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Hardness of Heat Treated 4140
Posted: 20 Dec 06
The maximum hardness that can be obtained in any steel depends on carbon content. The section size in which maximum hardness can be obtained depends on alloy content. 4140 has a nominal carbon content of .4% and this carbon content will yield a hardness of Rc 51 for a 90% martensitic structure (9th Edition of the ASM Material Handbook,Volume I). To find out if the center of a bar will fully harden (i.e. 90% martensite), you must convert the bar diameter into a jominy equivalent distance. Per the ASM Handbook, a 3/4" diameter round bar converts to a Jominy equivalent distance of 5. Inspection of a Jominy chart for 4140, which can be found in the SAE handbook, shows that for a Jominy distance of 5, the lower hardness limit is Rc 51 and the upper limit is Rc 59. Since the 90% martensite hardness of Rc 51 lies within the upper and lower hardness limits, the 3/4" bar will fully harden to the center. A larger diameter bar will not, unless the actual alloy content is on the high side of the spec range or a water quench is used instead of oil. These hardness values are for as-quenched material and final hardness will need to be reduced by tempering before the part can be put into use. A tempering temperature of 800F will result in a final hardness of Rc 40 minimum. The lowest practical tempering temperature, 400F, will result in a final hardness close to the as-quenched level. Although higher hardnesses may be obtained with surface techniques, such as induction hardening, there is no practical technique for predicting these values.
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