SWCH10B21 (10B21) — boron cold-heading steel for class 8.8 fasteners
How boron buys hardenability at low carbon, why free boron is the whole story, and where 10B21 stops being the right grade for a heat-treated fastener.
Grade data
| JIS designation | SWCH10B21 |
|---|---|
| Closest equivalents | AISI 10B21 |
| Carbon (%) | 0.18–0.23 + B |
| Steel family | Boron cold-heading steel |
| Heat-treatment route | Quench and temper |
| Typical surface hardness (HRC) | 40–48 HRC |
| Typical hardness specified in service (HRC) | 28–40 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
SWCH10B21 is a low-carbon cold-heading steel with a few parts per million of boron added. That trace is doing real work: boron segregates to grain boundaries and delays ferrite formation, so a steel that is soft enough to cold head still responds to a quench. It is the volume grade behind an enormous share of class 8.8 cold-formed screws and bolts.
How it is heat treated
Quench and temper. The catch is that only free boron counts — if nitrogen in the steel ties the boron up, the hardenability effect disappears and the parts come out soft with a perfectly normal-looking furnace chart. That is a mill and material-certificate question, which is why an unexplained soft lot on a boron grade starts with the material certificate, not the furnace.
Plating and hydrogen embrittlement
At class 8.8 the part is below the ISO 4042 trigger and baking is not normally required, but the drawing decides — some customers specify a bake for every plated fastener regardless of class, and that is their call to make.
Not normally indicated
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 fasteners at class 8.8 where cost per thousand pieces is the deciding factor and the sections are small.
When not to choose it
Do not choose it for 10.9 or 12.9 — the carbon is not there — or for heavy sections, or for service above about room-temperature-plus, where boron steels give back their advantage. It is also a poor choice for parts that will be re-austenitized several times.
What tends to go wrong
Frequently asked questions
Why did a boron-steel lot come out soft?+
On boron grades the first suspect is the steel, not the furnace: if the boron is tied up as nitride there is no hardenability effect to use. Check the material certificate for boron, nitrogen and titanium before re-running the heat treatment.
10B21 or SCM435?+
For class 8.8 in a cold-formed size, 10B21. For 10.9 or 12.9, SCM435. Choosing the alloy grade for an 8.8 part adds cost and an embrittlement risk you would then have to manage with a bake.