Austempering is an isothermal heat treatment that, when applied to ferrous materials, produces a structure that is stronger and tougher than comparable structures produced with conventional heat treatments. Conventional heat treaters heat the parts to "red heat" in a controlled atmosphere and then quench them in a bath of oil or water that is near room temperature. (Maybe even as high as a few hundred degrees Fahrenheit). This produces a crystalline structure known as Martensite, a hard, brittle phase. The parts are then tempered in another furnace at 350°F (177c) to 1100°F (593c) to decrease the "brittleness."

Austempering starts the same way. The parts are heated to red heat in a controlled atmosphere (so they don't scale) but then are quenched in a bath of molten salt at 450°F (232c) to 750°F (399c). The quench temperature is above the Martensite starting temperature. Therefore, a different structure (not Martensite) results. In Austempered Ductile Iron and Austempered Gray Iron the structure is Ausferrite, and in steel, it is Bainite.

Austempering Means Uniform Structure
During the process of quenching to Martensite, the Martensite reaction begins immediately. The result is that the outside of the part may already be transformed while the inside is still red hot. It is this "non-uniform phase transformation" that results in distortion and tiny micro cracks that lower the strength of the part.

By contrast, the Austempering reaction that produces Ausferrite or Bainite takes place over many minutes or hours. This results in uniform growth and a stronger (less disturbed) microstructure. Austempering truly is "a better mousetrap."

AP's understanding of the process is very thorough, and we keep pushing the envelope with R&D. Twenty percent of all heat treaters list Austempering as just one of the many processes they do. This means that they only do Austempering part time. Austempering is ALL that AP does...and we like to think we do it better than anyone.

Based on customer input, our proprietary computer models can make accurate material and process selections and predict final mechanical properties. This allows us to assist the design engineer in formulating the optimum and most efficient design, material, and process combination.

Out of the Lab and Into the War
The history of Austempering begins in the 1930's, when Grossman and Bain, working for the United States Steel Laboratories, were evaluating the metallurgical response of steels cooled rapidly from 1450°F (788C). to intermittently high temperatures and held for various times. The outcome of their pioneering research is what we now commonly call the "isothermal transformation diagram"

Grossman and Bain were familiar with the conventional metallurgical structures of ferrite, pearlite and Martensite. What they discovered, however, was another structure, formed above the Martensite start temperature (Ms) and below the pearlite formation region. In steels, this structure took the form of an acicular (plate-like) structure with a feathery appearance. X-ray diffraction later identified this structure as a combination of ferrite and metal carbide. The resultant structure, termed "Bainite," was found to be stronger and tougher than a comparable "quenched and tempered" structure (Fig. 2).

In 1937, while working for International Nickel, Flinn documented the microstructure of Austempered gray iron (Figure 3). By 1941 Inco and Climax Molybdenum collaborated on experiments with cast iron that produced a "tempered bainitic" microstructure with a 90 ksi (620 MPa) tensile strength.

During World War II the Austempering process was used extensively in the production of gun parts. It was found that the process resulted in low distortion and parts that were tougher than the quenched and tempered components they replaced. (Critical gun parts are still routinely Austempered today). However, the best equipment available for Austempering then was very inefficient. Therefore, the Austempering process was relatively expensive.

By the 1950's the Austempering process was routinely applied to steel and malleable iron parts. The relatively high cost of the process limited its use to only the highest performance parts.