S20C (AISI 1020) — case hardening, hardness and plating for small parts
How S20C is carburized rather than through-hardened, what the case does that the core cannot, and why case-hardened screws still fall under the ISO 4042 baking rule after zinc plating.
Grade data
| JIS designation | S20C |
|---|---|
| Closest equivalents | AISI 1020 / DIN C22 |
| Carbon (%) | 0.18–0.23 |
| Steel family | Plain carbon steel |
| Heat-treatment route | Carburizing (case hardening) |
| Typical surface hardness (HRC) | 55–62 HRC |
| Typical hardness specified in service (HRC) | 55–60 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
S20C is a plain low-carbon steel. There is not enough carbon in it to form useful martensite by quenching alone, so it is a case-hardening grade: carbon has to be put into the surface first. In exchange it is cheap, very cold-formable and forgiving in the header, which is why so many tapping screws and pins start life as S20C wire.
How it is heat treated
The route is carburizing followed by quench and temper. Carbon diffuses into the surface at carburizing temperature, the quench turns that enriched skin into martensite, and the low-carbon core stays tough underneath. Effective case depth is set by your drawing and measured per ISO 2639 — it is a specification, not a house default. Through-hardening S20C is not a route: without carbon there is nothing to harden.
Plating and hydrogen embrittlement
A case-hardened surface sits well above the ISO 4042 hardness trigger even though the core is soft, so electroplated case-hardened screws are normally baked, and the bake must start as soon as possible after plating. If the customer allows a non-electrolytic finish, zinc flake removes the acid and electrolytic hydrogen source altogether.
Indicated after electroplating
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 when you need a hard, wear-resistant and thread-forming surface on a cheap cold-headable steel, and the load is carried by a tough core — tapping screws, drive screws, small pins and clips.
When not to choose it
Do not choose it when the drawing calls for a through-section property class such as 8.8 or 10.9; carburized S20C has case hardness but no bulk strength, and a quench-and-temper grade is the right answer. It is also the wrong grade if the part is machined after hardening, because machining cuts the case off.
What tends to go wrong
Frequently asked questions
Can S20C be through-hardened?+
No. The carbon content is too low for a useful quench response, which is why the process has to add carbon first — carburizing or carbonitriding. If someone quotes you a through-hardened S20C bolt, ask what the core hardness is measured at.
Does a case-hardened S20C screw need baking after zinc plating?+
Normally yes. The surface is above the ISO 4042 hardness trigger even though the core is not, and case-hardened screws are a classic delayed-fracture population. Follow the drawing, and bake as soon as possible after plating.