Applications of Hardenability

In the previous Hot Seat column, the concept of hardenability was discussed. It was determined that the sole contributor to hardness was carbon content, but the depth of hardening was a function of grain size and composition. Methods for calculating hardenability were illustrated. In this column, applications of hardenability are described.

The classic method related to the ability of a quenchant to harden steel is to determine the Grossman H-value [1, 2] or severity of quench. The H-value is defined as [3]:

where α is the average heat transfer coefficient at the surface of the part, and λ is the thermal conductivity of the steel. For most steels, the thermal conductivity does not change appreciably over temperature or from alloy grade to grade, so it is directly approximate to the average heat transfer of the quenchant. A summary of the severity of quench for different media is shown in Table 1.

The Grossman H-value is determined experimentally by quenching a series of round bars. After quenching, the bars are sectioned, polished, and etched. The 50-percent martensite region is determined. This is readily achieved because the transition in etching between dark and light etching corresponds to 50-percent martensite.

Using the H-value, a correlation was developed [5] correlating the Grossman H-value to bar diameter and the Jominy distance. (See Figure 1.) Jominy curves are available from different sources [6]. A typical Jominy curve is shown in Figure 2.

Although this method has been used in the industry for many years, it is not without problems. The biggest issue with the application of the Grossman H-value is the difficulty in quantifying agitation rates. The terms describing the agitation are not quantifiable and can result in errors. There is really no understanding of what is meant by “mild” or “violent” agitation. Since most oils are tested without agitation, this results in a narrow range of possible oil values. Also, when the original testing was done, the oils used in the original paper were straight oils, devoid of speed improvers. Modern oils achieve much higher quench rates (nearly double) than those tested in the original paper. Further, the different methods of quenching, such as spray quenching, have no equivalent to the Grossman H-value. This method is only focused on the ability of the quenchant to harden steel and gives no indication regarding distortion. However, use of the Grossman H-value provides a method of determining the hardness of a steel alloy based on the quenchant used.

Previously, there has been no way to correlate cooling curve data to severity of quench (H). Recently [7], a correlation of the Grossman H-value to cooling curve data was presented. In this method, the average heat transfer coefficient at 705°C (W/m2·°K) was correlated to the Grossman H-value. Substituting the heat transfer coefficient into the aforementioned definition of the H-value and converting to English units, the Grossman H-value was determined. (See Figure 3.) Typical values for several common quench oils are shown in Table 2.

The calculations show a good similarity to the classical H-values of Grossman. Data shows that the calculated results correspond to Jominy data and predicted hardness distributions in quenched bars [8].

This method makes possible the calculation of the critical size in terms of a standardized quench and calculation of the critical size from a single test. Using charts, it is possible to predict how a known steel with a specific Jominy curve would behave and to predict the hardness distribution within the bar.

While this method provides a rough estimate of the hardness within a round bar, it is limited to cylindrical geometries. Unfortunately, parts do not always conform to simple cylindrical shapes. The concept of “equivalent round” can be used to estimate properties in different geometries than round bars (See Figure 4.)

Using the concept of equivalent round, the known Jominy curve of the desired alloy, and the Grossman H-value of the desired quenchant, the hardness distribution can be predicted for different sizes of materials through the use of Lamont charts [8]. An example of a Lamont chart correlating the Jominy distance from the quenched and the Grossman H-value is shown in Figure 5.

Conclusion

Here, the use of hardenability in predicting the hardness of a steel component based on the hardenability as shown in the Jominy end-quench curve and the Grossman H-value, as well as a method for determining the Grossman H-value as a function of the cooling curve measured by ASTM D6200, have been illustrated. This is a powerful method to either calculate the hardness in a given component given the quenchant used or to select a quenchant to achieve a desired hardness at the core or surface of a part.

References

1. M. A. Grossman and M. Asimov, “Hardenability and Quenching,” Iron Age, vol. 145, no. 17, pp. 25-29, 1940.
2. M. A. Grossman and M. Asimov, “Hardenability and Quenching,” Iron Age, vol. 145, no. 18, pp. 39-47, 1940.
3. A. V. Reddy, D. A. Akers, L. Chuzoy, M. A. Pershing and R. A. Woldrow, “A Simple Method Evaluates Quenches,” Heat Treating Progress, vol. 1, pp. 40-42, 2001.
4. American Society of Metals, Metals Handbook, T. Lyman, Ed., Cleveland, OH: ASM International, 1948, p. 1444.
5. M. A. Grossman, M. Asimov and M. F. Urban, “Hardenability, Its Relationship to Quenching and Some Quantitative Data,” ASM Transactions, pp. 124-190, 1939.
6. Timken Company, Timken Practical Guide for Metallurgists, 14th ed., Canton, OH: Timken Company, 2015.
7. D. S. MacKenzie, “Selection of Quenchants,” Journal of International Federation of Heat Treatment and Surface Engineering, vol. 8, pp. 8-14, 2014.
8. J. L. Lamont, “How to Estimate Hardening Depth in Bars,” Iron Age, no. 10, pp. 64-70, 1943.