Properties of Tool Steels — Hardness

Hardness is a measure of a steel’s resistance to deformation. Hardness in tool steels is most commonly measured using the Rockwell C test. Hardened cold work tool steels are generally about 58/64 HRC (hardness Rockwell C), depending on the grade. Most are typically about 60/62 HRC, although some are occasionally used up to about 66 HRC.

Hardness vs Compressive Yield Strength

Hardness testers work by using a standardized load to make an indentation in the test piece, then measuring the size of the indentation. A large indentation indicates low hardness (material is easily indented). A small indentation indicates high hardness (material resists being indented). Thus, the material’s resistance to deforming (compression, indentation) is indicated directly by its hardness. When different steels measure at similar hardnesses, it is because the hardness tester made the same size impression in each. Thus, at the same hardness, different steels have similar resistance to deformation. The hardness test is basically independent of the grade of steel tested.

Tools which plastically deform in service possess insufficient hardness. Permanent bending of cutting edges, mushrooming of punch faces, or indenting of die surfaces (peening) all indicate insufficient hardness. Because a steel’s resistance to indentation is directly related to the hardness, not the grade, corrective actions for deformation may include increasing hardness, or decreasing operating loads. Changing grades will not help a deformation problem, unless the new grade is capable of higher hardness.

Choosing for Hardness

Small differences in hardness do not usually have a significant effect on the wear life of tool steels. Different tool steels are used at similar hardnesses, yet offer significant differences in expected wear life. Thus, hardness is not usually a primary factor in wear resistance, only in deformation resistance. The wear resistance of tool steels is more directly affected by their chemical composition (grade) as discussed below.