Technically Speaking
Types of Tests Used to Characterize Springmaking Materials — Part 4: Hardness Testing By C. Richard Gordon
T
his is the fourth Springs magazine article in the series regarding mechanical tests for springmaking materials, the hardness test. Previous articles in the series are the tensile test (Winter 20201), the coiling and wrapping tests (Spring 20202), and the torsion test (Summer 20203). This article includes presentation materials from a Testing and Properties class4 that I have taught for a number of years for the Wire Association International as part of their Fundamentals of Wire Manufacturing program.
In all these techniques, an indenter is forced into the sample of interest by the application of a specific load or force, as shown in Figure 1. The depth or width of the impression left is evaluated to establish a measure of hardness. Force
Overview In general, the testing of materials represents an important part of all quality work. It can include the control of incoming raw materials, materials in production, and produced materials or components before delivery. Many different techniques are used, including chemical analysis; microscopy; nondestructive testing; mechanical tests such as tensile strength, hardness and fatigue; and technological tests such as bending, torsion, coiling, wrap and weldability. In this series of articles, we have focused on mechanical and technological tests used to characterize springmaking materials. In this article, several different types of hardness tests will be discussed. The uniformity of tensile strength and ductility of the wires used to produce springs are critical to successful spring production and end use. The hardness test can be good indicator of tensile strength.
Hardness Testing In general, the hardness of a material suggests a resistance to deformation. For metals, hardness is a measure of the resistance to permanent or plastic deformation. For materials testing, hardness can be thought of as a resistance to indentation. Hardness can be correlated with tensile properties of materials. It is an important quality parameter for assessing the suitability for many wire products and springs. It is included as a specification parameter for many sophisticated product standards worldwide. Hardness testing is a key element in many quality control procedures and research and development work. In this article, several different types of hardness tests will be discussed: Brinell, Rockwell, Vickers, and Knoop.
Indenter
Sample
Indentation
Figure 1. Schematic representation of hardness testing.
With the Rockwell hardness test, the depth of indenter penetration is measured. With the Brinell, Vickers and Knoop hardness tests, the size of the impression left by the indenter is measured. Figure 2 shows typical indentation marks from various hardness tests. There are essentially two basic types of indentation hardness tests: 1) macro load testing, using loads greater than or equal to 1,000 grams force and 2) micro load testing, using loads less than 1,000 grams force.
Rick Gordon is the technical director for SMI. He is available to help SMI members and non-members with metallurgical challenges such as fatigue life, corrosion, material and process-related problems. He is also available to help manage and oversee processes related to failure analysis. This includes sourcing reputable testing labs throughout North America, forwarding member requests to the appropriate lab and reporting results and recommendations. He can be reached at c.richard.gordon@gmail. com or 574-514-9367.
SPRINGS / Fall 2020 / 19
