Having the right welding equipment is only one ingredient required to make the best welds. Having access to the various methods of testing welds is also important. Welds are often tested when developing a procedure and occasionally in-process as a form of quality assurance. Hardness is one property of a completed weld, heat-affected zone, or unaffected base metal that may be analyzed.
Why Test Hardness
Hardness is an indicator of metal microstructure, which impacts how suitable a material or heat-treating process is to an application. In the world of structural welding, avoiding brittle microstructures is preferable, since excessively hard metals are not typically as ductile or tough as softer metals. Hardness may be desirable when welding certain metals, such as tool steel and hardfacing alloys. Through experimentation or modeling, hardness test results can be correlated to estimated properties such as strength—in the case of tool steels—and abrasion resistance—in the case of hardfacing alloys.
How is Hardness Tested? What Are Scales?
Rockwell Hardness Test
The Rockwell hardness test uses a calibrated instrument that measures the depth of an indent into a material when applying a load through an indenter predetermined by the testing scale. A harder material will provide a shallower indent while a softer material will provide a deeper indent. There are over 15 scales that govern testing ranging from plastics and coating to metals and ceramics. The most common scales for the steel industry are B and C. B is suitable for many lower-strength carbon steels, but C may be needed for high-strength, higher-alloy, and high-hardness metals.
All of these “scales” implement various indenters that, as the name implies, make an indent in the material surface. However, consistency in the shape, size, and material of the indenter is critical to achieving consistent results. Also important is the magnitude of the load and the time dedicated to applying the load. Since so many variables must be controlled, experimental test methods are standardized using various scales. The particular “scale” of the testing gives meaning to test values.
Softer materials, depending on composition, may fall within the E, F, G, or H scale. Again, each scale is an indicator of the test conditions that make the test results meaningful. A 65 Rockwell C-scale material is much harder than a 65 Rockwell B-Scale and may register “off the charts” of Rockwell A-Scale.
Vickers & Knoop
These two test methods are grouped together due to their similarities, but each test still demands conformance to its specific test method/specification. Both use a diamond indenter cut into a diamond shape and is used as an indenter. Here, the dimensions of the indent are used to correlate to hardness. Like the Rockwell test, a softer material will leave a deeper indent having a larger footprint.
A key advantage of both test methods is that very small indenter sizes can be used. Small indenters mean small areas of analysis; the test can be used to target specific areas where microstructure transformation and refinement may have occurred. Often the test equipment consists of the load application apparatus as well as a microscope having an eyepiece used to help measure the minuscule vertex-to-vertex distances needed to calculate the indent area.
For the Vickers hardness test, the scale most commonly used when analyzing carbon and low alloy steels in the HV10 scale, which indicates that a 10kg load is used. Other scales commonly used in the Vickers hardness test employ loads ranging from 1 gram (for very soft and/or very thin materials) to 30 kg (HV30, for very hard materials).
The Knoop hardness test also uses a diamond-shaped indenter which is used to correlate indent dimensions to hardness values. However, the indenter shape and load applied is well suited for thin materials and/or treated areas of minimal thickness. Values resulting from Knoop testing are indicated by placing “HK” after the calculated value (example 250HK). Like the Vickers test, accurate measurement of the indent is critical to achieving consistent and accurate results, so the equipment usually integrates a microscope for analysis.
Brinell
Brinell testing, indicated by “BHN” (Brinell Hardness Number) is somewhat opposite to the Knoop and Vickers tests. In the Brinell hardness test a spherical indenter having a diameter of 10mm is used with an impressive 3,000 kg (~6,600 lbs.). Like other testing methods or automation services, the diameter of the indent is measured, and the measured value is inputted into a calculation that factors in the applied load and indenter diameter to achieve a numerical result.
Mohs
The Mohs hardness test uses reference samples, each of which is assigned a number 1-10 that indicates a progressively higher hardness. Of the reference materials, talc is a 1, apatite is a 5, and diamond is a 10. Each sample is dragged along the surface of the metal to be tested. If the sample cannot scratch the metal to be tested, it is at least as hard as that Mohs number. The Mohs test provides much less precision than other testing methods, but a primary advantage is that it is quick and portable.
Conclusion
Some test methods are capable of testing hardness on a microscopic level, while others are designed to provide more of a “macroscopic” view. Each method has its own “scale” to help interpret the test results: examples include the Rockwell B and C scale or the HV10 scale for the Vickers hardness test. Understanding the test results requires some knowledge of the nature of the test and test equipment themselves, particularly when there must be correlation between test units or approximation of related properties such as tensile strength.
If this brief introduction has left your head spinning, our team of welding experts can help you better understand the practical application of many of these tests within the welding industry. Contact Us today to learn more about hardness testing, and the vast world of weld testing! Remember that knowledge and technology are the keys to success for any welding company
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