Heat Treatment Guide
A complete heat treatment reference: annealing, quench hardening, tempering, case hardening (carburizing, induction, flame), nitriding, aging/precipitation hardening, stress relief, furnace atmospheres, aluminum heat treatment, and a full comparison guide.
🔥Heat Treatment
Heat treatment — altering mechanical properties through heating/cooling cycles without changing chemical composition. Critical to the entire metals industry.
Full Annealing
830-900°CFurnace CoolHB≤180
Temp.830-900°C (austenitize)
CoolingFurnace — 20-50°C/h
ResultHB≤180, A%↑↑, RM↓
StructureCoarse pearlite
SteelAny carbon steel type
UseBefore heavy machining, bending, cutting
Restores maximum ductility. Coarse pearlite = soft and easy to machine.
✓ Maximum ductility. ↓RM. Easy to machine
✗ Very slow. ↓RM completely
▶ YouTube✓ Maximum ductility. ↓RM. Easy to machine
✗ Very slow. ↓RM completely
Process Annealing
550-650°CRecrystallize
Temp.550-650°C (below Ac1)
UseBetween cold-work passes: ↓work hardening
StructureRecrystallization — new grains
SteelLow carbon <0.3%C
Not austenitize! Only recrystallize below Ac1.
✓ Fast and economical. ↓work-hardened hardness
✗ Not for high-carbon steels
▶ YouTube✓ Fast and economical. ↓work-hardened hardness
✗ Not for high-carbon steels
Normalizing
870-940°CAir CoolUniform Structure
Temp.870-940°C (+55°C above Ac3)
CoolingAir — faster than full anneal
StructureFine pearlite — fine grain
ResultRM=550-700 MPa | HB 160-220
SteelAny C-steel. Forgings, castings
UseStructure normalization after forging/casting
Fine pearlite (vs. coarse in full anneal) = ↑RM + ↑mechanical properties
✓ Fast. ↑RM vs. full anneal. Uniform structure
✓ For forgings and castings — mandatory for normalization
▶ YouTube✓ Fast. ↑RM vs. full anneal. Uniform structure
✓ For forgings and castings — mandatory for normalization
Spheroidize Annealing
680-720°CCarbidesMaximum Machinability
Temp.680-720°C (below Ac1) + cycling
StructureSpheroidal cementite in ferrite matrix
ResultHB 150-200 — the softest!
UseTool steel before machining: D2, A2, H13, M2
Time4-24 hours
Spherical carbides = ↓↓ hardness = ↑↑ machinability.
✓ Optimal preparation before machining tool steel
✗ Long (4-24h). Not for low-carbon steels
▶ YouTube✓ Optimal preparation before machining tool steel
✗ Long (4-24h). Not for low-carbon steels
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Quench Hardening
MartensiteHRC 55-65Critical
Step 1Austenitize: Ac3+30-80°C (800-900°C)
Step 2Quench: water/oil/polymer/gas
StructureMartensite — hard+brittle!
HardnessHRC 55-65 (depends on %C)
MartensiteC% → HRC: 0.4%C=HRC 56-58 / 0.6%C=HRC62 / 0.8%C=HRC65
HRC as a function of %C: only the carbon determines maximum achievable HRC!
Hardenability (depth) = depends on Cr,Mo,Ni — not C!
✓ Maximum hardening
✗ Brittle martensite — tempering mandatory immediately!
✗ Not for complex parts without tempering
▶ YouTubeHardenability (depth) = depends on Cr,Mo,Ni — not C!
✓ Maximum hardening
✗ Brittle martensite — tempering mandatory immediately!
✗ Not for complex parts without tempering
Quench Media — Comparison
WaterOilGas
Water/BrineVery fast — ↑↑ severity. ↑crack risk. Low-carbon steels
Polymer PAGControlled. Aqua-Quench, Tenax — between water and oil
OilMedium. ↓distortion. Alloy steels
Gas N2/ArSlow. Vacuum furnace. ↓↓distortion. ↑price
Salt 160-220°CMarquench — ↓distortion. Bainite/martensite
Rule: ↑hardenability → ↑gentler quench (oil/gas)
✓ Gas = ↓↓distortion. Vacuum furnace
✗ Water = ↑cracking. Only for simple low-carbon steels
▶ YouTube✓ Gas = ↓↓distortion. Vacuum furnace
✗ Water = ↑cracking. Only for simple low-carbon steels
Jominy End-Quench — Hardenability
ASTM A255H-Band
PurposeMeasuring hardening depth
MethodJominy specimen: quench one end → measure HRC along the length
DIIdeal Critical Diameter — theoretical hardening depth
H-BandGuaranteed hardenability range in the standard
Steel 4140DI=50-80mm. HRC 40+ depth: ~25mm
SAE/AISI defines H-Band: guaranteed RM/HRC range along the Jominy bar
✓ Hardenability = a steel property, not a process!
▶ YouTube✓ Hardenability = a steel property, not a process!
Marquenching / Martempering
↓DistortionSalt 160-230°C
ProcessAustenitize → quench in salt at 200-230°C → hold → air cool
AdvantagePrevents thermal shock — ↓↓ distortion + cracking
StructureUniform martensite — fewer residual stresses
UseGears, cutting tools, complex parts
✓ ↓↓ distortion + cracking vs. direct quench
✗ Complex. ↑cost. Salt bath = safety hazard
▶ YouTube✗ Complex. ↑cost. Salt bath = safety hazard
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Tempering
150-700°C↓Hardness+↑ToughnessMandatory!
Low temp.150-200°C → HRC 58-64 → cutting tools, bearings
Medium temp.350-450°C → HRC 40-52 → springs, hand tools
High temp.550-650°C → HRC 28-40 → gears, drive shafts
Cycles2x temper at least — for tool steel!
Rule2h + 2h × supplier spec
Brittle martensite → tempered martensite = ↑ toughness
Temper immediately after quench — never delay!
✓ Restores toughness while controlling hardness reduction
✗ "Temper embrittlement" at 350-570°C — avoid!
▶ YouTubeTemper immediately after quench — never delay!
✓ Restores toughness while controlling hardness reduction
✗ "Temper embrittlement" at 350-570°C — avoid!
Double Tempering
Tool SteelRetained Austenite
Why?1st temper: retained austenite → new martensite
2nd TemperTempers the new martensite
Temp.Same in both cycles. 1h-2h each
UseH13, D2, M2, HSS — every tool steel!
Retained AusteniteIn high-C steels: >5% RA = ↓hardness!
+OK+ Single temper = retained austenite → a problem
+OK+ Double temper = RA converted → ↑↑ dimensional stability
▶ YouTube+OK+ Double temper = RA converted → ↑↑ dimensional stability
Secondary Hardening
HSSM2/M42Carbides
PhenomenonAt tempering 500-600°C: alloy carbides precipitate → HRC ↑!
SteelsM2, M42, H13, D2, H11
HRCM2 T6: HRC63-65 after 560°C temper
MechanismFine Mo,W,V carbides → pin dislocations
✓ HSS achieves HRC63-65 — higher than as-quenched!
✓ Hardness increases with tempering — unique to high-alloy steel
▶ YouTube✓ Hardness increases with tempering — unique to high-alloy steel
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Carburizing
Low CCase HRC 58-62Gas/Pack/Vacuum
MethodsGas (Endogas) · Pack · Vacuum (LPC)
Temp.900-960°C · 4-24h depending on depth
Case Depth0.5-3mm total case
ECDEffective Case Depth @ HRC50
Steels8620, 9310, 4320, 17CrNiMo6
StandardAMS 2759/7 · ISO 2639
Carbon diffuses per ATM = C% rises from 0.2% to 0.8%+
Core: HRC 25-40 (ductile) | Case: HRC 58-62 (hard)
✓ Ideal combination: hard outside + tough inside
✓ Vacuum LPC = ↑↑ results + ↓distortion
▶ YouTubeCore: HRC 25-40 (ductile) | Case: HRC 58-62 (hard)
✓ Ideal combination: hard outside + tough inside
✓ Vacuum LPC = ↑↑ results + ↓distortion
Vacuum Carburizing — LPC
Vacuum↓DistortionAerospace
ProcessVacuum 1-50 mbar + acetylene pulse
Temp.880-1050°C (higher than gas carb!)
Advantage↓↓ distortion, ↑↑ consistency, clean
CaseUniform! — even in blind holes
StandardAMS 2759/7
ManufacturersIpsen ALD, ECM, Solar Atmospheres
✓ ↓distortion vs. gas carburizing
✓ Penetrates complex blind holes
✗ High equipment cost
▶ YouTube✓ Penetrates complex blind holes
✗ High equipment cost
Gas Nitriding
N2/NH3HV 700-1100No Quench
ProcessNH3 decomposes → [N] penetrates ← white layer+diffusion
Temp.490-530°C — lower than carburize!
Depth0.1-0.8mm diffusion zone
White LayerFeN/Fe2N: 5-25µm — brittle! Sometimes removed
HardnessHV 700-1100 (depends on steel)
AdvantageNo quench → ↓↓ distortion!
Nitriding = low temp. + no quench = ↓↓ distortion
✓ For precision parts: drive shafts, machine guides
✗ Brittle white layer — sometimes needs grinding away
▶ YouTube✓ For precision parts: drive shafts, machine guides
✗ Brittle white layer — sometimes needs grinding away
Induction Hardening
FastLocalizedHRC 55-62
PrincipleAC current → magnetic field → self-heating (eddy currents)
Frequency10 kHz (deep) → 500 kHz (shallow)
Depth0.5-10mm depending on frequency
Cycle5-60 seconds — very fast!
Steels1045, 4140, 5160, 4340 — C>0.35%
ManufacturersEFD, Inductotherm, Ajax
✓ Focused and selective — only what's needed
✓ Fast + economical for volume
✓ Compressive residual stress = ↑fatigue life
✗ A unique coil needed for every shape
▶ YouTube✓ Fast + economical for volume
✓ Compressive residual stress = ↑fatigue life
✗ A unique coil needed for every shape
Flame Hardening
Oxy-FuelSimpleLarge Areas
PrincipleOxy-acetylene/propane flame → immediate quench
Depth1-6mm
UseLarge gears, rails, large parts
AdvantageSimple, portable equipment
SteelsC>0.35%
✓ Portable. Large areas. ↓cost
✗ Less uniform than induction. Operator-dependent
▶ YouTube✗ Less uniform than induction. Operator-dependent
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Plasma / Ion Nitriding
Plasma↓White LayerSS Possible
ProcessGlow discharge plasma: N+ → penetration
Temp.400-570°C
White Layer↓↓ — controllable!
AdvantageSS, Inconel — not possible with gas!
StandardAMS 2759/6
✓ ↓white layer + SS + alloy steel
✓ ↓↓ distortion. ↑↑ process flexibility
✗ Batch process. ↑equipment cost
▶ YouTube✓ ↓↓ distortion. ↑↑ process flexibility
✗ Batch process. ↑equipment cost
Ferritic Nitrocarburizing — FNC/Tufftriding
Salt BathFe2-3NFast
MethodsSalt bath (Tenifer/QPQ) · gas (Nitrotec) · plasma
Temp.560-580°C · 1-4h
LayerCompound layer 5-25µm + diffusion 0.1-0.3mm
Corrosion ResistanceSalt spray: 480h+!
UseDrive shafts, agricultural equipment, firearms, door handles
QPQ = Quench-Polish-Quench: black finish + ↑↑ corrosion resistance
✓ Fast (1-4h) + ↑corrosion resistance + ↓distortion
✓ Any steel including low-carbon
▶ YouTube✓ Fast (1-4h) + ↑corrosion resistance + ↓distortion
✓ Any steel including low-carbon
Boronizing
HV 1500-2000Maximum Wear Resistance
ProcessB4C + activator @ 700-950°C → Fe2B/FeB
HardnessHV 1500-2000!
Depth0.025-0.25mm
UseDie casting dies, extrusion, mining equipment
⚠️Fe2B preferred over FeB — less brittle
✓ Maximum abrasive wear resistance
✗ Very brittle. Not for impact!
▶ YouTube✗ Very brittle. Not for impact!
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Age Hardening (Aluminum)
T6T4/T73Solution+Age
Step 1Solution heat treatment: 480-560°C → quench
Step 2Aging: natural (T4) or artificial (T6/T7)
T4Natural age RT/96h+ → HV increases over time
T6Artificial age 120-175°C/6-24h → peak strength
T73/T76Over-age → ↓RM but ↑↑ SCC resistance
T8xStretch + age → ↓distortion + ↑strength
GP zones → theta prime → theta (equilibrium precipitate)
✓ T6 = peak strength
✓ T73 = ↑SCC resistance (7075-T73 vs T6)
▶ YouTube✓ T6 = peak strength
✓ T73 = ↑SCC resistance (7075-T73 vs T6)
PH Stainless — Precipitation Hardening SS
17-4PHAMS 2759
17-4PH H900480°C/1h → HRC 40-47. ↑↑ strength
17-4PH H1025550°C/4h → HRC 33-38. ↑toughness
17-4PH H1150620°C/4h → HRC 25-32. ↑↑ toughness
15-5PHSimilar to 17-4. ↑toughness vs. 17-4
Custom 455H900: HRC 48! ↑↑↑ strength
StandardAMS 2759 / AMS 5604
H = hardened @ temperature (F). H900 = 900°F = 480°C
✓ Excellent corrosion + high-strength combo
✗ H900 = ↓toughness. ↑temperature → ↑toughness
▶ YouTube✓ Excellent corrosion + high-strength combo
✗ H900 = ↓toughness. ↑temperature → ↑toughness
Nickel Superalloys
Inconel 718Gamma Prime
IN718 STASolution 980°C/1h → age 720°C/8h + 620°C/8h
MechanismGamma prime (Ni3Al) + gamma double prime (Ni3Nb)
ResultRM=1380 MPa @ RT. 900 MPa @ 650°C!
WaspaloySolution + double age → gamma prime
Mar-M247DS/SC casting → special HT
✓ Gamma prime = strongest known precipitate
✗ Complex HT. ±5°C temperature control mandatory
▶ YouTube✗ Complex HT. ±5°C temperature control mandatory
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Stress Relief
550-650°CNo HRC ChangeAfter Welding
PurposeReducing residual stress from welding/machining/casting
Steel Temp.550-650°C · 1h/25mm
SS Temp.850-1050°C (annealing)
Ti64 Temp.480-650°C · 1-4h
Al Temp.260-345°C (T2 temper)
StandardAWS D1.1 · ASME Sec. VIII
Rule: temp. < tempering temp! Don't degrade hardening
✓ ↓↓ distortion in subsequent machining
✓ ↑ fatigue life + dimensional stability
✗ Too-high temp. = over-temper = ↓HRC
▶ YouTube✓ ↓↓ distortion in subsequent machining
✓ ↑ fatigue life + dimensional stability
✗ Too-high temp. = over-temper = ↓HRC
Cryogenic Treatment
-185°C LN2↑WearRetained Austenite
ProcessLN2 → -185°C (-120°C deep cryo) · 24-36h
PurposeConverting retained austenite → martensite
Advantage↑20-30% tool life! ↑wear resistance
UseCutting tools, gears, firearms
CycleQuench → cryo → temper
✓ ↑20-30% tool life — research-proven
✓ ↑dimensional stability (↓retained austenite)
✗ Controversial — not universally accepted
▶ YouTube✓ ↑dimensional stability (↓retained austenite)
✗ Controversial — not universally accepted
Stabilizing — SS/Cast Iron
Al/SSDimensional
SS Stabilizing430-590°C: sensitization → Cr23C6 → correction
Cast Iron500-565°C: ↓residual stress from casting
Al260°C/T2: ↓residual stress from quench
PurposeLong-term dimensional stability
✓ Essential for precision parts (frames, guides)
✓ SS: prevents IGA (intergranular attack)
▶ YouTube✓ SS: prevents IGA (intergranular attack)
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Endogas — Protective Atmosphere
CO+H2+N2CarburizingCommon
CompositionCO 20% + H2 40% + N2 40%
Carbon PotentialCP = 0.4-1.2% — measurable and controllable!
UseCarburizing, hardening, bright annealing
ProductionEndothermic generator: CH4 + air @ Ni catalyst
SafetyCO = toxic! Explosive range 12-74%
✓ Measurable CP ← precise carbon control
✗ CO = toxic. Explosive. Safety procedures!
▶ YouTube✗ CO = toxic. Explosive. Safety procedures!
Vacuum Furnace
10-4 mbar↓DistortionClean
Vacuum1-10-4 mbar — ↓↓ oxidation!
CoolingN2/Ar gas quench — controlled intensity
AdvantageClean, ↓distortion, ↓decarb, ↓scale
UseAerospace, medical instruments, tool steel
ManufacturersIpsen, Seco/Warwick, Solar Atmospheres
✓ The best finish. ↓↓ distortion
✓ Clean — no cleaning processes needed
✗ Expensive. Batch process. ↓cooling rate vs. oil
▶ YouTube✓ Clean — no cleaning processes needed
✗ Expensive. Batch process. ↓cooling rate vs. oil
Salt Bath Furnace
Rapid HeatUniformMarquench
SaltsBaCl2 (1000-1300°C) · KNO3/NaNO3 (150-600°C)
AdvantageFast, uniform heating — ↓distortion
MarquenchingKNO3 @ 160-230°C — controlled martensite
⚠️BaCl2 = toxic! KNO3 = oxidizer
✓ Fast, uniform heating, ↓distortion
✗ BaCl2 toxic. KNO3 oxidizer. Waste disposal
▶ YouTube✗ BaCl2 toxic. KNO3 oxidizer. Waste disposal
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Al — Solution Heat Treatment
480-560°CImmediate Quench!Within 30 Seconds
PurposeRe-dissolving Mg2Si/CuAl2 → supersaturated SS
Temp. 6061527-543°C
Temp. 7075460-480°C
QuenchWater ≤80°C — within 30 seconds!
⚠️Transfer time is critical! >30s = ↓properties
Transfer time from furnace to quench: MAX 30 seconds!
✓ The basis of all Al heat treatment: T4/T6/T73
✗ Burning: 5-15°C above solidus = damage!
▶ YouTube✓ The basis of all Al heat treatment: T4/T6/T73
✗ Burning: 5-15°C above solidus = damage!
Al Artificial Aging — T6/T73/T76
T6T73Peak Strength
T6 — 6061160-175°C / 8-18h
T6 — 7075120°C/24h OR 160°C/8h
T73 — 7075120°C/8h + 165°C/12h — over-aged!
T76Partial over-age. ↑SCC vs. T6, ↓RM vs. T73
T8xStretch + age. ↓distortion + ↑strength
T73: ↓15% RM vs. T6 but ↑↑↑ SCC resistance — aerospace!
✓ T6 = peak strength for most applications
✓ T73/T76 = SCC-critical applications
▶ YouTube✓ T6 = peak strength for most applications
✓ T73/T76 = SCC-critical applications
Al — Annealing O Temper
330-415°CMaximum Ductility
Full Anneal330-415°C → cool ≤28°C/h → 260°C → air cool
PurposeO temper — maximum ductility for bending/cold cutting
5xxx260-345°C / H2 → H0
6xxx415°C / controlled cool → O
✓ O temper = completely soft. Bending and forming
✗ ↓↓ strength. Not for structural use
▶ YouTube✗ ↓↓ strength. Not for structural use
Room-Temperature Aging — Natural Aging T4
T4RT96h+
T4Solution → quench → RT aging
6061-T4Reaches 70% of T6 properties after 96h
2024-T4Peak natural age! ↑fatigue vs. T6
Advantage↑formability vs. T6 — for bending before aging
⚠️2024 — natural age is optimal! Not T6
✓ 2024-T4 = ↑fatigue ↑formability vs. T6
✓ Can be formed then artificially aged (warm forming)
▶ YouTube✓ Can be formed then artificially aged (warm forming)
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📊 Heat Treatment Comparison — Selection Guide
| Treatment | Temp. °C | Cooling | HRC/HV | Distortion | Main Use |
|---|---|---|---|---|---|
| Full Anneal | 830-900 | Furnace | HB≤180 | Minimal | Before heavy machining |
| Normalizing | 870-940 | Air | HB 160-220 | Low | Forgings, castings |
| Quench+Temper | 800-900→150-650 | Oil/Water | HRC 28-65 | Medium-high | Gears, drive shafts |
| Carburizing | 900-960 | Oil | Case HRC62 | Medium | 8620, 9310 — gears |
| Nitriding Gas | 490-530 | Air | HV 700-1100 | Minimal | Drive shafts, machine guides |
| Induction | Instant | Quench | HRC 55-62 | Low | Localized, drive shafts, rails |
| FNC/Tufftriding | 560-580 | Air/Oil | HV 500-700 | Very minimal | Salt spray! Firearms, automotive |
| Al T6 | 480-560+120-175 | Water+Furnace | HV 95-180 | Low | 6061, 7075 — aerospace, structure |
📌 Heat Treatment Selection Principles
1. Strength only? Quench + Temper (QT)
2. Hard outside + tough inside? Carburize / case harden
3. Maximum ↓distortion? Nitriding / FNC / vacuum
4. Localized/selective? Induction / flame
5. Aluminum? T6 (strength) / T73 (SCC) / O (forming)
6. PH stainless? H900 (strength) / H1150 (toughness)
7. ↑surface wear resistance? Boronizing / nitriding / PVD
2. Hard outside + tough inside? Carburize / case harden
3. Maximum ↓distortion? Nitriding / FNC / vacuum
4. Localized/selective? Induction / flame
5. Aluminum? T6 (strength) / T73 (SCC) / O (forming)
6. PH stainless? H900 (strength) / H1150 (toughness)
7. ↑surface wear resistance? Boronizing / nitriding / PVD