In this column, I will discuss a few different methods of empirically determining the hardenability of a steel.
As discussed in my last few articles, hardenability is the depth of hardening, and not the achievable hardness of the steel. In the last article, I demonstrated methods of calculating the hardenability of a steel from its chemistry. In this article, I will discuss methods of measuring hardenability.
Hardness Distribution
In Grossman’s Principles of Heat Treatment [1] in Chapter 11, on “Variations of Hardening”, he described a test, using different round bar diameters ranging from 0.5” (12.5mm) to 5” (125mm) of SAE 1045 and SAE 6140 steel. These bars were quenched in water. After austenitizing, and quenched in water, the bars were sectioned in half, perpendicular to the long axis of the bar. The sectioned surface was ground smooth, and Rockwell C hardnesses were taken at different radial distances. Several hardnesses were taken at each radial distance, and the data was plotted to create a “hardness penetration diagram.” The complete hardness penetration diagram for each of the steels is shown in Figure 1.
In Grossman’s original paper, he described a water quench. However, the paper did not describe the agitation used, or the temperature of the water used. However, the inference — based on an earlier description — is that the parts were quenched in room temperature water, with no agitation other than light swirling by hand.
This figure shows that maximum hardness of the SAE 1045 can only be achieved at very small diameters, and that the depth of hardness of the SAE 1045 decreases significantly as the diameter is increased. This alloy has very low hardenability.
On the other hand, the depth of hardening of the SAE 6140 steel is greatly increased. The 0.5” (25mm) diameter bar of the SAE 6140 bar has essentially the same hardness across the diameter of the section. There are still variations in hardness as the bar size is increased; however, the depth of hardening is greater than SAE 1045.
Critical Size
Grossman and Bain [2] developed a method based on the critical diameter, and the ideal diameter. I already discussed the calculation of the ideal diameter in my previous article. The critical diameter is the largest size of a bar quenched in a quenchant that contains no unhardened core after quenching. The definition of an unhardened core was based on 50% martensite. The reason for this criterion is that there is an etching transition at approximately 50% martensite. Martensite etches white or doesn’t etch, while pearlite and other non-martensitic transformation products etch darkly. At martensite concentrations below 50%, the matrix etches dark, while above 50% martensite, little etching occurs in nital. Further, if the quenched specimen is fractured, the 50% martensite zone correlates with the transition between an intergranular fracture along prior austenite grains, and a rough transgranular ductile fracture [3].
By comparing the etching response of different rounds of steel, in different quenching mediums, a relative ranking of hardenability could be accomplished.
Jominy End Quench
The Jominy end quench [4] [5] [6] is the most accepted test for determining hardenability. The Jominy end quench test consists of a simple bar 25mm in diameter, and 100mm long. A small flange is machined at the end of the specimen for support. The bar is heat treated at normal austenitizing temperatures for the specific alloy, for approximately one hour. The specimen is removed from the furnace and hung in a special fixture (Figure 2).
The test specimen is heated to the austenization temperature and transferred to a fixture. As soon as the specimen has been hung in the fixture, a water valve is turned on, allowing water to cool the bottom of the specimen (the quenched end). A gradient of quench rates occurs along the length of the sample, with the quenched end exhibiting the highest quench rate, and distances away from the quenched end showing progressively slower quench rates.
Once the specimen has cooled, two flats the length of the specimen are machined on opposite sides of the bar. Harness measurements are taken at either 1mm or 1/16” intervals along the length of the bar, starting from the quenched end. This data is then plotted as a function of distance from the quenched end. An example of the Jominy end quench plots for several alloys is shown in Figure 3.
In Figure 3, it can be observed that at identical quench rates (or distance from the quenched end of the Jominy end quench specimen), the hardness of each of the alloys is different. This very clearly shows the differences in hardenability of the different alloys. In the same fashion, after a material has been chosen, the data can also be used to select a quenchant to be used to achieve the desired surface and core properties. This is useful to ensuring that once the part has been designed, it can be manufactured and not be prone to cracking or distortion.
This data can be used to predict the hardness of a quenched part, using Lamont charts [8], or as input to different modeling programs.
Conclusion
In this article, different methods of empirically determining the hardenability of an alloy have been described. By far the most practical, and widely accepted method is the Jominy end quench. This test is used for material selection and procurement specifications.
Should there be any questions regarding this article, or suggestions for new articles, please contact the editor or myself.
References
- M. A. Grossman, Principles of Heat Treatment, Metals Park, OH: American Society of Metals, 1937.
- M. A. Grossman and E. C. Bain, Principles of Heat Treatment, 5th Edition ed., Cleveland, OH: American Society for Metals, 1964.
- G. Krauss, Steels – Processing, Structure, and Performance, 2nd ed., Metals Park, OH: ASM International, 2015.
- J. E. Jominy, “Standardization of Hardenability Tests,” Metals Progress, vol. 40, pp. 911-914, December 1941.
- W. E. Jominy, Hardenability of Alloy Steels, Cleveland: American Society for Metals, 1939.
- ASTM, “Standard Test Methods for Determining Hardenability of Steel,” ASTM International, West Conshocken, PA.
- Timken, Practical Data for Metallurgists, 17th Edition, North Canton, OH, 2014.
- J. L. Lamont, “How to Estimate Hardening Depth in Bars,” Iron Age, no. 10, pp. 64-70, 1943.
