Second in a series > Polymer quenchants, first introduced in the 1970s, have captured an increasingly large market share of quenchants at the expense of oil quenchants. In this series of articles, the quenchants (covered generally in the July 2024 issue) and individual polymer types will be examined in more detail.
In this series, I will discuss the different types of polymer quenchants available to the heat treater, and describe typical applications and advantages.
Introduction
Polyalkylene glycol (PAG) quenchants are the most-used polymer quenchants in the heat-treating market today. Polyalkylene glycols (PAG), or polyalkylene glycol ethers, were first introduced as quenchants in the early 1970s. PAG quenchants are an example of a copolymer. This quenchant is derived from two monomeric units, ethylene oxide and propylene oxide, to form polyalkylene glycol (Figure 1).
By varying the molecular weights and the ratio of oxides, polymers having broad applicability may be produced. Proper selection of the polymer composition, and its molecular weight, provides a PAG product that is completely soluble in water at room temperature.
However, the selected PAG molecules exhibit the unique behavior of inverse solubility in water, that is, water insolubility at elevated temperatures. This phenomenon provides a unique mechanism for quenching by surrounding the metal piece with a polymer-rich layer that serves to govern the rate of heat extraction into the surrounding aqueous solution. As the metal part approaches the quenchant temperature, the PAG polymer coating dissolves to again provide a uniform concentration in the quenchant bath. In PAG quenchants, this occurs in the temperature range of 60-85°C depending on the specific polymer. This is shown in Figure 2.
This mechanism of inverse solubility is limited to two polymer quenchant classes: PAG and PEOX. In these systems, as the temperature of the solution is raised, the thermal energy of the system becomes greater than the energy of the hydrogen bond interactions with water. When this occurs, a two-phase system develops, with one layer being water-rich and the other a polymer-rich layer (Figure 3). This is not a clean separation as both phases have some of the other components. The temperature at which this separation occurs is called the cloud point. In PAG quenchants, the ratio of the monomers used to produce PAG quenchants control the cloud point. In this case, the cloud temperature decreases as the propylene oxide monomer proportion increases.
While thermal separation can be used to recover the polymer, the water-soluble corrosion inhibitors will report to the water phase. Once the polymer solution is reconstituted with water, additions of corrosion inhibitors must be added to the solution to maintain proper corrosion inhibition.
While corrosion can be an issue when quenching with water or aqueous salt solutions, solutions of PAG quenchants may be inhibited to provide corrosion protection of the quench-system components. Corrosion inhibition of quenched parts will be of short duration, so that specific protection should be provided following the tempering operation.
Effect of Concentration, Temperature, and Agitation
As concentration is increased, the overall rate of quenching decreases (Figure 4). This is due to the increasing thickness of the polymer film as the concentration increases.
Different PAG quenchants will exhibit different cooling curves, as it is dependent on the molecular weight of the polymer. As the molecular weight increases, the cooling rate decreases.
Just as water exhibits a loss in quenching power as temperature is raised, so do PAG quenchants. As the temperature is increased, the quenching power of PAG quenchants decrease (Figure 5). In general, PAG quenchants are limited in bulk temperature rise to less than 25°C less than their cloud point. If the bulk temperature exceeds the cloud point temperature, then separation of the polymer could occur. If parts are extracted at temperatures above the cloud point, then excessive drag-out of the polymer would occur.
Agitation is always recommended for PAG polymer quenchants (or any other polymer quenchant). In general, low to moderate agitation is essential (a) to ensure that adequate replenishment of polymer occurs at the hot metal surface; and (b) to provide uniform heat transfer from the hot part to the surrounding reservoir of cooler quenchant. Vigorous agitation may be essential for achieving a rapid rate of cooling (for example, with a low hardenability steel) to avoid undesired transformation.
Application of PAG Quenchants
Immersion quenching of steel components
The primary reason for consideration of PAG quenchants for steel quenching is the elimination of the smoke, fume, and fire hazards associated with quenching oils, although the benefits of flexibility of quenching speed and improved process economics are also important.
PAG quenchants are suitable for a wide range of steels, including plain carbon steels, boron steels, spring steels, general engineering steels, martensitic stainless steels, low and medium alloy carburizing steels, and higher alloy steels of heavier section thickness.
Components that can be processed range from very small parts with section size as small as 1 mm (such as needles, circlips, screws, and fasteners) up to large shafts and forgings weighing 10 tons or more.
Between these extremes, a very wide range of components including bolts, bearings, crankshafts, springs, steel bars and coils, high-pressure gas cylinders, general forgings, and agricultural and automotive parts have been successfully quenched into PAGs.
Induction hardening applications of PAG quenchants
PAG quenchants are used widely for the quenching of components after induction or flame hardening as an alternative to water, soluble oil, or mineral oil. They are generally used at concentrations of 5-15 percent to eliminate spotty hardening associated with water quenching, to control distortion, and to provide corrosion protection to the induction hardening equipment.
Typical applications include the quenching of gears, crankshafts, camshafts, drive shafts, bearing rings, tubes, and bars (Figure 6 and Figure 7).
Quenching of aluminum alloys
PAG quenchants are used widely as an alternative to water or oil for the quenching of aluminum parts such as thin-section airframe and skin components, castings and extrusions for aerospace applications, and engine blocks, cylinder heads and wheels for the automotive industry. Typically, these quenchants are governed by AMS 3025 [2], and are either Type I or Type II quenchants. Type I quenchants are single polyalkylene glycol polymers, while Type II quenchants are multiple molecular weight polyalkylene glycol polymers. Each offers different benefits. Because of the higher molecular weight of the Type II PAG quenchants, lower concentrations can be used.
The controlled uniform quenching characteristics of PAGs can significantly reduce or even eliminate the distortion often associated with water quenching without impairing mechanical properties or corrosion resistance. This is particularly important with thin-section sheet aluminum airframe components in the aerospace industry and complex castings and forgings, which are often quenched into boiling water or mineral oil to minimize distortion.
Conclusions
In this short column, the use and application of polyalkylene glycol quenchants were discussed. This type of quenchant is widely used for immersion quenching of steel and aluminum, as well as spray applications in induction hardening.
Should you have any comments regarding this column, or topic suggestions for further columns, please contact the writer or the editor.
References
- G. Totten, C. Bates and N. Clinton, Eds., Handbook of Quenching and Quenchants, Metals Park, OH: ASM International, 1993.
- SAE, “AMS 3025E Polyalkalene Glycol Heat Treat Quenchant,” SAE, Warren, PA, 2018.
- P. M. Kavalco, L. C. Canale and G. E. Totten, “Distortion Reduction by Aqueous Polymer Quenching of Aluminum Alloys,” Industrial Heating, vol. 2, no. February, p. 39, 2011.
