Rockwell hardness testing is the most commonly used method for assessing the hardness of specimens. Under this method, the hardness of the specimen is measured by assessing the depth of the indentation made by the indenter under the test load on the specimen surface.
Hardness is the resistance of a material when subjected to deformities under test loads. It is defined as the resistance to indentation and is determined by measuring the depth of the indentation made on the specimen surface.
Introduction: Rockwell Hardness Testing
When an indenter works on a specimen surface under a fixed load, the smaller the indentation made, the harder the material is. Hardness testing is an important part of most engineering and technology industries.
It helps to determine the strength and structural integrity of the material and its suitability for a particular application. The material has to pass through certain tests for it to be considered safe for being used for specific applications. Materials which pass through hardness testing are deemed suitable for use in specific applications.
Hardness testing also helps to ensure compliance with safety standards prescribed by the relevant authorities. It also ensures that the final product manufactured is of the highest quality and meets the prescribed safety norms.
Rockwell hardness testing is a method of determining the relative hardness of a material.
This testing method is most simple among all other methods and does not make use of any optical equipment and systems which makes it cost-effective also.
Rockwell method also produces the quickest results as no optical instruments are involved and the specimen surface does not need to be worked on and prepared before testing its hardness.
Rockwell method of hardness testing has another added advantage that it is non-destructive, which means that the specimen is not destroyed during the testing process and it can be used for other purposes after the hardness testing is completed.
Rockwell hardness testing method has few drawbacks too.
It is not as accurate as other methods as the hardness value are determined from indentation depth and do not involve advanced optical equipment for measuring hardness.
The indentation depth made by the indenter on the specimen surface should be measured very carefully under this method as even a small deviation in measuring it can lead to erroneous hardness values.
The readings in this method are also impacted by any imperfections on the specimen surface like dirt and contamination and the testing environment.
The indenter is also a very important accessory in Rockwell hardness testing. A worn-out intender can give faulty and unreliable hardness values.
Measuring Hardness Using Rockwell Hardness Testing
The Rockwell method of hardness testing uses the depth of the indentation made by the indenter on the specimen surface. The depth of the indentation following the application of minor load and a major load is measured.
The indenter used in this method can either be a carbide tungsten ball of a particular diameter or a spherical diamond tipped cone at an angle of 120-degrees and 0.2mm tip radius, which is known as a Brave indenter.
In the Rockwell method of hardness testing, the indenter moves down on the specimen surface on the application of test load. A minor load is applied and the zero-reference position is established.
This is followed by the application of major load for a particular time period known as dwell time beyond zero.
The major load is applied to allow for elastic recovery. The major load is then released but leaving the minor load applied at the same time.
The Rockwell hardness valued is determined from the difference in the baseline and final depth measurements. The distance is then converted into a hardness number.
The preliminary test loads used can range from 3Kgf to 10Kgf. The total test loads can vary between 15Kgf to 150Kgf. There are many considerations that should be evaluated before selecting a Rockwell scale.
Generally, a scale that specifies the largest load and the largest indenter without affecting or exceeding the defined testing conditions and accounting for conditions that may alter the test results should be selected.
During testing, it is important to ensure that the test axis is within 2-degrees of perpendicular for precise loading. There should be no deflection of the specimen surface due to dirt or other contamination.
The indenter also should not deflect during the application of test load due to any reasons.
The specimen surface should be kept completely clean and free from any imperfections before using the Rockwell method of hardness testing.
There have been technological advancements in this field which have resulted in much improved and accurate hardness testing. The latest technologies allow for a high level of test point repeatability for better results.
A nose-mounted indenter allows access to and visibility of different test points which means that that there is no need for time-consuming and costly test sample sectioning.
This also helps to cut down the cost of testing. Testing the hardness of sheet metal can become difficult with the Rockwell method.
Sheet metal can be too thin and soft for testing with this method and it may not exceed the minimum thickness requirements which may also damage the test anvil. In such situations, a diamond test anvil can be used to provide consistent influence on the results.
Rockwell hardness valued is expressed as a combination of a hardness number and a scale symbol representing the indenter and the minor and major test loads.
The hardness number is represented by the symbol HR and the scale designation. There are nearly 30 different Rockwell scales.
Most of the applications are covered by Rockwell C and B scales for specimen testing of different types. However, if the test specimen is different than ferrous metals, brass, and other metals, then the Rockwell scale should be carefully selected.
The type of indenter is also important. The choice is generally between the diamond indenter and the steel ball indenters with varying diameters.
If the test specimen belongs to soft metal groups like copper alloys, soft steel, aluminum alloys, etc. then a 1/16-inch diameter steel ball coupled with 100Kgf test load is used and the hardness is represented as B on the scale.
For testing of harder metals, a diamond-tipped indenter is used which is tilted at 120-degrees and a test load of 150Kgf is applied and the hardness is expressed as C on the Rockwell scale.
The scales B and C are also known as the common scales under the Rockwell method.
A Rockwell hardness reading will have the hardness value followed by the HR (Hardness Rockwell) and the scaling letter. Hence, a reading of 60 HRB means that the specimen has a hardness reading of 60 on the B scale.
Factors Affecting the Selection of Rockwell Scales
The first consideration for Rockwell testing is identifying the proper scale for testing. There are different types of materials which require varying Rockwell scales for hardness testing.
As discussed already, the scales B and C are the most common ones in the Rockwell method and they cover most of the different types of test specimens.
However, there are many other scales in addition to the scales B and C and they should be used depending on the type of material to be tested.
The type of indenter to be used is also an important consideration for Rockwell hardness testing.
Before starting the hardness testing process, if there is any doubt about the selection of the appropriate Rockwell scale, then the following factors which affect the scale selection should be analyzed in details:
If the specific hardness scale is not available, then the material type to be tested should be identified and it should be compared with different tables which list the best-recommended scales for testing of that particular material.
Such tables are generally based on empirical testing information and historical data.
Generally, using the heaviest load which the specimen material can take is the best option.
It is so as the larger indent made on the specimen surface will provide the greatest integrity and will be least affected by the condition of the test surface.
It is important to note that knowledge of the material to be tested plays a critical role in selecting the ideal scale, but it is not the sole criteria for doing so and there are other factors at play which help in deciding the best scale.
The material thickness also influences the scale to be used. A force or load that is excessive for the material thickness will be influenced by the support anvil.
As a general rule, the material to be tested should be at least 10 times the depth of the indentation made while using a diamond indenter and at least 15 times the depth of indentation when using a steel ball indenter.
If required, the indentation depth can be calculated to confirm that these requirements are being fulfilled. Also, no deforming should be obvious on the supporting surface of the specimen material.
Support is a key factor in Rockwell hardness testing. The specimen surface should not be deflected or move during testing.
A slight movement of 0.001 inches of the specimen can lead to a difference of 10 Rockwell points in the final readings. The anvil should be selected to match the specimen geometry and provide it complete support for keeping it secure during hardness testing.
The supporting shoulder and the surface on which the specimen is placed should be parallel to each other. It is absolutely critical for the anvil to ensure perpendicularity of the specimen to the indenter.
For getting accurate and reliable hardness testing results, it is very important that the specimen surface should be presented perpendicularly to the indenter and the specimen surface should not move or deflect at all.
It is estimated that a tilt angle of 1 degree between the specimen surface and the indenter can cause an error of 5% in hardness.
To guarantee accurate and acceptable readings, the tilt angle should never be greater than 2 degrees in any condition.
The perpendicularity of the indenter to the specimen surface is also influenced by supporting anvil, opposing surfaces of the material, mechanical components of the tester, etc.
Indent Spacing readings
In order to get reliable hardness testing values, the spacing between indents and from the material edge must be maintained properly.
This helps to prevent any adjacent indents or worked edge from affecting the testing readings.
The distance from the center of any indentation should be at least three times the diameter of the indentation.
The distance from the center of indentation to the edge of the specimen should be at least 2.5 times the diameter of the indentation.
Cylindrical Testing and Correction Factors
Cylindrical specimen surfaces will show lower hardness value compared to flat surfaces due to the curvature.
A cylindrical surface will be characterized by a reduction in lateral support which means that the indenter will penetrate it more compared to a flat specimen surface.
This gives lower hardness results for cylindrical specimen surfaces.
If the diameter of the cylindrical surface to be tested is less than 1 inch, then correction factors need to be accounted for and added to the final test results for ensuring reliable and accurate hardness testing values.
Most of the digital Rockwell hardness testers will add this correction factor to the final test results automatically.
The specimen surface to be tested should be free of any imperfections.
To get the best results, the test surface should be smooth and clean. When the test load becomes lighter, the surface of the specimen plays an important role in hardness testing.
Under such conditions where the test load is about 15Kgf, the specimen surface needs to be smooth for reliable testing results.
Proper care should be taken before working on a surface and finishing it before testing to avoid the chances of causing a work-hardened condition to the specimen.
Cleanliness of the specimen material being tested, indenters, supporting anvils and any other contacting surfaces also become important factors for getting accurate hardness testing results.