Austempering Heat Treatment: The Bainitic Revolution for Steels, Irons, and Their Components

Welcome to our brief on austempering. Austempering is a highly specialized method of heat treatment. It is recognized for producing superior mechanical properties in steels and irons. This technique promises advanced strength, ductility, and reduced distortion.

Austempering Explained: Austempering is a Two-Step Process

  • Austenitization : The steel or iron material is heated to a temperature where it enters the fully austenitic phase. By controlling the temperature and time we soak at high heat ensures consistent properties for each part in the load leading to optimized properties after quenching
  • Quenching in Molten Salt Bath: Following the high heat soak, the material is quenched in a molten salt bath, usually between 400F and 750F. Here, the austenite undergoes an isothermal transformation to bainite (in steels), a microstructure that can out perform martensite in terms of toughness for the same tensile strength.

Key Technical Benefits of Austempering:

  • Enhanced Mechanical Qualities: The final microstructure results in a unique combination of strength and ductility, superior to the traditional tempered martensite.
  • Distortion Control: The lower temperature differential during the molten salt bath quench, coupled with the extended transformation time, minimizes distortion. This is especially crucial for steel components that have had historical distortion challenges during heat treatment.
  • Increased Wear and Fatigue Resistance: The bainitic structure is renowned for its resistance to wear and fatigue, prolonging the life and performance of treated components.
  • Uniform Microstructure: The controlled process assures a consistent microstructure, promising predictable performance characteristics.

Special Focus: Austempering Steel Components

Austempering is particularly beneficial for:

  • Long and Slender Components: Pieces that are slender or have thin cross sections.
  • Distortion-Prone Components: Steel parts that have been traditionally challenging with regard to distortion in other heat treatments.
  • Quench Crack Sensitive Components: Parts that risk quench cracking due to specific material properties or design features.

Notable Steel Grades for Austempering:

  • AISI 4140
  • AISI 4340
  • AISI 1045 – 1095

Austempering Ductile Iron (ADI): An Innovative Approach to Castings

The ADI process turns the conventional heat-treatment for ductile iron castings on its head by offering superior mechanical properties. This results in castings boasting an impressive strength-to-weight ratio in comparison to steel counterparts.

Process Breakdown: 


  • The material is heated to achieve complete austenitization, typically between 1575F to 1675F.
  • Following a sufficient soak, the castings are quenched in a molten salt bath at temperatures ranging from 450F to 750F, ensuring minimal distortion.
  • The chosen temperatures are influenced by the ductile iron’s chemistry and the customer’s specific mechanical and hardness requirements.

Choosing the Right Alloy for Austempering

Navigating the world of alloys for austempering can be daunting. It’s crucial to engage with experienced metallurgical staff early in the decision-making process. This helps to ensure a sufficient understanding of material options, costs, and final product requirements.

ThermTech: Leading the Austempering Frontier

ThermTech possesses the equipment and expertise to deliver top quality austempered steel and iron products. With both a variety of options available to us we are equipped to austemper parts ranging from steel stampings to iron castings with everything in between.
In the ever-evolving heat treat industry, austempering is helping to shape to future. Whether it’s to attain unmatched mechanical properties, reduce distortions, or guarantee component longevity, the austempering process holds the key.

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.