Heat treatment furnaces heat steel and other metals to set temperatures, sometimes in a controlled gas or vacuum, so they can be hardened, tempered, annealed or case hardened. Box and pit furnaces are simple and flexible; sealed quench furnaces carburise and harden in a protective atmosphere; vacuum furnaces give clean, bright parts; mesh belt furnaces treat small parts continuously.
Quick facts
- Temperature
- About 150–1,200 °C depending on type
- Atmosphere
- Air, endothermic gas, nitrogen, vacuum
- Quench
- Oil, water, polymer or gas
- Main types
- Box, pit, sealed quench, vacuum, mesh belt, tempering
- Control
- Thermocouples and controllers; carbon potential control for carburising
- Checks
- Hardness test, case depth, microstructure
Which furnace for which job
| Furnace | Used for |
|---|---|
| Box furnace | Hardening, annealing, small batches |
| Pit furnace | Long shafts, carburising, nitriding |
| Sealed quench | Carburising and hardening without scale |
| Vacuum furnace | Tool steels, bright parts, low distortion |
| Mesh belt furnace | Large quantities of small parts |
| Tempering furnace | Tempering after hardening, about 150–650 °C |
Good to know
- Temperature uniformity surveys check every part of the load heats evenly
- Protective atmosphere or vacuum prevents scale and decarburisation
- Loading pattern affects distortion
- Records of each batch help trace problems
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Questions
- Why do parts come out with scale?
- They were heated in air. Sealed quench or vacuum furnaces avoid scale.
- What is a temperature uniformity survey?
- A test with several thermocouples placed through the furnace to show the whole load reaches the set temperature.
- Can one furnace harden and temper?
- Hardening and tempering are usually done in separate furnaces because tempering needs a much lower temperature.
The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 29 October 2026.