SWCH10B33 (10B33) — medium-carbon boron steel for class 10.9 fasteners
Where 10B33 sits between 10B21 and SCM435: enough carbon for 10.9, boron for hardenability, and a hard requirement for de-embrittlement baking after electroplating.
Grade data
| JIS designation | SWCH10B33 |
|---|---|
| Closest equivalents | AISI 10B33 |
| Carbon (%) | 0.30–0.36 + B |
| Steel family | Boron cold-heading steel |
| Heat-treatment route | Quench and temper |
| Typical surface hardness (HRC) | 45–52 HRC |
| Typical hardness specified in service (HRC) | 32–42 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
SWCH10B33 is the medium-carbon member of the boron cold-heading family. The extra carbon over 10B21 is what makes class 10.9 reachable, while boron keeps the hardenability up without the cost of a full Cr–Mo alloy. It is still a cold-heading steel, so it is bought to be formed and then heat treated.
How it is heat treated
Quench and temper, with the same free-boron caveat as the rest of the family. At class 10.9 the minimum tempering temperature in ISO 898-1 applies and is worth respecting — a boron grade tempered below the minimum to hit a hardness reading is a brittle bolt with a compliant test report.
Plating and hydrogen embrittlement
At 10.9 the part is above the ISO 4042 trigger, so electroplated parts are baked, and started quickly. If the same grade is tempered back to 8.8 for a different order, the baking requirement follows the delivered class, not the grade — one more reason the class belongs on the job traveller.
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 high-volume cold-formed 10.9 fasteners where the section is small and the commercial gap to SCM435 matters.
When not to choose it
Do not choose it for 12.9, for heavy sections, or for elevated-temperature service. Where the requirement is maximum toughness at high strength, the Cr–Mo grade still wins.
What tends to go wrong
Frequently asked questions
10B33 or SCM435 for class 10.9?+
Both reach 10.9 in fastener sizes. 10B33 is normally the cheaper cold-formed route in small diameters; SCM435 has more hardenability and more temper resistance, which matters as the section grows or when toughness at strength is critical.
Does 10B33 need de-embrittlement baking?+
At class 10.9 with an electroplated finish, yes — it is above the ISO 4042 trigger. The specification sets the parameters and the maximum delay after plating.