⚡ Key takeaways
- A property class is a heat treatment result, not an alloy grade. 8.8, 10.9 and 12.9 describe the mechanical property combination (tensile strength and yield ratio) achieved after heat treatment.
- Quench and temper is the mainstream. Classes 8.8, 10.9 and 12.9 are all quenched and high-tempered to tempered sorbite, balancing strength and toughness.
- Carburising is for surface wear resistance. Low-carbon steel carburised to HRC 58–62 at the surface with a tough core is used for self-tapping screws and wood screws.
- Nitriding is for fatigue resistance. A hard compound layer raises fatigue strength 30–50 % and is used on parts in high-frequency vibration.
- Above class 12.9, hydrogen embrittlement must be controlled. The higher the strength, the greater the sensitivity (see the hydrogen guide).
- The sequence cannot be changed. Roll the thread, then quench, then finish. Reverse it and the thread cracks or cannot be formed at all.
- Tempering temperature sets toughness. Class 10.9 tempers at about 425 °C, class 12.9 at 425–450 °C. Hotter means tougher but slightly weaker.
Contents
1. What 8.8, 10.9 and 12.9 actually mean
2. Quench and temper: the mainstream route
3. Carburising: for self-tapping screws
4. Nitriding: fatigue resistance
5. Process sequence
6. Defects and control
2. Quench and temper: the mainstream route
3. Carburising: for self-tapping screws
4. Nitriding: fatigue resistance
5. Process sequence
6. Defects and control
1. What 8.8, 10.9 and 12.9 actually mean
ISO 898-1 defines the marking:
- First figure = tensile strength ÷ 100 MPa, so 8.8 means 800 MPa.
- Second figure = yield ratio × 10, so 8.8 means 0.8.
- Yield for 8.8 is therefore 800 × 0.8 = 640 MPa.
| Class | Tensile (MPa) | Yield (MPa) | Hardness (HV) | Typical material |
|---|---|---|---|---|
| 4.8 | 400 | 320 | 110–160 | Low-carbon steel |
| 8.8 | 800 | 640 | 250–320 | Quenched and tempered medium carbon |
| 10.9 | 1,040 | 940 | 320–380 | Quenched and tempered alloy steel |
| 12.9 | 1,220 | 1,100 | 380–420 | Quenched and tempered alloy steel |
2. Quench and temper: the mainstream route
Quench and temper means austenitise at 850–900 °C, oil quench, then high-temper at 425–550 °C:
- Structure: tempered sorbite.
- Result: a balance of strength and toughness.
- Every class 8.8, 10.9 and 12.9 bolt is made this way.
3. Carburising: for self-tapping screws
Low-carbon steel such as 1018 or 1022 is carburised at about 900 °C for two to four hours to a surface carbon of 0.8–1.0 %:
- After quenching the surface reaches HRC 58–62, giving wear resistance.
- The core stays at HRC 20–30, giving toughness.
- Used for self-tapping screws and wood screws, where the point must cut and the head must not snap.
4. Nitriding: fatigue resistance
Gas nitriding at 500–550 °C:
- Forms an ε-Fe₂₋₃N compound layer 10–20 μm thick.
- Hardness HV 900–1,200.
- Fatigue strength up 30–50 %.
- Distortion is minimal, so it is applied after finish machining.
5. Process sequence
- Cold heading.
- Thread rolling.
- Quench and temper.
- Surface finish.
- Hydrogen removal bake.
⚠️ The sequence cannot be reversed: roll before quenching, or the distortion from quenching puts the thread out of tolerance; quench before rolling, and the material is too hard to form.
6. Defects and control
- Quench cracking: cooling too fast, or inclusions in the steel.
- Decarburisation: air ingress into the furnace lowers surface strength.
- Under-tempering: poor toughness and risk of delayed fracture.
- Hydrogen embrittlement: no bake after plating (see the hydrogen guide).
FAQ
Q. How is a class 8.8 bolt heat treated? Quench and high temper to 800 MPa tensile, 22–32 HRC.
Q. Carburising or nitriding? Carburising gives a hard surface with a tough core for wear parts; nitriding distorts very little and suits precision parts.
Q. Why does temperature control matter for 12.9? High-strength steel has limited temper stability; the post-plating bake must stay below 220 °C or strength drops.
🛠️ Engineering support: Toprecision runs quench and temper, carburising and nitriding in-house and supplies classes 8.8–12.9 consistently, with a material certificate and hardness report per batch.
👉 Engineering services ｜ 👉 Request a technical quote
👉 Engineering services ｜ 👉 Request a technical quote