Heat treatment
Annealing vs Tempering
Annealing heats steel above its critical temperature, roughly 1,475 to 1,650 F, then cools it slowly in the furnace to soften it fully and erase prior stress. Tempering reheats already quenched steel to 300 to 1,200 F to trade some hardness for toughness. One resets the microstructure; the other tunes it.
| Annealing | Tempering | |
|---|---|---|
| Purpose | Full soften, stress relief, machinability | Toughen after quench hardening |
| Temperature | 1,475 to 1,650 F, above critical | 300 to 1,200 F, below critical |
| Cooling | Furnace cool, 50 to 100 F/hr | Still air cool |
| Resulting hardness | 4140 anneals to about 197 HB | 4140 tempers to 28 to 54 HRC |
| Cycle time | 8 to 24 hr including furnace cool | 1 hr per inch of section, 2 to 4 hr typical |
| Position in sequence | Before machining or forming | Immediately after quench, always |
| Commercial cost | $0.15 to $0.40/lb | $0.25 to $0.60/lb as quench and temper |
Choose Annealing when
- Softening stock before heavy machining
- Stress relief after welding or cold work
- Restoring formability mid process
Choose Tempering when
- Setting final hardness after quench
- Wear parts needing 40+ HRC with toughness
- Any quenched part, no exceptions
The verdict
This is a sequence question, not either or. Anneal when you need soft, machinable, stress free material going into the process. Quench and temper when you need final properties coming out. Never skip tempering on quenched steel; as quenched 4140 at 55+ HRC will crack in service, sometimes within hours.
Cost comparison
Commercial annealing runs $0.15 to $0.40/lb with lot minimums of $150 to $300, and the slow furnace cool ties up equipment 8 to 24 hours. Quench and temper packages run $0.25 to $0.60/lb. Buying pre annealed bar costs $0.05 to $0.10/lb over standard hot rolled and beats in house annealing below about 5,000 lb lots. The expensive mistake is machining hard stock because annealing got skipped: carbide spend and cycle times roughly triple.
Common questions
Can you temper steel without quenching it first?
No point. Tempering works by decomposing brittle martensite, which only exists after a quench. Heating annealed or normalized steel to 400 to 1,100 F just gives you a mild stress relieve with no hardness change. If a print calls quench and temper, both steps are mandatory and the temper must follow the quench within hours to avoid cracking.