Heat Treating: Selecting the Right Quenching Oil

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Quenching – Mastering the Process

D. Scott MacKenzie, PhD, FASMNovember 2011

Quenching Oil Selection

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• Real loads are complex– Important to meet properties

and reduce distortion– No quantitative method to

understand interaction of fluid flow and part

– Often understanding is “Trial and Error”

– Experience trims the Heuristic tree

– Heat transfer mechanism is complex

• Multiple heat transfer regimes

• Time and Temperature dependent phase transformations

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• Mechanism of Quenching– Quenching occurs in three stages

• Vapor Phase– Formation of vapor film around the part– Heat transfer is slow– Heat transfer occurs primarily through radiation and conduction through

vapor• Nucleate Boiling Phase

– High heat extraction rates– Heat removal by bubble formation and contact of cool quenchant on

part surface• Convection Phase

– Starts at below boiling temperature of quenchant– Slow heat transfer

– Governs Properties and Distortion

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Vapor Phase NucleateBoiling Phase

Convection Phase

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• Metallurgical Effects– Carbon Content and Hardenability

• Avoid the nose or knee of the TTT curve• Cooling rate depends on hardenability of steel• Maximum hardness attainable is dependant on % Carbon

present– Cooling Rates

• Cooling rate is limited by thickness of part– Limited by thermal diffusivity– Excessive cooling rates may cause cracking or distortion– Higher cooling rates yield higher thermal gradients

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• Quenching Mediums– Many different types

• Water• Brine• Caustic• Polymer• Oils• Molten Salts• Gases

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Quench Oil Composition - Overview

• Composition governed by– Price– Stability– Quenching performance

• Quenching performance governed by– Acceptable flash temperature– Low sludge forming tendency– Thermal and oxidation stability– Appropriate heat transfer

charectoristics• Best quench oils

– Maximum cooling rate to achieve maximum hardness

– Minimal deposition of sludge– Maximum thermal and oxidation

stability• Presently favors mineral oils

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Quench Oils Composition - Overview

• Composition affects quenching performance– Straight mineral oils exhibit wide variation in quenching

performance– Formulated oils can produce even wider range of cooling rates

• Volatility of quench oil– Decreases as average molecular size increases– Inversely proportional to flash point

• Quench severity directly related to “wettability”– Wettability is measured by contact angle– As viscosity increases

• As contact angle decreases Cooling rate is decreased– Additive packages also change wettability characteristics

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• Selection of proper quench oil is dependant of many factors:

– Part• Alloy• Geometry and section size• Metallurgical Properties• Distortion• Cleanliness

– Furnace• Type of Furnace• Method of Quenching• Temperature

– Cost• Initial Cost• Long Term Costs

– Additives– Staining– Drag-out– Environmental

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• Divided By operational temperature

– “Cold” Oils (80° - 200°F)– Martempering Oils (200° - 450°F)

• “Cold” Oils– Typically three different speeds

• Fast (7-9 GMQS)• Medium (10-13 GMQS)• Normal (14-16+ GMQS)

• Martempering Oils– Applicable to high alloy, or

carburized distortion prone parts– Slower than Cold Oils (Typically

20+ GMQS)

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• “Cold” Oils– Normal Speed

• Low rates of cooling• Applicable to high hardenability

steels– Tool and High-Alloyed Steels

– Medium Speed• Intermediate quenching

characteristics• Medium to high hardenability

requirements• Applicable to widest variety of

steels– High-Speed

• Applicable to low hardenability steels

• Carburized or Carbo-nitrided• Large cross-sections

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• Martempering Oils– Quenching workpiece into

quenchant maintained at about 100°-200°C

– Part maintained in quenchant until thermal equilibrium is established

– Substantially reduces distortion by reducing thermal gradients

– Special Formulations• Special Base Stocks

– High oxidation resistance– High thermal stability– High flash points

• Large variety of viscosities (200-2500 SUS)

• Complex anti-oxidant packages

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• Quench Oil Composition– Complex mixture of

constituents• Paraffinic• Naphthenic• Open-chain and Cyclic

Derivatives– Sulfur hetrocycles– Oxygen hetrocycles– Nitrogen hetrocycles

– Specific composition depends on source of crude (Western U.S., Eastern U.S., Middle East, North Sea, Venezuela, etc.)

– Compositional variation affects quenching performance

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Quench Oils Composition - Overview

• Naphthenic Grades– Exhibit inferior cooling

characteristics– Greater deposit-forming

tendency (staining)– High sludging tendency

• Oxidize at faster rate• Lower thermal stability

– Lower flash points• Paraffinic Grades

– Superior cooling curves– Lower staining tendency

• Oxidize at slower rates• Increased thermal stability

– Higher flash temperatures

References:

R-J Windgassen, Metalworking Fluids Today, Society of Tribologist and Lubrication Engineers, Feb 1989

G.E. Totton, C.E. Bates and N.A. Clinton, Handbook of Quenchants and Quenching Technology, ASM International, 1993

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Quench Oil Composition – Base Oils

• Base oils– Double Hydro-treated– Refined Paraffinic Mineral

Oils– Refined Napthenic Mineral

Oils– Re-refined mineral Oils– Reclaimed Mineral Oils– Vegetable and Ester-based

Oils• All are used in commercial

quenchants

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• Double Hydrotreated Oils– Advantages

• Thermal and oxidative stability

• Higher flash point• Infers longer life• Visual Appearance• Marginal improvement in

cooling curves for same viscosity

– Disadvantages• Cost

• Oil color not indicative of staining

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• Refined Paraffinic Base Oils– Most common used base oil– Advantages

• Better flash points than Napthenic oils• Good thermal and oxidative stability• Wide range of viscosities available (70-2500 SUS)• Lower staining (imparts light gray color – depends on additive package)• Hydrophobic

– Better splitting– Water displacing

– Disadvantages• Increasing cost

– Due to global demand– Local refinery shut-downs due to maintenance

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• Refined Napthenic Base Oils– Advantages

• Readily available• Cost• Wide variety of viscosities available (100-2500 SUS)

– Disadvantages• Less thermal and oxidative stability• Lower flash point temperatures• Lower life and quicker degradation• Hydrophillic

– Emulsifying– Not split well– Bad for quench oil due to water absorption

• Cost advantage decreasing due to global demand

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• Re-refined Paraffinic Base Oils– Oils are recovered and passed thru cracker

• Specific viscosities are recovered• Pulled from narrow heights from column

– Advantages• All advantages of paraffinic base oils

– Dependent on supplier/source and quality of re-refiner• Generally less expensive than virgin base oils

– Disadvantages• Limited source of supply

– High demand due to “eco-friendly”– Recycling reduces supply and availability– Dependent on supplier/source and quality of re-refiner

• Limited viscosity range (generally ± 100 SUS)• Cost advantage narrowing because of demand

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• Reclaimed Oils– Oils recovered from many sources

• Coolants• Motor oils• Greases

– Filtered and blended to give specific viscosity

– Advantage• Cheap

– Disadvantages• “You get what you pay for…”• Presence of heavy metals• Source unknown

– Multiple grades (including used motor oil, coolants, etc.)

– Different mix of light and heavy ends

– Viscosity is usually only requirement

• Lack of process repeatability• Poor life

– Precursors to oxidation present (free radicals)

– Mixture of blended products

• Overall process cost increases– More cleaning required– Higher drag-out– Reduced life

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• Vegetable and Ester-based Base Oils– Advantages

• “Eco-Friendly”• Renewable• Can be used at wide range of temperatures• Excellent cooling curve

– Unstable vapor phase– High nucleate boiling rates and temperatures– Curves often superior to mineral oils

• High flash temperatures• High thermal and oxidative stability

– Disadvantages• Limited supply

– Bio-fuels and cyclic availability (crop production)• Cleaning requires specialized cleaners• Different smell• Cost

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Quench Oil Composition – Additive Packages

• Desired results– High thermal and oxidative stability

• Low sludge and staining tendency• Formation of organic acids minimized as

oil ages– Low viscosity to minimize drag-out– Cooling curve

• High cooling rates to achieve maximum hardness

• Unstable vapor phase to achieve high cooling rates

– Avoid formation of upper transformation products

• High temperature transition between nucleate boiling and convection

– Minimize distortion and residual stresses

– Improve wetting

• Function of the additive package

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• Additive packages have two functions

– Speed improvers– Thermal and oxidative stability– Additives have dramatic effect on

quenchant properties– Magnitude of effect is dependent

on additive package• Packages usually proprietary• Not appropriate to mix additive

packages from different quenchant suppliers

• Some additives prone to selective drag-out after quenching

• May degrade during use, depending on robustness

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• Speed improvers– Quenching rates increase with

increasing wettability– Additives that increase wettability

improve quench speed– Different types of additives– Most common is sulfonate-type

• Barium sulfonate was most common– No longer used– Older quench tanks may still contain

Barium• Now Sodium and Potassium sulfonates

used• Good speed improvers and oxidation

stabilizers– Hydrocarbon based

• Cost• Used in high-end quenchants• Robust

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• Thermal and Oxidation Stability– Provided by additive package– Reduces production of sludge– Minimizes formation of

organic acids upon aging– Additive package usually

chosen to act as both speed improver and thermal stability

– May be consumed over time.– Oil generally dumped because

of staining

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• Oxidation– Results from build-up of

organic acids– Measured by several methods

• Precipitation Number• Total Acid Number• Sludge Content• Viscosity

– Changes cooling curve• Increases speed • Decreased vapor phase

stability– Increased staining

• Additive package minimizes changes

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Oil Degradation

• Quench oils degrade– Exposure to high temperatures– Aggravated by

• Residues on parts• Washer residues from reclaimed• High watt density heaters• Excessive peak temperatures

– Robust additive packages prolong quenchant life

• Exhibited by:– Part staining– Higher quench rates

• Monitored– TAN– Viscosity

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Oil Degradation

The presence of calcium and sulfur in the stain shows that the material is baked on quench oil. Oil is near end of life.

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Quenching Products from Houghton

Cold Quenching OilsHoughto-Quench KHoughto-Quench GHoughto-Quench 3440Houghto-Quench 3430Dasco Quench LPA 15Dasco Quench LBA 15

Aqueous QuenchantsAqua-Quench 140Aqua-Quench 145Aqua-Quench 245Aqua-Quench 251Aqua-Quench 260Aqua-Quench 3699Aqua-Quench C

Email [email protected] more information.

Hot Quenching OilsMar-Temp 355Dasco Quench MPA 60

Below is a sampling of Houghton quenchants. However, you should consult a Houghton expert for the right product for your application.

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