Men with the greatest resistance to the pressure often leave their footprints on the sand of time and allegorically diamond is the strongest mineral, which provides the greatest resistance to the deformation. Seems like human life is carbon copy of nature and vise a versa. Theoretically, hardness is defined as the resistance offered by the metal to plastic deformation. The term "hardness" may also refer to stiffness or resistance to abrasion or cutting.
Hardness testing can be defined on a macro, micro or nano scale according to the forces applied and displacements obtained. Testing of macro-hardness of materials is a quick and simple method, wherein mechanical property data for the bulk material is obtained from a small sample. Hardness testing is also widely used for the quality control of surface treatments processes. However, where materials have a fine microstructure, are multi-phase, non-homogeneous or prone to cracking, macro-hardness testing will be highly variable and will not identify individual surface features. Therefore here micro-hardness measurements are more appropriate. Micro-indenters works by pressing a tip into a sample metal and continuously measuring the reaction of the metal to the applied load, penetration depth and cycle time. Nano-indentation testing is the measure of the hardness by indenting using very small, on the order of 1 Nano-Newton, indentation forces and measuring the depth of the indention that was made on the metal.
The metals industry uses three types of tests with accuracy:
- Brinell Hardness Test
- Rockwell Hardness Test
- Vickers Hardness Test.
These tests determine the depth to which a non-deformable ball or cone will sink into the metal, under a given load, within a specific period of time.
What is a Portable Hardness Tester?
Hardness tester is required to measure the hardness of the metal. The inconvenience of going to a laboratory has been sorted almost completely by portable hardness tester, which has been developed so that it permits on location hardness measurements by offering quick and economical supplements to conventional, stationary testing machines. There are two main methods adopted worldwide, one is the UCI (Ultrasonic Contact Impedance) method in which the methodology is to measure the frequency shift of a resonating rod with a Vickers – diamond tip that occurs when the diamond penetrates into the test material by applying a specific test load. The frequency shift is evaluated and is electronically converted to a hardness value, which is displayed on the LCD.
Second well-known principle for portable hardness testers is the rebound method. In this method the device measures the velocity of a propelled impact body directly before and after the impact onto the test material’s surface. The ratio between both velocities indicates the hardness of the material that can be converted into different scales by using conversion tables stored in the instrument for different materials.
Many devices combine these two most successfully applied portable hardness-testing principles in one instrument. Whether one wants to use the UCI principle or the dynamic Rebound testing method, all one needs is just one instrument and all probes and impact devices can be plugged in and used. There are any products available in the market and one can easily choose as per his or her requirement. The potable hardness tester is very easy to use and requires no special knowledge of hardness testing in order to operate the instrument.
Portable hardness tester uses are immense and comes handy because of their special features like quick measurement, which takes hardly a few seconds. These devices have built in memory for data and calibration points.
ASTM International Committee A01 on Steel, Stainless Steel and Related Alloys have developed standards for portable hardness testing which are very useful. While both methods – UCI and Rebound are successfully used for hardness testing and solve many on-site hardness-testing applications, there are limitations concerning the kind of material under test and its size and weight, respectively. Furthermore, because of the influence of Young’s Modulus (which is a material property that describes its stiffness and is therefore one of the most important properties) most conventional testing methods do not allow to measure different materials without firstly calibrating or adjusting the instrument.
Today hardness portable testers are accepted tools for portable, on-site hardness testing applications. Mobile hardness testing instruments will not replace the conventional bench-top machines, but nevertheless, they solve plenty of mobile hardness testing tasks, however, each method is limited – more or less – to a specific application area. During the last decades several portable instruments based on different physical methods were developed and the search for the best is still on.