February 2021 Volume 3

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: What are the most common types of quench media used for heat treating forgings and what are the advantages/ disadvantages of each? Because forgings can be made of virtually any alloy system known to mankind with limitless part configurations, it follows that the entire world of quenchmedia becomes a possibility for this question. To effectively answer this, an overview of the most common quenchants will be provided, along with some decision tools that will describe the types of forgings that lend well to being processed in each quenchant. The various quench media discussed below are listed in order of their cooling speed, starting with the slowest. Air Many heat treating processes can be accomplished simply by cooling the forgings in air after they are heated to the austenitizing temperature. Air is obviously not nearly as effective a cooling media as are liquid quenchant options, however, its properties can be sufficient to accomplish certain processes. The cooling rate supplied by air can be significantly enhanced by fans. For carbon and alloy steel grades, the practice of air cooling is commonly referred to as normalizing, in which the goal is to homogenize and refine the grain structure of the forging after it is forged. In many cases, this process is not significantly increasing the hardness of the parts but will enhance their core properties due to the improved homogeneity of the microstructure. Another common heat treat process that may employ air cooling is a solution anneal treatment for precipitation hardening stainless steels such as 17-4 or 15-5. This process creates the necessary starting microstructure for the material to respond to the subsequent age hardening process. These grades are highly alloyed and often do not require an aggressive liquid quench to accomplish an effective solution treatment. Molten Salt Speciallydesignedheat treating systems have the capability toquench workpieces into a molten salt bath. These salts can typically be used in the temperature range of 375-750°F.The most common processes that use salt quenching media are martempering, marquenching, and austempering.Martempering andmarquenching are considered to be quench and temper heat treatments as they are intended to produce a martensitic microstructure, albeit with significantly Heat Treating Corner By Chuck Hartwig, P.E.

reduced risk of distortion and cracking. Austempering produces a bainitic microstructure which provides enhanced ductility and uses a higher quench temperature. Regardless of the salt quench process being used, the selection of this quench media would typically be driven by concern of distortion or cracking. This usually comes about through a combination of high hardenability base material (i.e. 4340, 4150, 4350) and other features on the forging that increase risk of cracking such as drastic change in cross-section. Salt is very effective at removing heat from workpieces in a uniform fashion, as it does not undergo a boiling transition phase. However, the relatively high temperature range that must be used to keep the salt from solidifying means that the thermodynamic driving force for heat removal is drastically lower than other liquid quench media that operate closer to ambient temperature. Therefore, its use is often restricted to high hardenability steels with thin cross-sections where the user can be confident that through hardening has been obtained. Oil Oil is perhaps the most common quench media used in heat treating throughout the world. This is because it provides a great balance between rapid heat removal and control over distortion. It can also be used on a wide variety of products and processes, from forgings to castings to intricately machined components. It must be stressed here that to simply use the term “oil quench” does not adequately describe the myriad of options that exist in the marketplace for quench oil products. Quench oils are rated by their quench speed performance on a quenchometer test in which a 12mm nickel ball is heated and quenched in the oil sample and the time it takes to cool to ball to under 352°C is measured and reported. Common quench oil varieties are 7 second, 15 second, and 20 second with 7 second providing the fastest cooling rate of the three. Additionally, some oils are designed to operate up to ~180°F for rapid quenching and others (so called martempering oils which bridge the gap between oil and molten salt) are designed to run at hotter temperatures from 200°F to over 300°F. The danger in oversimplifying oil quenching into an umbrella term is that some heat treating operations may choose to run a slower 15 or 20 second oil for certain distortion sensitive product lines and therefore not be well suited to attain maximum hardness depth on a heavy cross-section forged part. The converse is also true – a heat treating department running a 7 second quench oil may crack or distort intricate workpieces that require slower oils or hot oils. Thus, purchasers and operators of heat treating services should thoroughly understand the cooling properties of their quench oil products with respect to the needs of products being processed.

FIA MAGAZINE | FEBRUARY 2021 50