Glossary

The Rockwell Hardness Scale is one of the most widely used systems for measuring the hardness of metals, alloys, and other materials. Unlike the Mohs scale, which relies on scratch resistance, the Rockwell method determines hardness by measuring how deeply a steel or tungsten carbide indenter penetrates the surface of a material under a specific load. This makes it more precise, standardized, and applicable to a broad range of engineering materials.

The testing process begins by pressing an indenter—either a cone-shaped diamond, known as a Brale indenter, or a hardened steel ball—into the surface of the sample. First, a minor load is applied to seat the indenter and establish a reference point. A larger major load is then applied and held for a set time before being released. The depth of penetration relative to the initial position is measured, and the result is expressed as a Rockwell Hardness Number (HR). In this system, a shallower indentation indicates a harder material.

There are multiple Rockwell scales, each tailored to different materials and hardness ranges, and identified by a letter suffix after the HR value. For example, the Rockwell C scale (HRC) uses a diamond cone indenter and is typically applied to hardened steels, cutting tools, and high-strength fasteners. The Rockwell B scale (HRB) uses a steel ball indenter and is more suitable for softer metals such as copper, aluminum, and softer steels. Other variations—such as HRA, HRD, HRE, and HRF—are used for specific materials and testing conditions. As an example, a hardened steel bolt might fall in the range of HRC 35–45, while a softer structural steel may measure HRB 70–90.

The purpose and function of the Rockwell test is to provide a quick, reliable, and reproducible measure of a material’s resistance to indentation and deformation. This measurement correlates directly to the strength, wear resistance, and durability of the material being tested, making it highly valuable for both quality control and engineering design.

The Rockwell method offers several advantages. It is fast and easy to perform, requires minimal sample preparation, and is considered non-destructive for large parts since it only leaves a small indentation. It applies to a wide variety of metals and alloys and gives direct numerical results without the need for conversions.

There are also limitations. The test works best on flat or smoothly curved surfaces and is not suitable for rough or irregular geometries. It requires precise test conditions and proper calibration to ensure accuracy. It is less effective for very thin materials, coatings, or small parts where indentation depth may compromise the sample.

In applications, the Rockwell Hardness Scale is frequently used in fasteners and other industrial components to verify that they meet strength and wear resistance requirements. Automotive bolts, aerospace fasteners, and cutting tools are commonly measured on the Rockwell C scale, ensuring consistent quality and reliability in critical assemblies.