Metric carbon and alloy steel bolts are graded by property class under ISO 898-1. Common classes are 4.6, 5.8, 8.8, 10.9 and 12.9, and the higher the number, the stronger the bolt.
Nuts follow ISO 898-2, stainless steel fasteners follow ISO 3506, and US inch fasteners follow ASTM or SAE grades. Dimensions come from separate standards, so a full order names both.
How to read an ISO property class
The class has two numbers. The first, times 100, gives the nominal tensile strength in megapascals. The second, times 10, gives the ratio of yield strength to tensile strength as a percentage.
So class 8.8 has a nominal tensile strength of 800 MPa and a nominal yield of 80% of that, or 640 MPa. Class 10.9 is 1,000 MPa tensile with 90% yield ratio. The bolt head should carry the class mark and the maker's mark.
Other common grades
- ISO 898-2 nut classes, such as 8 and 10, must match or exceed the bolt class they are used with.
- ISO 3506 stainless grades, such as A2-70 and A4-80, give the steel group and the strength level.
- ASTM A193 B7 studs and ASTM A194 2H nuts are common for flanged pipework.
- ASTM F3125 covers structural bolts, including grades formerly known as A325 and A490.
- SAE J429 grades 2, 5 and 8 are common for inch bolts in vehicles and machinery.
Where DIN fits in
Many buyers still order to DIN numbers, such as DIN 931 and DIN 933 for hex bolts. Most of these have been replaced by ISO standards, such as ISO 4014 and ISO 4017. The dimensions are close but not always identical, especially across-flats sizes on some diameters.
State which standard you want. If your existing parts are DIN, say so, and ask the supplier to confirm any differences.
The same care applies to thread pitch and length. Metric coarse threads are the default, but fine-pitch bolts are common in vehicles. Inch bolts need UNC or UNF stated clearly.
Tests that prove the grade
- Hardness testing on samples from each lot.
- Tensile or wedge tensile testing on full-size bolts.
- Proof load testing on nuts and bolts.
- Chemical analysis of the steel, backed by a mill certificate.
- Coating thickness and, for plated high-strength bolts, a check that baking was done.
Certificates and independent tests
Ask for a lot certificate that shows these results against the standard. For critical bolts, have an independent lab test samples, as explained in reading a lab certificate. To see the product range, read what fasteners you can source.
Next step
We check grade markings, review lot certificates and have samples tested at an independent lab. Book a free sourcing review to discuss your fastener range.