In a previous white paper, we discussed the characteristics and types of hardness testers. Many readers inquired about What to consider when buying or choosing a durometer? Below are certain aspects to consider, at least to approximate the decision and obtain the best alternative for the available budget.
Of all the different kinds of measurements that are made in a metrology laboratory, hardness tests are among the most complex.
On the other hand, there are different measurement procedures and in addition one falls into the need to measure large, small, hard, soft, thin or thick samples. Considering the different methods and the large number of existing scales, it is understandable that even the most experienced people are challenged by hardness testing tasks.
As in many other application areas, electronic development has made hardness testing simpler. A computer-assisted durometer provides higher precision readings, as well as the possibility of being able to statistically store and process the information of the measurements, represent them graphically and document them.
Although automated durometers continue to emerge and penetrate the market, manual and mechanical tests are still applied and fully valid.
It is of fundamental importance to know what are the main points to take into account when buying a device to measure hardness. The most relevant factors are detailed below:
- Maximum test loadOn the one hand, there is a general rule that the test should be done at the highest possible load. This allows for greater precision in the reading, since the measurement is less sensitive to the texture of the sample surface when a large load is applied. At the other extreme, the indentation should not be deeper than 1/10 of the thickness of the specimen or the hardened surface to be tested. The degree of homogeneity of the material is also an important criterion. For example, a gray cast iron sample is usually tested with the highest possible load, except in those cases where it has been induction hardened, such as the bases of the tools of certain machines.
- Hardness rangeAbove a hardness of approximately 650 HB / 30, it is recommended to use a diamond indenter; below this value it is possible to use a steel or hardened steel indenter. The Brinell method, which does not allow diamond indenters, cannot be used with materials such as hardened steel. The more universal Rockwell method allows the use of diamond cone and steel ball indenters. The Vickers method allows the use of a pyramid diamond indenter and can be used across the entire range of hardness. However, it is more applicable for laboratory testing than on job sites.
- PrecisionThe precision of a measurement is highly dependent on the precision employed by the operator. This includes that the finished surfaces of the specimens are well roughened and polished, that the measurement periods are adequate and sufficient, and that frequent checks of the durometers are carried out in order to obtain reliable values. Static is preferable to dynamic ones. Using very low penetration loads is a restriction when it comes to obtaining precision in the measurements made.
- Flexible and adaptable durometers to the different shapes and dimensions of the samplesThe sample must be able to be placed in the durometer, or the durometer must be able to be positioned in the sample. For the first case, there are stationary durometers, whose advantage lies in their ability to hold the sample. They are suitable for testing small and medium-sized samples. Portable devices can be attached to the samples, using grab chains, jaws, magnets, among others, or, if it is very large and heavy samples, they must be good enough to simply lean on them. These portable hardness testers can only be used dynamically when the applied load is high enough. If the loads are small, the measurement should be done statically.
- Economic aspects
These include the following items:
- The purchase cost of the durometer.
- The universality of the application.
- The cost / benefit relationship with the equipment supplier’s after-sales services (calibrations, maintenance, spare parts)
- The period of time between measurements.
- The qualified operation of the device. The first two aspects are important when the samples have different shapes and different surface treatments. This is the case of small-scale industries that have a variety of products and it is necessary to find a solution that suits their needs.On the other hand, the supplier of the device could be required what are the after-sales services that are associated with the purchase , such as the detail of calibrations, repairs and supplies necessary for its correct operation. There are computers that are very expensive in terms of purchase cost, but are almost maintenance free. Knowing the post-sale costs should be part of the buyer’s final decision.In companies where it is necessary to do hardness tests serially, both the speed of the measurements and the qualified personnel are the predominant variables at the time choosing a durometer.
Advantages and Disadvantages of Different Hardness Measurement Methods
One of the main questions when buying a durometer is which one is the most suitable for the true needs of the business. To do this, a table will be listed with the pros and cons of each measurement method.
A single durometer for various materials
Finally, if the need is to have a single equipment whose use is transversal for different materials, it is necessary to know what are the equivalences between the different hardness scales according to the methods seen.
Below and as a summary, a comparison table between the existing hardness scales is presented.