Hardness Testing of Metallic Materials - NIST Recommended Practice Guide

The Rockwell hardness test continues to be applied as a tool for assessing the properties of a product while the tolerances on the acceptable material hardness have become tighter and tighter. Adhering to "good practice" procedures when performing Rockwell hardness measurements and calibrations is a beneficial step to reducing measurement errors. The purpose of this Guide is to explain the causes of variability in Rockwell hardness test results and to supplement the information given in test method standards with good practice recommendations. Although this Guide is directed more towards the users of Rockwell hardness having the greatest concern for accuracy in their measurements, much of the information given is also applicable for users that only require test results to be within wide tolerance bands, where high accuracy is not as critical. Working in a ball-bearing manufacturing plant in 1919, Stanley P. Rockwell invented the Rockwell hardness test as a tool for obtaining a rapid and more accurate measure of the hardness of ball races(1). Soon after, Charles H. Wilson expanded on Rockwell's invention, and he advanced the Rockwell hardness test into what is today the most widely used method for acceptance testing and process control of metals and metal products. Since its development, the popularity of the Rockwell hardness test has steadily grown. The Rockwell hardness test continues to be applied as a tool for assessing the properties of a product while the tolerances on the acceptable material hardness have become tighter and tighter.