Steel is a mixture of iron, carbon from 0.0 to 1.2 percent, and alloying elements. Carbon provides the hardness, and the alloying elements provide how deep this hardness will occur. This concept is called “hardenability.” Hardenability should not be confused with the maximum hardness after quenching, which is only dependent on the amount of carbon present and the percentage of martensite. Rather, hardenability is how deep a steel alloy can be hardened. Steels that deeply harden are called high hardenability steels, while steels that do not harden deeply are called low hardenability steels.
The major factors affecting hardenability and the rate of austenite transformation are carbon content, grain size, and alloying elements.
Transformations 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 the part 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 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.
The effect of grain size is similar to that of alloy additions. Increasing the grain size retards the diffusion of carbon (a 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. This is shown in Figure 2.
In general, the effect of grain size is independent of composition. For instance, an increase of ASTM grain size number (e.g., ASTM 6 to 5) will exhibit the same proportional increase in hardenability. Grain size can also have deleterious effects of decreasing toughness. Modern steel practice is to create a fine grain size to improve properties, especially toughness. While it is possible to increase the hardenability of a lean alloy by increasing the grain size, this is usually impractical due to the objectionable decrease in toughness. Modern steel mill practice has excellent grain size control, achieving, on a routine basis, grain sizes that exceed an ASTM number of 8 and often averaging 9 to 11. In mills where there is not good control of grain size, the grain size can vary from ASTM grain size number of 2 to 8. This can cause variability in heat treating response and problems with localized high hardenability. This combination can increase the tendency of cracking and distortion.
Composition (Carbon and Alloying Elements)
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 (see Figure 3). Additional alloying elements do not increase the achievable maximum as-quenched hardness of the steel. Alloying elements, such as nickel, chromium, and others, retard diffusion of carbon within the steel. 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.
Figure 4 shows the effect of hardening two different compositions of steels with equal carbon contents. Different diameter bars are quenched, and the hardness was measured across the diameter. The alloy steel is through-hardened at the 13 mm diameter, but only the surface of the carbon steel was hardened. The center of the carbon steel showed a significant drop in the hardness in the center of the 13-mm-diameter bar. As the diameter increased, the carbon steel hardness at the surface dropped significantly, and at the 125 mm diameter, the bar shows little hardening. In the case of the alloy steel bar, significant hardening still occurred to a measurable depth in the 125-mm-diameter bar.
Calculation of Hardenability
Grossman  defined DI as the ideal diameter of a given steel that would harden to 50-percent martensite when quenched in a bath where H = ∞. This is the hypothetical infinite cooling rate that is equivalent to instantly reducing the surface temperature of the steel bar to the quenchant temperature. The definition of DI also has the advantage of being easily calculated from heat transfer. The ideal diameter is a true measure of hardenability associated with a steel composition. The concept of the ideal diameter can be used to determine the critical size of steels quenched in quenchants of differing severity.
The calculation of the ideal diameter, DI, for non-boron containing steels relies on a series of multiplying factors. This base DI is determined from the grain size and carbon content and then is multiplied by the various factors from the composition:
These multiplying factors are tabulated in ASTM A255 . The base ideal diameter from carbon content and austenite grain size is shown in Figure 5. Multiplying factors for alloy content are shown in Figure 6.
Consider an alloy with the composition shown in Table 1, with an austenite grain size of 7.
From either the tabulated values from ASTM A255 or from Table 1, the base DI is determined to be 0.21 inches (5.3 mm). Taking into account the chemistry of the alloy, the multiplying factors are determined (see Figure 6). The resultant multiplying factors (shown in Table 1) are multiplied together to determine the DI of this alloy of 1.23 inches (31.2 mm).
From a known chemistry, the ideal diameter can be calculated. This is useful in comparing alloy grades and the specific alloy chemistries within an alloy grade. The concept of hardenability can be extended to predict expected microstructures for a specific quenchant, which will be discussed in subsequent Hot Seat columns.
- 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.
- ASTM, “Standard Test Methods for Determining Hardenability of Steel,” ASTM International, West Conshocken, PA.