This document discusses quench oil fundamentals, including how quench oil facilitates hardening of steel during quenching by enhancing wetting and minimizing thermal gradients. It describes how oil degradation and contaminants like water, soot, salt, and hydraulic fluid can impact the maximum cooling rate and temperature. The document recommends monitoring oil quality over time through tests and precautions like checking water content, flash point, viscosity, and acidity. It notes that contaminated oil can be reclaimed through filtration and vacuum dehydration to restore its performance.

1. Quench Oil Fundamentals
WHITE PAPER
© 2017 AGC Refining & Filtration LLC

2. AGC REFINING & FILTRATION
QUENCH OIL FUNDAMENTALS 2
Contents
Overview 3
Effects of Contaminants 3
Precautions 4
Reclamation 5
References 6

3. AGC REFINING & FILTRATION
QUENCH OIL FUNDAMENTALS 3
Overview
Quench oil serves two primary functions:
 It facilitates hardening of steel during quenching
 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

4. AGC REFINING & FILTRATION
QUENCH OIL FUNDAMENTALS 4
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.

5. AGC REFINING & FILTRATION
QUENCH OIL FUNDAMENTALS 5
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.

6. AGC REFINING & FILTRATION
QUENCH OIL FUNDAMENTALS 6
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.”

7. www.AGCInternational.com
3045 East Elm Street
Springfield, Missouri 65802, USA
Toll Free: +1 800 865 3208
Phone: +1 417 865 2844