Water-soluble quenching fluids are primarily composed of polyalkylene glycol (commonly referred to as PAG, or polyoxyethylene oxypropylene glycol), which is a non-flammable aqueous solution with a small amount of corrosion inhibitors added.

PAG is fully soluble in water at temperatures below approximately 74–88°C, but separates from water at higher temperatures.

When a pre-heated metal part is immersed in this solution, a PAG film instantly forms on the surface, suppressing the formation of a vapor blanket and evenly distributing heat. This enables uniform hardening and helps prevent cracking and deformation.

The cooling rate and concentration of water-soluble quenching fluids can be broadly controlled by the structure and performance of the agitation system. This allows adjustments based on the material, shape, and size of the heat-treated parts, enabling a wide range of treatment outcomes.

Advantages of Water-Soluble Quenching Fluids

1. Minimal deformation, low risk of cracking or hardness irregularities, reduced reprocessing costs, and no degreasing required before tempering.
2. Improved mechanical properties allow the use of lower-cost materials. Also, minimal fluid remains on products, reducing overall fluid consumption.
3. Adjustable cooling rate by modifying concentration, fluid temperature, and flow rate, making it possible to achieve results comparable to water or mineral oil quenching.
4. No fire hazard, smoke, or soot, making it safer and easier to use.

Issues and Management of Water-Soluble Quenching Fluids

1. Contamination by Sludge and Foreign Substances

a. Unlike mineral oils, water-soluble fluids are diluted with water, making them more susceptible to contamination from accumulated sludge and particles.

The most common contaminants are iron oxide scale, soil, and dust. Fine floating particles in the fluid can interfere with PAG film formation, leading to rust, wear on conveyors and elevators, and equipment failure.

b. Using low-quality groundwater or contaminated water for extended periods leads to salt accumulation, which causes product contamination during cooling, resulting in unsatisfactory outcomes.

c. Carbide residues from forged parts may contaminate the fluid, slow the cooling rate, reduce product hardness, and result in shallower hardening.

d. When equipment is idle for long periods, microorganisms may multiply and degrade the PAG. Circulating air in the storage tank can suppress the growth of anaerobic bacteria, and adding a small amount of biocide can help control microbial activity.

These problems can increase electrical conductivity, lower the pH, and cause foaming, leading to various operational issues.

Management:

• Perform regular cleaning to remove sludge and foreign matter.
• Avoid using contaminated water such as groundwater or rainwater.
• Operate the agitation pump periodically during equipment downtime to prevent microbial growth.

2. Concentration Control

If the fluid concentration is too low or equipment is idle for long periods, decomposition may occur, causing corrosion to both equipment and materials.

Concentration control is fundamental. Incorrect concentrations—whether too high or too low—can lead to cracking and other quality defects.

While refractometers are commonly used onsite for their convenience, their readings can be skewed by dissolved contaminants. Therefore, it’s recommended to periodically check with a hydrometer or viscometer for more accurate measurements. However, ionic substances like release agents and surfactants used in forging processes can also affect readings. For precise concentration measurements, testing should be outsourced to professional research institutions.

3. Foam Control

The main cause of foaming is often mechanical failure, but contaminated fluid also promotes foam generation.

Excessive agitation causes oxygen ingress, which leads to foaming. In clean fluid, the foam dissipates quickly, but in contaminated fluids, foam persists. In such cases, defoamers are typically added.

However, overuse of defoamers can degrade the performance of PAG and reduce cooling efficiency. Therefore, it is more effective to minimize foam generation through regular cleaning and fluid maintenance.

Final Recommendations

While proper fluid management and supplementation can delay or reduce contamination, it is impossible to fully restore contaminated fluid to its original condition.

Therefore, it is best to replace the entire fluid periodically, cleaning both the tank and related equipment at the same time.

Additionally, using SUS (stainless steel) materials for internal equipment components can help prevent contamination and corrosion.

Dyna Co., Ltd.
Industrial Lubricant Solution
E-Mail : dyna@dynachem.co.kr
Web : dyna.co.kr/en/