SKS3 (100MnCrW4 / near AISI O1) — oil-hardening tool steel for small precision parts
Why SKS3 is chosen for dimensional stability on an oil quench, how it is hardened and tempered, and the honest limits of a tool steel used for small parts.
Grade data
| JIS designation | SKS3 |
|---|---|
| Closest equivalents | AISI O1 (近) / DIN 100MnCrW4 |
| Carbon (%) | 0.90–1.00 |
| Steel family | Tool / high-carbon steel |
| Heat-treatment route | Quench and temper |
| Typical surface hardness (HRC) | 58–63 HRC |
| Typical hardness specified in service (HRC) | 56–62 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
SKS3 is an oil-hardening cold-work tool steel. It is not a fastener grade and it is not sold as one — it is specified when a small part has to be hard, wear-resistant and dimensionally predictable after hardening, such as punches, dies, gauges and precision pins. Within our small-parts scope it is a precision-part grade, not a bulk-fastener grade.
How it is heat treated
Harden from a relatively low austenitizing temperature, oil quench, then temper. The reason anyone pays for this grade is that the oil quench and the manganese content give far less size change than a water-hardening steel would, so a finished dimension survives the process. Tempering temperature is chosen against hardness and toughness together, and it must not be skipped.
Plating and hydrogen embrittlement
Tool-steel parts at working hardness are above the ISO 4042 trigger. They are normally protected with oil or a phosphate-and-oil system rather than electroplated; where a plated finish is specified, prompt de-embrittlement baking applies and the effect of coating thickness on a ground dimension has to be agreed first.
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 for small precision parts where post-hardening dimensional change has to be small and wear resistance matters — punches, precision pins, gauges and similar tooling.
When not to choose it
Do not choose it for fasteners, for hot-work service, or where corrosion resistance is needed — it has none. Do not choose it where a cheaper hardened carbon steel would meet the dimensional requirement, because you are paying for stability you do not need.
What tends to go wrong
Frequently asked questions
Why choose SKS3 over a plain high-carbon steel?+
For dimensional stability. An oil-hardening grade changes size less through the quench than a water-hardening one, so ground dimensions on a finished part are more likely to survive. If the part is rough and gets finished after hardening, the argument is much weaker.
Do you run tool steel as well as fasteners?+
We process small parts at bulk volume, and small tooling parts such as punches, precision pins and gauges are within that scope. Send the drawing with the required hardness and the dimensional tolerance and we will tell you whether it fits our process.