Polymer quenchants, first introduced in the 1970s, have captured an increasingly large market share of quenchants at the expense of oil quenchants. In a series of articles, I will present a general overview of polymer quenchants, followed by a more detailed examination of the individual polymer types.
Polymer quenchants
In the early history of the heat treatment of steel, water and brine were used to quench lean alloys that consisted of little more than iron, carbon, and a little manganese. As the use of additional alloying elements such as chromium, nickel, molybdenum, and vanadium increased, water and brine quenchants were simply too fast. Cracking and excessive distortion dictated that slower quenchants be used. Quench oils, with maximum cooling rates from less than 50°C/s to 100°C/s, ruled as the primary steel quenchants.
As environmental regulations, sustainability requirements, and oil prices increased, an alternative to oil was needed (Figure 1). With increased research, the creation of polymer quenchants has not only filled the gap between water and oil quenchants but has replaced oil in many instances. New polymers have been created that achieve the slow quench rate of oil quenchants. Polymer quenchants are being used to quench high hardenability steel forgings, patenting high carbon wire, high carbon railroad rail, and steel castings. Polymer quenchants have been used in mesh belt and integral quench furnaces, as well as typical open quench tanks. Application of polymer quenchants is now extensively used in the heat treatment of aluminum sheet, plate, forgings, and castings to reduce distortion and residual stresses. With proper control of operating variables, polymer quenchants can achieve quench rates that are nearly as fast as brine quenchants, or as slow as straight quench oil that contains no speed improvers. This is illustrated in Table 1.
There are three primary driving forces for the application of polymer quenchants in the heat-treating shop: reduction of fire hazards, environmental concerns, and cost. Water-based quenchants significantly reduce the risk of fire. Environmental concerns, such as disposal and regulations concerning Volatile Organic Compounds (VOC) are reduced due to the use of water and the low volatility of polymer quenchants. The ability of polymer quenchants to be concentrated by reducing water content significantly reduces the disposal costs. While the cost of polymers per gallon is high compared to oil, polymer quenchants are diluted with water, which drastically reduces their in-tank costs. As an example, a manufacturer had planned to fill his 50,000-gallon open quench tank with moderate speed oil. Filling this tank would cost more than $350,000. An appropriate choice of polymer quenchant at a concentration (15%) that would yield an equivalent quench rate achieved an in-tank cost savings of $175,000. The benefits of reduced fire risk (and lower attendant fire insurance premiums), environmental concerns, and cost have resulted in a much larger market share for polymer quenchants in the past 40 years.
For all polymer quenchants, the primary manufacturing variables to achieve a desired quenching rate are:
- Concentration.
- Agitation.
- Temperature.
As the concentration of the polymer is increased, the effective quench rate is reduced. As the concentration is increased, a limit will be reached where additional additions of polymer will not significantly reduce the cooling rate. This concentration is dependent on the molecular weight of the polymer, and the type of polymer chosen.
Polymer quenchants tend to be more sensitive to agitation than mineral oils. Increasing the agitation increases the cooling rate and reduces the polymer film thickness. However, decreasing the agitation can produce non-uniform quenching because of non-uniform film thickness. It also limits the transport of polymer to the part surface. As in every quenching operation, the magnitude and uniformity of agitation is extremely import. Racking of parts is also more critical in polymer quenchants than mineral oil because of the strong effects of temperature. Agitation tends to minimize these thermal gradients within the quenchant.
Because of the sensitivity of polymer quenchants to agitation, cooling curves of these quenchants are performed using agitation. They are not tested without agitation like oil quenchants or water. Specialized agitation devices are used, such as the Tensi agitation system (Figure 2) as described by ASTM D6482 [1].
The effective quench rate of polymer solutions is affected by temperature. As temperature increases, the quench rate is reduced. Increasing temperature also increases the oxidation and reduces thermal stability of the polymer, effectively shortening the life of the polymer. The amount of degradation is dependent on the amount of polymer present used, and the application temperature. Depending on the polymer used, there is also a limit on the bulk quench temperature of the quenchant, as some quenchants will tend to separate or precipitate from solution. In general, the typical operating temperature range of polymer quenchants is 20-45°C.
There are four predominant types of polymer quenchants in use today. They are:
- Polyalkylene Glycol (PAG).
- Polyethyl Oxazoline (PEOX).
- Polyvinyl Pyrrolidone (PVP).
- Sodium Polyacrylates (ACR).
In future articles, I will review the individual polymers and applications for each.
Should you have any comments regarding this article, or have suggestions for further columns, please contact the author or the editor.
References
- ASTM International, “Standard Test Method for Determination of Cooling Characteristics of Aqueous Polymer Quenchants by Cooling Curve Analysis with Agitation (Tensi Method),” ASTM International, Conshohocken, PA, 2016.
- Houghton International, Inc., “Houghton on Quenching,” 1991.
