Understanding Rockwell Hardness and Brinell Hardness Tests: Significance, Types, How to Calculate, Examples, and Applications

Hardness is a critical property of any material. It determines how resistant a material is to deformation, scratching, or penetration by other materials. Various methods are used to measure the hardness of a material. Brinell hardness and Rockwell hardness are two of the most common methods used for this purpose. In this article, we will discuss these two hardness testing methods, their significance, types, and applications.

Rockwell Hardness

Rockwell hardness is method of measuring the hardness of a material. It is named after American metallurgist Hugh M. Rockwell and Stanley P. Rockwell, who developed the method in the early 20th century. The Rockwell hardness test involves indenting the surface of a material with a diamond cone or steel ball indenter and measuring the depth of the resulting indentation.

Significance of Rockwell Hardness

Rockwell hardness is significant because it is a quick and easy method of measuring the hardness of a material. It is also a reliable method of measuring the hardness of materials with small or thin sections, such as sheet metal or thin-walled tubing.

Types of Rockwell Hardness Test

The Rockwell hardness test is categorized into three types:

1. Regular Rockwell hardness test: In this test, a load of 60, 100, or 150 kg is applied to the surface of the material for a few seconds, depending on the hardness of the material.
2. Superficial Rockwell hardness test: In this test, a load of 15, 30, or 45 kg is applied to the surface of the material for a few seconds, depending on the hardness of the material.
3. Rockwell hardness test for plastics: In this test, a load of 15 or 30 kg is applied to the surface of the plastic material for a few seconds.

Types of Rockwell scales (A, B, C, D, E, F, G, H, K)

The Rockwell hardness test uses a diamond cone or steel ball indenter to make an indentation in the material being tested. The depth of the indentation is measured and used to calculate the Rockwell hardness number (RHN) of the material. There are several different Rockwell scales, each designed for specific types of materials and applications. Here are the most common Rockwell scales:

1. Rockwell A scale (RHA): This scale is used for testing soft materials, such as lead, aluminum, and copper. The load applied is 60 kgf and the diamond cone indenter is used.
2. Rockwell B scale (RHB): This scale is used for testing harder materials, such as brass, mild steel, and annealed copper alloys. The load applied is 100 kgf and the diamond cone indenter is used.
3. Rockwell C scale (RHC): This scale is used for testing hard materials, such as hardened steels and cemented carbides. The load applied is 150 kgf and the diamond cone indenter is used.
4. Rockwell D scale (RHD): This scale is used for testing very hard materials, such as tempered steels and hard titanium alloys. The load applied is 100 kgf and the steel ball indenter is used.
5. Rockwell E scale (RHE): This scale is used for testing thin materials, such as foils, thin sheets, and films. The load applied is 100 kgf and the diamond cone indenter is used.
6. Rockwell F scale (RHF): This scale is used for testing very thin materials, such as thin foils and films. The load applied is 60 kgf and the diamond cone indenter is used.
7. Rockwell G scale (RHG): This scale is used for testing very hard and very thin materials, such as hardened steel and thin foils. The load applied is 150 kgf and the diamond cone indenter is used.
8. Rockwell H scale (RHH): This scale is used for testing hard and thick materials, such as cast iron, large forgings, and heavy parts. The load applied is 150 kgf and the steel ball indenter is used.
9. Rockwell K scale (RHK): This scale is used for testing very hard and thick materials, such as hardened steel and large forgings. The load applied is 150 kgf and the diamond cone indenter is used.

Rockwell Hardness test Indentation process, load, and duration of load application

• Indentation Process: The Rockwell hardness test uses an indenter, typically a diamond cone or steel ball, that is pressed into the surface of the material being tested. The depth of the indentation is measured to determine the hardness of the material.
• Load: The Rockwell hardness test uses a pre-load of 10 kgf, followed by the application of the main load, which varies depending on the Rockwell scale being used (e.g., 60 kgf for the Rockwell A scale, 150 kgf for the Rockwell C scale). The load is applied for a set duration of time.
• Duration of Load Application: The duration of the load application depends on the Rockwell scale being used. For example, the load is applied for 15 seconds on the Rockwell B and C scales, and for 30 seconds on the Rockwell A scale.

Calculation and reporting of Rockwell hardness number (RHN)

The calculation and reporting of Rockwell hardness number (RHN):

Calculation:

1. Measure the depth of the indentation made by the indenter in the material being tested, in millimeters (mm).
2. Determine the Rockwell scale being used and the load applied in kilograms-force (kgf).
3. Use the appropriate formula for the Rockwell scale being used to calculate the Rockwell hardness number (RHN):

• For the Rockwell A scale: RHN = 100 – (d / 0.002mm)
• For the Rockwell B scale: RHN = 100 – (d / 0.002mm)
• For the Rockwell C scale: RHN = 100 – (d / 0.002mm)
• For the Rockwell D scale: RHN = 130 – (d / 0.002mm)
• For the Rockwell E scale: RHN = 100 – (d / 0.002mm)
• For the Rockwell F scale: RHN = 60 – (d / 0.002mm)
• For the Rockwell G scale: RHN = 150 – (d / 0.002mm)
• For the Rockwell H scale: RHN = 100 – (d / 0.002mm)
• For the Rockwell K scale: RHN = 100 – (d / 0.002mm) Note: d is the depth of the indentation made by the indenter in millimeters (mm).

Reporting: The Rockwell hardness number (RHN) is typically reported with the Rockwell scale being used and the load applied. For example, “Rockwell C 50 kgf” would indicate the Rockwell C scale was used with a load of 50 kgf, and the hardness number obtained was the Rockwell hardness number (RHN).

RHN Example

Let’s say we’re testing a sample of steel using the Rockwell C scale with a load of 150 kgf. The depth of the indentation made by the indenter is measured as 0.8 mm. Using the formula for the Rockwell C scale, we can calculate the Rockwell hardness number (RHN) as: RHN = 100 – (d / 0.002mm) RHN = 100 – (0.8 mm / 0.002mm) RHN = 100 – 400 RHN = -300 Note: In this example, the RHN is negative because the depth of the indentation is greater than the maximum depth the Rockwell C scale is designed to measure. This is a common issue with Rockwell hardness testing on very hard materials.

In practice, if a Rockwell hardness number falls outside the range of the scale being used, a different scale or testing method should be used to obtain an accurate measurement.

Applications of Rockwell Hardness Test

The Rockwell hardness test is used in various industries, including automotive, aerospace, and construction. It is commonly used to determine the hardness of metals, including steel, aluminum, and copper. Additionally, Rockwell hardness testing is used in the manufacturing of tools, dies, and other components that require high strength and durability.

Brinell Hardness

Brinell hardness is a method of measuring the hardness of a material. It is named after Swedish engineer Johan August Brinell, who invented the method in 1900. The Brinell hardness test involves indenting the surface of a material with a spherical indenter, typically made of tungsten carbide or hardened steel, and measuring the diameter of the resulting indentation.

Significance of Brinell Hardness

Brinell hardness is significant because it can be used to determine the strength and durability of a material. It is particularly useful in industries such as automotive, aerospace, and construction, where materials need to withstand high stresses and forces. Additionally, Brinell hardness is a reliable method of measuring the hardness of materials with rough or irregular surfaces.

Types of Brinell Hardness Test

The Brinell hardness test is categorized into two types:

1. Standard Brinell hardness test: In this test, a load of 500 to 3000 kg is applied to the surface of the material for 10 to 30 seconds, depending on the hardness of the material.
2. Micro Brinell hardness test: In this test, a load of less than 1 kg is applied to the surface of the material for a longer duration, typically 15 to 60 seconds.

Brinell Hardness Test Indentation process, load, and duration of load application

• Indentation Process: The Brinell hardness test uses a hard steel ball indenter that is pressed into the surface of the material being tested. The diameter of the indentation is measured to determine the hardness of the material.
• Load: The Brinell hardness test uses a pre-load of 3 kgf, followed by the application of the main load, which ranges from 5 kgf to 3,000 kgf depending on the hardness of the material being tested. The load is applied for a set duration of time.
• Duration of Load Application: The duration of the load application for the Brinell hardness test is typically 10 to 30 seconds, depending on the hardness of the material being tested and the load used.

Calculation and reporting of Brinell hardness number (BHN)

The calculation and reporting of Brinell hardness number (BHN):

Calculation: To calculate the Brinell hardness number (BHN), the diameter of the indentation made by the indenter is measured and used in the following formula:

BHN = (2 * F) / (π * D * (D – √(D^2 – d^2)))

Where:

• F is the load applied to the indenter in kgf
• D is the diameter of the indenter in millimeters
• d is the diameter of the indentation made by the indenter in millimeters

The resulting number is the Brinell hardness number, which is reported as a unitless value.

Reporting: When reporting Brinell hardness numbers, it’s important to specify the load and duration of load application, as well as the diameter of the indenter used. For example, a Brinell hardness number of 200 with a 3000 kgf load and a 10 mm diameter indenter would be reported as “BHN 200/3000/10”.

It’s also important to note any special testing conditions or modifications made to the standard testing procedures, as these can affect the accuracy and reliability of the hardness measurements.

Overall, the calculation and reporting of Brinell hardness numbers involve measuring the diameter of the indentation made by the indenter and using a formula to calculate the hardness number. The resulting number is reported with specific details about the load, duration of load application, and diameter of the indenter.

Exampe of BHN

Example: Let’s say we want to determine the Brinell hardness number of a metal sample using a 1,000 kgf load and a 5 mm diameter indenter ball. After applying the load for 30 seconds and measuring the diameter of the resulting indentation, we find that the indentation diameter is 2.5 mm.

Using the formula above, we can calculate the Brinell hardness number as follows:

BHN = (2 * F) / (π * D * (D – √(D^2 – d^2))) BHN = (2 * 1,000) / (π * 5 * (5 – √(5^2 – 2.5^2))) BHN = 143.2

Therefore, the Brinell hardness number for this sample would be reported as “BHN 143/1000/5”.

Applications of Brinell Hardness Test

The Brinell hardness test is used in various industries, including automotive, aerospace, and construction. It is commonly used to determine the hardness of metals, including steel, aluminum, and copper. Additionally, Brinell hardness testing is used in the manufacturing of gears, bearings, and other components that require high strength and durability.

Brinell vs. Rockwell Hardness

While both Brinell and Rockwell hardness tests measure the hardness of a material, there are some key differences between the two methods.

One significant difference is the shape of the indenter used in each test. The Brinell hardness test uses a spherical indenter, while the Rockwell hardness test uses either a diamond cone or steel ball indenter.

Another difference is the amount of force applied to the surface of the material. The Brinell hardness test typically uses a higher load than the Rockwell hardness test.

The way the results are reported is also different. Brinell hardness is reported as the Brinell hardness number (BHN), which is calculated by dividing the applied load by the surface area of the resulting indentation. Rockwell hardness, on the other hand, is reported as a number on a scale, with higher numbers indicating higher hardness.

Both methods have their advantages and disadvantages, and the choice of which method to use depends on the specific application and the properties of the material being tested.

Difference between Brinell and Rockwell Hardness

Here is a table summarizing the differences between Brinell hardness and Rockwell hardness tests:

Applications of Hardness Testing

Hardness testing is used in various industries, including automotive, aerospace, and construction. It is used to determine the strength and durability of materials and components, as well as to assess the quality of manufacturing processes.

Hardness testing is also used in research and development to study the properties of materials and to develop new materials with improved properties.

Advantages and Disadvantages of Brinell hardness and Rockwell hardness tests

Advantages of Brinell Hardness Test:

1. Suitable for rough or irregular surfaces: The spherical indenter used in the Brinell hardness test can accommodate variations in the surface of the material being tested. This makes it suitable for testing materials with rough or irregular surfaces.
2. Can be used for hard and soft materials: The high load applied in the Brinell hardness test makes it suitable for testing very hard materials such as steels and alloys, as well as soft materials such as aluminum and copper.
3. Reliable and accurate results: The Brinell hardness test is known for its reliability and accuracy in measuring the hardness of materials. It is widely used in various industries for this reason.

Disadvantages of Brinell Hardness Test:

1. Time-consuming: The Brinell hardness test typically requires a longer duration of load application than other hardness testing methods. This can make it a time-consuming process.
2. Requires a larger sample size: The spherical indenter used in the Brinell hardness test creates a larger indentation on the material surface. This requires a larger sample size, which can be a limitation in some applications.
3. Not suitable for very hard materials: The high load applied in the Brinell hardness test can cause deformation and cracking in very hard materials, making it unsuitable for testing materials with very high hardness.

Advantages of Rockwell Hardness Test:

1. Quick and easy to perform: The Rockwell hardness test is a quick and easy method for measuring the hardness of materials. It requires minimal preparation and can be performed within a few seconds.
2. Suitable for thin or small sections: The Rockwell hardness test is suitable for testing thin or small sections of materials, making it a preferred method in applications where sample size is limited.
3. Provides a wide range of hardness values: The Rockwell hardness test provides a wide range of hardness values, which makes it suitable for testing a variety of materials.

Disadvantages of Rockwell Hardness Test:

1. Indentation may damage the material surface: The diamond cone or steel ball indenter used in the Rockwell hardness test can cause damage to the surface of the material being tested. This can be a limitation in applications where surface quality is critical.
2. Not suitable for very soft materials: The Rockwell hardness test is not suitable for testing very soft materials, as the indenter may cause deformation or damage to the material surface.
3. Not suitable for very hard materials: The Rockwell hardness test has a limit in terms of hardness values that can be measured accurately. It may not be suitable for testing materials with very high hardness values.

Conclusion

In summary, Brinell hardness and Rockwell hardness are two of the most common methods used to measure the hardness of a material. Both methods are significant in various industries, including automotive, aerospace, and construction, where materials need to withstand high stresses and forces.

While there are some differences between the two methods, both are reliable and accurate ways of measuring hardness. The choice of which method to use depends on the specific application and the properties of the material being tested.

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