Bolt Property Classes 8.8, 10.9 and 12.9: Steels and Heat Treatment

Reading the stamp

Under ISO 898-1, property class X.Y means:

• Nominal tensile strength ≈ X × 100 MPa
• Yield ratio ≈ Y/10 — so 10.9 yields at about 90% of its tensile strength

Class	Tensile (nominal)	Yield (nominal)	Typical hardness range
8.8	800 MPa	640 MPa	roughly 23–34 HRC
10.9	1000 MPa	900 MPa	roughly 32–39 HRC
12.9	1200 MPa	1080 MPa	roughly 39–44 HRC

Typical steels

• 8.8: medium-carbon steels (C35/1045 class) or boron steels for smaller diameters, quenched and tempered. The workhorse structural and machinery class.
• 10.9: alloy steels such as 42CrMo4 / SCM440 or boron-alloy grades (10B38 class) — needed because higher strength at larger diameters demands hardenability through the section.
• 12.9: alloy steels of the SCM435 / 4140 class, tightly controlled; the strength ceiling for standard commercial bolts.

Quench-and-temper logic

All three classes are made the same way — quench to martensite, then temper back to the target window — and the tempering temperature does the grading. ISO 898-1 sets minimum tempering temperatures (of the order of 425 °C for classes up to 10.9 and somewhat lower for 12.9) to guarantee toughness and stress-relaxation behaviour, because an under-tempered bolt can hit its hardness figure while remaining brittle.

Two cautions follow from the physics. First, strength is section-dependent: a large-diameter 12.9 is harder to make honestly than a small one, since the core must still transform. Second, 12.9 (and electroplated 10.9) is hydrogen-embrittlement sensitive — plating and acid pickling need baking controls, which is why critical 12.9 joints are often left unplated or mechanically coated.

What the class does not tell you

The property class certifies the bolt's tensile behaviour, nothing else. It does not certify the clamp load a joint will achieve: installed tension depends on tightening method and friction (thread and under-head), which is why torque tables always name a friction condition and why the same 10.9 bolt can deliver very different preloads dry, oiled or coated. Nor does the class stand alone — nuts are classed to match (a class 10 nut for a 10.9 bolt, and so on), and using a lower-class nut quietly demotes the assembly.

ISO 898-1 also limits decarburization at the thread surface and sets hardness differently for large diameters (above ~M39 the standard hands over to agreement), so very large high-strength fasteners are frequently specified to project or mill standards rather than the catalogue class. Finally, fatigue-critical joints often roll threads after heat treatment — the compressive residual stress from rolling raises fatigue strength markedly, and this manufacturing note appears on serious bolt specifications right beside the property class.