heat treating: the how and why of quenching metal parts

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Quenching – Mastering the Process

D. Scott MacKenzie, PhD, FASMDecember 2011

The How and Why of Quenching Metal Parts

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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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1. Moment of immersion – vapor film around probe2. After 5 seconds – boiling commences at corners3. After 10 seconds – boiling front moves along the probe4. After 15 seconds – showing vapor. Boiling and convection phases5. After 30 seconds – 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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• Austenite will transform if held at a lower constant temperature

– Lower than austenite stability temperature

– Percent transformation plotted by time versus temperature

– Performed at many temperatures– Summarized in a single diagram– Called TTT (Time-Temperature-

Transformation) Diagram• A “map” that charts austenite

transformation as a function of time and temperature

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• TTT Diagrams– Time-Temperature-Transformation– Also called Isothermal Transformation (IT) diagrams– Shows the time required for transformation

• Start and finish times• Diagram created

– Small specimens austenitized in molten salt– Quenched into molten salt at different temperature for a specific time– Quenched in water– Metallographically examined and volume fraction of constituents are

determined.• Curves widely available• Permits estimates of microstructure for times and temperature

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• Diagram is simplified– Really a mixture of curves

• Bainite, pearlite curves are usually shown as one curve• Really overlapping several curves should be shown

– Sufficient to understand microstructures• Isothermal transformations are understood• Pearlite, upper Bainite, lower Bainite and martensite

transformation fractions can be estimated.• Allows estimated times and temperatures to be estimated for

heat treatment

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• TTT Diagram for plain carbon eutectoid steel– A - 0.01 volume fraction

Pearlite– B – 0.99 volume fraction

Pearlite– C – 0.01 volume fraction upper

Bainite– D – 0.99 volume fraction upper

Bainite– E – 0.01 volume fraction lower

Bainite– F – 0.99 volume fraction lower

Bainite– G – 0.01 volume fraction

Martensite

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• Shape and Position of Curves– Depends on composition, grain size and alloying

elements• Increasing carbon tends to retard transformation• Increasing alloying elements tends to retard transformation• Increasing grain size tends to retard transformation

– Retarding transformation shoves “nose” or “knee” to the right

• Greater hardenability or deepen harder

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Atlas of Isothermal Transformation and Cooling Transformation Diagrams, ASM, 1977

AISI1020

AISI1050

AISI1080

AISI1095

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AISI 1060 AISI 5160

C-0.63% Mn–0.87% C-0.61% Mn–0.94% Cr-0.88%

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• Quenching and Tempering– Most common method of

heat treating• Heating to Austenite

region, then rapidly cooling to miss “knee” to avoid transformation

• Center and surface curves shown

• Transformation of austenite completely to Martensite

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• Martempering– Method is used to limit distortion and

residual stresses in parts– Process

• Part is quenched to intermediate temperature at approximately the Martensite (Ms) temperature

• Held until center of piece reaches bath temperature

• Cooled in air or convenient manner• Determines how long part is held at

temperature before formation of Bainite– Widely used to control distortion in

gears and other dimension critical parts

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• Austempering– Process of isothermal

transformation of Austenite to Bainite

– For this process, TTT diagram is indispensable

– Shows the minimum time for Austenite to Bainite transformation

– Used for planning austempering heat treatments, and determining time at temperature

– Method used for austempering ductile irons and similar

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• TTT Diagrams cover isothermal transformations– Real heat treat processes cover a range of temperatures– Mixture of isothermal transformation products– Useful in planning heat treatments– Can not be used to accurately predict course of transformation

during cooling• CT (Continuous Cooling Transformation) Curves

– Austenite transformations shifted lower and to the right– Determined primarily by empirical data

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• Derivation of CCT Diagrams– Early attempts were unsuccessful.– Originally superimposed end-

quench data on TTT diagram to show downward shift.

– Now determined by hardness and metallographic data from Jominy End Quench

– Dilatometer and Metallography used extensively

– Instrumented rounds of material also used

– Designed to estimate hardness of round bars

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• Alternatively– Time versus temperature plot

used.• Hardness is displayed at the

end of cooling curve• Difficult to predict time effect

of higher austenitizing temperatures

– Cooling rate at 1300°F (704°C) used

• Based on Jominy• Good for pearlite

transformations• Not as good for Bainitic

transformations

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• Martensitic microstructures are the hardest in any carbon steel– Only produced if ferrite and cementite is avoided

• Section discusses– Relationship of carbon to martensite hardness– Discusses hardness can be achieved throughout a part– Hardenability term intoduced

• Ease of martensite formation• Effect of section size, cooling rates and composition

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• Hardness– Function of Martensite content

• Hardness of martensite much higher than Pearlitic structure

• High hardness and high strength is reason why Martensitic structures are preferred

– Retained Austenite formed at high carbon contents

• Ms temperatures drop below room temperature

• Low temperature treatments to force transformation is done

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• Grain size effects attainable hardness– Smaller grain size

increases hardness– Associated with carbon

segregation making yielding more difficult

• Finer structures have shorter differences

• Less ability to yield

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• Hardenability– “Ability to harden deeply” not “Ability to Harden”– The depth and distribution of hardness produced by

quenching– Official Definition:

• “The capacity of a steel to transform partially or completely from Austenite to some percentage of Martensite at a given depth when cooled under some given conditions.

– Affected by cooling rates, composition and grain size

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• Factors Affecting Cooling Rates– Two factors

• Ability to diffuse heat out of the part (thermal diffusivity)• Ability of the quenching medium to remove heat.

– Thermal Diffusivity• Ability of steel to transfer heat• Changes as a function of temperature• Very little change as a function of composition

– Quenchant• Most important control over cooling rate• Complex process

– Depends on radiation, boiling and forced and unforced convection– Agitation, quenchant temperature and concentration (if polymer quenchant)

primary factors• Important practical considerations

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• Severity of Quench– Effectiveness of quenchant ranked by parameter called “Severity of

Quench”• Measure identified by “H”• Determined experimentally by quenching a series of round bars• 50% Martensite region determined (dark/light etch transition)

– Determination of “Severity of quench” makes possible:• Calculation of critical size in terms of standardized quench;• Calculation of critical size from a single test• Predicting how a know steel would behave under different severity of quench• Predicting hardness distribution

– How to do it?• Quenching varies more than suspected• No way to translate results from laboratory to field, or to other quench.

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• Different bar diameters quenched in different quenchants – Unhardened diameter to

bar diameter determined (Du/D)

– Ratios plotted as a function of bar diameter

– H-Value determined• Water quenching

generally is given as H=1

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Air Oil Water BrineNo circulation of fluid or piece 0.02 0.25-0.30 0.9-1.0 2Mild circulation or agitation 0.30-0.35 1.0-1.1 2-2.2Moderate agitation 0.35-0.40 1.2-1.3Good circulation 0.40-0.50 1.4-1.5Strong agitation 0.05 0.5 - 0.8 1.6-2.0Violent Agitation 0.8 - 1.1 4 5

Severity of Quench (H) for Various Quenching Media

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• Ideal Diameter– Requires definition of “Ideal Quench”

• Fastest possible quench, where surface of the bar would be cooled instantly to the temperature of the quenchant.

• Handled simply by heat transfer calculations– Fastest Quench, then the largest size obtainable with a

specific hardenability• Basis of comparison• Called “Ideal Critical Size”, or DI• Reliable method and used for hardenability calculations and

specification purposes.• Can be determined experimentally by quenching bars• Can be determined by composition.

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• Jominy End Quench– Developed by Jominy– Characterizes hardenability of

a steel from a single bar– Simple test

• Specimen cooled at one end• Provides cooling rates

between water quenching and air cooling

• After quenching, parallel flats are ground on each side

• Hardness readings taken at 1/16” intervals

– Hardness differences can be readily compared

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• Standardized Test– ASTM A 255– SAE J406

• Each position corresponds to a specific cooling rate– Cooling rate determines

percentage of Martensite– Data can be used to predict

hardness if cooling rates known

• Test is highly accurate and repeatable– Excellent method for selecting

steels

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Timken “Practical Guide for Metallurgists

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• Severity of Quench– Effectiveness of quenchant ranked by parameter called “Severity of

Quench”• Measure identified by “H”• Determined experimentally by quenching a series of round bars• 50% Martensite region determined (dark/light etch transition)

– Determination of “Severity of quench” makes possible:• Calculation of critical size in terms of standardized quench;• Calculation of critical size from a single test• Predicting how a know steel would behave under different severity of quench• Predicting hardness distribution

– How to do it?• Quenching varies more than suspected• No way to translate results from laboratory to field, or to other quench.

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From “Practical Data for Metallurgists”, Timken Company, 8th Edition

For ½” Bar, the expected values are:Surface: JEQ 1/16 = 62HRCCore: JEQ 2/16 = 59 HRC

For 2” Bar, the expected values are:Surface: JEQ 2/16 = 59HRCCore: JEQ 7/16 = 31 HRC

For 3” Bar, the expected values are:Surface: JEQ 4/16 = 38HRCCore: JEQ 12/16 = 21 HRC

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• Quenching Mediums– Many different types

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

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• Water– Approaches maximum cooling rate attainable– Inexpensive and readily available– Easily disposed– Widely used

• Non-ferrous parts• Stainless steels• Large forgings

– Agitation important to break up persistent vapor phase– Contamination can change cooling rate

• Emulsions, oils, soaps decrease speed• Salts or hard water increase speed

– Disadvantages• Rapid cooling near typical Ms temperatures• Usually restricted to small parts• Large parts when core hardness is important

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0

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0 10 20 30 40 50 60

Time, Seconds

Te

mp

era

ture

, De

gre

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C

20 C 40C 60C

20C 40C 60C

Hard WaterDistilled Water

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• Brine– Applies to water solutions of salt

(NaCl, KCl or CaCl2)– Advantages

• Faster cooling rate than water• Temperature less critical• Reduced soft spots from steam

pockets• Equipment is simplier

– Disadvantages• Corrosive• Hood required for corrosive

fumes• Increased cost• Increased testing for

concentration

From “Houghton on Quenching”

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• Quenching Systems– Equipment varies over a

wide range• Depends on size• Product requirements

– Typical system consists of:• Tank• Agitation equipment• Fixtures• Cooling Systems• Heaters• Filtration Equipment

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• Tanks– Rule of thumb

• “One pound of parts – one gallon of quenchant”

– Holds the quenchant– Auxiliary Equipment

• Agitation• Heaters• Filtration

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• Agitation– Critical for quenching

uniformity– Reduces surface to surface

thermal gradients– Must provide uniform flow

thru-out furnace load– Wipe vapor blanket from

parts to achieve quench uniformity

– Racking and Agitation work together

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• Agitation Equipment– Pumps

• Often selected• Simple• Easy to direct flow

– Propellers• Most efficient method of moving

quenchant• Compact• Require little piping

– Velocity Effect• Recommend 0.5-1.0 m/s• Dense loads may require up to 2

m/s– Number of Agitators

• Reduces dead spots• Provide more uniform agitation• Baffles improve flow uniformity

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• Achieving necessary agitation– Requires adequate horse

power to drive agitators– Most design rules based on

marine propellers– Polymers more sensitive to

agitation than oil

• Uniformity of agitation is just as important

Volume - Tank GallonsHP per Gallon

Oil Water

50-800 0.005 0.004

800-2000 0.006 0.004

2000-3000 0.006 0.005

Greater than 3000 0.007 0.005

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• Racking– Very critical to get part

properties and low distortion

– Must allow proper flow of quenchant around part

• Minimize oil hot spots• Uniform heat transfer on

all surfaces– Specialized racks and

fixtures often expensive• Life• Alloy cost• Well worth cost of

grinding and distortion

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• Quenchant Heating– Achieved by several methods:

• Gas• Electrical

– Maximum rating on heaters should not exceed 10 watts/in2

• Locally overheat quenchant• Cause premature failure of

quenchant and heating elements

• Good quench oil flow should be maintained around heaters

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• Quenchant Cooling– Use of water cooled heat

exchangers is not recommended

• Risk of water contamination

– Air-Oil Heat Exchangers recommended

• Recommended to be sized to recover maximum heat load in the time of one heat-treat cycle

• Filtration is suggested prior to heat exchanger

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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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heat treating: the how and why of quenching metal parts

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