Use and Maintenance of Quenching Media

Personnel engaged in heat treatment production shall properly maintain the quenching media used. Typical management tasks include: preventing contamination of the medium, ensuring the cooling system operates normally, maintaining the bath temperature within specified limits, regularly monitoring and controlling the concentration of water-based media, and periodically testing the cooling characteristics of the quenching medium. The following points should also be observed: w3U China Metallography Network

1. Before introducing new quenching medium, especially when performing a complete tank replacement in an existing quenching tank, the tank and cooling system must be thoroughly cleaned. Some organizations, seeking to save time and effort, merely drain the old quenching oil from the tank to a rough extent before pouring in the new oil during a full-tank change. As a result, oil sludge that had settled at the bottom of the tank, carbon-black sludge adhering to the tank walls, and residual oil contaminants remaining in the cooling system all mix into the new oil, contaminating the entire batch. Consequently, the newly introduced quenching oil becomes polluted, leading to workpieces that emerge from quenching heavily stained and difficult to clean. w3U China Metallography Network

2. If quenching oil is found to become easily ignitable, the cause must be promptly identified and eliminated. One possible cause is water contamination in the oil, particularly when hot oil is used. Another potential cause is a malfunction in oil-temperature measurement or display, resulting in the actual oil temperature being significantly higher than the indicated temperature. In addition, the presence of low-flash-point, highly volatile fluids in the oil can also increase the risk of ignition. w3U China Metallography Network

3. Prevent carbon-black contamination of quenching oil in the heating furnace. Carbon black is inevitably produced in carburizing and carbonitriding furnaces. When this carbon black enters the quenching oil, it contaminates the oil. Even small amounts of carbon black, when allowed to accumulate over time, first degrade the brightness of the quenched workpieces and subsequently impair the oil’s cooling characteristics. Because carbon-black particles are extremely fine and tend to remain suspended in the oil, they cannot normally be separated by filtration or sedimentation. The most effective current solution is to periodically burn off the accumulated carbon black inside the heating furnace. w3U China Metallography Network

4. Views on medium degradation and full-tank replacement. Both water-based and oil-based media have a limited service life; when that life is reached, the entire tank should be replaced. w3U China Metallography Network

The primary factors influencing the service life of quenching oil are the operating temperature of the oil, the total surface area of the workpieces being quenched, the quality of the oil itself, and the degree of external contamination. The lower the oil’s operating temperature—taking into account both its average temperature and the temperature rise experienced by the workpieces upon immersion—the longer the oil will last; conversely, the larger the total surface area of the quenched workpieces, the shorter the oil’s service life. Quenching oils used for small workpieces have a very short service life because the specific surface area per unit weight of the workpieces is extremely high; in contrast, quenching oils used for large workpieces have a much longer service life, as the specific surface area per unit weight is relatively low and the frequency of quenching cycles is also reduced. The quality of the oil depends on the quality of the base oil and additives employed; under identical operating conditions, poor-quality oil may last only a few months, whereas high-quality oil can often last several years. In addition, external contamination—particularly the ingress of water and the accumulation of carbon black—has a significant impact on the oil’s service life. w3U China Metallography Network

The service life of water-based quenching media is primarily determined by the type of medium. For example, polyvinyl alcohol–based quenchants typically have a service life of no more than a few months, whereas PAG-based media can generally be used for several years. External contamination also has a significant impact on the service life of water-based media. Consequently, the maintenance and management of water-based media require greater attention and are more labor-intensive than those of oil-based media. PAG quenching solutions can extend their full-tank replacement interval through degreasing and cleaning treatments. w3U China Metallography Network

Whether water-based or oil-based, quenching media will gradually degrade during use and are also susceptible to contamination. The accumulation of degradation products and extraneous contaminants will progressively impair the performance of the medium. After a certain period of service, the entire bath should be replaced. It is understood that, with the exception of oils used exclusively for quenching large workpieces, in most applications involving the heat treatment of general small- and medium-sized basic components, the service life of quenching oils—both domestically and internationally—is typically no more than three to five years. Even for PAG quenching media, if no decontamination treatment is performed, the recommended interval for complete bath replacement is usually shorter than three to five years. Timely replacement of the entire bath when the service limit is reached can help ensure consistent heat-treatment quality, enhance production efficiency, simplify operational management, and reduce medium consumption, thereby lowering overall production costs. w3U China Metallography Network