Feb 2026
Hardness Testing for Fasteners – Methods, Importance & Standards
What is Hardness?
Hardness is the measure of a material’s resistance to plastic deformation when subjected to indentation. In practical terms, it represents a material’s ability to resist scratching, abrasion, cutting, or permanent deformation when a load is applied. Generally, the harder a material is, the better it can withstand surface damage and wear.
In fasteners, hardness is closely linked to manufacturing and heat treatment processes such as annealing, quenching and tempering, and case hardening. High-strength fasteners that are quenched and tempered typically exhibit higher hardness compared to lower-strength, annealed fasteners.
Hardness is influenced by several material properties including strength, ductility, stiffness, toughness, and microstructure. For fasteners, hardness serves as a critical indicator of overall mechanical performance.
Why is Hardness Testing Important?
For many materials, especially steels, there is a strong correlation between hardness and tensile strength. Tensile strength defines the maximum tensile stress a material can withstand before failure.
While hardness testing does not directly measure yield strength or ductility, results that fall within specified limits provide a strong indication that other mechanical properties are also within acceptable ranges.
In ideal conditions, tensile testing is the preferred method to verify fastener strength. However, many fasteners are too small to be securely fixtured in tensile testing machines. In such cases, hardness testing becomes the most reliable predictor of tensile strength.
Additionally, hardness testing is:
• Fast and efficient
• Non-destructive or minimally destructive
• Suitable for in-process monitoring
Fastener manufacturers and heat-treatment facilities commonly use hardness testing during production to ensure consistent quality and compliance with specifications.
Types of Hardness Testing
Hardness testing is performed by pressing a precisely shaped indenter into the fastener surface under a controlled load. These tests are broadly classified into two categories:
Macro Hardness Testing
Macro hardness tests use relatively large loads and indenters, producing indentations visible to the naked eye. These methods are commonly used for standard fastener testing.
Microhardness Testing
Microhardness tests use very light loads and small indenters, creating indentations that are only visible under magnification. These tests are ideal for thin sections, surface-hardened layers, and case-hardened fasteners.
Common Hardness Testing Methods for Fasteners
Rockwell Hardness Test
The Rockwell hardness test is the most widely used method for fasteners, especially in North America. Steel fasteners are typically tested using either the Rockwell B or Rockwell C scale.
• Rockwell B (HRB):
Uses a 1/16” diameter steel ball with a 100 kgf load.
Suitable for annealed and non-heat-treated fasteners.
• Rockwell C (HRC):
Uses a 120° diamond cone with a 150 kgf load.
Suitable for hardened and heat-treated fasteners.
The Rockwell test measures the depth of indentation by applying a minor load followed by a major load and converting the depth difference into a hardness value.
Rockwell Superficial (N Scale)
Used for case-hardened fasteners with shallow hardened layers.
• 15N, 30N, and 45N scales
• Diamond cone indenter
• Lower test loads for accurate surface measurements
Vickers Hardness Test
The Vickers method is widely preferred outside North America and is known for its versatility. It uses a pyramidal diamond indenter and produces results expressed as a Vickers Hardness Value (HV).
The same indenter can be used for both soft and hard materials, making this method highly adaptable. Loads range from 1 gf to 100 kgf, allowing both macro and micro hardness testing.
Unlike Rockwell, Vickers hardness is calculated by measuring the diagonal lengths of the indentation and applying them to a standardized formula.
Knop Hardness Test
The Knop hardness test is a micro hardness method designed for very thin layers or brittle materials. It uses an elongated pyramidal diamond indenter with a light load, typically around 100 gf.
Only the long diagonal of the indentation is measured, making this method particularly suitable for case-hardened fasteners and surface coatings.
Brinell Hardness Test
The Brinell test is less commonly used for fasteners but may still be applied in certain cases. It uses a large 10 mm diameter steel ball with a load of up to 3000 kgf.
The diameter of the indentation is measured and converted into a Brinell Hardness Number (HB). This method is more suitable for softer materials and larger components.
Hardness Testing in Fastener Manufacturing
Hardness testing is commonly performed:
• During heat treatment processes
• As an in-process quality control check
• During final inspection
Typical applications include:
• Quench and tempered fasteners: Rockwell C or Macro-Vickers testing
• Case-hardened fasteners: Micro-Vickers or Knoop testing
• Annealed fasteners: Rockwell B testing
Each test method is selected based on fastener size, material, heat treatment, and application requirements.
Standards & Compliance
Hardness testing of fasteners is conducted in accordance with internationally recognized standards, primarily governed by organizations such as:
• ASTM
• ISO
Strict adherence to these standards ensures test accuracy, repeatability, and global acceptance of results.
Conclusion
Hardness testing is an essential and routine quality control process in fastener manufacturing and inspection. It provides a reliable indication of material strength, heat treatment effectiveness, and overall performance.
When conducted in accordance with ASTM and ISO standards, hardness testing ensures that fasteners meet specified requirements and perform safely in their intended applications.