August 2020 Volume 2

MATERIALS

I have seen certain heat-treating specifications require minimum values for “as-quenched hardness.” Can you explain what this refers to and why it is important? When we harden and temper forged products, it is most common to be concerned with the final hardness of the forgings and perhaps some validation of microstructure and mechanical properties. This testing is done after thermal processing is completed in order to validate the final result of the heat-treating process. When working with alloys that tend to respond well to typical heat treating processes in a variety of part configurations (e.g., 4140 or 4340), a specification may simply mandate a minimum tempering temperature along with some other simple process requirements. So long as these parameters are followed and the parts conform to final property requirements, no further controls are required. Things get more complex when a heat treater is working with what are described as “low hardenability” grades of steel forgings. Most commonly, this group of alloys includes 1040/45, 4130, 1144, and sometimes 15XX series steels, among others. Forgings made from these alloys tend to be sensitive to heat treating process parameters— most notably the quench rate that is obtained when cooling from the austenitizing temperature. Whereas in most cases a 4140 forging will achieve a robust hardening response in a conventional oil quench bath, the same forging produced using 1045 would likely require water quenching. Often, if the process for 4140 were applied to a 1045 forging, the result would be what heat treaters call a “slack quench” or “spotty hardening.” This is a term used to describe what occurs when the quenching process fails to produce the desired microstructural result. Ideally, the part is transformed entirely (at least on the surface) during the quench from an austenitic microstructure to a martensitic microstructure. If a slack quench occurs, the result is usually referred to as “mixed transformation products,” which usually consists of some percentage of martensite with the balance being pearlite or bainite. A uniformly martensitic microstructure will respond predictably to tempering, but mixed microstructures will not because the final hardness result (post tempering) will depend heavily on the exact percentage of martensite that was obtained in the quench. If a slack quench occurs because the quench rate was insufficient, it is also difficult to discern which areas of the part could be harder than others—this phenomena refers to “spotty hardening,” mentioned above, and there will be more discussion of this below. Given that low-hardenability steels are prone to the aforementioned quench rate/quench uniformity problems, some specifications will require the heat treater to verify the hardness of the parts immediately after the quenching process is completed, as a way of validating the effectiveness of the quench. The value obtained in Heat Treating Corner By Chuck Hartwig, P.E.

this check is referred to as the “as-quenched hardness.”The question then becomes: “How do we know that the quench was effective for a given alloy?” The simple answer to knowing what should be obtained for as quenched hardness lies in the carbon content of the alloy. If we know the carbon content of the steel, we know the maximum hardness that it can achieve. The metallurgical theory driving this concept is that when we cool austenite rapidly, we pin the carbon atoms in between the iron atoms in the metallic matrix – this forms the phase we have been referring to as “martensite,” inwhich entrapped carbon atoms make the movement or “slip” of the iron atoms very difficult, and thus produces a harder steel. The higher the carbon, the more atoms there are to pin between the iron atoms and the higher the maximum hardness. If the critical cooling rate is not achieved, the carbon atoms diffuse out of the iron matrix into the aforementioned “mixed transformation products,” and the iron atoms are now more free to slip past one another, resulting in softer steel. While it is theoretically possible to entrap all the carbon atoms in the atomic matrix to produce 100% martensite, it may be quite difficult, if not nearly impossible, to do it in practice. For example, 1045 steel has an allowable content range for carbon of 0.43-0.50%. Using the table that has been developed by ASTM in Figure 1, we should expect to obtain an at least 58 and up to 61 HRC for the as-quenched hardness value if we have achieved the pinnacle result of 100% martensitic transformation. In real-world practice, it is practical to expect at least a 90% transformation as a quality quench outcome, so perhaps a minimum as-quenched hardness of 54 or 55 HRC would be specified for 1045 steel.

FIA MAGAZINE | AUGUST 2020 40