Determining a metal’s hardness is crucial, as cutting hard metal with a softer blade can be an expensive mistake. Material testing, particularly for metal hardness, is vital for material and tool selection. The choice of tool for a task can depend on the metal’s hardness. Knowing how to test metal hardness—and more importantly, knowing what the individual hardness tests actually means—is important not only in choosing materials for a project, but also in choosing the tools that will cut and work that material.
How to Test Metal Hardness: The Different Types of Testing
Hardness is a metal’s resistance to forceful alteration, but not all alterations are related to hardness. Bending a metal measures its tensile strength or ductility. Hardness is best measured by a material’s ability to resist direct penetration. Rigorous hardness testing involves using a probe to indent or penetrate the surface of a material, with common tests using different probe shapes and formulas to derive a numeric value.
Here, we take a look at the most commonly used metal hardness tests.
|The Most Commonly Used Metal Hardness Tests
|Metal Hardness Test
|Rockwell Hardness Test
|Widely used due to its simplicity and speed. It employs a cone-shaped or spherical indenter. An initial minor load is applied to break through any surface oxidation. This is followed by a major load. The difference in penetration depths between the minor and major loads determines the Rockwell hardness value.
|The hardness test results are expressed using different scales that express the Rockwell hardness value based on the major load used during the test. The most common being the B (100 kilogram-force) and C (150 kilogram-force) scales.
|Brinell Hardness Test
|This hardness test is commonly used for testing large or coarse-grained materials like metals, including non-ferrous metals, castings, and forgings. It involves applying a known load to a spherical indenter, typically made of hardened steel, although a tungsten carbide ball may be used for harder material. The diameter of the resulting indentation is measured, and the hardness value is determined.
|2P/πD (D – √(D²-d²)
|When a Brinell Hardness Number is quoted the conditions of the hardness test must also be expressed. Whether a steel (S) or tungsten (W) ball was used, its diameter, and the force used during the test.
|Vickers Hardness Test
|The Vickers hardness test utilizes a pyramid-shaped diamond indenter to create a square-shaped indentation on the metal surface. The diagonal lengths of the indentation are measured, and the Vickers hardness value is calculated.
|HV = (1.854 × F) / (d2)
|This test method is suitable for various materials, including metals. It is known for its accuracy and precision, making it widely used in research and quality control applications.
|Knoop Hardness Test
|The Knoop hardness test is used for measuring the hardness of small and delicate materials, and thin sections and coatings. These test methods employ a rhombic-based diamond probe. The elongated shape allows for a precise diamond-shaped indentation on small and narrow surfaces.
|HK = (1.854 × F) / (I2)
|The advantages of the hardness test are that it can be used on very small materials, and requires a very small sample for testing. The disadvantage is that it can be a difficult test. It can be affected by environmental factors like temperature. Controlling for these factors makes setup and completion time-consuming.
For fabrication or machine shops, performing rigorous scientific hardness tests may not be possible. Instead, they can evaluate hardness using standardized charts or rely on trusted suppliers to provide the correct metal for the project.