Heat Treating

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• IQ three is running wet, dew point's up near fifty, so the 8620 rings came out lean on case depth.

  TranslationThe integral quench furnace atmosphere has too much moisture, which lowered its carbon potential, so the carburized gears did not pick up enough carbon and the hardened layer is too shallow.

• Load came up to heat at two, soak it hour per inch, then pull and dunk, ten seconds door to oil.

  TranslationStart timing when the furnace load reaches temperature, hold one hour for each inch of the thickest section, then remove it and quench it in oil within ten seconds.

• They're glass hard off the quench, snap temper them now and we'll draw them back to 58 on second shift.

  TranslationThe parts are fully hardened and brittle, so give them a quick low-temperature temper immediately to prevent cracking, then do the full temper to 58 HRC later.

• Probe says 0.85 but the shim came back 0.72, she's sooting up somewhere, schedule a burnout and reseason.

  TranslationThe oxygen probe reading of carbon potential disagrees with the shim stock test, carbon is depositing as soot, so the furnace needs its carbon burned off with air and then conditioning before production.

• Cut and etch one off the top tray, I want a traverse at the pitch line before we cert this lot.

  TranslationSection and etch a sample part from the load and run a microhardness profile at the gear tooth pitch line to confirm case depth before signing the certification.

• Whole lot is two points soft and the mount shows pearlite at the root, that's a slack quench, normalize and rehard.

  TranslationEvery part is two Rockwell numbers below spec and the microstructure shows the quench was too slow, so the parts must be normalized and hardened again.

• Clutch plates potato-chipped in the open tank, next lot goes in the press quench and gets flatten-tempered under weight.

  TranslationThe thin plates warped during quenching, so the next batch will be quenched in a die press and tempered while clamped flat.

The phases, diagrams and steel properties that explain why heat treating works at all.

• Austenitealso gamma iron
  The face-centered cubic form of iron that dissolves carbon and must form before steel can be hardened.
• Bainitealso upper bainite, lower bainite
  A tough mix of ferrite and fine carbides that forms between the pearlite and martensite temperature ranges.
• Carbidealso alloy carbide, primary carbide
  Any hard compound of carbon with iron or alloying elements such as chromium, vanadium, molybdenum or tungsten.
• CCT diagramalso continuous cooling transformation diagram, CCT curve
  Continuous-cooling-transformation chart predicting which structures form as steel cools at various steady rates.
• Cementitealso iron carbide, Fe3C
  Iron carbide, Fe3C: the hard, brittle compound that carries most of the carbon in slowly cooled steel.
• Critical temperaturealso transformation temperature, critical
  A temperature at which steel's structure changes phase on heating or cooling, such as Ac1 or Ac3.
• DIalso ideal critical diameter, ideal diameter
  Ideal critical diameter: the largest bar that would reach 50 percent martensite at its center in a perfect quench.
• Ferritealso alpha iron, free ferrite
  The soft, low-carbon, body-centered cubic phase of iron that makes up most of an annealed low-carbon steel.
• Grain sizealso prior austenite grain size, ASTM grain size
  The average size of the crystals in a metal, usually reported as an ASTM number where higher means finer.
• Hardenability
  A steel's ability to form martensite to a given depth, set by its chemistry and grain size, not its hardness.
• Iron-carbon diagramalso iron-carbon phase diagram, Fe-C diagram
  The phase map of iron and carbon that shows which structures are stable at each temperature and carbon level.
• Martensitealso mart
  The hard, needle-like phase that forms when austenite is quenched fast enough to trap its carbon.
• Ms temperaturealso Ms point, martensite start
  The temperature at which martensite starts to form as austenite cools during a quench.
• Pearlitealso fine pearlite, coarse pearlite
  A layered mix of ferrite and cementite that forms when austenite cools slowly through the eutectoid range.
• Retained austenitealso RA
  Austenite that fails to transform during the quench and survives at room temperature inside hardened steel.
• TTT diagramalso isothermal transformation diagram, IT diagram
  Time-temperature-transformation chart showing how long austenite takes to transform when held at a fixed temperature.

Getting steel hot enough to transform and cooling it fast enough to make martensite.

• Agitationalso quench agitation
  Forced movement of quenchant around parts by pumps, propellers or nozzles to speed up and even out cooling.
• Air hardeningalso air-hardening, air hard
  Describes steels with enough alloy to harden fully when cooled in still air or gas instead of a liquid.
• At heatalso up to heat, at temp
  Shop phrase meaning a load has reached its setpoint temperature, so its soak time can start counting. Shop talk
• Austemperingalso austemper
  Quenching steel into a salt bath above Ms and holding it until the austenite fully transforms to bainite.
• Austenitizingalso austenitize, hardening temperature
  Heating steel above its upper critical temperature and holding it until the structure becomes austenite.
• Cherry redalso cherry heat
  Color name for steel glowing at roughly 750 to 800 °C, near where most hardening heats begin. Shop talk
• Cryoalso cryogenic treatment, deep cryogenic treatment
  Shop name for deep cryogenic treatment, a slow cool to near liquid nitrogen temperature after hardening. Shop talk
• Dunkalso dunk time
  Heat-treat slang for quenching a part, especially by hand into an open oil or water tank. Shop talk
• Glass hardalso glass-hard, as-quenched
  Fully hardened and untempered, so hard and brittle that a file skates off and a drop could shatter it. Shop talk
• Hot oilalso marquench oil, martemp oil
  Shop name for quench oil run at about 120 to 230 °C for marquenching and low-distortion quenching. Shop talk
• Hour per inchalso an hour an inch, hour-per-inch rule
  Heat-treat rule of thumb that a part needs about one hour at temperature per inch of its thickest section. Shop talk
• Magnet testalso nonmagnetic check, magnet check
  Checking steel with a magnet while heating, since it stops being magnetic near its hardening temperature. Shop talk
• Marquenchingalso martempering, marquench
  Quenching steel into hot oil or salt just above Ms, holding to even out temperature, then air cooling.
• Polymer quenchantalso polymer quench, PAG quenchant
  A water solution of polymer that quenches at a speed between water and oil, tuned by its concentration.
• Pullalso pull the load
  To take a load or part out of the furnace, usually on its way to the quench or at the end of a cycle. Shop talk
• Q&Talso QT, quench and temper
  Quench and temper: hardening steel by quenching, then tempering it back to a set strength and toughness.
• Quenchalso quenching, quench hardening
  To cool hot metal rapidly in a liquid or gas so that it hardens, or the act of doing so.
• Quench severityalso H value, Grossmann number
  How fast a quenchant pulls heat out of steel, usually rated by the Grossmann H value.
• Quenchantalso quench medium, quench media
  The liquid or gas used to quench parts, chosen for its cooling speed and the distortion it produces.
• Slack quenchalso slack-quenched
  A quench too slow to make full martensite, leaving pearlite, bainite or ferrite and low hardness.
• Soakalso soaking, hold
  To hold parts at a set temperature until they are evenly heated and the intended change is complete.
• Sub-zero treatmentalso cold treat, freezer
  Chilling hardened steel well below room temperature to turn retained austenite into martensite.
• Through hardeningalso through-hardening, thru-hardening
  Hardening a steel part so martensite forms across the whole section, not just at the surface.
• Vapor blanketalso vapor phase, film boiling
  The film of vapor that wraps red-hot steel at the start of a liquid quench and slows its cooling.

Softening, toughening, stress relief and precipitation treatments that follow or replace hardening.

• Agingalso ageing, age hardening
  Holding a solution-treated alloy at room or moderate temperature so fine particles form and strengthen it.
• Annealingalso anneal, full anneal
  Heating metal and cooling it slowly to soften it, relieve stress and make it easier to machine or form.
• Dead softalso dead-soft, fully annealed
  Fully annealed to the lowest hardness the metal can reach, with no work hardening left in it. Shop talk
• Double temperingalso double temper, triple temper
  Tempering a part twice, cooling to room temperature in between, to temper martensite formed after the first.
• Drawalso draw back, drawing
  Shop word for tempering, as in drawing hardened steel back to a softer, tougher hardness. Shop talk
• Normalizingalso normalize, normalise
  Heating steel above its upper critical temperature and cooling it in still air to refine and even out grain.
• Precipitation hardeningalso PH, age hardening
  Strengthening an alloy by forming very fine particles inside its grains through solution treating and aging.
• Secondary hardeningalso secondary hardness peak
  A rise in hardness when certain alloy steels are tempered around 500 to 600 °C, from fine alloy carbides.
• Snap temperalso snap draw
  A quick, low-temperature temper given right after quenching to keep crack-prone parts from splitting. Shop talk
• Solution treatingalso solution heat treat, solution anneal
  Heating an alloy to dissolve its strengthening elements into one phase, then quenching to trap them there.
• Spheroidizingalso spheroidize anneal, spheroidize annealing
  Long holds near the lower critical temperature that turn carbides into small round particles.
• Stress relievingalso stress relief, stress relieve
  Heating a part below its transformation range to relax residual stress from welding, machining or forming.
• Temper colorsalso tempering colors, oxide colors
  Thin oxide colors, from pale straw to blue, that form on clean steel as it is heated in air.
• Temper embrittlementalso temper brittleness, reversible temper embrittlement
  Loss of toughness in some alloy steels held in, or slowly cooled through, roughly 375 to 575 °C.
• Temperingalso temper, draw
  Reheating hardened steel below its lower critical temperature to trade some hardness for toughness.

Carburizing, nitriding, induction and other ways to harden only the surface.

• Boost and diffusealso boost-diffuse, boost/diffuse
  Carburizing schedule that first floods the surface with carbon, then lowers the potential so it spreads inward.
• Carbonitridingalso carbonitride, CN
  Adding both carbon and nitrogen to a steel surface at a lower heat than carburizing, then quenching.
• Carburizingalso carburize, gas carburizing
  Diffusing carbon into the surface of low-carbon steel at high heat, then quenching for a hard case.
• Casealso hardened case
  The hardened outer layer of a case-hardened part, as opposed to its softer, tougher core.
• Case depthalso total case depth, TCD
  Thickness of a hardened surface layer, measured from the surface to a specified hardness or structure.
• Effective case depthalso ECD, effective case
  Depth below the surface where hardness falls to a set value, usually 50 HRC, on a case-hardened part.
• Flame hardeningalso flame harden, flame-hardened
  Surface hardening with an oxy-fuel torch followed by a water quench, used on large or one-off parts.
• FNCalso ferritic nitrocarburizing, nitrocarburizing
  Ferritic nitrocarburizing: adding nitrogen and carbon below the critical temperature for a thin, hard layer.
• Induction hardeningalso induction harden, IH
  Heating only the surface of a steel part with an alternating magnetic field, then quenching for a hard case.
• Ion nitridingalso plasma nitriding, glow discharge nitriding
  Nitriding with a glow-discharge plasma in a vacuum vessel, which allows close control of the white layer.
• Nitridingalso gas nitriding, nitride
  Diffusing nitrogen into steel below its critical temperature to form a hard case with no quench.
• Stop-offalso stopoff, maskant
  A paint, paste or plating applied to areas of a part that must not carburize or nitride.
• Vacuum carburizingalso LPC, low-pressure carburizing
  Carburizing at low pressure in a vacuum furnace using pulses of hydrocarbon gas, then a gas or oil quench.
• White layeralso compound layer, compound zone
  The thin, hard compound layer of iron nitrides that forms on the surface during nitriding.

The equipment, gases, controls and floor routines that run every heat-treat cycle.

• Alloyalso furnace alloy, baskets
  Heat-treat slang for the heat-resistant baskets, trays, grids and fixtures that carry parts through the furnace. Shop talk
• Brightalso bright annealed, bright hardened
  Describes parts that come out of the furnace clean and unoxidized, with no scale or discoloration. Shop talk
• Carbon potentialalso CP, carbon pot
  The carbon content a furnace atmosphere would give the surface of plain steel at equilibrium.
• Chargealso load, furnace load
  One batch of parts loaded into a furnace together, or the act of loading it.
• Dew pointalso DP
  The temperature at which water vapor in a furnace atmosphere condenses, used as a gauge of carbon potential.
• Endothermic gasalso endo, endo gas
  A carrier atmosphere of carbon monoxide, hydrogen and nitrogen made by reacting natural gas with air.
• Flame curtainalso door curtain, flame screen
  A sheet of burning gas across a furnace door that keeps air out while the door is open.
• Integral quench furnacealso IQ, IQ furnace
  A sealed batch furnace with a heating chamber and an attached oil quench, run under protective atmosphere.
• Mesh belt furnacealso belt furnace, belt line
  A continuous furnace that carries parts on a wire mesh belt through heating, quench and wash in one line.
• Pyrometryalso furnace pyrometry
  The practice of measuring and controlling furnace temperature: sensors, instruments, calibration and surveys.
• Recipealso cycle, program
  Shop word for the stored program of temperatures, times and atmosphere setpoints that runs a furnace cycle. Shop talk
• Running richalso rich, running heavy
  Shop talk for a furnace atmosphere carrying more carbon than its setpoint and heading toward soot. Shop talk
• Salt bathalso salt pot, salt
  A pot of molten salt used to heat, quench or case harden parts by quick, even immersion.
• Seasoningalso conditioning, reseasoning
  Running an empty furnace at carburizing setpoint after a burnout so walls and fixtures soak up carbon first. Shop talk
• Sootingalso soot, soot-up
  Free carbon depositing in a furnace or on parts when the atmosphere carries more carbon than it can hold.
• TUSalso temperature uniformity survey, uniformity survey
  Temperature uniformity survey: a test mapping how evenly a furnace holds temperature across its work zone.
• Vacuum furnacealso vac furnace, vacuum
  A sealed, pumped-down furnace that heats parts with no atmosphere, then quenches with pressurized gas or oil.
• Washeralso parts washer, prewash
  The parts washer that strips oil and chips before heat treat and quench oil after it. Shop talk
• Wetalso wet atmosphere
  Shop word for a furnace atmosphere with too much water vapor, meaning a high dew point and low carbon. Shop talk

Hardness checks, metallography, the things that go wrong and the documents that prove what was done.

• Certalso certs, heat treat cert
  Shop word for the certificate of heat treatment that states what was done to a lot and the results. Shop talk
• Cookedalso overheated, burnt
  Shop slang for parts that were overheated or held too long, ending up with coarse grain or burned structure. Shop talk
• Cut and etchalso section and etch, cut-up
  Shop phrase for sectioning a sample part and etching it to check case depth or structure. Shop talk
• Decarburizationalso decarb, decarbed
  Loss of carbon from the surface of steel during heating, leaving a soft skin that will not fully harden.
• Distortionalso movement, warpage
  Unwanted change in shape or size of a part during heat treatment, such as bowing, warping or ovality.
• File hardalso file-hard, skates a file
  Hard enough that a sharp file skates across the surface without biting, roughly 58 HRC and up. Shop talk
• Grinding burnalso grind burn, grinding abuse
  Heat damage to a hardened surface from abusive grinding, which softens it or creates brittle rehardened spots.
• Growthalso size change, heat treat growth
  The increase in a part's size after hardening, caused mainly by austenite turning into bulkier martensite.
• Hydrogen embrittlementalso HE, hydrogen-assisted cracking
  Loss of ductility in high-strength steel when absorbed hydrogen causes delayed cracking under stress.
• Jominy testalso end-quench test, Jominy
  End-quench test that measures a steel's hardenability by spraying water on one end of a hot bar.
• Microhardnessalso microhardness traverse, Vickers
  Hardness measured with very small loads under a microscope, used for thin cases and individual phases.
• Nital etchalso nital, temper etch
  An etch of nitric acid in alcohol that reveals steel microstructure and grinding damage.
• Pointsalso Rockwell points, points of carbon
  Shop shorthand for Rockwell numbers, as in a part running two points high on hardness. Shop talk
• Potato chipalso potato-chipped
  Shop name for a thin, flat part that warps into a dished or wavy shape during the quench. Shop talk
• Quench crackalso hardening crack, quench cracking
  A crack that forms during or soon after quenching from thermal and transformation stress in hardened steel.
• Rehardalso reharden, re-harden
  To harden a part again after it misses hardness, usually after normalizing or annealing it first. Shop talk
• Rockwell hardnessalso Rockwell, HRC
  The standard shop hardness test that presses a diamond or ball into a part and reads depth as a number.
• Scalealso furnace scale, oxide
  The oxide crust that forms on steel heated in air or in an oxidizing atmosphere.
• Soft spotalso soft spots
  A small area on a hardened surface that reads lower than the rest, usually from uneven quenching.
• Traveleralso router, shop traveler
  The paperwork that rides with a lot through the shop, recording each operation, furnace, time and result. Shop talk

Frequently asked questions

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