ISO 898 is a family of international standards defining the mechanical and physical properties of carbon-steel and alloy-steel fasteners. It establishes property classes such as 4.6, 8.8, 10.9 and 12.9 for bolts and Class 8, 10 and 12 for nuts.
It mainly answers:
- How strong must the fastener be?
- What material and heat-treatment requirements apply?
- What proof, tensile and hardness tests are required?
- How should the fastener be marked?
ISO 898 does not normally define the dimensions of the bolt or nut. Dimensions are controlled by product standards such as ISO 4014, ISO 4017, ISO 4032 or DIN equivalents.
Main Parts
ISO 898-1: Bolts, screws and studs made from carbon or alloy steel
ISO 898-2: Nuts made from non-alloy or alloy steel
ISO 898-3: Flat washers used with property-class fasteners
ISO 898-7: Torsional testing and minimum torque for small bolts and screws
ISO 898-1: Bolt Property Classes
A bolt property class contains two numbers, for example:
8.8
The first number indicates the nominal tensile strength: 8 × 100 = 800 MPa
The second number indicates the ratio between nominal yield strength and tensile strength: 0.8 × 800 = 640 MPa
Therefore, an 8.8 bolt has approximately:
- Nominal tensile strength: 800 MPa
- Nominal yield strength: 640 MPa
Common Property Classes
4.6 — 400 MPa tensile / 240 MPa yield — Low-strength mild steel
4.8 — 400 MPa tensile / 320 MPa yield — Low-strength commercial fastener
5.6 — 500 MPa tensile / 300 MPa yield — Medium-strength steel
5.8 — 500 MPa tensile / 400 MPa yield — General engineering
6.8 — 600 MPa tensile / 480 MPa yield — Medium-strength fastener
8.8 — 800 MPa tensile / 640 MPa yield — High-strength structural bolt
9.8 — 900 MPa tensile / 720 MPa yield — Often used in smaller diameters
10.9 — 1,000 MPa tensile / 900 MPa yield — High-strength, quenched and tempered
12.9 — 1,200 MPa tensile / 1,080 MPa yield — Very-high-strength socket screws
The exact minimum values can vary slightly depending on diameter, manufacturing method and the applicable table in ISO 898-1. ISO 898-1 applies to carbon- and alloy-steel bolts, screws and studs tested under standard laboratory conditions. The stated mechanical properties may not be retained at very high or very low service temperatures.
ISO 898-2: Nut Property Classes
Nuts use a single-number property class, such as Class 8. The class number broadly corresponds to the highest bolt property class with which the nut is intended to be paired.
Typical combinations: 4.6-5.8 bolts → Class 5 nut, 8.8 bolt → Class 8 nut, 10.9 bolt → Class 10 nut, 12.9 bolt → Class 12 nut.
A Class 8 nut is not described as 8.8. The correct designation is: Bolt = Property Class 8.8, Nut = Property Class 8. Nut performance is principally verified by a proof-load test.
Important Tests and Requirements
- Tensile strength, Yield strength or proof stress, Proof load
- Elongation after fracture, Reduction of area, Hardness
- Impact strength, Decarburisation and carburisation
- Head soundness, Wedge tensile testing
- Torsional strength for certain small fasteners
- Chemical composition and heat treatment, Surface integrity
For high-strength classes such as 8.8, 10.9 and 12.9, proper material selection and heat treatment are critical.
Marking Examples
10.9 XYZ
Where 10.9 = property class, XYZ = manufacturer's identification mark.
What ISO 898 Does Not Cover
ISO 898 is not appropriate for stainless-steel fasteners (ISO 3506), non-ferrous fasteners, fasteners requiring special corrosion resistance, fasteners for very high or cryogenic temperatures, or thread-forming screws.
Practical Specification Examples
M16 × 60 hexagon head bolt to ISO 4014, steel property class 8.8 to ISO 898-1, zinc plated.
M16 hexagon nut to ISO 4032, steel property class 8 to ISO 898-2, zinc plated.
The dimensional standard (ISO 4014/4032), mechanical-property standard (ISO 898-1/898-2), thread tolerance, and coating requirement should all be stated separately.