Considerations for Selecting a Hardness Tester

Across many different industries such as oil and gas, aerospace, auto manufacturing, and commercial construction, just to name a few, there is a need to have the hardness of the materials tested. This is to make sure the product or material in question will not critically fail down the line.  In order to test a material’s hardness, there is a device called a hardness tester.

A hardness tester is an instrument used to measure just how hard a material is. This is typically done by assessing the impact that penetration has on the surface of an object by a rounded or pointed indenter made of diamond, carbide, or hard steel. To ascertain the hardness of the material, the depth of the impression made by the pointed object is then measured.

There are several different kinds of hardness testers and test scales. So, we want touch on some of the considerations for selecting a hardness tester that would be right for you and your application.

Considerations for Selecting a Hardness Tester

Test load: the hardness of the material determines this. In the case of steel or alloys, the test load can be as much as 3,000 kgf. On the other hand, soft metals will only need a test load of 500 kgf. The higher the load used is, the higher the accuracy of the test will be. The impression made on an object during testing should not exceed 10% of its thickness.

Hardness range: this is the factor that dictates the material of the indenter. If you are working with a hardness of over 650 HB/30, you would need a diamond indenter. If you are testing the hardness of an object with a value less than this, steel or hard metal indenters should be effective.

Accuracy level: this is dependent on the surface that you will be measuring. Things such as the cleanliness, texture, system, etc. of the surface come into play.

Adaptability of the device: the shape and size of the material that will be tested is to be considered when selecting an appropriate hardness tester.

Types of Hardness Testers

The classification of hardness testers is based on the method they use in the testing process. Here are the 5 major types of hardness testers:

The Brinell Hardness Testers

The Brinell method is considered to be one of the more common, if not the most common hardness test considering it was the first hardness test. This test method uses a steel ball indenter with a diameter that varies according to the sample to be measured. This is the preferred method for rough samples and can test just about all types of metal. The Brinell method uses a wide range of indenters so the range of measuring is wide with a high-test load (3,000 Kg) as well.

There are a few other things though that you need to be made aware of before choosing a Brinell Hardness Tester. Measuring errors are possible since the impression made on the sample surface is measured with an optical instrument. It is however considered to be more accurate than the Rockwell method because the use of a spherical indenter distributes pressure evenly. You do also need a well prepared, clean flat sample in order to use this testing method, so a test of cylindrical surfaces is out of the question.

The Rockwell Hardness Testers

The Rockwell Method is one of the most widely known hardness testing techniques. It uses both a penetrating diamond cone and a penetrating steel ball to measure the hardness of a material. The diamond cone can only be used on hardened steel and materials though. Do not use it for materials below 785 N/mm² hardness. Instead, for softer materials, use the steel ball. You do want to keep the total load lower than the bigger the indenter you use. Large ball indenters are used to test plastics.

Unlike the Brinell test mentioned above, Rockwell Hardness Testers can test on cylindrical surfaces. Also, in contrast to the Brinell Hardness Testers, the Rockwell Hardness Testers offers measurements without optical equipment, The Rockwell Method also offers fast testing because of a short measurement cycle.

You will also want to note a couple of drawbacks though. The test load ranges are limited. The accuracy of the measurement depends on the contact between the indenter and the sample surface. So, it is not considered the most accurate for hard metals.

The Vickers Hardness Testers.

The Vickers method is a microhardness testing method. This method bears many similarities to the Brinell method. The difference is Vickers Hardness Testers use a diamond penetrating pyramid with a square base and a fixed load. Because of this, Vickers allows light test loads as well as macro loads up to 30 kg. It can test all types of materials, both soft and hard and is easy to perform. Vickers hardness testers are used a lot in laboratory testing. It can be a time-consuming process because the imprint on the surface is measured optically, and just like the Brinell test, needs a well-cleaned and prepared sample. It is also known that you can get an irregular distribution of the test load by the pyramid (the load is higher on the edges than on the sides).

Knoop Hardness Testers

Often combined with Vickers because it is another microhardness testing technique. Knoop Hardness Testers  uses a diamond-shaped indenter and is appropriate for light test loads. For example, to assess the risk of cracking in brittle materials such as ceramics or coatings.

Ultrasound Hardness Testers

This method is the top pick for hardness testing operations that want to avoid destroying the object being tested, also known as non-destructive testing. These hardness testers consist of a probe and an electronic module. There are two methods of ultra-sonic testing, contact and immersion. Contact testing allows for inspection of parts that cannot be transported and immersion immerses the material in a liquid in order to obtain better movement of the sound waves.

Benchtop Versus Portable Hardness Testers

Benchtop hardness testers are usually used in a testing lab and are traditional testing methods such as Brinell, Rockwell or Vickers. There are several advantages of portable hardness testers. For example, portable hardness testers are perfect for when you can’t take a sample back to a lab. Another example is that portable hardness testers can be used on much smaller surface areas than traditional hardness testers.

Final Words

It is not possible to accurately test the hardness of an object without considering the test load, hardness range, accuracy level, and the adaptability of the device. In order to test materials for their level of hardness, you must also know the most appropriate hardness tester and method. We hope this post has provided some insight into which hardness tester is right for your applications. Metallurgical Supply Company (METSUCO) has been a leading supplier of portable hardness testers for over 30 years. We serve not only industries such as oil and gas, aerospace, automotive, and heavy manufacturing, but also universities and quality testing labs. Because of our years of service across a wide range of industries, we know each of these hardness testers well. If you have any questions or inquiries, Call 713-827-0700 or email sales@metsuco.com