One of the concepts that always causes trouble in the field is the concept of hardenability. Hardenability is not to be confused with the achievable hardness of an alloy, but rather how deeply the alloy hardens. Steel contains iron, and carbon at a concentration up to about 1 percent. Various alloying elements such as chromium, nickel, and molybdenum are added depending on the steel alloy used. It is the carbon content that provides the ultimate hardness. It is the alloying elements that govern how deeply the alloy will harden.
Transformation of Austenite
Heat treating steel requires that the component be heated into the austenite phase range. Once the part is thoroughly heated into the austenite range, then it is cooled in a controlled fashion to achieve the desired microstructure. If the part is cooled slowly, the microstructure will consist of pearlite and ferrite; if cooled rapidly, the part will consist of martensite. If an intermediate cooling rate is achieved, bainite or a mixed microstructure of bainite and martensite will result. The required rates to achieve the desired microstructures are governed by the carbon and alloy content. This is shown schematically in Figure 1.
In general, for most heat-treated products, the desired microstructure is martensite. Martensite provides a hard microstructure for high wear and strength applications. It is also brittle and must be tempered to improve ductility. Non-martensitic transformation products such as pearlite and bainite are avoided because they reduce hardness. In case-carburized parts, the core may be a mixture of ferrite, pearlite, and bainite to provide additional ductility for the tooth loads.
Effect of Carbon Content
Increasing the carbon content tends to retard austenite transformation. This enables a slower quench for reduced distortion while maintaining hardness. The as-quenched hardness of an alloy is only dependent on the amount of carbon present. This is shown in Figure 2. Additional alloying elements do not increase the achievable maximum as-quenched hardness of the steel.
Grain Size
Increasing the grain size retards the diffusion of carbon (further distance to travel), promoting the formation of martensite. Nucleation of pearlite occurs at prior austenite grain boundaries. With a coarse grain size, there is less surface area at the grain boundaries to nucleate. Finer grain size increases the grain boundary area and promotes nucleation of pearlite.
The higher nucleation rate at finer grain sizes will result in the decrease of time needed to complete the formation of pearlite. As the grain size increases (ASTM grain size decreases), the hardenability increases.
While, in general, the effect of grain size is not as important anymore with the improvements in steel making, with most domestic, Japanese, and European steelmakers achieving fine grain size of ASTM 8 or finer, some other countries do not have grain size under strict control, especially in sheet or plate products.
In the case of forgings, grain size can vary due to the amount of deformation, time at temperature, and the temperatures used. Recrystallization, recovery, and grain growth can vary across a part depending on the amount of local deformation. This is why it is always recommended to normalize forging before heat treatment to achieve a uniform grain size throughout the part.
Alloying Elements
Alloying elements, such as nickel, chromium, and others, retard diffusion of carbon within the steel [2]. This diffusion of carbon is needed for the formation of pearlite. Martensite formation is promoted. Therefore, alloying elements promote the formation of martensite and allow martensite formation at lower quenching rates. This enables a part to be more deeply hardened [3].
The hardenability is governed almost completely by composition, and the calculation of hardenability based on composition will be described in a later column. There are many different alloying elements used in the formulation of steel. A brief description of the contribution of the various alloying elements used are described below.
Boron (B)
Boron is used as a substitution or an addition for carbon and used to boost hardenability. Boron steels will usually contain 0.0005-0.003% boron. It is used almost exclusively in low-alloy steels to great effect. However, it is not used in alloy steel because of limited effectiveness. Only soluble boron is effective in increasing hardenability.
Boron has no effect on the martensite start temperature (Ms) and has no effect on the stability of retained austenite. In terms of tempering response, there is a small increase in the susceptibility to temper embrittlement.
Silicon (Si)
Silicon is used in steel as a deoxidizer and is predominantly the most common alloying element in steel. It tends to promote high temperature oxidation resistance. It is a moderate hardenability alloying element.
Manganese (Mn)
Manganese is intentionally present in most grades of steel, or as a residual in others. It is commonly used to tie up excess sulfur (forming MnS inclusions). Manganese has a minor solid solution effect in austenite but can increase the hardness of ferrite. It strongly retards the transformation of austenite and promotes deep hardening. Manganese lowers the transformation temperature and eutectoid carbon content.
Because of the plentiful supply of manganese, it is a very cost-effective method to increase hardenability.
Nickel (Ni)
Nickel is a widely used alloying element in steels. It offers some increase in hardenability, but most importantly, increases toughness, especially at low temperatures. It is rarely used alone as an alloying element. For the most part nickel is used in combination with chromium, vanadium, and molybdenum to improve toughness. It retards both pearlite and bainite reactions, with the bainite reactions more severely affected. Because nickel is an austenite stabilizer, nickel alloys are particularly prone to creating retained austenite.
Chromium (Cr)
Chromium is not as effective a hardenability addition as manganese or molybdenum. However, it is very cost effective and is used extensively in alloy steel. Chromium forms strong and stable carbides (Cr3C2, Cr7C3 and Cr23C6), this results in some secondary hardening, or resistance to softening during tempering. However, because of this resistance to softening, higher tempering temperatures or longer tempering times are required during tempering.
Molybdenum (Mo)
Molybdenum is a strong hardenability alloying addition and retards softening at elevated temperatures. It is generally more effective than chromium or tungsten at similar concentrations. It is a high-cost hardenability additive, but very potent. Content will typically be held to less than 0.5 percent. Many steels are prone to temper embrittlement, but the addition of molybdenum can minimize or eliminate this effect.
Other alloying elements such as tungsten, vanadium and cobalt are also used, but these elements are usually limited to specialized applications such as tool steels.
Conclusions
In this article, I introduced the concept of hardenability and differentiated it from the maximum achievable hardness of a steel. Hardenability is the ability to deeply harden, and not the ability to get hard.
In the next column I will review methods of calculating hardenability from compositions and explore additional concepts in hardenability. Should there be any comments on this article, or suggestions for further articles, please contact myself or the editor.
References
- ASM International, “Introduction to Steel Heat Treatment,” in Steel Heat Treating Fundamentals and Processes, vol. 4A, J. Dossett and G. E. Totten, Eds., Materials Park, OH: ASM International, 2013, pp. 3-25.
- M. A. Grossman and E. C. Bain, Principles of Heat Treatment, 5th Edition ed., Cleveland, OH: American Society for Metals, 1964.
- M. A. Grossman, “The Nature of the Quenching Process,” in Elements of Hardenability, Metals Park, OH: American Society for Metals, 1952, pp. 61-91.
