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Austempering Heat Treatment for Superior Ductility in Steel Components
Austempering is a process for steel components which yields a bainitic microstructure. Compared to a standard quench and temper process, the austempering forging heat treatment is superior in terms of ductility and distortion.

Advantages of the Austempering Process:

Austempering is a two-step process which yields a tough and strong bainitic microstructure.

  • 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.
  • The parts are then quenched in a molten salt bath in the temperature range of 400F to 750F. During this part of the process, the austenite isothermally transforms to bainite. Bainite is tougher than martensite at the same strength levels.
  • As the parts are quenched in a molten salt bath, the difference in temperature between the work piece and the quench bath is lower as compared to oil quenching. This difference yields far less distortion due to stresses caused by quenching.
  • The transformation is isothermal and takes place over a longer period of time as compared to martensitic transformation, hence this component of distortion caused by quenching is lower.
  • The sum total of these facts leads to a real advantage with regard to distortion of the final product when austempering is compared to a traditional quench and temper process.

If you have a steel component that has been problematic with respect to distortion in heat treating, austempering may be an option to eliminate this problem!

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.

Typical steels that are austempered include:

  • AISI 4140
  • AISI 4340
  • AISI 1045 - 1095


Austempering Ductile Iron (ADI) - Best for Castings

Austempering Ductile Iron is a heat-treating process that can be applied to ductile iron castings. The resulting product is referred to as Austempered Ductile Iron (ADI). It 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.

ASTM A897/897M provides information about the ADI process and mechanical testing requirements.

The Austempering Ductile Iron Process

  • When austempering ductile iron for castings, the material must be heated to a specified temperature to achieve austenitization.
  • The ADI process brings the material to a fully austenitic state with typical temperatures ranging from 1575F to 1675F.
  • Next, the parts are soaked at a time sufficient to reach complete austenitization.
  • After soaking, parts are quenched in a molten salt bath at a temperature that minimizes the probability of distortion, which is inherent in all quenching operations. Quench temperatures range from 450F to 750F.
  • The temperatures employed are determined as a function of the chemistry of the ductile iron, as well as the required mechanical and hardness requirements specified by the customer.

Chemistry: When it comes to chemistry, if section size increases, then specific alloying elements must also be increased 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, it may be beneficial to receive support from our metallurgical staff. We recommend obtaining 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

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.

ThermTech offers both small and large batch capabilities for austempering steel and iron investment castings.