Overview

Quench oil serves two primary functions:

1. It facilitates hardening of steel during quenching
2. It enhances wetting of steel during quenching to minimize the formation of undesirable thermal and transformational gradients, which may lead to distortion or cracking

When hot metal is quenched, a vapor envelope is initially formed around the hot metal as it is immersed in the oil. The stability of this vapor envelope—and thus the ability of the oil to harden steel—is dependent on the metal surface irregularities, the presence of oxides, surface wetting agents (which accelerate the wetting process and destabilize the vapor envelope), and the presence of other oil degradation by-products.

Upon further cooling, the vapor envelope collapses, resulting in so-called nucleate boiling, which is the fastest heat transfer.

Nucleate boiling is a type of boiling that can take place under certain conditions. It is the process of forming steam bubbles within liquid in micro cavities adjacent to the wall if the wall temperature at the heat transfer surface rises above the saturation temperature while the bulk of the liquid is sub-cooled. The bubbles grow until they reach some critical size at which point they separate from the wall and are carried into the main fluid stream. There the bubbles collapse because the temperature of bulk fluid is not as high as at the heat transfer surface where the bubbles were created. Heat and mass transfer during nucleate boiling has a significant effect on the heat transfer rate. This heat transfer process helps to quickly and efficiently carry away the energy created at the heat transfer surface. When the temperature of the hot steel interface is less than the oil’s boiling point, nucleate boiling will stop and convective cooling will begin.

Oil degradation is often accompanied by sludge and varnish formation. These by-products do not adsorb uniformly on the steel surface as it is being quenched, resulting in cooling rate variations and thermal gradients.

Another source of non-uniform heat transfer is water contamination of the quench oil. Water causes thermal gradients and lower viscosity.

Effects of Contaminants

Viscosity

Of all the variables that can affect the maximum cooling rate during nucleate boiling, temperature has the most significant effect on the maximum cooling rate. Increasing the temperature increases the maximum cooling rate due to the change in viscosity. At room temperature, the oil is viscous and does not wet the surface of the part well. As the viscosity decreases with increased temperature, the result is better wetting of the part and consequently better heat transfer.

Soot

Soot has the second largest impact on maximum cooling rate. The maximum cooling rate increases as the amount of soot in the oil increases. This is due to the soot particles functioning as nucleation sites for bubble formation during nucleate boiling. Soot also causes the temperature of maximum cooling to increase.

Salt

Salt crystals have an effect similar to soot particles since they do not dissolve in oil and form nucleation sites for bubble formation during nucleate boiling.

Water

Water increases the maximum cooling rate and substantially decreases the temperature of maximum cooling. This increases the chances of distortion of the part by increasing the thermal gradients within the part.

Hydraulic Fluid

Contamination with hydraulic fluid increases the maximum cooling rate and the temperature at which maximum cooling occurs. Because hydraulic fluids are miscible in quench oil, the properties of the quench oil change. The boiling point of the mixture will likely increase, causing an increase in maximum cooling rate and the temperature at which maximum cooling rate occurs.

Oxidation

Oxidation causes the maximum cooling rate and the temperature of maximum cooling to decrease, which is caused by increases in viscosity of the quench oil. This in turn causes a decrease in wetting. Increase in viscosity also causes bubble formation to become more difficult while the maximum cooling rate and the temperature of maximum cooling is reduced.

Precautions

Percent Water

This contaminant in amounts as low as 1,000 parts per million (ppm) can cause foaming, fires, and explosions.

Flash Point

This value should be as high as possible. Changes usually indicate contamination or degradation. Low flash points increase the chance of fires.

Percent Sludge

This is the result of oxidation and polymerization.

Percent Ash

Increased inorganic ash content indicates degradation.

Kinematic Viscosity

As oil degrades, viscosity usually increases. Some contaminants reduce viscosity and flash point.

Neutralization Number

Increased oxidation causes the oil to become more acidic.

Quenching Speed

Either a GM Quenchometer test or a cooling rate curve should be used to evaluate the cooling/quenching characteristics of the oil.

Reclamation

The effects of contamination can cause significant changes in the maximum cooling rate and the temperature of maximum cooling. This can result in distortion, cracking, and non-uniformity of properties. A control program to monitor and track quench oil performance is necessary to ensure high quality parts.

Quench oil can be reclaimed even when it is severely contaminated. Today’s disposal problems and the eventual cycling of oil economics make the reclamation and revitalization processes extremely attractive. Reclamation of contaminated quench oil can be performed by using an Allen Oil Conditioner equipped with a water-cooled heat exchanger. We use a strainer to collect the solid particles and then cool the oil before it goes into the vacuum dehydration technology, which removes the water and gases and restores the quench oil to a like-new condition.

References

1. Herring, D. H. “Oil Quenching Part 1: How to Interpret Cooling Curves.” Industrial Heating, Aug 2007.
2. MacKensie, D.S. et al. “Effects of Contamination on Quench-oil Cooling Rate.” Houghton International, Inc. 2002.
3. Wachter, D.A. et al. “Quenchant Fundamentals-Quench Oil Bath Maintenance.”