The hardness of a material describes its resistance to permanent deformation, such as indentation or scratching. This characteristic is a fundamental property used by engineers and manufacturers to ensure components can withstand the stresses of their intended applications. Standardized methods are employed to measure this property, providing a reliable baseline for material quality and selection. The Rockwell Hardness Test is one of the most widely accepted and efficient methods for determining this measure in metallic materials.
Understanding the Indentation Depth Measurement
The Rockwell Hardness Test is an indentation method that determines a material’s resistance by measuring the depth of penetration. Unlike other tests that measure the diameter or area of an indentation, the Rockwell method focuses on the difference in depth caused by two distinct loads. The process begins with the application of a minor load, or preliminary test force, which seats the indenter and establishes a precise zero reference point. This initial step minimizes the influence of surface finish variations on the final measurement.
Once the minor load establishes the baseline depth, the major load is applied to the indenter. This load is maintained for a specific dwell time to allow the material to deform plastically. After the dwell time, the major load is removed, and the minor load is immediately reapplied and maintained. The machine then measures the depth difference between the initial baseline and the final depth remaining under the minor load.
The resulting Rockwell hardness number is inversely proportional to this measured difference in permanent indentation depth. A smaller permanent indentation indicates the material resisted the major load, resulting in a higher hardness number. The procedure is governed by established specifications for metallic materials, such as the standard ASTM E18.
Defining the Rockwell B Scale Parameters
The designation HRB (Hardness Rockwell B) identifies specific testing parameters designed for materials with intermediate hardness levels. The B scale is defined by the use of a \(1/16\)-inch diameter hardened steel ball indenter. This spherical indenter shape distributes the force over a wider area compared to a pointed diamond cone, making it suitable for softer metals where a sharp point might cause excessive localized deformation or pierce the material.
The testing sequence for the HRB scale begins with the standard minor load, or preliminary test force, of \(10\) kilograms-force (kgf). This load is applied first to hold the indenter in place and provide the initial reference depth measurement. The major load, which is the additional force applied, is set at \(90\) kgf, resulting in a total applied force of \(100\) kgf on the material.
The final HRB number is calculated using a formula that translates the measured permanent depth into a standardized unitless value. The scale uses a constant of \(130\) and subtracts the residual indentation depth increment, where each unit of depth is \(0.002\) millimeters. Using the \(130\) constant ensures that the resulting numbers are positive and fall within a usable range, where a higher HRB number signifies a harder material. Measurements falling outside the reliable range of \(0\) to \(100\) suggest that a different Rockwell scale should be selected for accuracy.
Common Applications and Scale Selection
The Rockwell B scale is utilized for testing softer metallic materials that do not require the forces of harder scales. It is the preferred scale for measuring the hardness of copper alloys, such as brass and phosphor bronze, as well as aluminum alloys and softer, unhardened steels. The \(1/16\)-inch steel ball indenter and the \(100\) kgf total load are optimized to produce a measurable indentation without causing the indenter to be permanently flattened or the test sample to deform excessively.
The necessity of the HRB scale becomes clear when contrasting it with the Rockwell C scale (HRC), which is intended for much harder materials. The HRC scale employs a diamond-tipped conical indenter and a higher total load of \(150\) kgf. This combination is necessary to penetrate materials like hardened tool steel or cast iron, which would barely be marked by the softer steel ball and lower load of the B scale.
Choosing the correct scale is required to ensure the validity of the test data. Using the HRC scale on a soft metal like brass would likely result in an excessively deep indentation that could damage the indenter or provide an unreliable reading. Conversely, using the HRB scale on a very hard material would produce a reading below the reliable range, indicating insufficient penetration. The B scale effectively bridges the gap for materials that are too hard for the Brinell test but too soft for the higher loads and sharp indenter of the C scale.