Fastener Property Classes Explained | 8.8 / 10.9 / 12.9 and A2-70 / A4-80
Fastener class numbers read straight off the mechanical properties: in 8.8 the leading 8 means 800 MPa nominal tensile strength and the .8 means a yield ratio of 0.8, i.e. 640 MPa yield. Class 10.9 is 1000 MPa tensile with 940 MPa minimum yield. Values follow GB/T 3098.1 (ISO 898-1); stainless classes follow ISO 3506. Every class is supplied as a matched bolt-and-nut set with a mill test certificate and tensile / hardness reports.
Properties & Specifications
| Property Class Standard | GB/T 3098.1 (= ISO 898-1) for carbon / alloy steel; GB/T 3098.6 / ISO 3506 for stainless |
| Tensile Strength | 4.8: 420 MPa class · 8.8: 800 MPa · 10.9: 1000 MPa · 12.9: 1200 MPa · A2-70: 700 MPa |
| Yield Strength | 8.8: 640 MPa · 10.9: 940 MPa (min) · 12.9: 1100 MPa (min) |
| Hardness | 8.8: 22 – 32 HRC · 10.9: 32 – 39 HRC · 12.9: 39 – 44 HRC |
| Common Steel Grades | 35# / 45# medium-carbon steel · 35CrMo · 42CrMo alloy steel (quenched and tempered) |
| Stainless Grades | 304 / 304L (A2-70, A2-80) · 316 / 316L (A4-70, A4-80) · duplex 2205 (special order) |
| Relative Price | 8.8 = 1 (baseline) · 10.9 ≈ 1.2 – 1.4 · 12.9 ≈ 1.5 – 1.8 |
| Nut Pairing Rule | Equal class or one below: 8.8 bolt → class 8 nut, 10.9 bolt → class 10 nut |
Material Advantages
The class number states the mechanics directly: first digit × 100 = nominal tensile strength, decimal digit = yield ratio — no back-and-forth on quotes or incoming inspection
Class 8.8 is the most universal high-strength grade, covering most static joints in machinery, steel structures and automotive work
Classes 10.9 / 12.9 use quenched-and-tempered alloy steel, allowing higher preload and giving more fatigue margin under alternating load and vibration
Stainless A2-70 / A4-80 follow ISO 3506 and prioritise corrosion resistance; their strength is below the equivalent carbon-steel class and must not be treated as a like-for-like substitute
Nuts ship from the same plant and batch as the bolts, with matched class and finish, avoiding the classic internal-thread stripping failure
Applications
Class 4.8 — light duty: furniture, general brackets
Class 8.8 — general high strength: machinery, steel structures, automotive chassis
Class 10.9 — high load and vibration: moulds, hydraulics, drive components, pressure-vessel accessories
Class 12.9 — maximum static load: mould fastening, power heads, heavy machining
A2-70 / A4-80 — food equipment, outdoor, coastal and chemical environments
ASTM A193 B7 / B7M — petrochemical flanges, valves, high temperature / pressure and cryogenic service
Grade Selection
| Class / Grade | Service temperature | Notes |
|---|---|---|
| 4.8 | ≤ 80°C（电镀锌） | Light duty: furniture and general brackets |
| 8.8 | ≤ 80°C（电镀锌）/ ≤ 250°C（热浸锌）/ ≤ 300°C（达克罗） | The most universal high-strength grade for machinery, structures and automotive |
| 10.9 | ≤ 80°C（电镀锌）/ ≤ 300°C（达克罗） | High load and vibration; quenched and tempered; hydrogen bake at 190–220°C after plating |
| 12.9 | ≤ 300°C（达克罗） | 1200 MPa tensile, 39–44 HRC; brittle, avoid impact, higher delayed-fracture sensitivity |
| A2-70 | 含氯水环境 > 60°C 有应力腐蚀风险 | 304, about 700 MPa tensile; the default for indoor and inland outdoor service |
| A4-80 | 同 A2，抗氯离子明显更好 | 316; coastal, salt-spray, chemical and marine environments |
Frequently Asked Questions
The first digit × 100 gives the nominal tensile strength in MPa; the digit after the decimal × the first digit gives the yield point — 0.8 × 800 = 640 MPa. This convention comes from the mechanical properties standard GB/T 3098.1 (equivalent to ISO 898-1). Class 10.9 follows the same rule: 1000 MPa tensile, 900 MPa nominal yield, 940 MPa measured minimum.
Class 8.8 is enough for ordinary static structures. Choose 10.9 for alternating load, vibration, or where joint reliability is critical. 10.9 typically costs 20–40% more. Under alternating load, stepping up one class is the right move, because insufficient preload is the leading cause of fatigue failure.
No. A2-70 has 700 MPa tensile and no yield guarantee equivalent to class 8.8, and treating stainless as a high-strength option is listed as one of the top selection errors. Where you need both strength and corrosion resistance, specify Delta-Tone coated carbon steel rather than stainless.
At most one class lower. An 8.8 bolt takes a class 8 nut; a 10.9 bolt takes class 10 or a Type 2 class 8 nut. If the nut is much softer, the internal thread strips first and the joint fails.
Get Grade Selection & Quotation
Submit your duty parameters (temperature, pressure, media, speed) — engineers reply with grade selection and quotation within 24 hours.