S45C (AISI 1045) — quench and temper hardness, plating and limits
What S45C actually does in quench and temper, why section size decides the result, when it reaches class 8.8, and when the hardness pushes it into the ISO 4042 baking rule.
Grade data
| JIS designation | S45C |
|---|---|
| Closest equivalents | AISI 1045 / DIN C45 |
| Carbon (%) | 0.42–0.48 |
| Steel family | Plain carbon steel |
| Heat-treatment route | Quench and temper |
| Typical surface hardness (HRC) | 50–58 HRC |
| Typical hardness specified in service (HRC) | 40–55 HRC |
| Hardenable by heat treatment | Yes |
Hardness values are typical industry ranges for the small sections we process, and depend on the part, the section size, the quench and the governing specification. They are not a guarantee and do not replace your drawing or standard.
Reviewed:
What it is
S45C is the medium-carbon workhorse: enough carbon to harden by quench and temper, no alloy content to pay for. Its limitation is hardenability, not hardness. Thin sections respond fully; as the section grows the centre cools too slowly and you get a hardness gradient instead of a uniform part.
How it is heat treated
Austenitize, quench, then temper to whatever the drawing asks for. A class 8.8 bolt is tempered high, so it lands soft and tough; a hardened washer or pin in the same grade is tempered low and lands hard. Because it is a shallow-hardening steel, the honest question on an S45C job is not "what hardness" but "at what depth, on what section".
Plating and hydrogen embrittlement
This grade sits on both sides of the ISO 4042 trigger. A high-tempered class 8.8 bolt is below it and is not normally baked; the same grade delivered as a hardened washer or pin above roughly 390 HV is above it and should be. Decide from the delivered hardness on the drawing, not from the grade name.
Depends on the delivered class or hardness
ISO 4042 calls for embrittlement-relief baking on electroplated fasteners at property class 10.9 and above, or at roughly 390 HV (≈ 40 HRC) and above. Many customer specifications set a lower trigger — the drawing decides, not this page.
Hydrogen de-embrittlement bake plannerTypical ISO 898-1 property classes
When to choose it
Choose it for the cheapest honest route to class 8.8 in small sizes, and for hardened washers, pins and simple machined parts where wear matters more than through-section strength.
When not to choose it
Do not choose it when the drawing says 10.9 or 12.9 — plain carbon steel cannot hold those properties after the tempering the standard requires. Also avoid it where the section is heavy or a uniform core hardness is specified; SCM435 or SCM440 exists for exactly that reason.
What tends to go wrong
Frequently asked questions
Can S45C be used for class 10.9 bolts?+
No, not honestly. ISO 898-1 expects an alloy steel or a carbon steel with additives, quenched and tempered above a minimum tempering temperature, for 10.9. Plain S45C will not hold the properties after that temper. Specify SCM435, SCM440 or a boron grade instead.
Water quench or oil quench for S45C?+
It depends on the section and the geometry, and it is a trade-off, not a preference. Water gives the cooling rate a shallow-hardening steel needs but raises the risk of quench cracking on sharp corners, threads and section changes; oil is safer but may not reach the required hardness on thicker parts.