Rockwell Hardness Test and Scale

The Rockwell hardness test is a widely used method for measuring the hardness of materials. It involves pressing a diamond or hard metal cone or ball into the surface of the material being tested and measuring the depth of the indentation. The hardness value of rockwell hardness test is then determined using a conversion chart that correlates the depth of the indentation to a hardness value on the Rockwell scale. The Rockwell scale is divided into several different ranges, with each range corresponding to a different type of material. Examples of scales include the A, B, C, and D scales.

The rockwell hardness test is performed by pressing a diamond or hard metal ball or cone into the surface of the material under a specific load, and measuring the depth of the indentation. This depth is then used to determine the hardness of the material on the Rockwell scale, which ranges from very soft materials such as lead (HRB = 20) to very hard materials such as tungsten carbide (HRB = 90). The Rockwell hardness test is widely used because it is simple to perform, does not damage the material, and can be applied to a wide range of materials including metals, plastics, and ceramics.

Types of Rockwell Hardness Tests

There are several types of Rockwell hardness tests, which are differentiated by the type of load and indenter used. The most common Rockwell hardness tests are:

1. Rockwell “B” Scale: This rockwell hardness test uses a 1/16″ diameter steel ball as an indenter and a load of 100 kgf (kilogram-force). It is used to test soft materials such as aluminum and copper alloys.
2. Rockwell “C” Scale: This test uses a diamond cone as an indenter and a load of 150 kgf. It is used to test a wide range of materials including steel, titanium, and hard plastics.
3. Rockwell “A” Scale: This test uses a 1/16″ diameter steel ball as an indenter and a load of 60 kgf. It is used to test thin materials and surface hardening of steels.
4. Rockwell “D” Scale: This rockwell hardness test uses a diamond cone as an indenter and a load of 100 kgf. It is used to test hard materials such as cemented carbides and heat-treated steels.
5. Rockwell “F” Scale: This rockwell hardness test uses a 1/8″ steel ball as an indenter and a load of 60 kgf. It is used for testing thin sheets and strips of soft materials such as gold, silver and lead.
6. Rockwell “G” Scale: This test uses a 1/16″ steel ball as an indenter and a load of 150 kgf. It is used for testing hard materials such as cemented carbides and heat-treated steels.
7. Rockwell “H” Scale: This rockwell hardness test uses a 1/16″ steel ball as an indenter and a load of 80 kgf. It is used for testing materials that are too hard to be tested by other scales.
8. Rockwell “K” Scale: This rockwell hardness test uses a 1/8″ steel ball as an indenter and a load of 150 kgf. It is used for testing the hardness of brittle materials such as glass and ceramics.
9. Rockwell “L” Scale: This rockwell hardness test uses a 1/16″ steel ball as an indenter and a load of 15 kgf. It is used for testing the hardness of thin sheets, wires and small parts.
10. Rockwell “M” Scale: This test uses a 1/8″ steel ball as an indenter and a load of 15 kgf. It is used for testing the hardness of thin sheets, wires and small parts.

Rockwell Hardness Test Procedure

The Rockwell hardness test procedure is relatively simple and can be performed using a Rockwell hardness tester, which is a specialized machine that applies the load and records the depth of the indentation. The general steps of the procedure are:

1. Clean the surface of the material to be tested to remove any dirt, oil or other contaminants.
2. Place the material on the tester’s base and align it with the indenter.
3. Apply the preliminary test load (minor load) to the indenter, typically 10 kgf.
4. Apply the major load to the indenter, which is specific to the type of Rockwell test being performed (e.g. 100 kgf for the “B” scale, 150 kgf for the “C” scale, etc.)
5. Record the depth of the indentation using the tester’s dial or digital readout.
6. Compare the indentation depth to the conversion chart provided with the tester to determine the hardness value on the Rockwell scale.
7. Repeat the test on at least two additional locations on the material and average the results to get a more accurate measurement.

It’s important to follow the manufacturer’s instructions and guidelines when performing a Rockwell hardness test to ensure accurate and consistent results.

Rockwell Hardness Test Formula

The Rockwell hardness test formula is used to convert the depth of the indentation made by the indenter into a hardness value on the Rockwell scale. The formula used to calculate the Rockwell hardness number (HR) is:

HR = E – (0.002 x D)

Where:

• E is the dial reading or digital readout of the tester
• D is the depth of the indentation in millimeters

For example, if the dial reading is 60 and the indentation depth is 0.04mm, the Rockwell hardness value would be:

HR = 60 – (0.002 x 0.04) = 59.92

It’s important to note that the formula is different for each Rockwell scale, as the preliminary test load and major test loads are different for each scale. The formula is usually provided by the manufacturer of the hardness tester. Also, the formula provided is just an approximation and the actual formula used in the hardness tester can be more complex.

Factors for Selecting Appropriate Rockwell Scale

There are several factors to consider when selecting the appropriate Rockwell scale for a rockwell hardness test:

1. Material type: Different Rockwell scales are designed for different types of materials. For example, the Rockwell “B” scale is commonly used for soft materials such as aluminum and copper alloys, while the Rockwell “C” scale is commonly used for harder materials such as steel and titanium.
2. Material thickness: The Rockwell “A” scale is commonly used for testing thin materials and surface hardening of steels, while the Rockwell “F” scale is commonly used for testing thin sheets and strips of soft materials such as gold, silver and lead.
3. Load capacity: The Rockwell scales are designed to apply different loads to the material being tested. The Rockwell “B” scale applies a load of 100 kgf, while the Rockwell “C” scale applies a load of 150 kgf. It’s important to select a scale that can apply the appropriate load for the material being tested.
4. Indenter: Different Rockwell scales use different indenters (diamond cone or steel ball). The choice of indenter depends on the material and the properties that need to be measured.
5. The intended use of the material: The choice of Rockwell scale can depend on the intended use of the material. For example, if the material will be used in a high-stress application, a Rockwell “C” scale may be more appropriate. If the material will be used in a low-stress application, a Rockwell “B” scale may be more appropriate.
6. The level of precision required: Different Rockwell scales have different levels of precision. The Rockwell “C” scale is more precise than the Rockwell “B” scale, but it requires more skill and experience to perform correctly.
7. The level of standardization: Different Rockwell scales have different levels of standardization. The Rockwell “C” scale is more standardized than the Rockwell “B” scale, but it requires more skill and experience to perform correctly.

It’s important to consider all these factors when selecting the appropriate Rockwell scale for a hardness test. It’s also important to consult with a metallurgist or engineer with experience in hardness testing to ensure that the right scale is selected.

Standards for Rockwell Hardness Testing

There are several standards for Rockwell hardness testing that provide guidelines for performing the test correctly and interpreting the results. Some of the most commonly used standards are:

1. ASTM E18 – Standard Test Methods for Rockwell Hardness of Metallic Materials: This standard provides guidelines for performing Rockwell hardness tests on metallic materials, including test procedures, test loads, indenters, and test conditions.
2. ISO 6508-1:2016 – Metallic materials – Rockwell hardness test – Part 1: Test method: This standard provides guidelines for performing Rockwell hardness tests on metallic materials, including test procedures, test loads, indenters, and test conditions.
3. JIS Z 2244:2010 – Rockwell hardness test for metallic materials: This standard provides guidelines for performing Rockwell hardness tests on metallic materials, including test procedures, test loads, indenters, and test conditions.
4. EN ISO 6508-1:2017 – Metallic materials – Rockwell hardness test – Part 1: Test method: This standard provides guidelines for performing Rockwell hardness tests on metallic materials, including test procedures, test loads, indenters, and test conditions.
5. ASTM D785 – Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials: This standard provides guidelines for performing Rockwell hardness tests on plastics and electrical insulating materials, including test procedures, test loads, indenters, and test conditions.

These standards provide guidelines for performing the test correctly, ensuring accurate and consistent results. It’s important to consult the appropriate standard when performing a Rockwell hardness test to ensure compliance with industry standards and best practices.