How to Choose the Right Fasteners? - Quanzhou Weforging Machinery Manufacturing Co., Ltd.

Detailed Explanation of the Hardness Grades of Nuts and Bolts: How to Choose the Appropriate Fasteners?

In industrial manufacturing and mechanical assembly, the Hardness Grade of nuts and bolts directly affects their strength, durability and applicable scenarios. Different hardness grades are suitable for different loads, environments and safety requirements. This article will introduce in detail the hardness classification standards of nuts and bolts (such as ISO, SAE, ASTM) to help customers correctly select non-standard fasteners that meet their requirements or customize by type.

Why is hardness grade important?

»»»The hardness grade determines the:

✔ Tensile Strength — the maximum Tensile force that can be withstood

✔ Yield Strength – The critical point at which permanent deformation begins

✔ Shear Resistance – Prevents fracture

✔ Durability – Resistance to fatigue and wear

»»»Choosing the wrong hardness grade may lead to:

∞ Bolt breakage (Excessive hardness may make it brittle)

∞ Thread slippage (deformation may occur if the hardness is too low

∞ Loosening or failure (not meeting the load requirements)

2. Hardness grade standards for bolts

»»»Different countries and regions adopt different standards.

→ The following are the main classifications:

(1) SAE standard (Society of Automotive Engineers)

It is mainly used for American-made bolts and is indicated by “Grade”, such as Grade 2, Grade 5, Grade 8.

SAE grade tensile strength (MPa) and yield strength (MPa) are commonly applied

Grade 2 400-550 250 Low-strength general-purpose bolts (Furniture, light structures)

Grade 5 800-900 650 Moderate strength (Automotive, Mechanical)

Grade 8 1,200-1,500 1,000 High strength (aerospace, heavy-duty equipment)

Identification method: The head of the bolt usually has concave point marks (such as Grade 5 = 3 lines, Grade 8 = 6 lines).

However, the strength of the above-mentioned nuts is related to the material to some extent. Different materials have different toughness and brittleness, so the manufacturing processes are also different. Even with the same strength grade, different materials will result in different usage effects, which are directly related to the materials and manufacturing processes. For more details, please contact our company. There are technical contact information at the bottom of this article!

(2) ISO standards (International Organization for Standardization)

Adopt “Property Class” (performance level), such as 4.6, 8.8, 10.9, 12.9.

Common applications of ISO grade tensile strength (MPa) and yield strength (MPa)

4.6 400 240 General low-strength bolts

8.8 800 640 Medium strength (Construction, Machinery)

10.9 1,000 900 High intensity (automobiles, heavy industry)

12.9 1,200 1,080 Ultra-high strength (Aerospace, racing)

Identification method: The head of the bolt will be engraved with a number (such as “8.8”).

(3) ASTM standards (American Society for Testing and Materials)

Mainly used in special environments (such as high temperatures and corrosion resistance), common grades:

ASTM A193 (High-temperature bolts) : Such as B7, B8 (stainless steel)

ASTM A320 (low-temperature bolts) : such as L7, L43

ASTM A490 (High-strength structural bolts) : Similar to SAE Grade 8

3. The hardness grade of the nut matches

The hardness grade of the nut must match that of the bolt; otherwise, it may lead to:

Thread damage (the nut is too soft)

Loose failure (the nut is too hard)

↔ SAE nut grade

Grade 2 (Matching SAE Grade 2 bolts)

Grade 5 (Matching SAE Grade 5 bolts)

Grade 8 (Matching SAE Grade 8 bolts)

↔ ISO nut grade

Grade 4 (Matching 4.6 bolts)

Grade 8 (Matching 8.8 bolts)

Grade 10 (matching 10.9 bolts)

Grade 12 (matching 12.9 bolts)

Note: High-grade nuts are usually marked with numbers (such as “10”).

4. How to choose the appropriate hardness grade?

(1) according to the load demand Low load (<400MPa) :

SAE Grade 2 / ISO 4.6

Medium load (800-1000MPa) :

SAE Grade 5 / ISO 8.8

High load (>1000MPa) :

SAE Grade 8 / ISO 10.9-12.9

(2) According to environmental factors

High-temperature environment:

ASTM A193 B7

Corrosive environment: Stainless steel (A2/A4)

Vibration environment:

With Nylon Insert Lock Nut (Nylon Insert Lock Nut)

(3) In accordance with safety regulations

Automotive industry:

SAE Grade 5 or 8 is usually required

Construction industry:

It must comply with ASTM A325 / A490

Aerospace:

Ultra-high strength (12.9 or special alloys)

5. 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.

6. 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:[email protected]