Austempering – Process, Benefits and History
Austempering is an isothermal heat treatment and is applied on ferrous metals that contain carbon. Steel is of course among the materials that are suitable for the treatment. In steels with medium carbon content, it produced a distinct bainite microstructure. Ferrous materials with high levels of carbon, such as cast iron, form ausferrite, a stabilized type of austenite.
The process is used to improve mechanical properties of the treated workpiece.
Austempering can only be referred to as such if the material displays bainite or ausferrite microstructure as a result of the heat treatment. However, not all materials are suitable for this process, as only certain metals will react to being austempered by forming these microstructures.
Why is bainite desired? The bainitic structure consists of cementite and dislocation-rich ferrite. Therefore, it is similar to pearlite, a microstructure made of alternating layers of ferrite and cementite. However, bainite is harder than pearlite while being softer than martensite. In terms of hardness, bainite can be seen as an intermediate of the two.
Why is ausferrite desired? Ausferrite is a mixture that consists of ferrite and stabilized austenites. Due to its chaotic structure, materials with ausferrite microstructures display increased toughness in comparison to untreated cast iron.
Benefits and Disadvantages
Austempered materials show improvements of the following mechanical properties compared to conventional materials:
- Destortion reduction
However, there are limits when it comes to austempering. For one, it does not work with every type of steel. When it comes to low carbon steels, austempering does not work as well. Futhermore, austempering is limited to thin sections (about 5 mm depth). If not executed correctly, austempering can result in the formation of pearlite instead of bainite. Steels with a pearlite structure are very prone to cracking and brittleness when placed under challenging conditions.
A good alternative to austempering is boronizing. Our established hardening process is suitable for almost every type of steel: unalloyed, and low alloyed steels as well as stainless steels (austenitic, ferritic, martensitic and duplex steels), tool steels and many more. The layer hardness we can achieve by means of BoroCoat® is impressive as well: it reaches up to 2800 HK. If you are looking for an alternative to austempering, boronizing is your best option.
The first step of austempering is transforming the surface microstructure into an austenite structure. To do this, the material has to be heated to austenitizing temperatures, which range from 1455 to 1680 °F (790-915 °C). The material will be held in this heat until a fully austenitic microstructure with enough carbon content is formed. When working with steel, this step typically only takes a few minutes. There are many ways to perform austenitizing: by induction heating, direct flame (typically used for partial austempering) or any type of furnace.
Quenching and Holding
Austenitizing must be immediately followed by quenching, typically in a bath of molten salt, in order to avoid the formation of pearlite. The speed of cooling down is determined by several factors: medium, temperature, agitation, thickness of the part and load. The material is then held at an austempering temperature that is above martensitic temperature.
Once quenched, the material is finally cooled down to room temperature by air or water. Unlike many other heat treatment methods, austempering does not necessarily require further tempering. It would be both cost-intensive and ineffective because the mechanical properties would not improve much further than they already have.
Austempering first began in the 1930s. Edgar C. Bain and Edmund S. Davenport pioneered the heat treatment for ferrous materials. While the bainite structure was not a novelty exactly, it had not been properly labelled or studied due to the limited research techniques available. Bain studied isothermal phase transformation and observed the behavior of austenites, which led to the discovery of a new microstructure with the help of Davenport.
InWorld War II, austempered steel was first used in a bigger capacity. Rifle blots were made of the hardened material because of its high impact strength. Plus, the small size and surface area of the components made it cheaper to apply austempering to these parts. Today, austempered steel is used in many areas of applications, the most important being the automotive industry. There, austempering is applied to safety components such as car seat brackets.