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Which Hardness Tester Scale Is More Accurate
There are 3 common material hardness tests, Vickers, Rockwell, and Brinell, each with their different ways of testing and scale. That begs the question, which hardness tester scale Is more accurate?
What is Material Hardness?
Hardness describes a characteristic quality of a material, not an intrinsic physical property. In other words, you might change the hardness of a steel plate by heating or hammering it without changing its molecular composition. A material’s hardness is defined by its resistance to indentation, which can be found by measuring the depth of a permanent indentation. When you apply a known, predefined force using a particular indenter, the depth of the indentation will be inversely proportional to the material’s hardness.
What is Hardness Testing?
Hardness testers are machines that test materials to verify their current physical characteristics and performance under predictable circumstances. Over the years, several methods have been developed to test the hardness of industrial materials. Because materials can vary in sample size, thickness, regularity, texture, scale, and so on, the suitability and accuracy of a given testing method depend on how well it is matched to a material. For instance, materials with a small grain or very small parts may respond differently to a cone-shaped indenter than they do to a spherical indenter.
What Differentiates the Hardness Testing Methods?
If we consider the Vickers, Rockwell, and Brinell hardness testing methods, we will see that each has a distinctive indenter and means of applying force to a given material. We do a deep dive into the differences in our post Brinell, Rockwell, and Vickers Hardness Tests: The Differences Explained. However, below is a summary of each testing method.
Vickers Hardness Testing
The Vickers hardness test is often described as a microhardness test. The ASTM E-384 test procedure applies a square pyramid-shaped diamond indenter in conjunction with a relatively light load up to 1 kgf to a sample, then takes an optical measurement of the depression to determine the material’s hardness. The Vickers method can accurately test the hardness of small, highly polished metal, ceramic, or composite parts. It is easy to implement, and the same indenter can be applied to different materials with varying hardness degrees.
Although Vickers hardness testing can be used for a wide range of applications, there are certain factors that should be considered for each test case. Preparing and testing the sample can be a time-consuming endeavor. The sample needs to be sectioned to fit and carefully mounted to be perpendicular to the testing platform to ensure the test load will be evenly distributed. It also needs to be polished so that the depression will be regularly shaped and measurable. The surface area of the indentation should be as large as possible. Because many of the preparation, testing, and measuring steps are performed by human operators, it can be challenging to keep the process free from error.
Rockwell Hardness Testing
The most commonly applied and generally accurate hardness test is the Rockwell test, as defined in ASTM E-18. This test measures the permanent depth of an indentation made by applying a designated force to the indenter, then derives the sample’s hardness by calculating the difference between the baseline and final depth measurements and converting this figure to a hardness number. This test is usually easy to perform on any soft or hard metal with a relatively uniform surface, unless the indenter is too large for the sample size or the shape of the sample is unsuitable.
The Rockwell method uses a conical, round-tipped diamond or a ball indenter to apply a preliminary force, or “minor load,” of 3 to 10 kgf to a sample for a specified dwell time. This step diminishes the effects that a surface finish could have on perceived or performative hardness. After holding the preliminary test force for a specified dwell time, the depression is measured as a baseline. Once this figure is recorded, a secondary force, called the “major load,” is applied and held for a predetermined dwell time so that the sample can undergo elastic recovery. Together, the minor and major loads comprise the total necessary test load, which can range from 15 to 150 kgf or 500 to 3000 kgf, depending on the particularities of the test.
When the major load is released, the tester returns to the preliminary load and holds it for a designated dwell time, after which the tester removes the load and measures the final depth of the depression. Since the hardness value corresponds to the maximum depth of the depression, it is directly readable and does not require a diagonal scale or instrumental optical evaluation. The simplified measuring technique reduces the chance of a reading error when assessing test results.
The depth of the indentation must not exceed the thickness of the test specimen, and the indenter cannot press too close to the edge of the sample. The surface of the specimen and test anvil must be free of dirt and debris. The indenter should be perpendicular to the test platform to ensure proper loading and impression. If the hardness of the sample is not uniform, the test will measure the average hardness of the sample, and the overall accuracy of the test may be called into question.
Brinell Hardness Testing
As defined in ASTM E10, the Brinell hardness testing method is generally used to test cast or forged materials with rough or coarse surfaces. This test utilizes a high load 500 to 3000 kgf load and a large ball indenter up to 10mm diameter to create an impression. The ball indenter that is pressed into the sample is held for a fixed period of time and then removed. The indentation is optically measured across at least two diameters using a specially designed Brinell microscope or other optical systems. The indentation is measured to produce an average result that accounts for the sample’s textural inconsistencies. These results are then converted to their corresponding Brinell hardness numbers.
When testing coarse materials, the Brinell hardness test can be more accurate than the Rockwell test because the ball indenter evenly distributes pressure across the sample. It remains essential to carefully prepare the sample and clean the test surface to produce viable indentations. The test also has the disadvantage of being somewhat time-consuming, and since different operators may cause inconsistencies in a range of measurements, Brinell test results can vary even under ideal conditions. Poor preparation and technique can greatly increase the potential for error. Automatic optical Brinell scopes and computer image analysis can help operators ensure that they are taking consistent, standardized readings.
For Accurate Results, Match Your Hardness Testing Method to Your Material
In the end, there really is no consensus on which hardness testing is the most accurate. A Brinell hardness tester can test all types of metals. Some consider the results of the Brinell method more accurate and reliable than those obtained by the Rockwell method because of the use of a spherical indenter that distributes pressure evenly. However, others think the Rockwell test is generally easier to perform, and more accurate than other types of hardness testing methods. Objectively speaking, the accuracy of a testing method largely depends on the suitability of the relationship between the test method and the characteristics of the sample material. Good results also depend on the work of qualified operators who can consistently manage the testing protocol and procedures and deliver reliable results. When you are aware of the properties of the materials that need testing, you should easily be able to select the hardness test that is best suited for your application.
METSUCO has been a leading supplier of hardness testing machines and blocks 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. Call 713-827-0700 or email sales@metsuco.com with inquiries.