Heating → Soaking → Cooling
Processes
Overview
Heat treatment is the controlled heating, holding (soaking) and cooling of metals to change their properties — hardness, strength, toughness, ductility, machinability or internal stress — without changing the shape of the part.
Every process has three stages: heat → soak → cool.
The furnace controls the first two; the cooling method (furnace, air, oil, water, polymer, gas) decides the final result.
Process & Methods
Core processes
~650–900 °C (steel)
Slow furnace cooling
WHAT IT DOES
Heats the metal and cools it very slowly inside the furnace to soften it and refine its grain structure.
WHY
Improves machinability and ductility, relieves internal stress, prepares material for further forming or hardening.
VARIANTS
Full annealing, process (sub-critical) annealing, spheroidise annealing (~650–750 °C, for highcarbon steels), bright annealing (in protective atmosphere).
APPLICATIONS
Forgings, castings, wire, sheet, tool steel blanks before machining.
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~850–950 °C (steel)
Air cooling
WHAT IT DOES
Heats steel above its critical temperature and cools it in still air.
WHY
Gives a uniform, fine grain structure with better strength than annealing; removes casting and forging inconsistencies.
APPLICATIONS
Forgings, castings, structural components, gear blanks.
~800–1050 °C (depends on steel)
Quench: oil / water / polymer / gas
WHAT IT DOES
Heats steel to the hardening temperature, soaks it, then cools it rapidly to form a hard martensitic structure.
WHY
Maximum hardness and wear resistance. Always followed by tempering, as hardened steel alone is brittle.
APPLICATIONS
Tools, dies, shafts, bearings, fasteners, machine components.
~150–650 °C
Air cooling
WHAT IT DOES
Reheats hardened steel to a lower temperature and holds it.
WHY
Reduces brittleness and quench stress, trading a little hardness for much better toughness.
Lower temperatures keep more hardness; higher temperatures give more toughness.
APPLICATIONS
Every hardened part — springs, tools, gears, shafts, hand tools.
~550–650 °C (steel)
Slow cooling
WHAT IT DOES
Heats the part below its transformation range and cools slowly to release locked-in stresses.
WHY
Prevents distortion and cracking after welding, machining, cold working or casting.
APPLICATIONS
Weldments, fabricated structures, heavy machined parts, pressure vessel components.
Surface (case) hardening processes
These give a hard, wear-resistant surface (case) while the core stays tough — ideal for parts that face friction and shock together.
~900–950 °C
Carbon-rich atmosphere, then quench
WHAT IT DOES
Adds carbon to the surface of low-carbon steel, then hardens it by quenching.
APPLICATIONS
Gears, camshafts, pinions, crankshafts, bearing races. Typically done in sealed-quench or integral-quench furnaces.
~800–880 °C
Carbon + nitrogen, then quench
WHAT IT DOES
Diffuses both carbon and nitrogen into the surface at a lower temperature than carburizing.
WHY
Shallow, hard case with less distortion; good for thin or small parts.
APPLICATIONS
Small gears, fasteners, stampings, light-duty shafts.
~500–580 °C
No quench required
WHAT IT DOES
Diffuses nitrogen into the surface at a low temperature.
WHY
Very hard, wear- and fatigue-resistant surface with minimal distortion because no quenching is needed.
APPLICATIONS
Dies, moulds, extrusion screws, precision gears, crankshafts.
Which process for which goal?
| Customer goal | Process to suggest |
|---|---|
| Make metal softer / easier to machine | Annealing, Normalizing |
| Maximum hardness and wear resistance | Hardening + Tempering |
| Hard surface, tough core | Carburizing, Carbonitriding, Nitriding |
| Fix distortion / cracking after welding or machining | Stress relieving |
| Reduce brittleness after hardening | Tempering |
| Minimal distortion, high toughness | Austempering, Martempering, Nitriding |
| Strengthen aluminium alloys | Solution treatment + Ageing |
| Foundry: protect ladle lining, cut thermal shock | Ladle preheating |
| Coating / paint / core drying | Industrial ovens |
Furnace types vs processes
| Furnace type | Typical processes | Best suited for |
|---|---|---|
| Box / Batch furnace | Annealing, normalizing, hardening, tempering | Mixed jobs, tool rooms, small to medium batches |
| Pit furnace | Annealing, hardening, nitriding, stress relieving | Long shafts, rods, tubes, heavy vertical parts |
| Bogie hearth furnace | Annealing, normalizing, stress relieving | Very heavy forgings, castings, fabrications |
| Integral quench furnace | Carburizing, carbonitriding, neutral hardening | Gears, shafts, high-volume case hardening |
| Continuous furnace (mesh belt / roller hearth) | Hardening, tempering, annealing | High-volume small parts, fasteners |
| Tempering / ageing furnace | Tempering, ageing, stress relieving | Low-temperature, high-uniformity work |
| Ladle pre-heater | Ladle and refractory preheating | Foundries, steel plants |
| Industrial ovens | Drying, curing, baking | Coating, core/mould drying, pre-weld heating |
