THE MECHANISM OF QUENCH OIL OXIDATION

D. Scott MacKenzie, PhD, FASM1

1Houghton International, Valley Forge, PA, USA 19482

smackenzie@houghtonintl.com

Keywords: Oxidation, Quench, Oil, Viscosity, Total Acid Number

ABSTRACT:

The life of quench oil is dependant upon its thermal stability. The thermal stability is a

function of the quality of the base oil, the antioxidant package used, and the presence of heat

and catalysts. In this paper, the mechanism of quench oil degradation and the function of

antioxidants will be explained. An example of oxidation testing to determine the role of the

base oil and antioxidants is illustrated.

1. INTRODUCTION

With the fluctuations in the price of oil, manufacturers see their quench oil purchases as

an investment not a commodity purchase. The cost of the quench oil, as well as the cost of

disposing poorly performing quench oil as dramatically increased in the last decade. The

choice of poorly performing quench oil can be disastrous in terms of cost and quality.

It is not known how long oils have been used in the hardening of ferrous alloys. Many

types of oils have been used; including vegetable, fish and animal oils, and particular sperm

whale oil have been used for quenching operations. The first petroleum-based quenching oils

were developed around 1880 by E.F. Houghton in Philadelphia. Since that time, much

advancement has been made in the development of quenching oils to provide highly

specialized products for specific applications.

A wide range of quenching characteristics can be obtained through careful formulation

and blending. High quality quenching oils are formulated from refined base stocks of high

thermal stability. Selected wetting agents and accelerators are added to achieve specific

quenching characteristics. The additions of complex anti-oxidant packages are included to

maintain performance for long periods of continued use – particularly at elevated

temperatures. Emulsifiers may be added to enable easy cleaning after quenching.

Petroleum-based quench oils can be divided into several categories, depending on the

operational requirements. These requirements include quenching speed, operating

temperatures and ease of removal.

The quenching speed is important because it influences the hardness and the depth of

hardening. This is probably the most common method of classifying quench oils. Quench

oils can be classified as normal, medium or high speed quench oils.

Normal speed quench oils have relatively low rates of heat extraction, and are used in

applications where the material being quenched has a high hardenability. Highly alloyed

steels such as AISI 4340 or tool steels are typical examples of steels quenched in normal

speed oils.

Medium speed quench oils provide intermediate quenching characteristics and are widely

used for medium to high hardenability applications where dependable, consistent

metallurgical properties are required.

High speed quench oils are used for applications such as low hardenability alloys,

carburized and carbonitrided components, or large cross-sections of medium hardenability

steels where high rates of cooling are required to ensure maximum mechanical properties.

Mar-quenching oils are a special case where the part is quenched into a quenchant at

elevated temperature, typically 100-200ºC. Since mar-quenching oils are used at relatively

high temperatures, their formulation and physical properties are different from cold

quenching oils. They are formulated from very carefully selected base stocks with high

oxidation resistance and thermal stability. They have high flash points and viscosities, and

contain complex anti-oxidant packages to provide long life. Selection of the mar-quenching

oil is based on the operating temperature and quenching characteristics. A minimum of 50ºC

should be maintained between the operating temperature of the oil and its flash point.

1.1. Oil Composition

Engineered quench oil is governed by the desired quenching performance; the necessary

thermal and oxidative stability, and finally price or market considerations. Quenching

performance is generally considered to be the heat transfer characteristics and the thermal

stability of the oil. This includes having an acceptable flash temperature that is a minimum

of 50ºC above the expected use temperature; low sludge forming tendency; long life; and the

necessary quenching speed. The best quench oils have the following characteristics:

• The maximum cooling rate to achieve maximum hardness and depth of

hardening;

• Minimal deposition of sludge; and

• Maximum thermal and oxidation stability (achieved through the additive

package).

• Control over distortion and residual stresses

At present, this currently favors mineral oils.

The base stocks of oils used in the formulation of quench oils are complex. They contain

various ratios of paraffinic and napthenic constituents, as well as numerous open chain and

cyclic derivatives such as sulfur, oxygen and nitrogen hetrocycles. The specific composition

of the base oil depends on the source of the crude oil. This means that the composition of the

crude is different in the western United States than in the eastern United States; Middle East;

North Sea, Venezuela, etc. From a supplier’s perspective, this means that different additive

packages, or different ratios of thermal stability and speed improvers may be necessary to

provide an oil that behaves similarly across the world. What this means to a heat treater is

that there will be minor differences in quenching performance of a quench oil in different

locations across the world because of differences in local feed stock.

As was inferred from the previous paragraph, the composition of quench oil affects the

quenching performance. Because of local differences in composition, quench oil can exhibit

a wide variation in thermal and oxidative stability, and a wide variation in quenching speed.

When you add the thermal stability and speed improver additive packages, an even wider

variation in cooling rates and thermal stability occurs.

Base oils are generally divided into two groups: napthenic and paraffinic grades.

Napthenic oils have a high proportion of cyclic hydrocarbons. Napthenic grades contain very

low portion of n-alkanes, which are chemical compounds that consist of only carbon and

hydrogen. Alkanes are also known as paraffins. The low temperature behavior of napthenic

oils is better than same viscosity paraffinic oils, so the napthenic oils tend to have a lower

pour point. The degradation products of napthenic oils tend to be more soluble in oil, which

can contribute to greater staining. For the same viscosity, napthenic oils tend to oxidize at a

faster rate and have a lower thermal stability. Napthenic grades, at the same viscosity as a

paraffinic grade, also tend to have a lower flash temperature.

Paraffinic oils are oils that consist predominately of long chains of hydrocarbons, called

alkanes. Because of these long chains, when compared to napthenic oils, paraffinic oils are

widely used for lubrication base stocks. They are also the preferred base stock for oil-based

quenchants. Comparing identical viscosities of napthenic and paraffinic oils, paraffinic oils

have higher flash temperatures, superior cooling curves, and better oxidation and thermal

stability. This means that they tend to stain less and last longer.

Of the base oil used for quenchants, there are many types:

• Double hydro-treated mineral oils;

• Refined paraffinic mineral oils;

• Refined napthenic mineral oils;

• Re-refined mineral oils; and

• Reclaimed mineral oils.

Double hydro-treated mineral oils are those that are treated with hydrogen to remove

carbon-carbon double bonds. These double bonds can occur in aromatics and in impure

paraffinics. Because these double bonds are removed, they have greater thermal and

oxidative stability. They have a higher flash temperature, compared to other grades, and have

a marginal improvement in the cooling curve for the same viscosity. These oils are generally

clear to very light amber. However, the color is not indicative of staining tendency – that is

the function of the additive package. The primary disadvantage of double hydro-treated oils

is availability and cost.

The refined paraffinic oils are the most commonly used base oil in quenchants. Besides

the advantages cited earlier, they also have a wide range of viscosities available (70-

2500SUS). They are hydrophobic, which means that they displace water. This means better

washing for the heat treater, and better splitting for improved reclamation. Unfortunately,

refined paraffinic mineral oil is getting more expensive because of global demand, as well as

local refinery shut-downs due to maintenance and other factors.

1.2. Additive Packages

Additive packages have two functions: first to act as a speed improver, and second to

increase thermal and oxidative stability. These additives can have a dramatic effect on the

quenching properties. The magnitude of the effect is dependant on the additive package.

These additive packages are usually proprietary. Depending on the robustness of the additive

package, they may tend to deplete during use. Some additive packages are prone to selective

drag-out over time. It generally not appropriate to mix additive packages from different

quenchant suppliers because of the unintended consequences of precipitation; increased

staining and generally unpredictable response.

Speed improvers increase the quenching rates by increasing the wettability of the oil.

There are different types of additives. The most common is the sulfonate-type. Barium

sulfonate used to be the most common, but is no longer used by most quenchant suppliers.

The presence of barium can cause an issue with disposal and special precautions during

disposal. Potassium and sodium sulfonates are commonly used. These sulfonates provide

excellent speed improvement and can double as thermal oxidation stabilizers. Hydrocarbon

based additive packages are used in high-end quenchants. They are very robust and have a

long life and are not prone to needing replenishment. The primary disadvantage is cost.

The other purpose of an additive package is to improve the thermal and oxidative

stability of the base oil. These packages reduce the formation of sludge and minimize the

formation of organic acids upon thermal aging of the oil.

2. OIL CHEMISTRY AND THE DETERIORATION PROCESS

Premium quality heat-treating quenching and martempering oils are formulated from

refined base stocks (usually paraffinic) of high thermal stability with additives to improve

performance and increase tank life. These additives are a combination of specially chosen

ingredients compatible with the base oil; in particular, carefully selected and tested

antioxidants, which retard the aging process.

Quench oils degrade from four primary reasons:

• Oxidation

• Thermal Degradation

• Contamination

• Additive Depletion

This is aggravated by residues on parts, washer residues from oil reclaimed from

washers; high energy density heater or radiant tubes, and excessive peak temperatures. The

addition of robust additive packages prolongs a quenchant life and provides for repeatable

quenching.

Oxidation of quench oil is caused by exposure to oxygen. As operating temperatures

increase, the kinetics of oxidation approximately double with each 10°C. This is especially

true with mar-tempering oils because of their elevated temperature of use.

Thermal degradation is from exposure to temperatures that cause the base oil and

additive package to change. This results in the formation of insoluble products of reaction

that can cause deposits on parts and sludge the quench tank.

Contamination can be from many sources. Water, dust, scale and soot are not the direct

result of oil degradation, but can contribute to other degradation issues. Soot can act as

nucleation sites for thermal degradation products, and can mimic oxidized oil.

Additive depletion is normal and expected. The anti-oxidants are consumed as part of

their function. Anti-oxidants are replenished as make-up oil is added.

Oil degradation is manifested by a viscosity increase, acidity increase (as measured by

Total Acid Number), varnish and lacquer deposits, sludge, and changes in the quench speed1.

Examples of staining of parts are shown in Figure 1. Changes to the cooling curve are shown

in Figure 2.

Figure 1. Deposits on ring gears from severely degraded quench oil.

Figure 2. Comparison of cooling rate profiles before and after aging for a straight

mineral oil and a high performance quenching oil (after Totten et al [1]).

There are many papers covering the mechanism of oxidation of oils and the function of

antioxidants2,3,4,5,6

.

There are three primary steps in the oxidation of oil:

• Initiation

• Propagation

• Termination

2.1. Chain Initiation

The oxidation mechanism of quenching oils is very complex. The presence of Iron and

Copper catalyze the reactions and play and important part in chain initiation reaction.

Typically reactions are slow at room temperatures but become increasingly faster above

100°C. This is why, for high quality quenching oil, “cold” oils (those used below 80°) do not

experience the severe oxidation with attendant increases in viscosity and Total Acid Number

(TAN) than Mar-tempering oils experience.

There are generally two types of initiation reactions:

•+•→+ HOORORH 2 (1)

222 OHR2ORH2 +•→+ (2)

There is a second class of reactions that are catalyzed by the presence of iron of copper.

These are hydroperoxide decomposition initiation reactions:

+++ +•→+ HROOFeROOHFe 23 (3)

−++ +•+→+ HOROFeROOHFe 32 (4)

2.2. Chain Propagation and Branching

The chain propagation step involves the reaction to produce additional radicals that

propagate the oxidation sequence. Alkyl radicals (R•) react with oxygen in the oil and create

peroxy radicals (ROO•). These peroxy radicals react with additional hydrocarbon molecules

to produce hydroperoxides (ROOH) and additional akyl radicals:

•→+• ROOOR 2 (5)

•+→+• RROOHRHROO (6)

Additional chain branching occurs my several different reactions that can occur. This is

dependant on the base oil and the temperature. Examples are show below:

•+•→ HOROROOH (7)

RROHRHRO +→+• (8)

•+→+• ROHRHHO 2 (9)

There are many other initiation, propagation and chain branching reaction that have been

reviewed elsewhere7,8.

2.3. Formation of Aldehydes and Ketones

The side reactions of the formation of aldehydes and ketones are probably the most

important in maintenance of quench oils because their subsequent reactions eventually form

sludge and deposits. While there are a number of different paths that lead to the formation of

aldehydes and ketones, the most accepted mechanisms are shown below:

'' RORCHHCORR +=→• (10)

'''''' ROCRRCORRR +=→ (11)

2ORCHOHRORCRCHOORR2 ++=→• ''' (12)

The first two reactions (Eqs. 10 and 11) are a chain scission step that forms two lower

molecular weight hydrocarbon fragments and the formation of an alkyl radical. These

reactions affect the physical properties of the oil quenchant by decreasing the viscosity,

increasing the volatility and decreasing the flashpoint, and finally, by increasing the polarity

of the quenchant. The alkyl radials are then free to react with oxygen in the propagation steps

above. The scission that occurs is very rare in quenchants, but can occur in other lubricants.

The last reaction above (Eq. 12) is a chain termination that consumes two peroxy radicals

and produces an alcohol and a carbonyl compound. The polarity of the hydrocarbon will

increase, but the molecular weight will be unchanged. This also means that the viscosity,

flashpoint will also be unchanged.

2.4. Formation of Carboxylic Acids

As the oxidation increases, the acid levels from the formation of carboxylic acids

increase. This increase leads to further oxidation because the carboxylic acids promote

oxidation. This is why, once oil starts to oxidize, it does so in an exponential fashion, with

the oxidation rapidly increasing. Carboxylic acids are formed by the oxidation of aldehydes

and ketones. This is illustrated in Figure 3. This is measured by ASTM D664. It is a

measure of the amount of organic acids present in the oil, and is useful for determining when

staining is likely, or the oil is reaching the end of its useful life. For most oil quenchants,

when the TAN (Total Acid Number) is greater than 1.5-2.0, it is indicative of of staining or

deposits on parts being quenched.

Figure 3. Oxidation of aldehydes and ketones to carboxylic acids. After Gatto et al[6]

2.5. Condensation Reactions

As oils become increasingly oxidized, whether in the quench tank, or in oxidation tests,

the viscosity increases. This occurs by condensation reactions that become important as the

levels of aldehydes and ketones increase. These reactions are known as Aldol

Condensations9, and are illustrated in Figure 4.

Figure 4. Condensation of ketones during quench oil oxidation. After [6]

These reactions are the primary source of viscosity increases in quench oils. The

formation of low molecular weigh polymers and other hydrocarbons are the result of the

“Aldol Condensations”, and can result in staining and other deposits. The viscosity of the

quenchant increases as the number of condensation reactions increase. This would occur

with severely oxidized mar-tempering oil.

2.6. Sludge and Deposit Formation

The condensation products described in the above section have a limited solubility in the

quenchant. These are high molecular weight oligomers. These are molecules that have a few

monomer units, in contrast to a polymer that can have an unlimited number of monomers. As

oil oxidizes the amount of carboxylic acids will increase. These acids are very effective

catalysts for Aldol condensation reactions. These then convert the low molecular weight

carbonyl compounds into higher molecular weight oligomers:

( ) ( ) ( ) ( ) OHRCHRCCHRCOCROCCHRR2 222 +==→= ''' (13)

As the reactions progress, chemical changes in the oligomers will result in making them

insoluble in the quench oil. At this point the insoluble oligomers will precipitate from the

quench oil and create sludge on the bottom of the quench tank, and deposits on the hot metal

part.

The higher kinetic rate of aromatic group oxidation increases sludge and deposits.

Because paraffinic oils have fewer aromatic groups than napthenic oils, paraffinic oils are

preferred for quenchants. Metal scale and soot, base oil sulfur, additive sulfur can also

promote the formation of sludge and deposits. Soot can also act as a nucleation site for the

formation of oligomers formation, resulting in oligomers coated soot particles. These

accumulate at the bottom of the quench tank in low velocity areas, and are deposited on parts.

2.7. Solvent-Refined versus Hydro-cracked Oil Oxidation

Both solvent-refined oils and hydro-cracked oils are used as the base stock for oil

quenchants. These differences result in differences in the oxidation process. As a general

rule, hydro-cracked oils have very low aromatic content, and very low sulfur and nitrogen

contents. Removing the aromatics, nitrogen and sulfur changes the oil oxidation in the

following ways:

• From above, the removal of aromatic compounds improves the oxidative stability

of the quench oil because aromatic compounds are more easily oxidized than

saturated hydrocarbons.

• Polar oligomers are more soluble in solvent-refined oils than in saturated hydro-

cracked oils. This means that sludging and the formation of deposits is easier in

hydro-cracked oils. They will also form sludge and deposits more readily.

• Removal of the sulfur and nitrogen hetrocycles improves the oxidative stability

because these hetrocycles can catalyze further oxidation.

• While aromatics, sulfur and nitrogen increase the susceptibility to oxidation, they

also oxidize into by-products that inhibit oxidation. This means that solvent

refined oil will oxidize more rapidly, but will not form oligomers as quickly. The

viscosity of the oil will not thicken as quickly.

• Solvent-refined oil will increase in viscosity at a gradual rate. This is because

oxidation is partially suppressed by naturally occurring anti-oxidants present in

the oil. Hydro-cracked oil will show very little initial oxidation or viscosity

increase. However, one oxidation begins; very rapid viscosity increases are

observed.

3. EXPERIMENTAL PROCEDURE

The purpose of this work was to demonstrate the effect of base oil on the oxidation

behavior, and to illustrate the power of additive packages to slow down the oxidation of

quench oil.

In this work, we used two different base stocks described in Table 1. Separate

formulations, containing identical anti-oxidant packages at identical concentrations were also

prepared.

Table 1. Paraffinic Base oils used for oxidation testing

Property Test Method Base Oil A Base Oil B

Type N/A Severely

Hydrotreated

Solvent-

Refined

Color ASTM D1500 0.5 4.0

Viscosity, cSt @ 40°C ASTM D445 101.6 112.3

Viscosity Index ASTM D2270 101.0 95.0

Flash Point, °C ASTM D92 270 266

Oxidation testing was conducted using a modified oxidation test after a similar AFNOR

used for oxidation testing of quench oils. In this test, a special high temperature Erlenmeyer

flask containing 300 ml of sample is immersed in a silicone oil bath at 180°C. No copper or

iron catalyst is used. The high temperatures used results in a very rapid test for oxidation.

This way a true understanding of the oxidation of the quench oil can be obtained without the

presence of catalysts. The apparatus used is shown in Figure 5.

Oxidation was measured by Viscosity

(ASTM D445), Total Acid Number (ASTM

D664), and by FTIR. Cooling curves at 121°

were compared for each of the two oils

containing additives.

Samples were taken at 24 hour intervals.

4. RESULTS

The results of the cooling curve testing

are shown in Figure 6. It can be seen that

the cooling curves are virtually identical.

The base oils selected and additive packages

used provide identical results.

The results of the oxidation testing by

viscosity are shown in Figure 7. The results

of the Total Acid determination resulting from oxidation testing are shown in Figure 8.

As can be seen from Figure 7, Base Oil A showed a delayed onset of viscosity increase,

followed by a rapid increase in viscosity. This type of behavior is typical of hydro-treated oil.

Base Oil B showed a gradual increase in viscosity, typical of solvent refined oil, with a lower

ending viscosity than Base Oil A.

The effects of the addition of the antioxidant package resulted in a gradual increase in

viscosity, and showed similar ending results.

Figure 5. Apparatus used for oxidation

testing.

The TAN testing mirrored the viscosity testing. Base oil A showed a delay and an initial

resistance to carboxylic acid formation, followed by rapid generation of these organic acids.

Base Oil B showed a gradual increase in build-up of carboxylic acids. The addition of the

antioxidant packages to both Base oil A and B, showed a delayed onset in the formation of

carboxylic acids, followed by a slow gradual increase in TAN. The resultant ending TAN of

both oils containing the additive packages was similar, with Base oil B having a slightly

lower ending TAN.

5. CONCLUSIONS

In this paper the mechanism of the oxidation of quench oil was discussed. An example

of oxidation testing showing the effect of base oil selection on the oxidation of oil, and the

proper additive package was illustrated.

Figure 6. Results of cooling curve testing per ASTM D6200. Testing was

performed at 121°C with no agitation.

Figure 7. Viscosity increase of the two base oils and the two base oils with identical

additive packages.

Figure 8. Effect of oxidation on the increase in Total Acid Number of two different base

oils and the two base oils with identical additive packages.

6. ACKNOWLEDGEMENTS

I wish to thank the management of Houghton International, Valley Forge for allowing

me to present this paper on oil oxidation. I would also like to thank Remy Napierala

(Houghton-Spain) for performing the oxidation, FTIR, and viscosity testing. I would also

like to thank the Croatian Society for Heat Treatment and Surface Engineering for inviting

me to Dubrovnik to present this work.

7. REFERENCES

1 G. Totten, C. Bates, N. Clinton, “Handbook of Quenchants and Quenching Technology”, ASM International,

Metals Park, OH, 1993 2 Scott, G., Atmospheric Oxidation and Antioxidants, Elsevier Publishing Company, Amsterdam, 1965.

3 Shlyapnikov, Y. A., “Antioxidant Stabilization of Polymers,” Russ. Chem. Rev., Vol. 50, 1981, pp.581–600.

4 Colclough, T., “Lubricating Oil Oxidation and Stabilization,” Atmospheric Oxidation and AntioxidantsVolume

II, G. Scott, Ed., Elsevier Science Publishers B. V., Amsterdam, 1993, pp. 1–70. 5 Rasberger, M., “Oxidative Degradation and Stabilization of Mineral Oil Based Lubricants,” Chemistry and

Technology of Lubricants, R. M. Motier and S. T. Orszulik, Eds., Blackie Academic and Professional, London,

1997, pp. 98–143. 6 V. Gatto, W. Moehle, T. Cobb, and E. Schneller, “Oxidation Fundamentals and Its Application to Turbine Oil

Testing”, J. ASTM, April 2006, V3 #4, p.1 7 AL-Malaika, S., “Autoxidation,” Atmospheric Oxidation and Antioxidants Volume I, G. Scott, Ed., Elsevier

Science Publishers B. V., Amsterdam, 1993, pp. 45–82. 8 Uri, N., “Mechanism of Autoxidation,” Autoxidation and Antioxidants Volume I, W. O. Lundberg, Ed., John

Wiley & Sons, Inc., New York, 1961, pp. 133–170. 9 March, J., “The Aldol Condensation,” Advanced Organic Chemistry, 3rd ed., John Wiley & Sons,

Inc., New York, 1985, pp. 829–834.