CHE 333 Hardness Testing of Materials.
Introduction.
Hardness testing is employed as a non destructive examination (NDE) technique on materials. It can be used to check if the correct heat treatment was conducted on steels and for age hardening of alloys. Cold working and annealing can also be investigated by employing hardness testing to determine the hardness values after different conditions. It is a valuable tool that is employed in several laboratory experiments in the CHE 333 laboratory section.
Process.
The most common hardness measurement technique is a direct measurement by forcing an indenter into the surface of the material at a fixed pressure or force. The amount of plastic deformation below the indenter is then used as a measure of the hardness of a material. The more plastic deformation for the same load then the softer the material. A common piece of measurement equipment is the Instron hardness tester, shown in the figure below. The system indents the material under precise conditions and returns a number on the digital readout of the machine.
In operation a preload is placed on the indenter to try to ensure that any surface features do not affect the final data. The preload is a function of the major load, which is applied after the preload. Once the correct preload is applied, then the major load is automatically placed onto the indenter. After the full major load is applied then the depth of penetration of the indenter is measured. This depth is a function of the plastic deformation below the indenter, and so is also a measure of the hardness of the material.
Indenters.
Clearly, the indenters have to be somewhat harder then the material under examination. For 70-30 brass, a hardened steel ball 1/16in diameter is employed as the indenter. For a hardened steel a conical diamond indenter is used. Some indenters are shown below.
Force.
On modern equipment, a chart exists of force required along with indenter type and major load for several different ranges or hardness scales. For example, the Rockwell “C” hardness scale requires a “C” brale indenter with a force of 150 kg/mm2 . A Rockwell “30T” hardness scale requires a 1/16in diameter steel ball and a force of 30 kg/mm2 . There are many other scales that require different combinations of indenter and major load. The Rockwell “C” scale is used on hard materials such as heat treated steels, while the Rockwell “30T” scale can be used on softer materials such as aluminum alloys and brass.
Samples.
Pretty much and sample shape can be accommodated in the Instron hardness tester as the mounting base can be interchanged to accommodate flat plate, square bar, round bar and other shapes. The surface has to be known, for example, paint or grease would distort readings as would oxidation scale from heat treatment, so these are sometimes removed by grinding. Metallographic samples can be tested.
Test Procedure.
The equipment is adjusted for the correct combination of indenter and force required for the Rockwell scale employed for measurement. Dials on the right hand side of the equipment set the force and a dial just above the indenter set the preload. The correct scale is also set digitally by scrolling through the possible combinations on the keypad on the front of the equipment. The sample is placed on the base opposite the indenter. The mounting base is on a threaded rod, and so can be moved up or down by the rotating the handle at the bottom of the machine. The surface of the sample to be tested is moved up to close proximity to the indenter. The sample must be moved slowly at this stage as the digital display will pick up the motion of the sample as it moves into contact with the indenter. When the indenter has reached a position in the sample when the correct preload is applied, then the machine automatically takes over and applies the major load. At this stage the base should not be moved as the indenter is moving into the sample and so any further manually allied motion will lead to an incorrect result. After the hardness tester has applied the major load, the distance the indenter penetrated into the surface is measured and a hardness value calculated from the range of force applied.
Knowing the hardness scale employed is vitally important as the same number will be available, but is only meaningful when the Rockwell scale is also known.
Errors.
1. Wrong Rockwell scale used.
2. Sample surface inconsistent due to contamination.
3. Sample too thin – plastic deformation goes through the thickness of the material.
4. Indents too close together – at least three indenter diameters apart.