November 2020 Volume 2

MATERIALS

Welcome to the Heat Treating Corner, where we discuss practical metallurgical topics pertaining to the heat treatment of forged products. If you have questions that you would like to see answered in a future issue, please email the author at chuck.hartwig@ thermtech.net. QUESTION: Please explain why different alloys have different tempering temperatures used to achieve the same final hardness. How are tempering processes determined? This question is a great follow-up to the as-quenched hardness discussion that can be found in the immediately preceding issue of this publication. The first aspect of tempering temperatures that must be understood as a caveat to the rest of this discussion is that the selection of a tempering temperature assumes that an effective quench has occurred. This means that the hardening process employed has, in fact, achieved the expected as-quenched hardness result for the alloy under consideration. If the hardening process has not been effective (see previous Heat Treating Corner article for reasons why this may occur), the tempering response of any particular alloy will be unpredictable. This behavior presents a heat treat quality concern in and of itself. Tempering is a sub-critical (below the austenitization temperature) re-heating process that is carried out immediately after quench hardening to relieve the stress that exists in the as-quenched martensitic microstructure. Recall from previous columns that untempered martensite is an extremely high stress/high energy phase in steel that, while necessary as a waypoint in successful heat treatment, is practically dangerous to the material and would result in extremely brittle behavior or spontaneous cracking if left in untempered state. Tempering thus results in lower hardness/ strength but improved toughness and ductility. The mechanism for this change in physical properties is the heat-assisted diffusion of carbon atoms out of the martensitic atomic matrix where they were previously pinned between iron atoms. Upon diffusion, the carbon atoms join with other elements to form either iron carbide or various other alloy carbides in the microstructure that greatly enhance the toughness of the material. The diffusional reaction described above does not occur at the same rate in all alloys. In fact, there can be drastic differences in the tempering protocol even if the carbon level (and therefore as quenched hardness) is the same between different alloys. The reason for this difference across various alloys is that the energy needed to impart atomic mobility to the carbon atoms and the subsequent properties of the temper carbides varies according to the amount of certain alloys present in the material. The ability of an alloy to maintain its hardness as the tempering temperature (or service temperature) increases is often referred to as tempering resistance . The primary drivers of temper resistance (in order of potency) are Heat Treating Corner By Chuck Hartwig, P.E.

vanadium, molybdenum, phosphorous, manganese, chromium, silicon, and to a lesser extent, silicon and nickel. It should be noted that the effects of different elements will vary according to the tempering temperature and the list discussed here pertains to a tempering temperature of at least 1000°F, which is common for most hardened and tempered forgings. In practical terms then, consider two common forging steels— AISI 1045 and AISI 4140. Both are commonly heat treated to a final hardness range of 30-34 HRC. While the base carbon in these two grades is similar, it will take a tempering temperature of approximately 1100°F to draw back the hardness of 4140 to 30 34HRC, but only around 1000°F for the 1045. This difference is because 4140 contains intentional additions of molybdenum (.15- .25%) and chromium (.80-1.10%) and may have a greater level of manganese as well. The only temper resistant elements in 1045 would be silicon and a relatively low level of manganese, along with anything obtained from incidental trace amounts of other alloying elements. It should be noted that the same elements that work to increase the hardenability of steel also tend to increase the tempering resistance. The mechanism of action in both cases is similar. In both cases, the larger alloying element atoms deter the atomic mobility of carbon. In the case of tempering, formation of carbides is an additional factor at play. The selection of tempering temperatures for a given hardness level is usually a simple decision based on empirical data—either from published alloy data or internally developed process information within a heat treating operation. While selecting the desired temperature canbe a relatively straight forward exercise, it is critically important that work pieces are soaked at the prescribed tempering temperature in a uniform fashion and for a long enough time to ensure that the entire cross section of each part has been heated and held sufficiently. Recall that tempering is a diffusional process—as such, it is highly dependent on both time and temperature. Because of the time-temperature codependency in tempering, it is commonplace for tempering soak times to be longer than they are for austenitization heating prior to quenching. Additionally, because tempering temperatures are lower than austenitizing temperatures, there is less thermodynamic driving force to bring the temperature of the work pieces up to equilibrium with the ambient furnace temperature. Tempering soak times must therefore take load density and weight into account in addition to the cross section of the part in question. There are many practical design implications for heat treating equipment and departments based on this discussion. First, heat treating departments are often configured such that there is double

FIA MAGAZINE | NOVEMBER 2020 14