Why the grade sets the hardness
Peak attainable hardness is governed mainly by carbon content. Alloying elements such as chromium (Cr), molybdenum (Mo) and manganese (Mn) improve hardenability—the ability to harden right through the core, not just the surface.
Steel below roughly 0.25%C cannot be through-hardened and must be surface-hardened (carburized) instead. Medium-carbon steel and up can take a quench-and-temper cycle.
Final HRC also depends on tempering temperature: the higher the temper, the lower the hardness but the tougher the part.
Approximate hardness by grade
- S45C (~0.45% C) — reaches ~52–58 HRC; general-purpose pins, small shafts, bolts.
- SCM435 / SCM440 (Cr-Mo) — better hardenability, ~55–60 HRC even in thicker sections; ideal for class 10.9 / 12.9 bolts.
- SCM415 / SCr420 (low carbon) — surface-hardened by carburizing to ~58–62 HRC with a tough core.
- SK5 (carbon tool steel) — ~58–62 HRC; flat springs, spring washers.
- SUP9 (spring steel) — ~44–50 HRC; tuned for fatigue and spring-back.
How to choose cost-effectively
Need uniform hardness in thicker parts? Pick the Cr-Mo group (SCM435/440) for higher hardenability than S45C.
Thin parts on a tight budget? S45C is usually enough.
Need a hard, wear-resistant skin over a tough impact-resistant core? Use low-carbon steel plus carburizing.
Not sure of the grade? Send a spec or a sample—we verify composition/hardness and recommend a process that hits your target HRC.
FAQ
What is the maximum HRC?+
It depends on the grade; typical hardened screws/bolts land around 55–62 HRC. Tell us your target and we control it via temper temperature.
How do I know the grade?+
Check the material certificate, or send a sample to our lab for composition analysis and a hardness test.
Can you hold a narrow HRC band?+
Yes, e.g. 58–60 HRC. We sample-test HV/HRC every lot and issue a certificate with the job.
