Imagine you have a gear or a rolling bearing! It is the hardness and the respective strength of these components that should be above the threshold limit to ensure quality. So, remember that hardness is the important aspect that decides the quality or performance of the given object. So, let us see the exact meaning of the term ‘hardness.’
What is hardness?
Hardness is the ability of a sample object to resist deformation due to abrasion or a mechanical indentation.
This hardness is basically characterized by strong intermolecular bonds. That’s how we call materials such as beryllium or titanium harder compared to plastics or metallic tin.
Hardness usually depends on the following characteristics listed below:
- Elastic stiffness
Examples of hard materials include certain metals, concrete, and ceramics.
What is the reason for testing the hardness of a sample object?
The hardness test is the easy, fastest, and cost-effective method to determine the mechanical characteristics of sample material.
Specifically, the hardness of a sample object is tested for two reasons:
- Examine the properties of the sample
- Check material quality and determine its hardness
- Determine tensile strength
- Resistance to the force
- Wear resistance
- Assures the ability to function as designed
What are the factors to be considered before selecting the hardness testing method?
Here is the list of features to be checked before opting for the hardness testing method:
- Sample size – If the size of the sample is small, just the lighter load is enough to create the indentation. So, it is mandatory to follow the regulations on thickness and spacing from both inside and outside edges. Similarly, samples with larger areas can be fixtured properly to assure a safe placement without any slip.
- Scale – There are certain conversions with appropriate validation. Hence it is mandatory to follow ASTM scale conversion charts, especially ASTM standard E140.
- Material – You can decide the testing method by looking at the type of a given material and the expected hardness. Samples like steel have smaller grain sizes that require diamond indenter, and the Rockwell scale is enough to measure it. Cast iron or powder metals require a large indenter and the Brinell scale is used to measure them. In the case of very smaller parts, the Vickers scale is enough.
- Sample thickness – To use Rockwell methods, the sample should remain thicker by a minimum of ten times the expected indentation depth.
- Sample shape – For testing a cylindrical shape with few diameters, you require correction to the final result. For convex cylinder surface diameter, add a roundness correction factor to the test result. Also, maintain minimal space of 2~1/2times the indentation’s diameter from one edge.
- Gage Repeatability and Reproducibility studies – It helps to determine the ability of the instrument to determine the tolerance limit of an object.
How do we measure hardness?
There are three major types of hardness measurements:
- Indentation hardness
- Scratch hardness
- Rebound hardness
- Indentation hardness – It measures the resistance level of an object to deformation because of the continuous compression load from a sharp entity. It precisely measures the depth of an indentation left by the loaded and dimensioned indenter. The lesser the indentation, the harder the sample. This test is used in the metallurgy and engineering sectors. The prevalent indentation hardness scales include Vickers, Brinell, Rockwell, and Shore.
- Scratch hardness – It determines how resistant a sample is to a permanent plastic deformation because of the friction from a sharp material. Imagine if you are going to test the coatings, it is the measure of the force that is used to cut through the film to the substratum. This test is used in the mineralogy sector. The prevalent scratch hardness scales include the Mohs scale using a sclerometer.
- Rebound hardness or dynamic hardness – It determines the height of the ‘bounce’ of a hammer, which has a diamond tip, fallen from a pre-determined height on the sample. The prevalent rebound hardness scales include the Bennett hardness scale and the Leeb rebound hardness scale using a stereoscope.
What are the major testing methods of hardness?
There are two types of hardness testing of a given sample.
- Static method – It is a slow and impact-free method. It is possible to detect the various dimensions of the indentation including its depth, area size, and projection area. This method includes Rockwell, Brinell, Leeb, Vickers, Wechsler, Babbitt, etc. Among all these test methods, Rockwell, Leeb, and Brinell are the widely used hardness testing methods. Especially, the Rockwell tester detects nearly 70 percent of the metals for hardness according to the present statistics.
- Dynamic method – It is dynamic and creates a test force with more impact. This method can assess the hardness of a large, immobile workpiece. This method includes Leeb and Shore hardness tests. Leeb hardness tester is used more than the Shore hardness tester.
Let us go through the major hardness testers one by one:
- Rockwell hardness testing – It assesses the depth of plastic distortion of an indentation. This hardness method is measured in a 0.002 mm hardness unit. The application of test forces and indents in this hardness tester can appear in various combinations corresponding to the appropriate Rockwell scales.
- Vickers hardness testing – It can be determined using a diamond-edged indenter at an angle of 136 degrees between the opposite surfaces. Press into the surface of the sample with load F, hold it statically for some time after unloading, measure the diagonal length d of the indentation, and determine the indentation surface area along with the average pressure. The final Vickers hardness value of a sample will be expressed with the HV symbol.
- Brinell hardness testing – It can also determine the hardness of the sample object, which is expressed as an HB symbol. Constrain a steel ball with diameter D into the metal surface to be determined with a load size P, held for some time, and unloaded. Here, the ratio of P and the F (surface area of the indentation) is the final Brinell hardness value.
- Leeb hardness testing – An exact body mass under the influence of the elastic force at a precise speed impact on the surface of the sample, with a punch at a distance of 1mm from the sample surface rebound velocity and the impact velocity of the determined value.
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