Fastener Class & Material Technical Parameters
Engineering data for the classes we stock: tensile and yield strength, hardness or PREN, service temperature and typical duty, plus the torque and friction basis we quote from.
| Class | Material & Standard | Service Temp | Service Pressure | Size Range |
|---|---|---|---|---|
| 8.8 | Medium-carbon steel (35#/45#), GB/T 3098.1 | — | — | — |
| 10.9 | Medium-carbon alloy steel (35CrMo), quenched & tempered | — | — | — |
| 12.9 | Alloy steel (42CrMo etc.), quenched & tempered | — | — | — |
| A2-70 | Austenitic stainless 304 / 304L (ISO 3506) | — | — | — |
| A4-80 | Austenitic stainless 316 / 316L (ISO 3506) | — | — | — |
| Class | Tensile Rm | Yield Rp0.2 | Hardness / PREN |
|---|---|---|---|
| 8.8 | Rm ≥ 800 MPa | Rp0.2 ≥ 640 MPa | 22–32 HRC |
| 10.9 | Rm ≥ 1000 MPa | Rp0.2 ≥ 940 MPa | 32–39 HRC |
| 12.9 | Rm ≥ 1200 MPa | Rp0.2 ≥ 1100 MPa | 39–44 HRC |
| A2-70 | Rm ≥ 700 MPa | Rp0.2 ≥ 450 MPa | PREN ≈ 18–20 |
| A4-80 | Rm ≥ 800 MPa | Rp0.2 ≥ 600 MPa | PREN ≈ 24–26 |
The most common general-purpose high-strength class: machinery, steel structures, automotive. Adequate for ordinary static joints; 800 MPa nominal tensile, 640 MPa minimum yield; quench and temper not mandatory.
High-load and vibrating service: choose 10.9 under alternating loads or high reliability requirements - it tolerates higher assembly preload and resists micro-movement fatigue. Quench and temper mandatory; electroplated parts must be de-embrittled at 190-220°C.
Dies, hydraulics and other high static-load uses. The higher the strength the more brittle - avoid impact, higher delayed-fracture sensitivity; de-embrittlement mandatory after plating.
Corrosion-resistant general service: food equipment, indoor and inland outdoor. 700 MPa tensile - slightly below 8.8, and must not substitute for it as "high strength"; under chlorides (seaside, de-icing salt) it pits.
Marine and chemical severe-corrosion service: seaside projects, chemical plants, ships. With 2-3% molybdenum its chloride pitting resistance sits one full environment class above 304; above 60°C in chlorided media under stress, switch to duplex or nickel alloys.
Use the simplified formula T = F x (0.16P + 0.58d2mu + 0.775dmu): preload F = 0.7 x yield strength x tensile stress area As (d2 = d - 0.6495P, d3 = d - 1.2269P, As = pi/4 x ((d2+d3)/2)^2), mu = overall friction coefficient. For quick field checks use T = 0.2 x F x d.
8.8 (800 MPa tensile, 640 MPa yield) covers ordinary static structures. Choose 10.9 (1000/940 MPa) under alternating loads, vibration or high reliability demands - it allows higher preload and fights micro-movement fatigue. 10.9 typically costs 20-40% more.
Three steps: well lubricated (oil / phosphated + soaped) mu about 0.10; lightly oiled (normal as-shipped) mu about 0.14; dry assembly mu about 0.20. Dropping mu from 0.20 to 0.10 nearly doubles the preload from the same torque - so standardize lubrication and use torque wrenches in production.
Acid pickling during plating drives hydrogen into the steel, and high-strength quenched-and-tempered parts are sensitive to hydrogen embrittlement. Skipping the 190-220°C bake plants a delayed-fracture risk - sudden failure hours to days after assembly. Confirm the bake when ordering plated high-strength fasteners.
Not in load-bearing service. A2-70 provides 700 MPa tensile and 450 MPa yield, below 8.8 (800/640 MPa), with no 8.8-level yield guarantee; its strength advantage is corrosion resistance. For high strength plus corrosion resistance, use alloy steel Q&T with a hydrogen-free coating such as Dacromet.
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