Bolt Grades, Nut Grades & Stainless Steel Fastener Strength Classes Explained
Introduction
Choosing the correct fastener strength class is essential for ensuring safe, reliable, and long-lasting bolted joints. Whether you are specifying fasteners for electrical panels, industrial machinery, structural steelwork, automotive assemblies, or general manufacturing, understanding the meaning behind markings such as 8.8, 10.9, 12.9, A2-70, and A4-80 helps prevent costly failures.
Steel fasteners are classified according to ISO 898, while stainless steel fasteners follow ISO 3506. These standards define mechanical properties including tensile strength, yield strength, hardness, and elongation.
This guide explains how fastener grades are designated, how to interpret the markings, and how to select the appropriate strength class for your application.
Why Fastener Grades Matter
Fasteners of identical dimensions can have vastly different mechanical strengths.
For example:
- An M10 Grade 4.6 bolt is suitable for light-duty applications.
- An M10 Grade 8.8 bolt is widely used in industrial equipment.
- An M10 Grade 10.9 bolt is used where high preload and heavy loads are required.
- An M10 Grade 12.9 socket screw provides extremely high tensile strength but lower ductility.
Selecting an incorrect grade may result in:
- Bolt stretching
- Permanent deformation
- Joint loosening
- Fatigue failure
- Catastrophic structural failure
Understanding Steel Bolt Grades (ISO 898-1)
Steel bolts use two numbers separated by a decimal.
Example: Grade 8.8
These numbers indicate the mechanical properties.
First Number
The first number multiplied by 100 gives the minimum tensile strength.
Example:
Grade 8.8
8 × 100 = 800 MPa
This means the bolt will withstand a tensile stress of at least 800 N/mm² before failure.
Second Number
The second number indicates the ratio of yield strength to tensile strength.
Yield Strength = Tensile Strength × Second Number × 0.1
Example
Grade 8.8
800 × 0.8 = 640 MPa
This means the bolt begins to plastically deform at approximately 640 MPa.
Example: Grade 10.9
Tensile Strength
10 ×100 = 1000 MPa
Yield Strength
1000 ×0.9 = 900 MPa
This explains why Grade 10.9 bolts are widely used in demanding engineering applications.
Steel Bolt Strength Comparison
| Grade | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Typical Applications |
|---|---|---|---|---|
| 4.6 | 400 | 240 | 22 | Light fabrication |
| 5.8 | 500 | 400 | 10 | General engineering |
| 6.8 | 600 | 480 | 8 | Medium-duty machinery |
| 8.8 | 800 | 640 | 12 | Industrial equipment |
| 10.9 | 1000 | 900 | 9 | Heavy machinery |
| 12.9 | 1200 | 1080 | 8 | High-strength socket screws |
Understanding Nut Strength Classes (ISO 898-2)
Unlike bolts, steel nuts have only one strength class number.
Examples:
- Class 6
- Class 8
- Class 10
- Class 12
This number indicates the proof load (test stress) the nut can withstand without thread stripping.
| Nut Class | Suitable Bolt Grade |
|---|---|
| 6 | 6.8 |
| 8 | 8.8 |
| 10 | 10.9 |
| 12 | 12.9 |
Using a lower-grade nut with a higher-grade bolt can lead to thread failure before the bolt reaches its design preload.
Understanding Grub Screw Grades (ISO 898-5)
Grub screws use hardness rather than tensile strength.
Example:
45H
- 45 ×10
- Minimum hardness = 450 HV
The “H” stands for Hardness.
These screws are designed primarily for torque transmission and shaft locking rather than tensile loading.
Stainless Steel Fastener Grades (ISO 3506)
Stainless steel fasteners use a different designation.
Example:
A2-70
The marking has three parts.
Letter
Indicates the stainless steel family.
| Letter | Stainless Structure |
|---|---|
| A | Austenitic |
| F | Ferritic |
Number Before Hyphen
Indicates the alloy group.
| Grade | Composition |
|---|---|
| A2 | Chromium + Nickel |
| A4 | Chromium + Nickel + Molybdenum |
A4 offers superior corrosion resistance, especially in marine and chemical environments.
Number After Hyphen
Represents one-tenth of the minimum tensile strength.
Example
A2-70
70 ×10 = 700 MPa
Stainless Steel Strength Comparison
| Grade | Tensile Strength (MPa) | Yield Strength (MPa) | Typical Use |
|---|---|---|---|
| A2-50 | 500 | 210–240 | General indoor applications |
| A2-70 | 700 | 450 | Industrial equipment |
| A4-50 | 500 | 210–240 | Marine environments |
| A4-70 | 700 | 450 | Chemical plants & offshore |
| A4-80* | 800 | 600 | High-strength stainless applications |
*Available from selected manufacturers.
Steel vs Stainless Steel Fasteners
| Property | Carbon Steel | Stainless Steel |
|---|---|---|
| Strength | Higher (up to 12.9) | Moderate |
| Corrosion Resistance | Requires coating | Excellent |
| Magnetic | Usually Yes | Mostly Non-magnetic (A2/A4) |
| Cost | Lower | Higher |
| Outdoor Applications | With coating | Excellent |
| Marine Applications | Not recommended | A4 preferred |
Common Applications
Grade 4.6
- Sheet metal fabrication
- Furniture
- General hardware
Grade 8.8
- Industrial machinery
- Electrical panels
- Automotive
- Heavy fabrication
Grade 10.9
- Machine tools
- Heavy equipment
- High-load assemblies
Grade 12.9
- Socket head cap screws
- Precision engineering
- Tooling
A2 Stainless Steel
- Food processing
- Pharmaceutical equipment
- Indoor installations
A4 Stainless Steel
- Marine
- Coastal installations
- Chemical processing
- Offshore equipment
Key Takeaways
- Bolt grades indicate tensile and yield strength.
- Nut grades specify proof load capacity.
- Stainless steel grades follow ISO 3506 rather than ISO 898.
- Higher grade does not always mean better—select the strength class based on the application.
- Always match bolt and nut grades appropriately to achieve the required preload and joint performance.
Frequently Asked Questions
What does Grade 8.8 mean on a bolt?
Grade 8.8 indicates a minimum tensile strength of 800 MPa and a minimum yield strength of 640 MPa, as defined by ISO 898-1.
What is the difference between Grade 10.9 and 12.9 bolts?
Grade 12.9 bolts offer higher tensile and yield strength than Grade 10.9 but generally have lower ductility. They are commonly used in high-strength socket head cap screw applications.
What does A2-70 mean on a stainless steel bolt?
A2-70 refers to an austenitic stainless steel fastener made from an A2 alloy (chromium-nickel) with a minimum tensile strength of 700 MPa according to ISO 3506.
Can I use an 8.8 bolt with a Class 6 nut?
It is not recommended. The nut should have a proof load rating compatible with the bolt’s strength class to prevent thread stripping under preload.
Which is stronger: Grade 8.8 steel or A2-70 stainless steel?
Grade 8.8 steel typically has a higher yield strength than A2-70 stainless steel, making it more suitable for high-load applications. However, A2-70 offers significantly better corrosion resistance.
