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Austempering is a through hardening technique that uses a salt quenchant to significantly slow down the cooling process during quenching, ultimately maximizing toughness and minimizing distortion in parts. This technique has become increasingly important in recent years as the automotive industry shifts towards electric vehicles (EVs) and parts are adapted to handle harsher environments. 

EVs produce instant torque which puts immense stress on components that carry energy from the motor to the wheels, such as bearings, axle shaft, and steering components. Compared to components in vehicles with internal combustion engines (ICEs), EV components responsible for transferring energy to the wheels require increased toughness—that’s where austempering comes into play.

In this article, we discuss the benefits of austempering for specific components, material considerations, along with the type of equipment that’s best geared toward high-volume, safety-critical production.

Benefits of Austempering

The primary benefits of austempering is that it allows parts to achieve superior properties, including higher impact and fatigue strength, increased ductility, and resistance to embrittlement while providing dimensional stability during the cooling process. This makes it a preferred technique for parts that need to perform under constant stress, such as complex stampings, springs, clips, seat belt loops, J Nuts, wheel well clips, and agricultural components. It is also highly effective for mining, oil, and gas components, primarily digging tools that are required to withstand heavy service.

Springs are a component that benefits greatly from austempering. Heavily loaded springs under the hood of a car, for example, may be exposed to salt and other harsh environments that could cause corrosion. Quenching is needed for added toughness, but because springs are dimensionally thin, an oil quenchant would cause significant distortion because it causes the parts to cool at a much faster rate. Austempering using a salt quenchant is the obvious choice in scenarios like this.

Austempered Components in EVs

  • Stampings
  • Springs
  • Clips
  • Seat belt loops
  • J Nuts
  • Wheel well clips
  • Bearings 

Quench-and-Hold for Distortion Control

Austempering employs a quench-and-hold technique that submerges parts in molten salt at elevated temperatures (about 600 degrees Fahrenheit) to slow down the cooling process and minimize thermal expansion. The molten salt used for austempering reaches a much higher temperature than any other quenching media and reduces the temperature change that parts experience when going from heat treating furnace to quenching solution. 

This method is especially beneficial for thin components that run the risk of distortion. If you were to use other quenching mediums such as oil-based quenchants, the temperature change is much more drastic due to the thermal range of other quenching solutions. Because molten salt is commonly 600 degrees Fahrenheit, it’s the primary quenchant used for austempering.

Thermal Range of Different Quenching Media

Salt Oil Gas
585-680F150-300F Ambient

Determining the Hold Time for a Part

In general, the higher the carbon and alloy content a part contains, the longer it will take to reach bainite—the crystalline microstructure achieved through austempering. For example, to achieve 45 HRC in a 4140 part, you would have to hold it in the salt for about 7 minutes. Whereas achieving the same hardness level (45 HRC) in a 4340 part would require 20 minutes in the salt. And, to bring alloyed ductile iron to 45 HRC would take over 2 hours in the salt to form austempered products.

Innovative Equipment for High-Carbon Parts

High-carbon, high alloy components require significantly more time in the salt which can cause production challenges if you’re not austempering with the right equipment. At Paulo, we designed high-capacity austemper furnaces specifically for parts like these. While many heat treaters still rely solely on the traditional salt pot for austempering, we’ve adapted the process to make it more efficient than ever.

The continuous mesh belt austemper furnaces located in our Murfreesboro Division feeds parts directly from the furnace into a 250,000-pound vat of molten salt—one of the largest salt quenching systems commercially available. Not only does this allow us to hold components for longer in our salt quenchant (up to 1.5 hours), but we’re also able to austemper up to 3,000 pounds of parts per furnace per hour.

High-Quality Heat Treating Partners

Whether your parts require austempering or not, it’s important to find a heat treating partner with a demonstrated commitment to providing high-quality thermal processing, along with innovating processes to fit the needs of the market and making heat treating more efficient than ever. Our innovative Production Information and Customer Service system (PICS) enables precise, computer-controlled operating temperatures throughout our continuous mesh belt austemper furnace to deliver consistent, reliable results and drive your supply chains forward. 
If you have questions about austempering or any other heat treating process for your next project, contact a Paulo expert today.

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