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.