State-of-the-art microprocessor-controlled hardness testers enable Vickers and Knoop hardness tests to be performed rapidly, accurately and reliably in both industrial research (on metals, sintered materials, ceramic products, integrated circuits, coatings, as well as grain microstructure analyses) and in quality-control environments (e.g., heat-treated surfaces, cutting tools, wires and small-scale, precision-engineered components).
What is hardness?
Hardness is a simple-to-measure material attribute with which to differentiate between and describe materials and their physical condition during processing. Hardness is defined as the resistance of a material to indentation by a body made of a harder material. Accordingly, most conventional hardness tests involve a hard indenter (one made of diamond, for example) that is pressed vertically into the surface of the sample being tested.
The method most commonly used in test laboratories is the Vickers hardness test. The indenter used in this technique is a square-base diamond pyramid having included face angles of 136?. The advantage of this indenter geometry is that the law of proportional resistance is obeyed. That is, the applied test force is directly proportional to the indentation area, making a Vickers hardness value fundamentally independent of the selected test force (load). A minor dependence on the applied force is found for very small indentation depths, but this is attributed to other causes.
The Vickers hardness value is calculated according to the formula: HV = 0.102 (F/A) where F is the applied force and A is the contact surface area of the resultant indentation after the indenter has been withdrawn. The area is computed from the mean diagonal (d) of the indentation; d = mean average of the two measured diagonals d1 and d2 (see schematic of diamond indenter and indenter impression).
Testing process automation
Computer-controlled hardness testing systems are now an established part of a modern materials testing laboratory. Important reasons for this development include:
- Time savings with large test series and frequently recurring test procedures
- Objective evaluation of measurement parameters (not observer dependent)
- Reduction in operator errors
- Reduction in the amount of physical effort required from laboratory personnel
- Acquisition and storage of measurement data for components for which documentation is obligatory
- Simple production of measurement logs
The number of separate measurements needed to establish hardness varies over a large range and depends on the sample and the definition of the particular measurement problem. For example, 6 to 10 measurements typically are sufficient to determine local case hardness. However, the number can be many times greater to determine the hardness profile across a weldment cross section.
Hardness tests carried out manually using conventional hardness testers makes the process highly cost and time intensive. In addition, the process can be tiring for test personnel because despite being a routine operation, the measurements must be performed to a high level of accuracy. Thus, measurement values often are dependent on the experience of the operator and on the day-to-day variations in operator accuracy.
This source of operator error can be reduced to a minimum by using automated image analysis and by automating the test procedure wherever possible. Using a modern automated tester, the operator can concentrate on setting up the sample and then invoking a predefined test sequence. Therefore, the development of modern hardness testing systems at Struers places emphasis on ergonomics, user friendliness and the greatest possible degree of automation.
Automated testers are controlled via the serial interface of a computer running special control and image-processing software. The system enables indentations to be made according to a preset series of patterns, which are then subsequently measured using the system's image-analysis function. The high degree of automation means a significant improvement in the reproducibility of the measurements and an almost total eradication of operator errors.
Automated hardness testing is particularly suited for:
- Examining different phases in heterogeneous alloys (e.g., distinguishing between microstructure hardness, polycrystalline hardness and monocrystalline hardness)
- Demonstrating diffusion processes
- Verifying increasing hardness with increasing alloy concentration in a mixed crystal
- Mapping hardness zones from the surface into the body of a sample
- Measuring hardness on thin films
- Measuring hardness on components having a very small test surface area
- Measuring the hardness of parts whose surface must not be damaged to any large extent
Typical applications using automated microhardness testers include measuring carburized-case hardness, determining hardness profiles across weldment cross sections and scanning surfaces to achieve a topographic representation of surface hardness.
For more information: Michael Schurmann, Struers GmbH, Linsellesstra¿ 142, 47877 Willich-Schiefbahn, Germany; tel: +49 2154 818 150; fax: +49 2154 818 134; e-mail: verkauf.struers@ struers.de
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