The heat treatment of steel often involves heating a component part into the austenite region (aka austenite phase field) in order to perform a thermal treatment such as normalizing, hardening, case hardening, etc. But what is austenite, how does it form and what temperature range is most conducive to the process we are running? Let’s learn more.
What is austenite?
In 1901 Floris Osmund, an early French pioneer in metallography, proposed naming the high-temperature crystal structure of steel after Sir William Chandler Roberts-Austen, a metallurgist noted for his research on the physical properties of metals and their alloys.
Austenite (aka gamma iron, g-Fe) is a metallic, nonmagnetic allotrope (a material that can exist in more than one crystal form depending on temperature) of iron. Austenite is a solid solution often combining iron with various alloying elements (e.g., carbon). It can be said that the science of steel processing is based on understanding the austenite phase field in the iron-iron carbide phase diagram (Fig. 1).
As Krause observes, “controlled transformation of austenite to other phases on cooling is responsible for a great variety of microstructures and properties attainable by heat treatment of steels.”
Austenitic Grain Growth
Austenite formation in microstructures (Fig. 2) has been extensively studied.[1-9] For plain-carbon steels, austenite can form from pearlite or even a highly spheroidized structure in a very short time period – in the range of 4-25 seconds and be complete after 60 seconds – but in alloy steels the time may increase a hundredfold or more (Fig. 3) since the alloying elements and carbides require more time for diffusion to occur.
Prior Austenitic Grain Size and Mechanical Properties
Austenite grain size is important because it influences the transformation products formed on cooling and, as such, properties related to hardenability and microstructure. The austenite grain size is commonly referred to as the prior austenite grain size since retained austenite present at room temperature does not influence the parent austenite. The prior austenitic grain size can significantly influence properties such as toughness, which is lowered as the grain size increases. It should be noted here that revealing this structure involves rather sophisticated etching techniques (c.f., “Grain Size and Its Influence on Material Properties”).
Small additions (in the order of 0.1%) of certain alloying elements (Nb, V, Ti) produce carbides, carbonitrides or nitrides; influence grain-size control and strengthening; and form the basis for microalloying steels. Finely dispersed microalloying particles retard austenitic grain growth, especially at higher temperatures (by so-called pinning of the grain boundaries).
Austenitizing Temperature – A Practical Approach
In practical terms, one wants to run at the lowest austenitizing temperature for the shortest amount of time in order to limit grain growth, minimize the influence of creep and unwanted surface effects (e.g., oxidation, IGO/IGA), reduce maintenance, extend the life of heat-treatment furnaces, reduce alloy fixture costs, and minimize distortion by reducing the temperature differential between the temperature of the part and the quench medium.
The choice of austenitizing temperature depends on both carbon and alloy content – a lower-carbon steel requires a higher temperature than a high-carbon steel. Alloy content plays a role as well by influencing the boundaries of the austenite field (c.f., “Influence of Alloying Elements on the Austenite Phase Field”).
In addition, alloy carbides often require higher temperatures to dissolve and disperse due to lower diffusion rates than carbon. In addition, varying the heating rate to austenitizing temperature influences the rate of transformation and dissolution of the various alloying constituents present (Fig. 4). In Fig. 4, for example, the third curve from the right represents a heating rate of about 3˚C (5˚F) per minute.
The iron-carbon equilibrium diagram outlines the austenite phase field for irons and steels. As the carbon content increases, the A3 temperature (the lower limit of the austenite field) decreases until the eutectoid composition is reached – 725˚C (1340˚F) at 0.80% C. For a 0.40% carbon steel, the austenite phase field begins at 915˚C (1500˚F). By contrast, the austenite phase field in pure iron begins at 912˚C (1674˚F) and ends at 1394˚C (2541˚F).
Finally, it should be noted that the workability of steels (e.g., rolling, forging) is enhanced in their austenitic state, responding to hot working by deformation, recovery, recrystallization and grain growth. The austenitic grain size decreases as hot working temperatures are reduced.
Austenite plays a unique role in steel heat treatment, and the subject requires a deeper understanding than presented here. The references listed contain a wealth of good information on austenite, and the reader is encouraged to study this topic in greater depth.
- Krause, G., Steels: Processing, Structure, and Performance, ASM International, 2005
- Grossman, M. A., and Bain, E. C., Principles of Heat Treatment, 5th Edition, ASM International, 1964
- Brooks, C. E., Principles of the Austenization of Steels, Elsevier Applied Science, 1992
- Herring, Daniel H, “A Comprehensive Guide to Heat Treatment, Volume 2,” Industrial Heating, 2018
- Dossett, Jon L., Practical Heat Treating, 2nd Edition, ASM International, 2006
- Herring, Daniel H, “A Comprehensive Guide to Heat Treatment, Volume 1,” Industrial Heating, 2018
- Thelning, K-E, Steel and Its Heat Treatment: Bofors Handbook, Butterworths, 1975
- Brooks, Charles R., Principles of the Heat Treatment of Plain Carbon and Low-Alloy Steels, ASM International, 1996
- Sinha, A.K., Ferrous Physical Metallurgy, Butterworths, 1989