Alloy Steel Class 10.9 High-Strength Bolts | 1000 MPa Tensile · 940 MPa Yield · Quenched & Tempered 35CrMo
Class 10.9 is made from medium-carbon alloy steel 35CrMo and is quenched and tempered to 1000 MPa nominal tensile strength, 940 MPa minimum yield and 32–39 HRC. It is the workhorse class for high load, vibration and alternating-load duty. Because it accepts a higher preload than 8.8, the joint faces are clamped harder and micro-movement fatigue resistance improves markedly. Price runs about 1.2–1.4× class 8.8. Note that plated parts at 10.9 and above must be hydrogen baked at 190–220°C, otherwise pickling-induced hydrogen leaves a delayed-fracture risk.
Properties & Specifications
| Tensile Strength | 1000 MPa (nominal) |
| Yield Strength | 940 MPa (measured minimum) |
| Hardness | 32 – 39 HRC |
| Base Material | Medium-carbon alloy steel 35CrMo |
| Heat Treatment | Quenched and tempered — mandatory at this class |
| Fracture Toughness | Moderate |
| Relative Price | ≈ 1.2 – 1.4 × class 8.8 |
| Nut Counterpart | Class 10 nut, or Type 2 class 8 |
| Plating Requirement | Hydrogen bake at 190–220°C after electroplating |
Material Advantages
Higher preload from the same diameter: insufficient preload sits behind almost every fatigue failure, and moving up to 10.9 is the most direct way to clamp the faces down
Quenched-and-tempered alloy steel gives a well-rounded property set, so life under alternating load and vibration is clearly longer than class 8.8
Increases joint stiffness and slip resistance without enlarging the bolt — the value shows where envelope is limited
Paired in-line with class 10 nuts, Type 2 heavy nuts and A194 2H nuts so a whole container matches classes
Delta-Tone has no electroplating step and introduces no hydrogen, which makes it the default corrosion route for class 10.9
Applications
Mould fastening — clamp plates and locators on injection and die-casting moulds
Hydraulics and pneumatics — manifold blocks, flanges and high-pressure line joints
Drive components — gearbox housings, couplings and speed reducers
Pressure vessels and flange sets where the design specifies class 10.9
Heavy-load joints on trucks and off-highway machinery
Studs — quenched-and-tempered 35CrMo M12–M64 for high-load equipment
Grade Selection
| Class / Grade | Service temperature | Notes |
|---|---|---|
| 8.8 | ≤ 300°C（达克罗） | Baseline class; prefer it for static duty rather than paying for unused strength |
| 10.9 | ≤ 80°C（电镀锌）/ ≤ 300°C（达克罗） | Quenched and tempered 35CrMo; the vibration and alternating-load class, hydrogen bake mandatory after plating |
| 12.9 | ≤ 300°C（达克罗） | 1200 MPa tensile, stronger but more brittle; avoid under impact |
| ASTM A193 B7 | 高温高压法兰工况 | US-standard Cr-Mo; inspected to the ASTM A193 system rather than metric classes, mated with A194 2H |
Frequently Asked Questions
Yes. Under GB/T 3098.1 the 10.9 class uses medium-carbon alloy steel such as 35CrMo and must be quenched and tempered to reach 1000 MPa tensile and 940 MPa minimum yield, whereas 8.8 does not require it. That is one reason 10.9 costs 20–40% more.
Pickling and electroplating drive hydrogen into the steel, and susceptibility rises with strength. In one of our fracture investigations a bolt failed intergranularly within three days of assembly because the customer had re-plated it without baking. Confirm whether the supplier's plating route includes a 190–220°C bake, or specify Delta-Tone, which has no electroplating step and introduces no hydrogen.
A class 10 nut, or a Type 2 heavy class 8 nut. The rule is equal class or one below; with too large a gap the internal thread strips before the bolt reaches its capacity.
Class 12.9 reaches 1200 MPa tensile and 39–44 HRC from 42CrMo-type alloy steel, also quenched and tempered, but it is more brittle, intolerant of impact, more sensitive to delayed fracture and about 1.5–1.8× the price. Unless the design calls for it, secure your preload in 10.9 first.
Get Grade Selection & Quotation
Submit your duty parameters (temperature, pressure, media, speed) — engineers reply with grade selection and quotation within 24 hours.