Hardness Testing Basics
Hardness is a feature of a given object, and not a basic physical property. Hardness is defined as the resistance to indentation, which can be detected by measuring the depth of the indentation. In a nutshell, the lesser the indentation, the harder the object. Indentation hardness value can be calculated by determining the depth of the indentation using one of the below listed test methods.
While hardness is considered as part of many materials’ characteristics, it is, however, not seen as being a critical physical property. In any case, it is often mentioned in relation to the resistance of a material’s indentation due applied loads. Still, the measurements of the permanent depth caused by these loads gives the hardness of the material. Furthermore, given the application of a constant load on different materials, the material with the smallest indentation is considered the hardest.
I. What is the definition of hardness testing?
Hardness testing is a critical procedure for ascertaining the suitability of a material for a specific production process as it helps to determine material properties that include wear and tear resistance, strength, and a material’s ductility. Yet, hardness in itself is not fundamental to any material as a property. Hardness testing can therefore be explained as the resistance of a material to the application of a force by another harder object. Ultimately, the hardness of a material is often related to certain measurable quantities that include the time of application of a load, the load size, and the shape of an indenter.
II. What is the main purpose of the hardness test?
The main purpose of a hardness test is to ascertain the strength of a material and to check its suitability for the purpose for which it was intended. Usually in manufacturing materials used in production must meet a tensile strength requirement as this shows if it can resist the specified or designed force usually encountered by the final product it is intended to produce. Ultimately, the hardness values obtained from the hardness test are often converted into an equivalent tensile strength using the appropriate scale based on the hardness test that was carried out.
III. How do hardness testing work?
To carry out a hardness test, you’ll employ an indenter which typically consists of a specified load borne by a dimensioned object to press into the sample material’s surface. The depth of the indenter penetration from the measurements you take determines the hardness of the material as a small depth signifies good resistance to the load and thus a harder material compared to one with very deep indentation. While hardness test including instrumented indentation testing, Rockwell, and ball indentation hardness measures the indenter penetration depth. Others like the Brinell, Vickers, and Knoop hardness tests measure the impression size of the indenter.
IV. What are the reasons for performing a hardness test?
Hardness testing is performed for two reasons.
- Test hardness
- Check material
- To confirm process
- Predict tensile strength
- Wear resistance
- Resistance to impact
- Confirm the capability to function as designed
V. What are the most common types of hardness tests?
Here are the most common hardness tests found in the industry.
- The Rockwell hardness test – This hardness test is very suitable for a wide variety of materials including thin steels, hard cast irons, cemented carbides, titanium, and hardened steel. With an applicable load range of 15 – 150kgf, this makes it one of the most applied hardness testing methods in the industry. It employs a conical diamond indenter that complies with EN ISO standard. The Rockwell hardness test is calculated by measuring the indentation depth caused by the application of an indenter with a specified load for a given time. It’s important to ensure that the shape of the sample is put into consideration for appropriate geometric correction. Also, the minimum thickness should be ten times the expected depth of indentation.
- The Vickers hardness test – This hardness test has a load range of 10gf – 10kgf which makes it suitable for both micro and macro hardness testing. It is often applicable to small and thin sample material including ceramics and it is used in welding, hardness progression, and sintered metals. When carrying out Vickers hardness test consideration should be given to material surface smoothness and should be polished for microhardness testing. The sample must also be less than 50kg in weight. While it is calculated by measuring the diagonal of the indentation caused by the diamond pyramid indenter, a geometric correction is often applied for small diameter cylindrical samples. Importantly, diagonal length must be greater than 20mm to reduce inaccuracies.
- The Brinell hardness test – For this hardness test the measurement of the depth of indentation caused by the preload gives the hardness value of the material. However, the load range is between 5 – 250kgf often applied for testing the hardness of plastics using a tungsten carbide ball of between 1 – 5mm diameter. The conversion of the value of the Brinell hardness test is not the same for every material. Yet, the Brinell hardness test has certain advantages which includes fast testing and direct result readout with little sample preparation.
- The Leeb hardness test – This is a non-destructive hardness test as it relies on the rebound nature of the testing device’s impact on the sample. It is also affected by three factors including the indenter shape, impact velocity, and indenter material type. These factors generally influence the categories of the Leeb hardness test. Furthermore, the Leeb hardness tester can be both stationary and portable which makes it versatile for both laboratory and field hardness testing. It is also suitable for ferrous and non-ferrous material hardness.
VI. How to select your test method?
The following features are considered before choosing the hardness testing method:
- Sample Size
- Sample Thickness
- Gage R & R studies
- The shape of the sample – flat, cylindrical, round, irregular
- Sample Size
It is important to consider the size of the test sample as this affects the load size required to cause an appreciable indentation in the sample. In other words, the smaller the sample material the smaller will be the required indentation load. Also, the minimum thickness requirement and indentation spacing from both interior and exterior edges are very critical for small material parts. However, it is also essential to avoid slippages and movements in larger parts during the test by ensuring they are tightly secured before the commencement of the test. It is inappropriate to have parts with overhanging excesses on the anvil as this may introduce error to the results. Hence, adequate support should be given to these parts before beginning the test.
To ensure accuracy in the result of a hardness test it might be essential to report the result on a different scale. However, an important consideration is to ensure that tests have validity after completing a test correlation in different scales despite having an established scale conversion. This is important since the reliability of established conversion is not guaranteed. ASTM conversion charts are often the best reference for the high and low hardness range of non-austenitic metals. Additionally, ASTM standard and E140 have more scale conversion for reference.
A critical criterion for selecting a suitable testing method for a material hardness is the type of the material. For materials with fine grained structure like hardened bearing steel, a testing method like the Rockwell method is often suitable as a diamond indenter and high loading is utilized. Brinell test may be required for material like powder metals and cast irons which require larger indenters. Also, the Knoop or Vickers scale may be essential for small parts which are often measured on a microhardness scale. A standard rule is to choose a scale that provides the largest indenter and load while staying within required testing conditions when selecting a hardness scale.
- Sample Thickness
The standard thickness recommended for samples is a minimum of ten times the depth of indentation required during the test procedures. Also, standard, and superficial Rockwell methods have their typical minimum thickness allowable for accurate results.
- Gage R&R Studies
To ensure that operators and their testing equipment operate within the limits of a testing piece is why the Gage Repeatability and Reproducibility Studies were introduced. Integral variables exist in hardness testing that inhibits the use of Gage R&R processes and techniques with real test samples. Two important factors that affect the Gage R&R result include the failure of depth measuring testers to test the same area twice and the difference in materials. Carrying out the Gage R&R studies on a very consistent test block will help to reduce the effects of built-in differences.
- Sample Shape – Cylindrical
The variation in radial and axial material flow is critical for cylindrical samples having small diameters and as such often require an adjustment for the test result. Depending on the cylinder convex diameter of the surface, a correction factor for roundness is often included in the test results. Also, a minimum spacing of 2.5 times the edge to indentation diameter or from another indentation is recommended.
VII. Factors that influence the accuracy and repeatability in hardness test
Like most processes, hardness testing methods have certain factors that influence its accuracy and repeatability especially at the preparation and execution stages. But more importantly, the load application is often the first critical factor to consider as the lower loads often require more careful control of other factors. Other critical factors of note can include
- Dirt, temperature, illumination, humidity, vibrations and other external factors that can influence the sample or the accuracy of the result should be carefully monitored
- Sample firmness and support should be controlled such that the sample is rigid and secured in the anvil when carrying out the test. Also, the platform must be perfectly horizontal.
- The indenter should maintain a perfect verticality to the test sample during the test procedure
- For Brinell, Knoop, or Vickers hardness test light setting must be adequate to ensure appropriate visibility
- For every time you replace the indenter, a recalibration of the tester should be carried out to ensure the accuracy of the result.
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