"Hardness is the mechanical resistance that a material opposes the mechanical penetration of a harder body."
Thus, the hardness is an essential ingredient of component calculations, drawing specifications or in the characterization of materials. Modern metal materials offer a very wide range of different operation purposes. Starting from very soft non-ferrous metals such as Pb or Cu, over (surface layer) hardened steels up to very hard coatings such as tungsten carbide. Our according to DIN EN ISO 17025 accredited lab offers the usual stationary methods of hardness testing, which are used in the field of metallic materials: Vickers, Rockwell and Brinell.
From the hardness test procedures, the Vickers method is the most universally applicable test. Hereby a pyramidal diamond indenter is pressed with a defined stress in the workpiece. The typical load range extends from 0.01 to 30 kg. A determination of the hardniss is carried out, depending on the selected test load, on the by plastic deformation caused dent by average calculation of the diagonal.
The spectrum of this method allows, for example, smallest measuring points with diagonals of approximately 10 µm for the testing of individual structural components, closely spaced dents for processes (hardening depth) or extensive dents on porous materials.
The GWP is equipped with the most modern hardness test devices, which offer the possibilities of manual, semiautomatic and fully automatic evaluation.
Normen: DIN EN ISO 6507-1 (Vickers) , DIN EN ISO 2639 (CHD) , DIN 50190-3 Nht
The Brinell hardness test is used for soft to medium hard metals, and very porous and inhomogeneous materials such as cast or light metal alloys.
In the tests hard metal balls with different diameters (1, 2.5, 5 and 10 mm) are pressed with into the surface with a defined force. Similar to the Vickers method the calculation of the hardness is based on the ratio of the average diagonal diameter for the used ball and the test load. According to DIN EN ISO 6506-1 wear levels are defined for various materials. These apply to the expected hardness, ball diameter and test load.
The GWP covers with their equipment park the typical testing areas. Thus, e.g. also tests with a load of 3000 kg can be performed.
The Rockwell hardness test is divided into several methods, which differ in the test loads and indenters. In the two most common methods HRA and HRC there is a test piece with diamond tip. In contrast to the Vickers and Brinell method, the hardness is determined not by the lasting dent diagonal but by the depth of penetration. Because of this kind of determination the hardness value can be read from a scale without optical measurement devices and the requirements regarding surface quality are lower.
The Rockwell method is preferably used for very hard materials or heat treatment conditions, e.g. surface hardened or coated components.
Norm: DIN EN ISO 6508-1 (Rockwell)
Surface layer hardenings are common methods for improving the wear resistance of components. For the examination next to metallographic investigations also hardness profiles are used. Via these measurements next to the hardness difference compared to the core hardness also the depth can be determined. For this purpose several Vickers-test indentations are set and measured in defined intervals to the component edge.
Usually the examinations take place on metallographic prepared samples. Due to different hardness and processing methods the testing methods can be differed: nitriding (Nht / NHD), input hardening depth (chd / CHD), welding seam profile testing, ... .
Also in the field of damage investigation hardness profile measurements are used, to e.g. characterize thermal effects.
Norm: DIN EN ISO 2639 (CHD) , DIN 50190-3 Nht
For the characterization of different materials the hardness test offers a very wide spectrum of possibilities. With the micro hardness test (usually load ranges of 1 to 200g) e.g. different phases of a joining and very thin layers (>10 µm) can be tested. These evaluations lead to conclusions about e.g. wear resistance and possible causes for damages. By these small impressions also very close contiguous impressions can be realized e.g. for hardness profiles or mappings.
Norm: DIN EN ISO 6507-1; DIN 50190-1-2-3
Filling the sample with indents (hardness mapping) can be useful for case-hardened parts or to check sample homogeneity. This feature is used to detect even the smallest local hardness increase in large samples. Hardness mapping are often applied to verify welded joints. Thermal and mechanical stress can induce microstructural changes in materials, which can be detected with a hardness test.
Particularly in the case of multi-layered welds, it is thus possible to identify specific areas, in which the heat input has significantly affected the microstructure, and as consequence affects the quality of the weld joint.
Hardness mapping are performed according to Vickers. 500-2000 indents are automatically placed on the selected sample area. The software automatically assigns colors for the minimum and maximum hardness values.
The instrumented penetration testing for the determination of hardness and other material parameter (also universal hardness or hardness martens) also offers the possibility to determine and estimate the elastic distortion of materials in contrast to the classical methods like Rockwell and Vickers. Furthermore it offers the possibility to determine values, which are equivalent compared to the traditional methods. Different materials like plastics, metals, coatings and ceramics can be tested.
Norm: DIN EN ISO 14577-1 / ASTM E2546
Shore hardness, named after Albert Shore, is a material parameter for elastomers and plastics. It’s defined in the norms DIN 53505 (ISO 7619), ISO 868 and ASTM D 2240. The Shore hardness is given on a scale of 0 - 100. 0 corresponds to the lowest hardness and 100 to the greatest hardness.
In order to determine the hardness of test specimens and plastic products, a steel tip of a certain shape and with a defined spring force is pressed into the surface with the aid of a depth gauge.
Shore A is used for soft elastomers. The penetrator consists of a truncated cone with a diameter of 0.79 mm and an opening angle of 35°, the contact force is 12.5 Newtons, the holding time 15 seconds. The Shore A hardness is determined in the range of 10 to 90 hardness units (softer plastics).
Shore D is used for relatively stiff plastics. The penetrator consists of a needle with an angle of 30° and a spherical tip (R=0.1 mm). The contact force is 50 Newton, the holding time 15 s. The Shore D hardness is determined in the range of 30 to 90 hardness units (harder plastics).