Many products and equipment entering the market today are made from new materials and processes. The manufacturing process of these high-tech products calls for a thorough understanding of the materials’ properties. One of the most important mechanical and optical ways used to determine if a material is suitable for a given application is hardness.
Hardness is defined as the material's ability to resist or withstand plastic deformation. This is crucial because the materials will be exposed to loads, extreme temperatures and pressures in the manufacturing process. The hardness test will, therefore, tell you if the material can handle the beat down and perform its intended purpose in the long run.
Among the various types of hardness testing methods is the Rockwell hardness test, which is also subdivided into two types; standard Rockwell test method and superficial Rockwell test method.
What is the Superficial Rockwell hardness test?
The Rockwell hardness test was invented by Stanley P. Rockwell, a metallurgist who wanted a fast and non-destructive way of measuring the effect of heat treatment on bearing races. At the time, the only hardness testing methods available were Vickers, Brinell and Sceleroscope. The Vickers test was too time-consuming, Brinell indentations were too big for the bearings, and the Sceleroscope method was too difficult to conduct.
Years later, the Superficial Rockwell hardness test was invented to test materials that are too small or thin. In this method, an accurately shaped spheroconical diamond or hard metal ball is used to make an indentation in the test sample. The Rockwell hardness tester has a depth indicating device that measures the vertical movement of the indenter into the test specimen.
Under an initial minor load of only 3kg-f, the indenter is forced into the test material and left there, while the indicating device marks a zero reference point.
A major load of 15, 30 or 45kgf is then added, forcing the penetration to go even deeper and the indicating device notes that position too.
The major load is then released, leaving the minor load in place.
The superficial Rockwell number is therefore the difference in depth from zero reference point after the application of the major load.
What's the difference between Superficial and Standard Rockwell Hardness tests?
The main idea and process in both Rockwell and Superficial Rockwell methods are the same. They both use a diamond or metal ball indenter to measure hardness. However, three slight characteristics differentiate the two methods;
Standard Rockwell test uses an initial minor load of 10kgf and a major load of 60, 100 or 150 kgf while superficial Rockwell method uses only 3kgf as its preliminary minor load and 15, 30 or 45 kgf as the major force.
The Superficial Rockwell test method was invented in the USA, but it’s mostly used in Europe.
The superficial test method is primarily used to evaluate small or thin components or materials whose hardness value is a little outside the standard Rockwell scale. For example, it can test coatings, carburized surfaces and thin strips.
Superficial hardness testing gives the finest and most accurate results of the thinnest or tiniest materials. This ensures their quality before they are incorporated in a more significant production process. It’s important to note though that the values determined by superficial Rockwell test can be converted to standard Rockwell test values and vice versa.
Without hardness testing, it would be challenging to choose materials that go well together in a manufacturing process. More importantly, products would be wearing out in no time because the wrong materials were used. Superficial Rockwell hardness test is used extensively to measure the hardness of soft and thin materials that need specific detailing of the specimen. Today, there are motorized versions of Rockwell hardness testers, including one that can conduct both standard and superficial tests.