Rockwell Hardness Testing
The Rockwell hardness test method, as properly defined in ASTM E-18 standards, is the most commonly used hardness test method among all the other methods. The customer should get a copy of these standards and understand them well before attempting a Rockwell test.
The Rockwell test is comparatively easier to perform and more precise than other hardness testing methods. The Rockwell test method can be used on all metals, except in the condition where the test sample structure or surface conditions would bring too many variations; where the indentations can be too large; or where the sample size or shape may prohibit its use.
This Rockwell hardness testing method measures the permanent depth of indentation created by a force/load on an indenter. At first, a preliminary test force (preload or minor load) is applied upon the sample with the help of a diamond or ball indenter. This preload breaks through the surface to decrease the effects of surface finish. After holding this preliminary test force for a specific period of dwell time, it is possible to measure the baseline depth of indentation.
Understanding the different types of hardness tests, including the Rockwell hardness test can be daunting. This statement is often true where there is a lack or insufficient information about these hardness testing techniques.
However, since Rockwell is a popular hardness testing method in many manufacturing and metal forging factories, it is essential then that quality inspectors and manufacturers understand its operations and limitations.
In this article we have put together all the information you will need to grasp the workings of the Rockwell hardness testing methods as well as its many advantages and applications. So, if you are ready to know more, then read on.
I. What is the Rockwell Hardness Test?
Rockwell hardness testing is a method that determines the hardness of metals and polymers, and it is very common in most metallurgy and metal forging industry activities. Hardness testing does not test performance attributes. Yet, a material’s hardness correlates with its strength, wear resistance, and other properties. Furthermore, hardness testing is also suitable for material assessment. However, its simplicity and cheap cost compared to direct measurement of many parameters make this material assessment possible. For structural alloys, such as steel and aluminum, conversion charts from Rockwell hardness to tensile strength are often the norm.
II. Types of Rockwell Hardness Test
There are two variations of the Rockwell hardness tests, which includes the standard and the superficial Rockwell tests. You will find a great difference in the applications of these two types of Rockwell hardness tests.
- Standard Rockwell Hardness Tests
This type of Rockwell hardness test assesses the test sample material hardness using different scales. These scales are often different for plastics, ferrous, and nonferrous materials, making them unique and even preferable for quality inspectors. Additionally, metals have A, B, C, and F as the popular Rockwell hardness scales, while polymers use M and R scales
- Superficial Rockwell Hardness Testing
This variation of the Rockwell hardness test is considered more sensitive to the nature of the material’s surface than is obtainable with regular Rockwell testing. Small sample areas and thin surfaces, as well as surfaces with hardness gradient, benefit more from this method than with the regular Rockwell testing. However, it uses a different scale for ascertaining the Rockwell hardness, including W, X, and Y for nonferrous and soft-coated materials, while T and N are for ferrous materials.
III. How to Perform a Rockwell Hardness Test?
Performing a hardness test is often more accurate and simpler with a Rockwell test than most of the other hardness tests. One major advantage it has over other tests is its ability to test all metals. The exception to this case only occurs when the surface conditions of the test sample result in several variations. Sometimes, these variations show up as very large indentation or as complex shapes and sizes of test samples which can hinder the smooth operation of the testing process.
Rockwell hardness tests determine the depth of depression caused by an applied load or force by an indenter on the test sample. Applying a preliminary small load or force often called a minor load is the first step. You apply this minor load with the use of a diamond indenter or sometimes a ball indenter. The essence of the minor load application is usually to break down the resistance of the sample surface. A dwell period is required to hold down this minor load after which you can determine the baseline depression depth which finalizes the preload process.
But that’s not all.
The next loading often referred to as the major load is required to achieve the total load required to accomplish the test. Also, a dwell period is also required for the major load (as it was done for the minor load) after which you can determine the final depth of indentation. The difference between the measurement taken from the baseline and the final depth of indentation is used to calculate the Rockwell hardness value. You can convert this value to the Rockwell hardness number.
Related Article: Rockwell Test Procedure: How to Perform a Rockwell Hardness Test?
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Related Article: Vickers Test Procedure: How to Perform a Vickers Hardness Test?
IV. How to Calculate Rockwell Hardness?
To calculate the Rockwell hardness number, an applied force ranging from 3kgf to 10kgf is often used as the standard for the preliminary force or load. This range of loads is often applicable for both superficial and normal Rockwell scales. For the total test loads required, this can also range from 15kgf to 150kgf and can sometimes be as high 500kgf to 3000kgf.
The parameters required to calculate the Rockwell hardness test include:
A = Depth of the indenter after removing the minor load
B = Indenter’s position for Total load, Minor, and Major loads
C = Indenter’s final point after the elastic recovery of test sample
D = The difference in heights between the indentation caused by major load and minor load. This difference in height helps to determine the Rockwell Hardness Number.
V. Limitations of Rockwell Hardness Test
With all the huge benefits of easy set-up and speed of testing and result acquisition that Rockwell hardness testing offers, its information will be incomplete without stating its limitations. One major limitation is the indenter travel which is often limited to 0.2mm or 100 Rockwell points. This indenter travel limit implies that it must combine various load sizes to ascertain the hardness level of the various test samples.
Also, as the hardness of the test samples increases, it becomes continuously challenging to use Rockwell hardness testing in distinguishing various materials. As a result, Rockwell hardness testing is suitable for distinguishing materials up to a certain point. In contrast, the hardness value beyond that point is only used to ascertain the tensile strength of the test sample.
VI. Factors Affecting Rockwell Test Accuracy, Precision, and Bias
For every process and procedure, certain factors tend to affect the reliability and effectiveness of such processes, and Rockwell hardness testing couldn’t be exempted from such factors. Top on the list of these issues is the equipment’s irregularities, operators’ inconsistencies, and certain environmental situations.
A procedure known as GRR (Gauge Repeatability and Reproducibility) is undertaken to measure the level of equipment irregularities. This can reveal a machine’s inconsistencies and the tendency to produce inaccurate results. A high GRR value often shows high irregularities in the machines and consequently results in high inaccuracies. To compare different machines to arrive at the most suitable one for the test, a GRR test is carried out for the machines assuming that the machines all perform the same test. The test result shows the most effective and accurate machine considering that the machine with the lowest GRR has the best performance rating. Also, experience underscores other design factors, equipment build quality, and loading systems all contribute to the efficiency of the Rockwell hardness testing machines.
While Rockwell hardness testing is seen as a very fast and easy-to-achieve testing method, it is also easily the most error-prone testing method. Even though these errors are easily avoidable, the responsibility rests on the operator to ensure that the set-up, test sample, equipment, and other necessary conditions ensure the high accuracy of the test results.
For example, ensuring that the machine is properly calibrated and the calibration certificates are valid during the testing period is one way to ensure the high accuracy of Rockwell hardness tests. Also, the required periodic and routine maintenance of the machines is done as scheduled is another way to ensure correct results of tests.
A damaged indenter is another common source of error in Rockwell hardness testing, which is usually due to the brittleness of the diamond indenter despite its toughness. Hence, high impact loads or forces can damage the diamond indenter, leading to a high penetration resistance and high hardness reading on account of wear and tear of the indenter tip.
Where a loading train experiences deflection because of a damaged or indented anvil, an error in the reading of the Rockwell hardness machine can occur. Likewise, surface impediments such as dirt or grease can obstruct perfect contact between the elevating screw and the anvil resulting in deflection and consequently an erroneous reading.
Aside from equipment anomalies, the test sample can also become a source of inaccuracies as a well-prepared sample surface is more likely to give a highly accurate result than an ill-prepared surface. The anvil and the test sample should have good contact, which you should be able to achieve with good sample cleaning. Additionally, firm placement of the test sample is required for an accurate result as vibrations, and excessive movements of the test sample during the test can result in a highly inaccurate result. Hence, the operator has a huge responsibility to ensure that all these requirements are met before commencing the test to achieve accuracy.
To wrap, Rockwell hardness testing is very beneficial in many manufacturing processes that involve ascertaining the tensile strength of production materials. One of such benefits is the fast operation time and result generation as well as its cost-effectiveness. However, knowing its limitations and the factors affecting the accuracy of its result can help you identify possible causes to help you take the necessary steps to avoid them. Specimen preparation and cleaning, equipment maintenance and calibration, and operator training are all contributors to the accuracy of Rockwell hardness testing methods. Moreover, finding a reliable manufacturer of Rockwell hardness test machines is one way to avoid some major causes of inaccurate results when conducting Rockwell hardness testing. The ball is now in your court. What is your next move?
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