Understanding Hardness in Metalworking

Quick take: Hardness data matters because it changes tool choice, wear behavior, and whether a shop should machine a part before or after heat treatment. This page should be read with the hardening and tapping references that turn hardness values into process decisions.

Related references: Case Hardening Do-It-Yourself: Quick Guide, Tips and Tools for Tapping Threads in Hardened Materials, and Coefficient of Linear Thermal Expansion: Machining Guide.

What is Hardness?

Hardness defines a material’s ability to withstand localized plastic deformation due to mechanical force or abrasion, quantifying how resistant a surface is to dents or scratches. A harder material is stronger, but increased hardness increases brittleness, making the material more sensitive to cracking under load. This property significantly affects material selection and impacts mechanical behavior, longevity and manufacturing costs. In machining applications, knowing the hardness of the workpiece material is crucial — it directly impacts tool selection, carbide grades and optimal cutting conditions, which we’ll discuss in this article.

Units for Measuring Hardness

There are several established methods and corresponding units used to measure hardness. The Rockwell, Brinell and Vickers scales are the most common in machining. Below, we outline each method, explain the application of each, and learn how to convert them from one to another.

Rockwell (HRC/HRB/HRA): Rockwell is the most frequently used unit in engineering drawings. The test compares penetration depth from a large load against a minor preload using a spherical indenter. It consists of nine scales labeled A through K, each employing different forces and ball sizes. The Rockwell C scale (HRC) is the standard for most metallic workpieces in machining, typically falling between 10 and 65 HRC.

Brinell [HB]: The Brinell hardness scale is the most common unit used to measure the hardness of machined raw materials. (For example, in cutting condition tables in the supplier’s catalogs). This method uses a 10 mm steel ball with a force of 3,000 kgf (6,614 lbf). Figures for machined metals usually range from 100 HB for softer materials to 650 HB for hardened steels. The Rockwell C scale is only valid for materials above 180 HB; for softer metals, Rockwell B is used. Brinell, however, provides a continuous scale across the entire range.

Vickers [HV]: The Vickers system is typically used to describe cutting material hardness, like carbide grades, ceramics, CBN and PCD. The test uses a diamond-shaped pyramid, and the HV value results from dividing the applied load by the indentation’s surface area. Carbide typically falls between 1,300 – 1,900 HV. Ceramics reach 2,000 HV, CBN up to 3,000 HV, and PCD as high as 6,000 HV.