Resources

Introduction

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. The once-thought-of manufacturing tool has developed into a metrological instrument. To achieve meaningful measurement results in these circumstances, it is important that the user make every effort to reduce measurement errors. This is more easily accomplished when the influences contributing to the error in a Rockwell hardness test are known, and there is an understanding of what can be done to reduce these errors. Adhering to “good practice" procedures when performing Rockwell hardness measurements and calibrations is a crucial step to reducing measurement errors. The purpose of this Guide is not to specify the requirements for conducting a Rockwell hardness test. Test method standards published by national and international standards writing organizations, such as the American Society for Testing and Materials (ASTM) and the International Standards Organization (ISO), provide specific requirements and procedures for Rockwell hardness testing. The intention 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. It is recognized that Rockwell hardness is often used for testing non-metallic materials such as plastics; however, this Guide is primarily applicable to the testing of metallic materials. This Guide also provides recommendations for conducting verifications of Rockwell hardness machines based on the procedures specified by the test method standards. Some procedures recommended by this Guide exceed current requirements of the test methods; however, they can be very useful in helping to determine and limit sources of measurement error.