What are hardness testers and how do you use them?
Hardness testers are used in the inspection and measurement of workpieces where hardness levels are important to its proper function and application. This is particularly important, for example, when predicting the life of gears, shafts and splines, as this will determine when these components need changing and so preventing disruptive operational failure. Testing the hardness of case hardened and through hardened materials is also useful when determining the tensile strength and durability of a metal or metal alloy.
Most commonly, a component or part will be placed on a stage located below an indenter, which will itself be attached to the hardness testing unit. To measure the hardness of the component, the indenter (usually a ball or pointed edge) will press into the material at a given force and make a mark (or indent) into it. Different types of hardness tester will measure different criteria to determine the hardness from this indentation; some will measure the size of the indentation whilst others will use the force (or resistance) applied to the indenter to determine this.
Whilst this method is the most common, often due to the size or type of workpiece this isn’t possible. Another method used on portable hardness testers involves firing a ball bearing down a shaft into a workpiece, on which it will bounce off. The speed or force at which this ball bearing bounces from the component will determine its hardness, which will be fed back to the display output of the unit. This method, whilst ideal for workpieces which cannot be placed on a stage and tested in the traditional sense, is often less accurate and therefore produces a wider range of possible hardness values.
What hardness scales can hardness testers measure?
Several scales are used to give a numerical value to hardness which can be accurately compared with other materials of similar hardness levels. Hardness testers often have a specific scale by which they measure hardness in, although some of the more advanced models will feature multiple interchangeable scales.
The most frequently used hardness scale is Rockwell C (HRC). This scale is the most common measurement of hardness and is recognised in most engineering contexts when describing material hardness. Tooling manufacturers, for instance, utilise the Rockwell hardness scale when stating the types of material each tool is compatible or best suited for. For instance, YG-1’s X5070 Blue cutter is designed for milling steels between HRc50 and HRc70. Generally, the higher the Rockwell level, the harder the material will be. Depending on the hardness and type of material, hardness testers measure in different hardness scales including HRC, HRA, HRB, HRF, HR15N, HR45T and HRG.
Another common hardness scale used in testing processes is the Brinell hardness scale. What is the difference between Rockwell and Brinell? There are a few key distinctions here:
- The indenter used in Brinell hardness testing is much larger than when testing in Rockwell hardness scales.
- Brinell hardness testers only feature 1 type of indenter (ball-shaped) whilst Rockwell testers feature both cone and ball shaped indenters (this will depend on the material and hardness scale being measured in).
- Because of the larger indenter, Brinell hardness is often less accurate than Rockwell hardness.
- Brinell testers also takes much longer (from approx. 30-90 seconds) than Rockwell testers (10-15 seconds) to measure hardness.
Brinell hardness is usually measured using its own hardness tester unit, though sometimes multi-scale units exist which can do both. This hardness scale is better associated with larger workpieces that have a high flatness value, as Brinell hardness is very sensitive to imperfections on the workpiece surface. A key reason for this is that Brinell (HB) scales use much larger values than Rockwell; HRc 72 equates to HB 800 on the Brinell scale.
Other common hardness scales used include Superficial Rockwell, Vickers and Micro-Vickers hardness scales, which all use their own scales and hardness testing units.
What different types of hardness tester are available?
Hardness testers, whilst all performing very similar functions, come in a variety of shapes, sizes and feature lists, which will depend on your application, accuracy required and output method. Already touched on was the hardness scale, which will likely be the first consideration when choosing a hardness tester. Whether you need to measure hardness in Rockwell, Brinell or another scale, different variations of units exist to cover all scales. Of course, conversion charts are available which will allow the user to calculate their hardness values in different scales should more than one scale be needed.
Next, the user should consider the workpiece or workpieces that are to be tested. If the workpiece cannot fit on a stage housed within a traditional hardness tester unit, then a portable hardness tester may be the best option. These units commonly use the ball bearing method of hardness measurement (referred to as the Leeb Rebound Hardness Test (LRHT), making them the less accurate option. Test force will also be important, as this will determine the range of hardness a tester can measure. Sample blocks, with specified hardness levels, can also help calibrate the machine and ensure hardness readings are accurate.
Both portable and large-scale hardness testers range in features, including readout, accuracy and captured datapoints. Most modern hardness testers will feature a digital readout, which will display the most accurate possible hardness reading. Lower value testers are fitted with dial readouts, which some users prefer as they do not require power to operate. Accuracy will also be determined by the unit’s monetary value, as well as the size of the range the unit operates within. Larger ranged units often have lower levels of accuracy.
Data output can often be performed via a data cable, allowing hardness values to be outputted to a PC via either bespoke statistical software or to Microsoft Excel. Many engineers also want a print-out containing hardness values, which is why many models either come with or are compatible with a printer. Furthermore, some models feature a digital camera next to the indenter to take photographs of the indentation, which can be used for further analysis.