How to Choose the Appropriate Fasteners?
Hardness Grade of Nuts and Bolts in Industrial Manufacturing
In mechanical assembly and industrial manufacturing, the hardness grade of nuts and bolts is a critical factor that determines their:
Strength
Durability
Applicable scenarios
Different hardness grades are designed for varying:
Load requirements
Environmental conditions
Safety standards
Industry Standards Overview
This article details hardness classification systems including:
ISO (International Standards)
SAE (Society of Automotive Engineers)
ASTM (American Society for Testing and Materials)
Hardness grade standards for bolts
(1) SAE Standard (Society of Automotive Engineers)
| Grade | Tensile Strength (MPa) | Yield Strength (MPa) | Application |
|---|---|---|---|
| Grade 2 | 400–550 | 250 | Low-strength (Furniture, Light Structures) |
| Grade 5 | 800–900 | 650 | Moderate strength (Automotive, Mechanical) |
| Grade 8 | 1,200–1,500 | 1,000 | High strength (Aerospace, Heavy Equipment) |
(2) ISO Standard (International Organization for Standardization)
| Property Class | Tensile Strength (MPa) | Yield Strength (MPa) | Application |
|---|---|---|---|
| 4.6 | 400 | 240 | General low-strength bolts |
| 8.8 | 800 | 640 | Medium strength (Construction, Machinery) |
| 10.9 | 1,000 | 900 | High strength (Automotive, Heavy Industry) |
| 12.9 | 1,200 | 1,080 | Ultra-high strength (Aerospace, Racing) |
(3) ASTM Standard (American Society for Testing and Materials)
| Standard | Grade | Application |
|---|---|---|
| ASTM A193 | B7, B8 | High-temperature bolts (Stainless steel) |
| ASTM A320 | L7, L43 | Low-temperature bolts |
| ASTM A490 | — | High-strength structural bolts |
The hardness grade of the nut matches
Nut Hardness Grade Matching Table
| Standard | Nut Grade | Matching Bolt Grade | Notes |
|---|---|---|---|
| SAE | Grade 2 | SAE Grade 2 | For low-strength applications |
| Grade 5 | SAE Grade 5 | For moderate-strength applications | |
| Grade 8 | SAE Grade 8 | For high-strength applications | |
| ISO | Grade 4 | ISO 4.6 | General-purpose |
| Grade 8 | ISO 8.8 | Medium strength | |
| Grade 10 | ISO 10.9 | High strength | |
| Grade 12 | ISO 12.9 | Ultra-high strength |
How to choose the appropriate hardness grade?
(1) Based on Load Demand
| Load Type | Typical Load Range | Recommended Grades | Standards |
|---|---|---|---|
| Low Load | ＜ 400 MPa | SAE Grade 2 / ISO 4.6 | SAE / ISO |
| Medium Load | 800–1,000 MPa | SAE Grade 5 / ISO 8.8 | SAE / ISO |
| High Load | ＞ 1,000 MPa | SAE Grade 8 / ISO 10.9–12.9 | SAE / ISO |
(2) Based on Environmental Conditions
| Environment | Recommended Material/Grade | Standard / Type |
|---|---|---|
| High Temperature | ASTM A193 B7 | Heat-resistant alloy steel bolts |
| Corrosive (Chemical/Marine) | Stainless Steel A2 / A4 | ISO / DIN / ASTM |
| High Vibration | Nylon Insert Lock Nut | Locking type nut |
(3) Based on Industry Safety Regulations
| Industry | Required Grade / Standard | Application Example |
|---|---|---|
| Automotive | SAE Grade 5 or Grade 8 | Engine parts, chassis connection |
| Construction | ASTM A325 / A490 | Steel structures, bridge bolts |
| Aerospace | ISO 12.9 or Special Aerospace Alloys | Aircraft fasteners, load-bearing joints |
Frequently Asked Questions -FAQ
Q1: Can bolts and nuts of different standards be mixed and used?
Not recommended. Different standards of thread tolerances may vary, leading to loosening or damage.
Q2: How to measure the hardness of bolts?
Use a Rockwell Hardness Tester or refer to the certification report provided by the manufacturer.
Q3: Are high-hardness bolts necessarily better?
Not necessarily. Excessively high hardness may make the bolt brittle and prone to breakage under impact loads.
Conclusion
Choosing the correct hardness grade is of vital importance.The hardness grade of Nuts and bolts directly affects their performance and safety. The correct selection of the matching level can ensure:
✔ Higher structural stability
✔ Longer service life
Lower maintenance costs:If you need high-strength, corrosion-resistant or customized fasteners, please feel free to contact us. We offer high-quality nuts and bolts that comply with ISO, SAE and ASTM standards to meet your industrial requirements!
Please contact Weforging right now.
Email:wf-kevin@outlook.com