***Update: this standard has been revised. ASTM E18-19: Standard Test Methods For Rockwell Hardness Of Metallic Materials is available on the ANSI Webstore.
As we have previously discussed, hardness often is attached to a multitude of definitions, including resistance to local penetration, scratching, and abrasion, yet it is not quite a fundamental property of a material but more of a characteristic, serving as a composite understanding of tensile strength, elasticity, and ductility. While there are various test methods for assessing material hardness, the Rockwell hardness test is the most widely used and versatile of the practiced methods.
Rockwell hardness test methods are standardized in ASTM E18-17: Standard Test Methods for Rockwell Hardness of Metallic Materials. This document provides guidance for Rockwell hardness machines and the procedures for performing Rockwell hardness tests of metallic materials, detailing the necessary principles, procedures, calculations, and apparatus for the tests. The standard is useful for determining related physical characteristics of the metallic materials undergoing testing, as well as meeting acceptance testing of commercial shipments, as long as adherence to any national requirements is maintained.
The Rockwell hardness test, according to ASTM E18-17, is an indentation hardness test that involves the use of a verified machine to force a diamond spheroconical indenter or tungsten carbide (or steel) ball indenter into the surface of a material. Hardness is evaluated by measuring the difference in depth of the indentation as the force on the indenter is increased from a specified preliminary test force to a specified total test force and then returned to the preliminary test force.
Rockwell hardness testing is somewhat comparable to Brinell hardness testing, in that it measures a metal’s resistance to penetration, however, it differs in that the depth of the impression is measured, rather than the diametric area in the case of Brinell. Due to its speed and simplicity, as well as the small size of the indentation and its ability to distinguish small differences in hardness, the Rockwell test is used extensively in industry, for which it is also frequently automated.
Hardness numbers have no units but commonly are given lettered scales. The scales given in ASTM E18-17 are B, C, A, D, E, F, G, H, K, L, M, P, R, S, V, and each undergoes a specific indenter and total test force, with some overlap among the bunch. For example, B scale, which uses a 1/16 inch ball diamond indenter under a total test force of 100 kgf, is applicable to copper alloys, soft steels, aluminum alloys, and malleable iron.
ASTM E18-17 is the most recent edition of the standard for Rockwell testing of metallic materials and contains some changes from the 2016 revision. These include the addition of Note A1.4 on indirect verification and the substitution of the word “signature” with “identification” in several sections of the standard document. In addition, it is worth noting that there was one substantial change that took place in the 2016 edition. This was a revision of Section A184.108.40.206 on the cone and spherical tips of Class B Diamond Indenters.
ASTM E18-19: Standard Test Methods for Rockwell Hardness of Metallic Materials is available on the ANSI Webstore.