Microbiological contamination and biostability in polymer quenchants

Proper control of the polymer quench tank is important to ensure long, consistent bath life and even case hardness and depth.


Biological issues such as bacteria and fungi can be a major issue when using polymer quenchants. Bacteria and fungi float in the air. When they land in a moist place, they tend to grow. Fungus tends to grow at difficult-to-reach places where there is little motion of the fluid. Oftentimes this is at the waterline of the quench tank. Bacteria like to grow in stagnant, oxygen-depleted, food-rich locations. Scale and other debris are wonderful food for anaerobic bacteria found in the typical quench tank. Bacteria and fungi can change the cooling curve of the polymer. This is done either by degradation of the polymer or by other mechanisms [1]. In addition to changes to the cooling curve of the quenchant, they can produce very unpleasant odors — such as a rotten egg smell, or a badly mildewed locker room. An extreme example of bacteria and fungi grown on a dip slide after 24 hours is shown in Figure 1. Additionally, the presence of bacteria and fungi lowers the pH, which can increase problems with corrosion or rusting.

Figure 1: Extreme example of bacteria, left, and fungi, right, growing on a dip slide that was exposed to a quench fluid and allowed to incubate for 24 hours.

Cleanliness and oxygen are important controlling factors to minimize the occurrence of bacteria and fungi. Keeping the quenchant moving will entrain air and will minimize the occurrence of bacteria. However, excessive agitation can result in splashing. This splashing can settle on difficult-to-reach locations where fungi or bacteria can grow.

Contamination from outside sources is often a prime driver for bacterial and fungal growth in a polymer quench tank. Machining coolants are often not cleaned from parts prior to induction hardening. They often contain large quantities of swarf, bacteria, and fungi. This is bad practice. Quench tanks are not parts washers — they collect everything (Figure 2). Make sure that skimmers, agitators, and associated equipment are working properly. Coolant should be removed from parts by proper cleaning and rinsing prior to induction hardening. For instance, if a part contains just 5 ml of coolant (one teaspoon), and there are 3,000 parts produced per day, this means that 15 liters of contaminate are dragged into the typical 500-liter quench tank per day. It does not take long before the quench tank containing only water and polymer becomes a coolant-contaminated mess (Figure 3).

Figure 2: Quench tanks collect everything. They should not be used as part washers. Skimmers and similar equipment should be inspected and used.
Figure 3: Samples taken from a badly contaminated polymer quenchant solution used for hardening critical suspension components.

Filtration is also effective in reducing bacteria growth. Filtration serves two purposes. First, it eliminates particulate, scale, and other debris from the quenchant that can act as a food source. Secondly, it maintains the quenchant cleanliness so proper quenching can occur. Bag and cartridge filters are commonly used in this application. Typically, bag sizes down to approximately 15-20 microns are used. However, this type of filter is not recommended for this application, as the low fluid flows through the filters and the high concentration of food sources can contribute to bacteria growth. Further, this type of filtration, once contaminated, can spread the contamination to other locations in the quench tank.

Sand filters are highly effective filtration systems for polymer quenchants. The nature of sand filtration has very high flow velocities, so stagnant solutions do not occur. Further, they are also very effective at filtration, with filtration levels often at 6 – 8 microns or better. They are also cost-effective, as the filtration media is inexpensive clean white sand.

Control of Fungus and Bacteria

To control biological contamination, biocides are often used to kill bacteria and fungi [2]. These biocides are used to control and kill unwanted microorganisms. Biocides are usually applied to a quench tank when the bacteria count exceeds 106 CFU/ml or fungi counts exceeds 10 CFU/ml. Biocides and fungicides should only be used sparingly, and only when needed. The dosage used should be a “kill” dose, and not a maintenance dose. Use of constant doses of biocides or fungicides can result in resistant bacteria that is very difficult to control.

Not only do biocides kill pathogens, but they also kill non-pathogens. This means that they can be dangerous to humans. Only tiny amounts are necessary to kill the bacteria and fungi present. For a typical 500-liter quench tank, only 40 ml are required to kill the bacteria and fungi. Biocides are severe skin irritants and are corrosive to the eyes. They may also cause burning of the skin and cause skin sensitization (contact dermatitis) [3]. Proper personal protection equipment, including face shields, goggles, rubber gloves, protective suits, and other PPE are necessary to prevent personnel injury. Not only are biocides hazardous, but they are also very expensive.

Biocides only last for two-four weeks depending on the contamination, and the number of new bacteria and fungi brought into the system. Constant dosage of new biocide may initially keep bacteria and fungi low, but eventually, resistant bacteria and fungi will be created. Generally, anything that the heat treater can do to avoid using biocides is a good idea.

Quenchants have been developed that will not sustain bacteria growth — and “will not stink” under most circumstances. These quenchants are specially designed for induction hardening applications with enhanced corrosion inhibitor packages. These are proprietary quenchants designed to have long life and not support bacterial or fungal growth. Examples are the Quaker Houghton’s Aqua-Quench™ biostable polymer quenchants. However, minimum concentrations must be maintained (typically 5 percent) to remain biostable.


Proper control of the polymer quench tank is important to ensure long, consistent bath life. Contamination must be held to a minimum for proper quenching. Contamination is controlled by appropriate washing of parts prior to heat treatment. This yields consistent case hardness and depth, while extending bath life.

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  1. J. Barberi, C. Faulkner and D. S. MacKenzie, “Extending the Life of Polymer Quenchants: Cause and Effect of Microbiological Issues,” in Proceedings from the 6th International Quenching and Control of Distortion Conference, 09-12 September, Chicago, 2012.
  2. J. L. Shennan, “Selection and evaluation of biocides for aqueous metal-working fluids,” Tribology International, vol. 16, no. 6, pp. 317-330, 1983.
  3. Rohm and Haas, “KATHON™ 886 MW Microbicide,” Rohm and Haas, 2006.