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Presented by Weldon Mak Makela

Senior Failure Analysis Engineer

Materials Testing & Analysis Group, Element St. Paul

Heat Treating Basics

Heat Treating 1July 26, 2012


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Future Topics for webinars

Metallurgical Failure Analysis for Problem Solving-Dec. 4, 2011

Carbon and Low-Alloy Steels-April 26, 2012

Heat Treating-July 26, 2012

Stainless Steels

Tool Steels

Aluminum Alloys Surface Engineering

Corrosion

Heat Treating 2


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Heat Treatment

What is heat treatment?

Hardenability.

Heat treatments to strengthen or harden an alloy.

Through hardening.

Surface hardening.

Precipitation hardening. Tempering.

Heat treatments to lower strength or soften an alloy.

Heat treatments for welding.

Heat Treating 3


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Heat Treating 4

Sources

Metals Handbooks, 10thEdition, Volume 4: Heat Treating, ASM

International, 1991. Isothermal Transformation Diagrams, United States Steel Corporation,

3rd Edition, 1963.

Grossman, M. A. and Bain, E. C., Principals of Heat Treatment,

American Society for Metals, 1968.

Welding Handbook, 8thEdition, Volume 1: Welding Technology,American Welding Society, 1991.

Metals Handbook, 9thEdition, Volume 6: Welding, Brazing and

Soldering, ASM International, 1983.


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Heat Treatment Definition

Any thermal treatment to alter the existing mechanical properties of a

metal or alloy.- Increase strength harden the material.

- Decrease strength soften the material.

- Through harden or surface harden a material.

- Toughen the material-tempering.

- Stress relieve to remove residual stress.

- Intermediate anneals after cold working to soften the material or for

grain refinement.

- Pre-heating or post-heating for welding processes.

Heat Treating 5


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Iron-Carbon Phase Diagram

Heat Treating 6


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Heat Treating 7

Iron-Carbon Phase Diagram


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Heat Treating

Hardenability of Steels

The ability to harden or strengthen a steel through heat treatment by

quenching from the upper critical temperature to:

- Form martensite.- Form bainite.

- Quenching is a rapid cool from the upper critical temperature intended to

miss the nose of the time-temperature-transformation curve.

Hardenability is measured as the distance below the surface where:- The metal exhibits a specific hardness.

- The microstructure contains 50% martensite.

Hardenability varies as a function of:

- Carbon content.

- Manganese content.

- Other elements such as chromium, nickel and molybdenum.

- Quench media and cooling rate.

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Heat Treating

Hardening Steels by Quenching and Tempering

Time-Temperature Transformation Diagram:

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Quench and Tempering to Strengthen Steel

Quench and temper to form martensite-the strongest possible structure insteels.

- Austenizing, at a temperature above A3, transforms the bcc structure to fcc.Ferrite and pearlite are dissolved to form austenite.

- Rapidly quenching to below the Ms temperature starts to form martensite.Cool to below MF to allow complete transformation to martensite.

- Martensite formation is a diffusionless transformation. The structure istetragonal and forms rapidly.

- Complete transformation requires cooling through the transformationtemperature range, to below MF. The resulting martensite is brittle and calleduntempered martensite.

- Tempering is a process of reheating to a low temperature to toughen thesteel.

Quenching can be in oil, plain water, salt water, polymers, salt baths, or airdepending on the composition or alloy content of the steel.

- Carbon steels will almost always require a water quench to form martensite.

- Alloy steels are usually quenched in oil, polymers or salt baths.

- Some tool steels harden by cooling in still or agitated air.

Heat Treating 10


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Heat Treating 11

Martempering to Strengthen Steel

Martempering, or marquenching, is a variation of the quench and

temper and consists of austenizing, quenching and tempering.- The quench is interrupted to hold the part(s) just above the Ms

temperature to allow parts to equalize in temperature. This reduces

stress and distortion. The parts are then quenched below Ms to form

martensite.

- Tempering is necessary to toughen the martensite.


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Heat Treating 12

Martempering Transformation Diagram


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Heat Treating 13

Annealed SAE 1045 Medium-Carbon Steel


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Heat Treating 14

Tempered Martensite


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Austempering to Strengthen Steel

Austempering forms bainite instead of martensite.

- Bainite is a slow isothermal transformation from austenite.

- Bainite has increased ductility and toughness at the same strength levels as martensite.- Bainite has reduced distortion and residual stress which lowers subsequent processing

costs.

- Austempering provides the shortest cycle time to through-harden within the hardness

range of Rockwell C 35-55 HRC.

- Not all steels can be austempered.

Austempering consists of the following processing steps.- Heating to a temperature above A3 to transform the microstructure to austenite.

- Quenching to a temperature above the Ms temperature and holding for a period of time to

transform the austenite to bainite.

- No tempering is required.

Steels for austempering:

- Plain carbon steels with carbon between 0.50-1.00%, and manganese 0.60%.

- Carbon steels with manganese 1.00% and carbon slightly less than 0.50%.

- Alloy steels with carbon 0.30% such as 5100 series.

- Alloy steels with carbon 0.40% such as 1300 to 4000 series and 4140, 6145, 9440.

Heat Treating 15


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Heat Treating 16

Austempering Transformation Diagram


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Heat Treating 17

Precipitation Hardening

Precipitation hardening is the process of heating an alloy to a high

temperature to transform all alloying elements and dissolve all

compounds in the microstructure to a single homogeneous phase. The alloy is then rapidly quenched to room temperature, retaining all

alloying and compound forming elements in a metastable condition.

Strengthening, or hardening, occurs by low temperature aging where

sub-microscopic particles are uniformly precipitated throughout the

microstructure.

- These particles substantially strengthen the material.

- Precipitation can occur over time at room temperature in some

alloys.

Carbon and alloy steels can not be precipitation hardened.

Some aluminum, titanium, nickel, cobalt and copper base alloys are

precipitation hardenable.

One group of stainless steels are precipitation hardenable.


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Surface Hardening to Increase Strength

Case hardening - a process that hardens the surface of steel without altering the surface

chemistry. The carbon content remains constant.

Diffusion hardening - carburizing, nitriding, carbo-nitriding are processes that harden the

surface by changing the chemical composition of the surface. Either carbon, nitrogen, or

carbon and nitrogen are diffused into the surface, thus altering the surface chemistry.

De-carburizing - a process where carbon has diffused from the surface resulting in a

lower carbon content, thus softening the surface of steel. The hardenability of thesurface is reduced.

We wont discuss coatings that increase the surface hardness such as:

- Plating.

- Hard facing.

- Thermal spraying.

- Vapor deposition.

Heat Treating 18


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Case Hardening

Localized heating, quenching and tempering of the surface to produce a hard layer ofmartensite relative to the interior of the part.

- The material could be through hardened.

- The chemical composition of the material is not changed.

Flame hardening-heating is accomplished using oxyacetylene or similar torches touniformly heat the surface to the austenizing temperature.

- Useful for hardening large parts or specific areas of parts.

- Underlying metal structure is not altered.- Requires operator skill.

Induction hardening-heating the surface of a material with induced magnetic fields.

- Frequency determines depth of heating.

- Very uniform and repetitive.

- Easy to automate.

Laser or electron beam energy can also be utilized to surface harden steels.

Heat Treating 19


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Heat Treating

Diffusion Hardening All diffusion hardening processes produce a thin, hard, wear resistant case at the surface

of a carbon or alloy steel. The chemical composition of the surface is altered.

Carburizing - the process of diffusing carbon into steel at a temperature above A3 to

increase the carbon content at the surface.

- Usually low-carbon content steels are carburized. Carburizing changes the surface

chemistry of a low-carbon content steel to a medium or high carbon content steel.

- Requires a high-carbon source to be in intimate contact with the surface.

- Carburizing is time-temperature-concentration dependent.

- Occurs at 1600-2000F when steel is austenitic. After carburizing, the steel is

quenched and tempered to produce the hard surface layer.

- Gas or liquids are common sources of carbon during carburizing.

- Increased surface hardness improves wear resistance and fatigue strength.

De-carburizing - the process where carbon diffuses out of the surface of steel.

- Carbon at the surface reacts with oxygen to form carbon dioxide.- The hardenability is lowered resulting in a soft surface.

- Fatigue strength and wear resistance of the material are reduced.

- Results in poor response to heat treatment.

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Heat Treating 21

Carburized and De-carburized Microstructures

Carburized Steel 100X De-carburized Steel 200X


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Diffusion Hardening continued

Gas Nitriding - the diffusion of nitrogen into the surface to form stable nitrides

with alloying elements.

- Steels that contain chromium, vanadium, tungsten, molybdenum oraluminum will allow forming of stable nitrides.

- Plain carbon steels are not well suited for gas nitriding because the iron

nitrides form a brittle case that will easily crack and spall. The hardness

increase is slight.

- Nitriding produces a white layer* on the surface which is very hard and

brittle.

- Low temperature process occurs at 925-1050F below any transformation

temperature.

- No phase change occurs, the structure is ferrite and pearlite.

- Since the process is at a low temperature, distortion is usually minimal.

*The white layer is only visible by metallographic examination of the microstructure.

Heat Treating 22


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Heat Treating 23

Gas Nitriding continued

Steels amenable to gas nitriding:

- Medium-carbon, chromium containing low-alloy steels such as the4100, 4300, 5100, 6100, 8600, 8700, and 9800 series.

- Hot-work die steels containing 5% chromium.

- Low-carbon, chromium-containing low-alloy steels such as the 3300,

8600, and 9300 series.

- Air-hardening tool steels such as A-2, A-6, D-2, D-3, and S-7.

- High-speed tool steels such as M-2 and M-4.

- Most stainless steel compositions.


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Heat Treating 24

Diffusion Hardening continued

Carbo-nitriding - the diffusion of carbon and nitrogen into the surface.

There are three variations of carbo-nitriding:- Between 1400-1600F, the low-alloy steel has transformed to

austenite.

- Between 1250-1450F, the steel can be partially austenitic but mostly

consisting of ferrite and pearlite.

- Between 1050-1250F, the steel consists of ferrite and pearlite. Theprocess is called nitro-carburizing.


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Tempering of Steels Tempering is a low-temperature heat treatment to substantially toughen

untempered martensite.

Untempered martensite is brittle with very low toughness.

- Low ductility exhibiting low elongation and reduction of area.

- Exhibits no necking prior to fracture.

Tempering is a time-temperature relationship:

- Low temperature-longer tempering time.- Higher temperature-shorter tempering time.

Mechanical properties after tempering are affected by:

- Tempering temperature.

- Time at temperature.

- Composition of the steel - carbon content, alloy content.

Tempering also relieves residual stress from quenching, welding, and

cold working.

Heat Treating 25


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Quenched & Tempered Hardness vs. Carbon Content

Rockwell C

Hardness, HRC

Ultimate Tensile

Strength, ksi.

55 301

50 255

45 21440 182

35 157

30 136

25 120

20 108

Heat Treating 26


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Tempering Problems

Temper embrittlement can occur if:

- The steel is cooled slowly from temperatures above 1065F.

- The steel is held between 700-1065F for long time periods.

- The result is a reduction in impact strength.

- The brittleness may be caused by precipitation of trash elements tothe grain boundaries. Trash elements are P, S, Sn, Se, As, etc.

- The original properties may be recovered through re-heat treatment.

Blue brittleness is caused by heating carbon and some alloy steels tothe temperature range between 450-700F. A precipitation hardeningeffect occurs.

- Results in increased tensile and yield strength.

- Results in lower ductility and impact strength.

- May be recovered by re-heat treatment.

Heat Treating 27


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Heat Treating 28

Tempering Problems continued

Tempered martensite embrittlement can occur if:

- Impurities, such as P, segregate to grain boundaries.- Cementite segregates to grain boundaries during tempering.

- Cementite forms between parallel martensite laths.

- Occurs between 480-570F.

- Avoid tempering between 390-700F if the alloy composition contains

high phosphorous or contains chromium as an alloy addition.

- Re-heat treatment and tempering outside the zone will recover full

properties.


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Heat Treating 29

Tempered Martensite Embrittlement


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Heat Treating

Cold/Cryogenic Treatments of Steel Cold temperature treatment after martensite transformation:

- Transforms remaining austenite to martensite.

- Optimum temperature is -120F.- Cold treating is typically done after tempering.

- Time at temperature (1 hr/inch of thickness) and warm-up rate are not critical.

- Different steels and part sizes/shapes can be mixed.

- Further tempering improves toughness and stress relieving of parts.

- Improves wear resistance because there is no retained austenite.

Cryogenic treatment of steels:

- Temperature is much lower, approaching -320F. Cool down must be slow.

- Soak time is approximately 24 hours. Warm up rate is not critical.

- Incomplete understanding, therefore some disagreement, of mechanismsoccurring in steels.

- The process appears to enhance wear resistance significantly.

- Evidence shows some improvement in corrosion resistance.

- Each application must be tested.

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Heat Treating 31

Heat Treatments to Soften Steel

Annealing A generic term to describe the heating, holding and coolingat appropriate rates to soften steel.

- Cooling occurs in the furnace at slow controlled rates.- Yields a coarse ferrite-pearlite-cementite structure.

- Facilitates machining and cold working.

- Relieves residual stress.

- Variations are full anneal, spherodize anneal, and process anneal.

- An annealed steel, or other alloy, is at its minimum mechanicalproperties.

Normalizing An austenizing heat treatment followed by controlledcooling in still or agitated air.

- The part must be heated above the critical A3 temperature.- Used to refine grain structure, improve machineability, reduceresidual stress, and homogenize the structure.


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Heat Treating 32

T-T-T Diagram for Annealing Steel


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Heat Treating33

Annealed Steel

Annealed SAE 1144 100X Annealed SAE 1144 500X


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Heat Treating 34

Heat Treatment for Welding Applications

Heat treatment for welding should always be considered if the carbon

content of the steel is greater than 0.3%.

Pre-heating:

- Slows the cooling rate after welding.

- Reduces distortion caused by steep temperature gradients in the

work piece.

- Reduces residual stress in the weld and/or heat affected zone.

- Reduces the potential for weld or base metal cracking caused by

distortion and residual stress.

- Reduces the potential for unintended martensite formation in the

weld area. Unintended martensite in the weld area can create astress-riser.

- Slower cooling rate can insure a consistent microstructure across the

weld zone.


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Heat Treating 35

Heat Treating for Welding Applications continued

Post weld heat treating:

- Post weld heat treating reduces distortion and residual stress.- Allows for straightening of welded assemblies.

- Reduces residual stress.

- Allows for more uniform mechanical properties across the weld, heat

affected zone and base metal.

- Reduces distortion when machining after welding.

- Reduces potential for post weld cracks.

Specific information on pre-heat or post weld heating of specific metals

and alloys can be found through the Welding Research Council or theAmerican Welding Society.


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Contact us for further information

Weldon Mak Makela

Senior Failure Analyst

651 659 7275

[email protected]

Josh Schwantes

Metallurgical Engineering Manager

651 659 7205

[email protected]

Craig Stolpestad

Sales Manager

651 659 7206

[email protected]

Mark Eggers

Inside Sales, NDT & Metals

651 659 7349

[email protected]

Heat Treating 36

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