What is Austempering?

Austempering is a preferred heat treatment process for steel components and ductile iron because it serves to improve the mechanical properties of these ferrous metals and also minimizes or in some cases, eliminates distortion.


Two-Step Austempering Process for Steel Components

  1.  The first step in the process is to heat the steel component to a temperature at which it is in the fully austenitic phase and soaking at this temperature for a period of time. This ensures temperature uniformity throughout the entire section.
  2. In the next step, the parts are quenched in a molten salt bath in the temperature range of 400°F to 750°F. During this part of the process, the austenite isothermally transforms to bainite. Bainite is tougher than martensite at the same strength levels.

Recommendations for Austempering Applications

Long slender parts or steel components that are thin in the cross section are excellent candidates for austempering. Another excellent application for the austempering process are parts that are sensitive to quench cracking due to material or design. Hear what one of our customers has to say about it.
Typical steels that are austempered include:

  • AISI 4140
  • AISI 4340
  • AISI 1045 – 1095


Why is Austempering Ductile Iron the Best Choice for Castings?

Austempering Ductile Iron is the recommended heat-treating process by industry experts, applied to ductile iron castings. The resulting product, Austempered Ductile Iron (ADI), provides superior mechanical properties relative to conventional heat-treating processes for cast iron. In certain applications, austempered castings feature superior strength-to-weight ratio when compared to steel parts. Several scientific studies, including a study by the Los Alamos National Laboratory, and the Advanced Steel Processing and Products Research Center, Colorado School of Mines, demonstrate these advantages and benefits of austempering for castings.
ASTM A897/897M provides information about the ADI process and mechanical testing requirements.

Overview of the Austempering Ductile Iron Process

When austempering ductile iron for castings, heat treating the material to a specified temperature enables it to achieve austenitization.
  • The ADI process brings the material to a fully austenitic state with typical temperatures ranging from 1575°F to 1675°F.
  • Next, soaking the parts for a sufficient length of time allows them to reach complete austenitization.
  • After soaking, the parts undergo quenching in a molten salt bath at a temperature that minimizes the probability of distortion, which is inherent in all quenching operations. Quench temperatures typically range from 450°F to 750°F.
  • To determine the ideal temperatures, skilled heat treatment specialists take into account the chemistry of the ductile iron, as well as the required mechanical and hardness requirements specified by the customer.

Important Considerations for Austempered Castings

Chemistry: If section size increases, then it is important to also increase specific alloying elements to achieve desired properties.

Foundries: Pearlite rating, nodule count, and nodularity rating are critical factors the foundry must control in order to maximize the end properties of austempered castings.

Recommendations for Austempering Alloys

If you are in the alloy consideration stage, our metallurgical staff can offer you valuable support. We recommend obtaining our team’s recommendations prior to the quoting process to ensure you have a full understanding of material options and costs. Our process development team will read and interpret the specifications you provide to certify that the final product will meet the necessary requirements.

ThermTech—Experts in Austempering Stampings, Machined Parts, and Investment Castings

ThermTech, Inc. offers both small and large batch capabilities for austempering steel and iron investment castings. Between our neutral salt bath furnaces and our larger austempering furnaces, we have the capabilities to austemper a wide variety of stamping, machined components, and investment castings. We have run austempering on alloys from 1060 through 4340 to ductile iron machined parts.

Our dedicated team works with you to ensure we meet your specifications. Count on our decades of experience with hardening, tempering, surface heat treatments, vacuum treatments, and annealing. To learn more about our services, please contact us today.

Learn more about austempering and how ThermTech can serve you by watching these videos.

Request a free quote online or call us at 800-752-6917. You can also contact us online for all your austempering and heat treating needs.


    ThermTech’s quality policy is ZERO DEFECTS for all customer products and in all quality management system activities. Through our efforts to continually improve the quality of the work we offer, we’ve been recognized as being able to meet the rigorous quality standards required by several of our major customers. ThermTech is certified for ISO 9001 and AS9100 in both Metal Heat Treat and Metal Finishing.