Why tempering is needed
After quenching, steel is hard but brittle martensite — too brittle to use as-is. Tempering reheats it to a moderate temperature (~150–650°C) to cut brittleness and restore toughness.
You trade a little hardness for durability — almost every hardened part is tempered.
Temper temperature sets the hardness
Low tempering temperature = very hard, less tough (wear parts like blades).
High tempering temperature = tougher, lower hardness (impact parts like bolts).
Controlling the temper temperature is how the final HRC is set to your spec.
Temper colors
Heating clean steel in air grows a thin oxide film that changes color with temperature: straw → brown → purple → blue (see the photo).
Smiths use these colors to estimate temperature, but real QC measures furnace temperature and hardness — colors alone are not enough.
Watch for "temper embrittlement" — some steels turn brittle if tempered in certain temperature ranges, which must be avoided.
FAQ
Why temper after hardening?+
As-quenched martensite is very brittle; tempering reduces brittleness and restores usable toughness.
How reliable are temper colors?+
They estimate temperature, but quality work controls furnace temperature and measures HRC — not color alone.
Can you hit a final HRC?+
Yes — we set the temper temperature to reach your HRC band and test every lot with a certificate.
Free Engineering Tools
- Hardness Converter HRC ↔ HV ↔ HB
- Which Steel Grades Harden — and How Far
- Fastener Weight: pcs ↔ kg
- Finishing Specification Builder for Purchasing and QC
- Salt Spray Test Plan Builder
- Coating thickness → thread fit (GO/NO-GO)
- Hydrogen Embrittlement — is baking required?
- Bolt Tightening Torque & Proof Load
- Thread Engagement Length
- Bolt Size Estimator (from load)
- Bolt Property Class — Mechanical Properties (ISO 898-1)
What this article covers at our plant
Related comparison guides
- Case hardening vs Through hardeningCase hardening vs through hardening: which one for screws and bolts?
- Annealing vs Tempering vs NormalizingAnnealing vs tempering vs normalizing: which one does your part need?
- Quenching vs TemperingQuenching vs tempering: two halves of one process
- Carburising vs NitridingCarburising vs nitriding: what a screw, nut or bolt actually gets
- Carbonitriding vs CarburizingCarbonitriding vs carburizing: which case-hardening for fasteners?
- S45C vs SCM440S45C vs SCM440: which steel should you harden?
- SCM435 vs SCM440SCM435 vs SCM440: one alloy step apart, and it shows
- Grade 8.8 vs Grade 10.9 vs Grade 12.9Grade 8.8 vs 10.9 vs 12.9 bolts: what actually differs?
- Class 10.9 vs Class 12.9Class 10.9 vs 12.9: what you gain, and what you take on
Technical content review
Reviewed by the V.S. Heat Treatment QA and production team—heat-treatment and finishing operations since 1994 under an ISO 9001:2015 quality system.
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