austempering the heat treat load is quenched to a temperature which is typically above the Martensite start of the austenite and held.

The quench and holding temperature are primary processing parameters that control the final hardness, and thus properties of the material.

After quenching and holding there is no danger of cracking; parts are typically air cooled or put directly into a room temperature wash system.

[5] Tempering adds another stage and thus cost to the process; it does not provide the same property modification and stress relief in Bainite or ausferrite that it does for virgin Martensite.

There are many such savings possible in the specific case of converting a quench-and-tempered steel component to austempered ductile iron (ADI).