Differences Between High-Strength Bolts And Ordinary Bolts

In engineering applications, carbon-steel fasteners are classified into two primary categories: high-strength bolts and ordinary bolts. While stainless-steel bolts (SUS304/SUS316) have clear material-based distinctions, carbon-steel bolts are often misunderstood. This article systematically analyzes their differences from material, manufacturing, and performance perspectives.

I. Material & Manufacturing Process

1. High-Strength Bolts

 

Materials:

Medium-carbon steel (45#, 35#)

Alloy steel (40CrMo, 35CrMo)

Key Processes:

Quenching + Tempering (forms tempered sorbite structure)

Surface treatment (e.g., zinc plating with dehydrogenation)

Grades:

8.8: 45# steel quenched and tempered

10.9: 40CrMo alloy quenched and tempered

12.9: 35CrMo alloy quenched and tempered

2. Ordinary Bolts

 

Materials:

Low-carbon steel (Q235, A3)

Mild steel (10#, 20#)

Key Processes:

Cold heading forming

Simple surface treatment (e.g., blackening)

Grades:

4.8: Q235 cold-formed

5.6: 10# cold-formed

6.8: 35# normalized

II. Mechanical Property Comparison

 

Property	High-Strength (8.8)	Ordinary (4.8)
Tensile Strength	≥830 MPa	≥400 MPa
Yield Strength	≥660 MPa	≥240 MPa
Elongation	≥12%	≥22%
Hardness (HRC)	24–31	12–18
Fatigue Limit	450 MPa	200 MPa

III. Application Scenarios

High-Strength Applications

 

Structural Engineering:

Bridge connections (M20–M30)

Steel frame joints (twist-off type bolts)

Heavy Machinery:

Wind turbine anchor bolts (M36+)

Pressure vessel closures (B7 grade)

Ordinary Applications

 

General Fastening:

Furniture assembly (M6–M10)

Pipe clamps (galvanized)

Temporary Use:

Construction scaffolding

Automotive interior fittings

IV. Identification & Testing

Head Markings

 

High-strength: Grade marking (e.g., 8.8, 10.9)

Ordinary: Usually unmarked or with manufacturer logos

Testing Requirements

 

High-strength:

Heat treatment records

Proof load testing

Hydrogen embrittlement test

Ordinary:

Thread gauge inspection

Zinc coating thickness measurement

V. Common Misconceptions

 

Material Myth: High-carbon steel alone doesn't guarantee high strength. Proper heat treatment is essential.

Grade Misinterpretation: 8.8 grade requires minimum 830 MPa tensile strength, not just alloy content.

Corrosion Misunderstanding: High-strength bolts need post-plating dehydrogenation (e.g., 200°C × 24h).