Quenching oil and heat treatment fluids are designed for rapid or controlled cooling of steel or other metals as part of a hardening, tempering or other heat-treating process. Quenching oil serves two primary functions. It facilitates hardening of steel by controlling heat transfer during quenching, and it enhances wetting of steel during quenching to minimize the formation of undesirable thermal and transformational gradients which may lead to increased distortion and cracking.

Oil has a major advantage over water due to its higher boiling range. A typical oil has a boiling range between 450ºF (230ºC) and 900ºF (480ºC). This causes the slower convective cooling stage to start sooner, enabling the release of transformation stresses which is the major problem with rapid water cooling. Oil is, therefore, able to quench intricate shapes and high-hardenability alloys successfully.

The Quenching Process

When heat treatment fluids are used to quench metals, cooling occurs in three distinct stages: film boiling, nucleate boiling and convective heat transfer.

Film Boiling

Film boiling, also known as the "vapor blanket" stage, occurs upon initial immersion. Contact between the hot metal surface and quenchant creates a layer of vapor (known as the Leidenfrost phenomenon) due to the supply of heat being greater than that which is carried off. The stability of the vapor layer, and thus the ability of the oil to harden steel, is dependent on the metal's surface irregularities, oxides present, surface-wetting additives (which accelerate wetting and destabilize the layer), and the quench oil's molecular composition (including the presence of more volatile oil degradation by-products). Cooling in this stage is a function of conduction through the vapor envelope and is relatively slow since the vapor blanket acts as an insulator.

Nucleate Boiling

As the part cools, the vapor blanket collapses at points and nucleate boiling (violent boiling of the quenchant) results. Heat transfer is fastest during this stage, with heat transfer coefficients sometimes over two orders of magnitude higher than during film boiling, largely due to the heat of vaporization. The boiling point of the quenchant determines the conclusion of this stage. The points at which this transition occurs and the rate of heat transfer in this region depend on the oil's overall molecular composition.

Convective Heat Transfer

When the part has cooled below the boiling point of the quenchant, slow cooling occurs by convection and conduction (also called the "liquid" stage). Cooling rate during this stage is slow, and is exponentially dependent on the oil's viscosity, which varies with the degree of oil decomposition. Heat-transfer rates increase with lower viscosities and decrease with increasing viscosity.

Figure 1 - Typical cooling curves and cooling-rate curves for new oils. Image credit: Vac Aero International Inc.

The ideal quenchant is one that exhibits little or no vapor stage, a rapid nucleated boiling stage and a slow rate during convective cooling. The high initial cooling rates allow for the development of full hardness by quenching faster than the so-called critical transformation rate and then cooling at a slower rate as the metal continues to cool. This allows stress equalization, reducing distortion and cracking in the workpiece.

This video shows the oil quenching of alloy steel:

Video credit: groves28

Oil Selection

When selecting quenching oils, industrial buyers will need to consider the chemistry, properties, and features of the fluid that are needed for the application.

Chemistry

The chemistry of the quenching media is the primary consideration in selecting the best fluid for the application.

• Straight oils are non-emulsifiable products used in machining operations in an undiluted form. They are composed of base mineral or petroleum oils, and often contain polar lubricants like fats, vegetable oils, and esters, as well as extreme pressure additives such as chlorine, sulfur, and phosphorus. Straight oils provide the best lubrication and the poorest cooling characteristics among quenching fluids. They are also generally the most economical.
• Water soluble and emulsion fluids are highly diluted oils, also known as high-water content fluids (HWCF). Soluble oil fluids form an emulsion when mixed with water. The concentrate consists of a base mineral oil and emulsifiers to help produce a stable emulsion. These fluids are used in a diluted form with concentrations ranging from 3% to 10%, and provide good lubrication and heat transfer performance. They are used widely in industry and are the least expensive among all quenching fluids. Water-soluble fluids are used as water-oil emulsions or oil-water emulsions. Water-in-oil emulsions have a continuous phase of oil, and superior lubricating and friction reduction qualities (i.e. metal forming and drawing). Oil-water emulsions consist of droplets of oil in a continuous water phase and have better cooling characteristics (i.e. metal cutting fluids and grinding coolants).
• Synthetic or semi-synthetic fluids or greases are based on synthetic compounds like silicone, polyglycol, esters, diesters, chlorofluorocarbons (CFCs),and mixtures of synthetic fluids and water. Synthetic fluids tend to have the highest fire resistance and cost.They contain no petroleum or mineral oil base, but are instead formulated from organic and inorganic alkaline compounds with additives for corrosion inhibition. Synthetic fluids are generally used in a diluted form with concentrations ranging from 3% to 10%. They often provide the best cooling performance among all heat treatment fluids. Some synthetics, such as phosphate esters, react or dissolve paint, pipe thread compounds, and electrical insulation. Semi-synthetic fluids are essentially a combination of synthetic and soluble petroleum or mineral oil fluids. The characteristics, cost, and heat transfer performance of semi-synthetic fluids fall between those of synthetic and soluble oil fluids.
• Micro-dispersion oils contain a dispersion of solid lubricant particles such as PTFE (Teflon®), graphite, and molybdenum disulfide or boron nitride in a mineral, petroleum, or synthetic oil base. Teflon® is a registered trademark of DuPont.

Properties

Properties for describing heat treating fluids can be classified as either primary or secondary.

Primary

Primary properties are those which describe the performance of the fluid. These include cooling rate, thermal conductivity, viscosity, water content, and sludge formation.

• Cooling rate / quenching speed - the rate at which a quenching fluid can cool a workpiece. This specification is given either as a ratio in comparison to water or as a number based on the GM quenchometer test. The GM test (also called the "nickel ball" test) measures how long it takes for a nickel ball to be cooled to the point at which it becomes magnetic. The figure below gives an example of the setup for such a test.

Figure 2 - GM quenchometer test apparatus. Image credit: Machinery Lubrication

This property does not give any information about the cooling pathway, however (as demonstrated in figure 3); it merely gives the time required to cool to a certain temperature.

Figure 3 - Cooling curves for 3 different quenching oils with the same GM results. Image Credit: Machinery Lubrication

• Thermal conductivity - the measure of a fluid's ability to transfer heat. Quenching fluids with higher thermal conductivity will cool metals faster than those with low thermal conductivity.
• Viscosity - the thickness of a fluid, commonly measured in centistokes (cSt). Heat transfer during the convective stage is exponentially dependent on the oil's viscosity, which will vary with the degree of oil decomposition. Oil decomposition (formation of sludge and varnish) will result initially in a reduction of oil viscosity followed by continually increasing viscosity as the degradation continues. Heat transfer rates increase with lower viscosities and decrease with increasing viscosity. Figure 4 shows viscosity change over time.

Figure 3 - Viscosity of a Martempering Oil as a Function of Time. Image Credit: Machinery Lubrication

• Water content - the amount of water in the quenching fluid. Water, because it is not compatible with oil and possesses different physical properties such as viscosity and boiling point, will cause increases in thermal gradients and may cause soft spots, uneven hardness, or staining on the workpiece. When water-contaminated oil is heated, a crackling sound may be heard; the basis of a qualitative field test for water in quench oil. Many automated moisture detectors typically measure as low as 0.5 percent, which is inadequate for the moisture content levels allowed for quench oils (typically less than 0.1 percent).

• Sludge content - the amount of sludge and varnish in the quenching fluid as a result of thermal and oxidative degradation. These by-products typically do not adsorb uniformly on the metal's surface as it is being quenched, resulting in non-uniform heat transfer, increased thermal gradients, cracking, and distortion. Sludge may also plug filters and foul heat-exchanger surfaces, causing overheating, excessive foaming, and fires. The relative amount of sludge in quench oil may be quantified by the precipitation number. This number can be used to estimate the remaining life of used oil by comparing it to the levels in new oil.

These images show the differences in analytical spectra of new and degraded oils:

Figure 4A - IR Spectra of a New vs. Moderately Degraded Quench Oil. Image credit: Machinery Lubrication

Figure 4B - IR Spectra of a New vs. Severely Degraded Quench Oil. Image credit: Machinery Lubrication

Secondary

Secondary properties are those which describe a fluid's operating parameters. They include operating temperature, pour point, and flash point.

• Operating temperature - the normal range of temperatures for which the fluid is designed, or the maximum temperature of material the fluid can cool safely or effectively.
• Pour point - the lowest temperature at which fluid or oil flows. The pour point is typically 15°F to 20°F below the system's lowest end-use temperature to prevent pump damage through cavitation.
• Flash point - the temperature at which the fluid produces sufficient vapors to form an ignitable mixture in air near the surface. The lower the flash point, the easier it is to ignite the material. Operating temperatures and procedures need to be considered along with an oil's flash point to ensure a safe quenching process.

Features

Quenching oils and heat treatment fluids can include a number of additional features which add versatility and functionality. Among these are biodegradable, low foaming, and water displacement characteristics.

• Biodegradable - fluids are designed or suitable to decompose or break down into harmless chemicals when released into the environment. This is useful for high volume operations where disposal costs for degraded oils could otherwise be very high.
• Low foaming - fluids do not produce foam or produce only small amounts of foam. Non-foaming characteristics are achieved through the use of additives that break out entrained air. Leaks which introduce air into a system can cause pump damage due to cavitation. Foaming can also reduce the cooling ability and the bulk modulus (or stiffness) of the fluid.
• Water displacement - fluids have the ability to displace water from a surface based on wetting or surface energy characteristics. Fluids with low surface energy or interfacial tension compared to water will flow under the water or moisture on a surface.

References

Vac Aero International Inc. - Oil Quenching Technology

Machinery Lubrication - Quenchant Fundamentals: Condition Monitoring of Quench Oils

Image credits:

Magic Lubricants Pvt. Ltd. | Excel Petroleum Pvt. Ltd.