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HARDENABILITY Factors That Affect Hardenability and the Rate Of D. SCOTT MACKENZIE HOT RESEARCH SCIENTIST – METALLURGY SEAT HOUGHTON INTERNATIONAL INC. HARDENABILITY Factors that affect hardenability and the rate of austenite transformation — carbon content, grain size, and alloying elements — can be used to calculate hardenability for comparing alloy grades. 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 Figure 2: Effect of ASTM grain size on the hardenability of four shallow-hardening steels Heat treating steel requires that the component be heated into the austenite phase range. Once the part is thoroughly heated into the there is less surface area at the grain boundaries to nucleate. Finer austenite range, then the part is cooled in a controlled fashion to grain size increases the grain boundary area and promotes nucleation achieve the desired microstructure. If the part is cooled slowly, the of pearlite. The higher nucleation rate at finer grain sizes will result microstructure will consist of pearlite and ferrite; if cooled rapidly, in the decrease of time needed to complete the formation of pearlite. the part will consist of martensite. If an intermediate cooling rate is As the grain size increases (ASTM grain size decreases), the harden- achieved, bainite or a mixed microstructure will result. The required ability increases. This is shown in Figure 2. rates to achieve the desired microstructures are governed by the In general, the effect of grain size is independent of composition. carbon and alloy content. This is shown schematically in Figure 1. 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 tough- ness. 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 Figure 1: Decomposition of austenite into different microstructures tendency of cracking and distortion. In general, for most heat-treated products, the desired microstruc- ture is martensite. Martensite provides a hard microstructure for high COMPOSITION (CARBON AND ALLOYING ELEMENTS) wear and strength applications. It is also brittle and must be tempered Increasing the carbon content tends to retard austenite transformation. to improve ductility. Non-martensitic transformation products such This enables a slower quench for reduced distortion while maintaining as pearlite and bainite are avoided because they reduce hardness. In hardness. The as-quenched hardness of an alloy is only dependent case-carburized parts, the core may be a mixture of ferrite, pearlite, on the amount of carbon present (see Figure 3). Additional alloying and bainite to provide additional ductility for the tooth loads. elements do not increase the achievable maximum as-quenched hard- ness of the steel. Alloying elements, such as nickel, chromium, and GRAIN SIZE others, retard diffusion of carbon within the steel. This diffusion of The effect of grain size is similar to that of alloy additions. Increasing carbon is needed for the formation of pearlite. Martensite formation the grain size retards the diffusion of carbon (a further distance to is promoted. Therefore, alloying elements promote the formation of travel), promoting the formation of martensite. Nucleation of pearlite martensite and allow martensite formation at lower quenching rates. occurs at prior austenite grain boundaries. With a coarse grain size, This enables a part to be more deeply hardened. 20 gearsolutions.com GS-2017-02.indb 20 1/24/17 10:58 AM Figure 4: Hardness distribution of AISI 1040 and AISI 6140 in various bar diameters, Figure 3: Maximum hardness as a function of carbon in plain carbon steels and alloy steels [1] quenched in water [2] 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 Figure 5: Hardenability, expressed as ideal critical diameter, as a function of austenite grain size and carbon content for steels Grossman [3] defined DI as the ideal diameter of a given steel that would harden to 50-percent mar- tensite when quenched in a bath where H = ∞. This is the hypo- thetical infinite cooling rate that is equivalent to instantly reducing the surface temperature of the steel bar to the quenchant tem- perature. The definition of DI also has the advantage of being Table 1: Example illustration of the easily calculated from heat trans- calculation of the ideal diameter calculation fer. The ideal diameter is a true measure of hardenability associated with a steel composition. The concept of the ideal diameter can be Figure 6: Multiplying factors as a function of concentration of various common used to determine the critical size of steels quenched in quenchants alloying elements in alloy steels of differing severity. From a known chemistry, the ideal diameter can be calculated. This The calculation of the ideal diameter, DI, for non-boron containing is useful in comparing alloy grades and the specific alloy chemistries steels relies on a series of multiplying factors. This base DI is deter- within an alloy grade. The concept of hardenability can be extended mined from the grain size and carbon content and then is multiplied to predict expected microstructures for a specific quenchant, which by the various factors from the composition: will be discussed in subsequent Hot Seat columns. REFERENCES These multiplying factors are tabulated in ASTM A255 [4]. The base 1. ASM International, “Introduction to Steel Heat Treatment,” ideal diameter from carbon content and austenite grain size is shown in in Steel Heat Treating Fundamentals and Processes, vol. 4A, Figure 5. Multiplying factors for alloy content are shown in Figure 6. J. Dossett and G. E. Totten, Eds., Materials Park, OH: ASM Consider an alloy with the composition shown in Table 1, with an International, 2013, pp. 3-25. austenite grain size of 7. 2. M. A. Grossman and E. C. Bain, Principles of Heat Treatment, 5th From either the tabulated values from ASTM A255 or from Table Edition ed., Cleveland, OH: American Society for Metals, 1964. 1, the base DI is determined to be 0.21 inches (5.3 mm). Taking 3. M. A. Grossman, “The Nature of the Quenching Process,” in into account the chemistry of the alloy, the multiplying factors are Elements of Hardenability, Metals Park, OH: American Society for determined (see Figure 6). The resultant multiplying factors (shown Metals, 1952, pp. 61-91. in Table 1) are multiplied together to determine the DI of this alloy 4. ASTM, “Standard Test Methods for Determining Hardenability of of 1.23 inches (31.2 mm). Steel,” ASTM International, West Conshocken, PA. ABOUT THE AUTHOR: D. Scott MacKenzie, Ph.D., FASM, is a research scientist of metallurgy at Houghton International, a global metalworking fluids supplier. He obtained his B.S. from The Ohio State University in 1981 and his Ph.D. from the University of Missouri-Rolla in 2000. He is the author of several books and over 100 papers, articles, and chapters, and he is a member of ASM International. MacKenzie can be reached at [email protected]. FEBRUARY 2017 21 GS-2017-02.indb 21 1/24/17 10:59 AM.
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