S45C vs SCM440: which steel should you harden?
A practical decision page for design, purchasing and QC, with realistic service ranges and section-size limits.
S45C vs SCM440
Short answer: SCM440 is not automatically “harder” than S45C
Both grades can reach similar surface hardness after quenching. SCM440 contains chromium and molybdenum, so its hardenability is higher and it holds hardness more uniformly through thicker sections. S45C is often the economical choice for small, general-purpose parts.
S45C versus SCM440 comparison
| Decision factor | S45C | SCM440 |
|---|---|---|
| Steel family | Medium-carbon steel | Cr–Mo alloy steel |
| Closest equivalents | AISI 1045 / C45 | AISI 4140 / 42CrMo4 |
| Typical carbon | 0.42–0.48% | 0.38–0.43% |
| Primary route | Quench & temper | Quench & temper |
| Typical as-hardened surface | About 50–58 HRC | About 50–58 HRC |
| Typical tempered service range | About 40–55 HRC | About 28–48 HRC |
| Hardenability | Moderate; section-size sensitive | Higher; more uniform in thicker sections |
| Typical use | Pins, small shafts, general bolts | High-strength bolts, shafts, pins, loaded parts |
How to choose
S45C
Choose S45C for modest section sizes, general loads and cost-sensitive parts where extreme strength and core hardenability are unnecessary.
SCM440
Choose SCM440 for thicker sections, more uniform core hardness, cyclic or impact loading, and tighter strength/toughness control after tempering.
Limits before putting a number on the drawing
- HRC ranges are typical small-part guidance, not guarantees.
- Core hardness depends on section, geometry, quenchant and actual heat chemistry.
- Specify an HRC window, test location and method—not only “harden”.
- Evaluate hydrogen-embrittlement risk if high-strength parts are zinc-plated afterward.
What to send the heat treater
- 1Drawing and material grade/certificate
- 2Critical section size and lot quantity/weight
- 3Hardness window, test location and method
- 4Distortion, microstructure or decarburization limits
- 5Downstream finish and due date
Frequently asked questions
Is SCM440 always stronger than S45C?
No. Both reach similar surface hardness after quenching. SCM440’s chromium and molybdenum give higher hardenability, so it holds hardness more uniformly through thicker sections; for small parts S45C is often the more economical choice.
Which should I pick for a small shaft or general bolt?
S45C is usually enough for modest sections and general loads at lower material cost. Choose SCM440 for thicker sections, uniform core hardness, or cyclic and impact loading.
What do you need to quote heat treatment?
The drawing and material grade or certificate, the critical section size, the hardness window with test location and method, and the lot quantity or weight.