Mainstream Hardness Scales and Their Conversion

Hardness is a measure of a material’s resistance to localized plastic deformation, such as a dent or a scratch. Unlike properties such as mass or length, hardness is not a fundamental property but is defined by the method of measurement. Consequently, there are numerous hardness scales, each suitable for different materials and applications. Direct conversion between scales is not always precise, as the tests measure different physical properties under different conditions.

The most common hardness tests can be categorized into three main types: Indentation Hardness, Rebound (Dynamic) Hardness, and Scratch Hardness. We will focus on the mainstream Indentation Hardness scales.

1. Rockwell Hardness (HR)

This is the most widely used method due to its speed, simplicity, and ability to test a wide range of materials directly on the production floor.

Principle: Measures the permanent depth of indentation produced by a force applied through an indenter. The test uses a minor pre-load (to seat the indenter and break through any surface scale) followed by a major load.

Common Scales: Rockwell uses multiple scales defined by the type of indenter and the load applied. The most common are:

HRC: Uses a 120° diamond cone (Brale indenter) and a 150 kgf major load. Used for hard steels, deep case hardened parts, and other hard materials.

HRB: Uses a 1/16-inch diameter steel ball and a 100 kgf major load. Used for softer steels, copper alloys, aluminum, and softer materials.

HRA: Uses a Brale indenter and a 60 kgf load. Used for very hard materials like tungsten carbide and thin, hard steel.

Unit: Reported as a dimensionless number (e.g., 60 HRC). A higher number indicates greater hardness.

2. Brinell Hardness (HB)

This is one of the oldest methods and is known for its reliability, especially for materials with a coarse or non-uniform grain structure.

Principle: A hard carbide ball (typically 10 mm in diameter) is pressed into the material’s surface with a specified force (e.g., 3000 kgf for steel). The diameter of the resulting indentation is measured under a microscope.

Unit: The Brinell Hardness Number (HB or BHN) is calculated as the load divided by the surface area of the indentation. It is often written with the test conditions, for example: 250 HB 10/3000 (250 Hardness, 10mm ball, 3000 kgf load).

Application: Ideal for cast iron, aluminum, copper alloys, and soft/annealed steels. The large indentation averages out variations in the material’s microstructure.

3. Vickers Hardness (HV)

The Vickers test is known for its high accuracy and is used for a very wide range of materials, from very soft to extremely hard.

Principle: Uses a pyramid-shaped diamond indenter with a square base and a face angle of 136°. The indentation is always geometrically similar, regardless of load. The diagonals of the square indentation are measured.

Unit: The Vickers Hardness Number (HV) is calculated as the load divided by the surface area of the indentation. The load is specified, e.g., 640 HV 30 (640 Hardness Vickers, 30 kgf load).

Application: Excellent for thin materials, surface-treated layers (like nitriding), and very hard, brittle materials. It can also be used for microhardness testing with very low loads (as Vickers Microhardness).

4. Knoop Hardness (HK)

A microindentation test primarily used for brittle materials or thin sheets where a Vickers impression might cause cracking.

Principle: Uses an elongated pyramidal diamond indenter that produces a rhombic-based indentation. The long diagonal is measured. The Knoop indenter creates a shallow indentation for a given load compared to Vickers.

Unit: The Knoop Hardness Number (HK) is calculated similarly to Vickers.

Application: Primarily used in microhardness testing for ceramics, glass, and thin coatings.

5. Shore Hardness (HS)

This method is predominantly used for polymers, elastomers, and rubbers.

Principle: Measures the resistance of a material to the penetration of a spring-loaded indenter. The two common scales are:

Shore A: Uses a truncated cone indenter. Used for softer plastics and rubbers (e.g., tire tread, shoe sole).

Shore D: Uses a sharp, pointy indenter. Used for harder plastics (e.g., hard hats, PVC pipes).

Unit: Reported on a scale of 0-100 (e.g., 70 Shore A).

Hardness Conversion

Crucial Point: There is no exact, universal mathematical formula to convert between different hardness scales. The relationship is empirical and depends on the material’s specific properties (e.g., work-hardening behavior, elasticity).

Conversion is done using standardized conversion tables published by organizations like ASTM International (ASTM E140). These tables are derived from extensive experimental data collected by testing the same sample with different methods.

Approximate Conversion Chart (for Steel)

Note: This is a simplified table for illustrative purposes. Always refer to the official ASTM E140 standard for precise conversions.

Approximate Conversion Formulas

For rough estimations, especially within a specific material group (like steel), some approximate formulas exist:

HV ≈ 1.08 * HB (Approximately valid for steel with hardness less than 400 HB)

HRC ≈ (HB – 70) / 3.2 (A very rough approximation for hardened steels)

For Steel: Tensile Strength (in psi) ≈ 500 * HB (A widely used rule of thumb for estimating ultimate tensile strength from Brinell hardness for carbon and low-alloy steels).

Summary and Key Takeaways

Scale Dependency: The “hardness” value is meaningless without specifying the scale (e.g., “60 HRC” is correct; “60 Hardness” is not).

Empirical Conversion: Conversion between scales is not a calculation but a lookup in standardized tables based on experimental data.

Material Suitability: Choose the hardness test based on the material:

Rockwell: Fast, general-purpose for metals.

Brinell: Best for coarse-grained or heterogeneous materials.

Vickers/Knoop: Excellent for thin sections, hard/brittle materials, and microstructural analysis.

Shore: For polymers and elastomers.

For any critical application, always consult the official conversion standards or perform direct testing with the required method.