There are several portable hardness testers available, with some more appropriate to certain situations than others. The particular hardness tester for the job would depend on what you needed to find out and why.
For example, a Barcol hardness tester for use on Al alloys is generally used as a comparative tool; to indicate a loss in strength in damaged regions of a structure compared with undamaged regions or virgin material. Barcol hardness testers are very portable and are good for this type of comparative work, but the conversion to more widely recognized hardness scales is only approximate.
Telebrineller testers. These types of tester determine the hardness by comparison of steel ball indentation in a standardized bar with the indentation in the test material. The standardized bars are consumable and should be close to the hardness of the test material. The method is relatively straightforward but would be subject to similar limitations to the rebound testing method.
The three main principles behind many other types of portable hardness testers are:
1. Ultrasonic Contact Impedance (UCI) – e.g. Krautkrämer Microdur models
2. Rebound methods – e.g. Equotip or Krautkrämer Dyna models
3. Rockwell principle – e.g. Equostat models
The principle of UCI is based on the shift in resonant ultrasonic frequency arising from the contact of a Vickers diamond indenter on the end of the probe, and the material under test. The shift in frequency arises from the additional mass on the end of the probe, upon contact with the test material. The frequency shift is proportional to the area of contact, which is the area of indentation generated by the Vickers diamond. The frequency shift also depends on the Young’s Modulus of the material, and calibration on a material with a similar Young’s modulus to the test material must be carried out before testing. The following equations are employed in the UCI method:
Where f is the frequency shift,
E elastic is the Young’s Modulus of the material
A is the area of indentation
F is the Force applied in the test (probe rating)
HV is the value of hardness to the Vickers scale.
The Rebound, or Leeb’s method, determines the indenter velocity immediately before impact by using a magnet in the indenter to generate an induction voltage during its travel toward the test material. The voltage is proportional to the velocity. The indenter rebounds and the velocity immediately after impact can be determined in the same way. These values are used to calculate hardness on the Leeb scale (HL):
where V I is the velocity of the indenter before impact
V R is the velocity of the indenter after the impact.
The value of Leeb hardness is then converted to the more widely used scales by means of conversion charts.
The Rockwell principle involves applying a preliminary test force (fo), and determining the depth to which that force indents the surface. The test load (fo + f1) is then applied and the material indented. The test load is removed and the preliminary force (fo) is held on and the depth of indentation measured. The difference between the depths determined on application of the test force is used to determine a hardness value. The indenter used for Equostat portable testers is a diamond cone, and the measurement is in HRZ, which can be converted to other scales.
where to is the depth of indent on applying force fo, t1 is the depth of indent after application of load (fo + f1) and removal of load f1.
The following table lists some of the appropriate applications for the methods.
|Application||UCI testing||Rebound testing||Rockwell Principle|
|Solid parts||Well suited||Very well suited||Well suited|
|Coarse grain materials||Not recommended||Very well suited||Not recommended|
|Steel and Aluminium cast alloys||Sometimes suited||Very well suited||Sometimes suited|
|HAZ regions in welds||Very well suited||Not recommended||Well suited|
|Tubes, wall thickness >20mm||Very well suited||Very well suited||Well suited|
|Tubes, wall thickness <20mm||Very well suited||Not recommended||Well suited|
For all methods there are limitations, arising from the size of the indentation, the thickness and the surface quality of the material.
Where local variations in hardness need to be evaluated, such as in heat affected zones (HAZs), special techniques need to be employed. TWI has evaluated, as part of some Group Sponsored Projects (GSPs), the use of the Krautkrämer Microdur MIC2 for measuring HAZ hardness. The results from the GSP (5672 – On site hardness testing of weld heat affected zones in steel) can be obtained from TWI for a buy back fee.