Fastener Decarburization and Carburization Control Solutions for High Strength Bolts and Nuts

Ensuring Mechanical Performance, Fatigue Resistance and Long-Term Reliability of Industrial Fasteners

High-strength fasteners such as bolts, screws, studs, and nuts are critical components in demanding industrial applications. Their mechanical performance depends not only on material selection but also on precise control of heat treatment processes, carbon content, surface hardness, and metallurgical structure.

During fastener manufacturing, improper heat treatment conditions may cause two common metallurgical defects:

• Decarburization – loss of carbon from the steel surface

• Carburization – excessive carbon enrichment on the fastener surface

Both conditions can negatively affect hardness, tensile strength, fatigue resistance, thread performance, and long-term reliability.

As an experienced industrial fastener manufacturer, Juxin Fasteners provides customized high strength fastener solutions, heat treatment quality control, and metallurgical inspection support for global OEM customers.

Our fasteners are widely used in:

• Automotive Manufacturing

• Electric Vehicle (EV) Systems

• Battery Pack Assembly

• Rail Transit Equipment

• Aerospace Components

• Electrical Cabinets

• HVAC Systems

• Medical Equipment

• Construction Machinery

• Industrial Machinery

• Robotics

• Sheet Metal Fabrication

Understanding Carbon Control in Steel Fasteners

Steel is an iron-carbon alloy containing approximately 0.0218% to 2.11% carbon.

Carbon plays a critical role in determining fastener performance:

Higher carbon content improves:

• Yield strength

• Tensile strength

• Surface hardness

• Wear resistance

However, excessive carbon may reduce:

• Ductility

• Impact resistance

• Fatigue performance

Therefore, controlling carbon content during fastener production is essential to achieve the correct balance between strength and toughness.

For high performance fasteners, especially Grade 8.8, Grade 10.9, and Grade 12.9 bolts and screws, precise heat treatment control is required to maintain consistent mechanical properties.

Fastener Decarburization: Causes, Risks and Prevention

What Is Decarburization?

Fastener decarburization occurs when carbon is removed from the surface layer of steel during heating processes.

This reduces surface hardness and weakens the fastener's ability to withstand:

• High tensile loads

• Cyclic loading

• Thread stress

• Fatigue conditions

Decarburization can be classified into:

Full Decarburization

The carbon content is completely removed from the surface area, resulting in a ferrite structure during metallographic examination.

Partial Decarburization

Carbon loss occurs only in a surface layer, causing:

• Reduced hardness

• Lower strength

• Different metallographic structure compared with the core material

A fastener is generally considered decarburized when surface hardness is approximately 30 HV lower than the core hardness.

Causes of Fastener Decarburization During Manufacturing

Common causes include:

1. Improper Heat Treatment Atmosphere

During quenching and tempering, uncontrolled furnace gases may introduce oxidation reactions.

Carbon loss can occur when:

• Furnace atmosphere is unstable

• Carbon potential is not properly controlled

• Oxygen-containing gases enter the furnace

2. Raw Material Surface Conditions

Rust or contamination on steel wire and bars may generate:

• CO₂

• H₂O vapor

during heating, accelerating carbon loss.

3. Improper Annealing Process

Incorrect annealing parameters may deepen the decarburized layer before cold forming.

When cold forging is performed, this weakened surface layer may remain on thread areas and reduce final fastener performance.

Effects of Decarburization on Fastener Performance

Surface decarburization may result in:

• Reduced thread strength

• Thread stripping failure

• Lower wear resistance

• Surface cracking during quenching

• Reduced fatigue strength

Studies show that bolts with surface decarburization may have significantly lower fatigue resistance compared with properly heat-treated fasteners.

For safety-critical applications, decarburization inspection is essential.

Fastener Carburization: Causes and Quality Risks

What Is Carburization?

Fastener carburization occurs when excessive carbon enters the surface layer of steel during heat treatment.

Although higher carbon content increases hardness, excessive surface carbon creates a brittle surface layer.

This may cause:

• Reduced fatigue resistance

• Increased cracking risk

• Poor impact performance

• Premature fastener failure

Causes of Fastener Carburization

Typical causes include:

Incorrect Furnace Carbon Potential

The protective atmosphere inside the heat treatment furnace must match the original carbon content of the steel.

Incorrect carbon potential control may increase surface carbon levels.

Excessive Carbon Restoration Treatment

When restoring decarburized fasteners, excessive carburization may occur if process parameters are not accurately controlled.

Proper carburization must avoid:

• Excessive surface hardness

• Grain growth

• Reduced toughness

Carburization Requirements for High Strength Fasteners

According to international fastener requirements, high strength bolts such as:

• Grade 8.8

• Grade 10.9

• Grade 12.9

require strict control of surface hardness.

Typical requirements include:

• Surface hardness should not exceed core hardness by more than approximately 30 HV

• Grade 10.9 fasteners should maintain controlled hardness levels

• Grade 12.9 fasteners require even stricter metallurgical control

Fastener Decarburization and Carburization Testing Methods

Juxin Fasteners applies professional inspection methods to ensure reliable fastener performance.

1. Metallographic Testing

Metallographic analysis is suitable for:

• Grade 8.8 fasteners

• Grade 10.9 fasteners

• Grade 12.9 fasteners

This method evaluates:

• Full decarburized layer depth

• Partial decarburized layer height

• Metallurgical structure changes

Testing references include:

• ISO 4014

• ISO 4017

2. Vickers Hardness Testing (HV)

Vickers hardness testing is widely used for evaluating carbon changes.

Testing parameters include:

• HV0.3 Surface hardness testing

• HV2 hardness comparison

• HV3 hardness evaluation

For decarburization:

HV2 ≥ HV1 - 30 HV

indicates no significant decarburization.

For carburization:

HV3 ≤ HV1 + 30 HV

indicates no excessive carburization.

3. Core Hardness Testing

Core hardness represents the original hardness of the fastener material before surface carbon changes occur.

The measurement area is normally located between:

• Thread radius area

• Fastener centerline

This ensures accurate evaluation of internal mechanical properties.

International Fastener Hardness Testing Standards

Juxin Fasteners follows internationally recognized testing methods including:

Brinell Hardness Test

ISO 6506-1

Rockwell Hardness Test

ISO 6508-1

Vickers Hardness Test

ISO 6507-1

These inspection methods help ensure consistent quality for industrial fasteners used in demanding environments.

Engineering Solutions for Different Industries

Automotive & EV Fastener Solutions

Automotive manufacturers require:

• High fatigue resistance

• Stable torque performance

• Reliable thread strength

Applications:

• Engine components

• Chassis systems

• EV battery structures

• Electric motor assemblies

Battery Pack Fastening Solutions

EV battery packs require fasteners with:

• High strength

• Corrosion resistance

• Stable mechanical performance

Juxin provides:

• Custom bolts

• Nuts

• Rivet nuts

• Threaded inserts

• Specialty fasteners

Rail Transit Fastener Solutions

Rail applications require:

• Long service life

• Vibration resistance

• High fatigue performance

Heat-treated fasteners with controlled metallurgy improve safety and reliability.

Aerospace Fastener Solutions

Aerospace applications demand:

• Lightweight design

• High strength-to-weight ratio

• Strict quality control

Electrical Cabinet & Sheet Metal Fastening Solutions

For electrical enclosures and sheet metal fabrication:

• Self-clinching fasteners

• Rivet nuts

• Weld nuts

• Thread inserts

provide reliable fastening without damaging thin materials.

HVAC, Medical Equipment, Machinery and Robotics

Precision fasteners support:

• Equipment assembly

• Structural connections

• Automated production systems

• High reliability applications

Why Choose Juxin Fasteners?

Juxin Fasteners provides:

✔ OEM customized fastener manufacturing
✔ High strength bolts and nuts
✔ Heat treatment controlled production
✔ Metallographic inspection
✔ Hardness testing
✔ ISO standard compliance
✔ Automotive and industrial fastening solutions
✔ Global OEM supply capability

We support engineers, purchasing teams, and manufacturers worldwide with reliable fastening solutions designed for demanding applications.

Contact Juxin Fasteners

For custom fasteners, heat treatment requirements, and engineering support:

Email: info@juxinfasteners.com

Website:
https://www.juxinfasteners.com/