A daily task in every heat-treating company as part of quality control is to compare whether the results match the required parameters on the drawing for the respective component. This is often done by checking the depth of hardening and additional control of the structure.
Constant development of machines and products requires high-quality materials that can withstand heavy use. The manufacturing components should have defined strength or wear characteristics. The characteristics specified by the developer for the respective area of the application must be tested during the production process, particularly in the case of high-risk components. However, the investigation, usually by means of hardness testing, should not only be performed quickly during production, it should also be carried out in a reproducible manner.
To meet the demands of today’s heat-treating industry, hardness testers are used for quality assurance during manufacturing processes, in laboratories for research-and-development purposes and during damage analysis.
What is hardness?
Hardness is defined as the resistance of one body to the penetration of another harder body. This definition gives rise to the possibility of hardness testing. In principle, a defined indenter is pressed into a surface with a defined force. From the depth to which the indenter penetrates the surface, the resistance to penetration, or the hardness, can be deduced.
Based on the ideas of Smith and Sandland, a hardness testing method was developed in 1925 in the English Vickers works, using a diamond pyramid as the indenter.
A regular four-sided diamond pyramid with a surface angle of 136 degrees is pressed vertically into the preferably polished surface of the test specimen with a certain test load. After a certain exposure time, the test specimen is removed again and the two diagonals (d1 and d2) of the indentation are measured with a measuring microscope. The average length (d) is calculated. The Vickers hardness is then determined according to a given formula.
The Hardness Test
The hardness test according to Vickers is the method used to perform hardness depth measurements.
Since fluctuations in the quality of components can be associated with a safety risk, the use of a precise, reliable and automated measuring device for hardness testing is an important prerequisite for achieving high product quality. By largely automating the measurement process, the influencing “human” factor on the results of such a measurement is minimized. In addition, due to the automated testing of several samples in one measurement run, the employee can devote attention to other tasks in the laboratory in the meantime.
In addition to hardness as an indicator of strength, a visual assessment of the material structure or microstructure is one of the main tasks in the materialographic quality laboratory of a heat-treatment company.
To accomplish these tasks, employees will typically need a hardness testing device as well as an additional microscope. However, modern hardness testers seamlessly combine two quality-control requirements – micro/low load hardness testing and analysis – in one device with full automation.
In addition to the precise force control and software, the optical measuring system forms the heart of a reliable hardness tester. A Köhler lighting system can also be integrated into a hardness tester. The specially developed optical design achieves an optimal, uniform illumination of the entire image field with an additional increase in the depth of field, which brings significant advantages both in hardness testing on contrasted (etched) ground surfaces and in the use of systems for image analysis.
With the Köhler lighting principle, the light source can be imaged by a collector lens. An adjustable iris is installed at the location of this figure. Any part of the image of the light source can be hidden with the aperture. As a result, the opening angle of the illuminating beam cone, or the aperture, can be adjusted. This iris is called the aperture stop. A second iris diaphragm is installed behind it. The “illuminated field diaphragm,” allows the size of the illuminated specimen area to be changed within certain limits. This size should always be selected in such a way that only the part of the object that is visible in the eyepiece or on the imaging medium is illuminated. This optimizes the object lighting and makes it automatically adaptable to any test and analysis situation.
An integrated overview camera is used to capture the entire sample at once. Up to eight different tools, test diamonds or lenses can be accommodated in the tool turret. A compact design of the tool turret, which is inclined by 20 degrees, also ensures excellent visibility into the test area. An optional XL table is available to allow a maximum of 16 samples in sample holders to be tested in one test process.
Software with intelligent measuring tools for lengths and angles makes it suitable for template creation as well as for measuring weld samples. Additional and optional software modules for phase analysis, layer thickness measurement and particle size determination are also available.
In Vickers hardness testing, a regular four-sided diamond pyramid with a surface angle of 136 degrees is pressed vertically into the surface of a test specimen with a certain test load. After exposure, the test specimen is removed and the two diagonals (d1 and d2) of the indentation are measured with a measuring microscope. The average length (d) is calculated to provide the hardness.
Since fluctuations in the quality of components can be associated with a safety risk, the use of a precise, reliable and automated measuring device for hardness testing is an important prerequisite for achieving high product quality.
Using these measurement modules, which are integrated into a hardness tester, image analysis methods can be carried out along with the hardness test in a measurement process and output automatically in a measurement report. The measuring modules include the automated measurement of layer thickness, grain size and phase fractions with the help of gray or color threshold setting and also the partially automated measurement of layer thickness and grain size.
With the help of these optional measuring modules, the nitriding hardness depth and the compound layer thickness, for example, can be measured on one device and output in a report without having to change the measuring device.
Modern hardness testers can process hundreds, even thousands, of test and analysis points automatically. The analysis functions are also integrated in such a way that the operator is guided during the manual evaluation of the sample properties to obtain the desired results in an uncomplicated manner.
The measuring modules include the automated measurement of layer thickness, grain size and phase fractions with the help of gray or color threshold setting and also the partially automated measurement of layer thickness and grain size.
A prerequisite for automated hardness testing is a well-prepared sample surface. You must choose the right cutting device for this purpose in order to carve out samples from the components. Cutting robots are currently used for automated cutting processes at defined positions.
For mounting the sample pieces that have been cut out, users can use hot mounting presses or cold mounting methods. Light curing processes allow users to reduce the mounting process to one minute, making it suitable for testing hardness depths in large numbers.
Grinding and polishing can be carried out with manual, semi-automatic or fully automatic grinding and polishing devices for high sample throughput.
Of high importance today is the data connection of the test devices to customer-specific networks for simple data acquisition. With specified solutions, it is possible to connect any kind of data-collection software directly to modern hardness testers and preparation devices. The combination of modern technologies and automation allows hardness testing to be incorporated into the process in a timely and cost-effective manner.
For more information: Matthias Ziegenhagen is Head of QATM Academy and international sales manager for Verder Scientific in Newtown, Pa. He can be reached at 267-757-0351 or email@example.com.
All photos/graphics provided by the author.