High-Strength Bolts (Grade 8.8 / 10.9)
High-strength bolts carry the main connections in steel structures, tunnel support frames and heavy equipment. The two things that decide whether they perform are the material and heat treatment behind the property class, and the surface condition that sets the torque–preload relationship.
Products in this range
- Hex bolts, partly threaded — DIN 931 / ISO 4014 / GB/T 5782
- Hex bolts, fully threaded — DIN 933 / ISO 4017 / GB/T 5783
- Large hex head bolts for steel structure connections
- Torsion-shear (twist-off) connection sets — bolt, nut and washer supplied as a set, tightened until the splined end shears
- Studs and threaded rods to drawing for anchor and equipment fixing
Typical specification range
| Item | Range / options |
|---|---|
| Diameter | M12 – M36 common; larger to drawing |
| Grade | 8.8 / 10.9 (metric property class) |
| Material | Medium carbon or alloy steel suitable for the grade required |
| Heat treatment | Quenched and tempered; de-hydrogen treatment applied where the process and grade require it |
| Coating | Dacromet / zinc flake, hot-dip galvanized, electroplated, phosphated, black oxide, plain |
| Thread tolerance | 6g external as standard; adjusted for coating thickness where required |
| Marking | Grade and manufacturer mark on the head |
Hydrogen embrittlement and grade 10.9
Higher strength means higher susceptibility to delayed fracture when hydrogen is introduced during pickling or electroplating. For grade 10.9, the process route matters: the coating method should be chosen with de-hydrogen treatment and, where required, a wedge-load or ductility check after treatment. Zinc-flake coatings are often selected for 10.9 for this reason, since the process introduces less hydrogen than electroplating.
Torque and preload
Assembly torque comes from the design preload, calculated as T = K × F × d, where K is the torque coefficient. K is not a fixed 0.2 — it changes with coating and lubrication. Zinc-flake and lubricated surfaces are lower friction than a dry hot-dip galvanized surface, so the same torque gives a higher preload. Always confirm the torque value against the coating actually supplied.
What to send for a quotation
- Standard or drawing, diameter, pitch and length
- Grade and material
- Coating, with the salt-spray requirement if specified
- Head form, thread length and whether the connection is supplied as a set
- Quantity, marking requirement and delivery schedule
Request a quotation or send your drawing
Tell us the specification, grade, coating and quantity — or send a PDF/DWG drawing and we will come back with price and lead time.
Tel / WhatsApp / WeChat: +86 185 3100 8199
FAQ
- Is grade 8.8 the same as SAE Grade 8?
- No. 8.8 is a metric property class to ISO 898-1; SAE Grade 8 is an inch-series grade to SAE J429. They differ in material, strength and dimensions, so the two must not be substituted without engineering approval.
- What torque should be used for a 10.9 bolt?
- The torque follows the design preload: T = K × F × d. Because K depends on the coating, ask us for the torque coefficient of the batch supplied, or confirm the torque value specified by the designer for that surface condition.
- How do you avoid hydrogen embrittlement?
- By controlling the process route: coating method, de-hydrogen treatment where required, and verification by wedge-load or ductility testing. Zinc-flake coatings introduce less hydrogen than electroplating, which is why they are often chosen for grade 10.9.
- Can you supply bolt, nut and washer as a set?
- Yes. Sets are supplied with matched properties and the same coating system so the torque coefficient is consistent across the assembly.