When manufacturing metal components of any kind, it’s important to have control over the properties of a finished part. To ensure that your finished product meets all the necessary specifications, you might need to perform a Rockwell hardness test on a metal part or test sample at different points during production. It is useful to check the hardness of steel and other metals because it indicates how much stress they can withstand before deformation and because different metals have different Rockwell hardness scales. Read on to learn more about how to perform a Rockwell Hardness Test.
I. What is the Rockwell Hardness Test?
A Rockwell hardness test is used to determine the hardness of metals. During the test, a metal component or metal sample is subjected to a controlled amount of stress. The depth of penetration into the metal is measured when it resumes its original shape. The larger the penetration depth, the softer or weaker the material; conversely, if no sign of penetration occurs, that indicates that the material is hard and strong. Metals with high Rockwell hardness are resistant to wear and can be used in applications with high loads. The Rockwell Hardness Test is the most accurate method of measuring the hardness of a material, and it is easier to perform than the Brinell or Vickers tests.01
II. Theory and Principle of the Rockwell Hardness Test
The Rockwell Hardness Test uses a depth-differential method to test for hardness. A predetermined minor load is applied to the test sample, and the depth measurement is taken. Then a major load is applied to the same spot, which creates a deeper indentation. The major load is then removed, and the minor load remains. The difference in depth of the indentations made by the major and minor loads is then used to determine the Rockwell Hardness. The principle behind measuring hardness is that as the material gets harder, it will take more force to deform the metal component than how much it would from its original shape.
Apart from being consistent among different testing facilities and equipment, another advantage of the Rockwell hardness test is that it can be applied over a large range of loads and pressures.
III. Standards of Rockwell Hardness Test
- ASTM E18 for testing the Hardness of Metals
- ISO 6508 for testing the Hardness of Metals
- ASTM D785 for testing the Hardness of Plastics
- ISO 2039 for testing the Hardness of Plastics
IV. Rockwell Hardness Test Procedure/ How is the Rockwell Hardness Test Performed?
To perform a Rockwell hardness test on the material, first, you need to prepare:
- Setting the test sample: The test sample is placed on a flat surface that is smooth and solid.
- Apply the minor load: A minor load of predetermined size and weight is applied to the test sample using a diamond or an indenter ball. In the case of a diamond, it is used as a hammer. In the case of an indenter ball, it is used as a plunger to make the minor load.
- Taking the minor load measurement: Using a dial or a measuring gauge, measure the depth of the indent made by the minor load.
- Apply the major load: A major load of predetermined size and weight is then applied on the same spot using the same instrument for applying the minor load. Hold the major load in place for some time to make a deeper indentation.
- Taking the major load measurement: Using a dial or a measuring gauge, measure the depth of the indentation made by the major load.
- Remove the major load, leaving the minor load in place.
- Measure the final indentation: Measure the final indentation from the initial indentation made by the minor load initially.
- Using the penetration depth and the Rockwell Hardness Scale, find the Rockwell Hardness number.
V. How to Calculate Rockwell Hardness?
The Rockwell Hardness is obtained by dividing the smaller indentation between the two indentations made by the major and minor loads.
Rockwell hardness number (R) = Smaller indentation – Initial indentation
R = a – b
where ‘a’ and ‘b’ are initial and final penetration depths in mm.
For example, if the initial penetration depth is 2.0mm, it becomes 0.2mm after the application of major load, which makes a bigger indentation of 2.1mm.
The Rockwell hardness test is widely used to standardize the hardness of metals. Although it is possible and easy to perform, it can be time-consuming and expensive. It is also important to note that since the hardness of a material is determined by the Rockwell Hardness number, it doesn’t indicate whether the material with that number will be able to withstand the loads placed on it over a longer period.