Heat Treating Basics

Presented by Weldon ‘Mak’ Makela

Senior Failure Analysis Engineer

Materials Testing & Analysis Group, Element St. Paul

Heat Treating Basics

Heat Treating 1

July 26, 2012

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

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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.

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Sources

• Metals Handbooks, 10th Edition, 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, 8th Edition, Volume 1: “Welding Technology”, American Welding Society, 1991.

• Metals Handbook, 9th Edition, Volume 6: “Welding, Brazing and Soldering”, ASM International, 1983.

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.

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Iron-Carbon Phase Diagram

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Iron-Carbon Phase Diagram

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 in steels.

- 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 is tetragonal and forms rapidly.

- Complete transformation requires cooling through the transformation temperature range, to below MF. The resulting martensite is brittle and called untempered martensite.

- Tempering is a process of reheating to a low temperature to toughen the steel.• Quenching can be in oil, plain water, salt water, polymers, salt baths, or air

depending 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.

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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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Martempering Transformation Diagram

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Annealed SAE 1045 Medium-Carbon Steel

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Tempered Martensite

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.

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Austempering Transformation Diagram

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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.

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 the surface is reduced.

• We won’t discuss coatings that increase the surface hardness such as:

- Plating.

- Hard facing.

- Thermal spraying.

- Vapor deposition.

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Case Hardening

• Localized heating, quenching and tempering of the surface to produce a hard layer of martensite 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 to uniformly 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.

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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 1600-2000°F 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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Carburized and De-carburized Microstructures

Carburized Steel – 100X De-carburized Steel – 200X

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 or aluminum 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 925-1050°F 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.

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Gas Nitriding continued

• Steels amenable to gas nitriding:

- Medium-carbon, chromium containing low-alloy steels such as the 4100, 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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Diffusion Hardening continued

• Carbo-nitriding - the diffusion of carbon and nitrogen into the surface. There are three variations of carbo-nitriding:

- Between 1400-1600°F, the low-alloy steel has transformed to austenite.

- Between 1250-1450°F, the steel can be partially austenitic but mostly consisting of ferrite and pearlite.

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

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.

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Quenched & Tempered Hardness vs. Carbon Content

Rockwell C Hardness, HRC

Ultimate Tensile Strength, ksi.

55 30150 25545 21440 18235 15730 13625 12020 108

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Tempering Problems

• Temper embrittlement can occur if:

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

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

- The result is a reduction in impact strength.

- The brittleness may be caused by precipitation of trash elements to the 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 to the temperature range between 450-700°F. A precipitation hardening effect occurs.

- Results in increased tensile and yield strength.

- Results in lower ductility and impact strength.

- May be recovered by re-heat treatment.

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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 480-570°F.

- Avoid tempering between 390-700°F 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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Tempered Martensite Embrittlement

Heat Treating

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

- Transforms remaining austenite to martensite.

- Optimum temperature is -120°F.

- 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 -320°F. Cool down must be slow.

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

- Incomplete understanding, therefore some disagreement, of mechanisms occurring 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 Treatments to Soften Steel• Annealing – A generic term to describe the heating, holding and

cooling at 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 mechanical properties.

• Normalizing – An austenizing heat treatment followed by controlled cooling in still or agitated air.

- The part must be heated above the critical A3 temperature.

- Used to refine grain structure, improve machineability, reduce residual stress, and homogenize the structure.

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T-T-T Diagram for Annealing Steel

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Annealed Steel

Annealed SAE 1144 – 100X Annealed SAE 1144 – 500X

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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 a stress-riser.

- Slower cooling rate can insure a consistent microstructure across the weld zone.

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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 the American Welding Society.

Contact us for further information

Weldon ‘Mak’ MakelaSenior Failure Analyst651 659 [email protected]

Josh SchwantesMetallurgical Engineering Manager651 659 [email protected]

Craig StolpestadSales Manager651 659 [email protected]

Mark EggersInside Sales, NDT & Metals651 659 [email protected]

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