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Making the right decisions in material selection is not only the essence of engineering but also the heart of manufacturing. Because a wrongly chosen material can shorten the product's lifespan, increase costs, or reduce performance. At this very point, one of the key concepts involved is "hardness."
But wait, what exactly is hardness?
Hardness is the resistance a material shows against deformation. In other words, it determines how resistant its surface is to an applied force. This property is critically important for preventing situations such as scratching, denting, or permanent deformation. However, hardness is not a property that can be determined by visual inspection alone. Therefore, scientists and engineers have developed various test methods to measure this resistance. One of the oldest and most commonly used of these tests is the Brinell hardness test.
So, why the Brinell method?
The Brinell hardness test provides consistent and reliable results especially for metals with thick and heterogeneous structures. Developed in the early 1900s, this method is still considered a fundamental test standard in many industries today. By applying force over a wide surface area, it offers a more "realistic" average hardness value.
The importance of this test is not only due to its accuracy. Its ease of application and wide range of uses have made it indispensable, particularly in the following sectors:
• Metallurgy
• Automotive
• Aerospace
• Construction
• Heavy industry manufacturing
In these sectors, it is widely preferred both for quality control during production processes and for evaluating new materials in R&D studies.
So, on which materials can the Brinell method be applied?
This test is generally used on metals and alloys with a homogeneous structure. Specifically:
• Steel and cast iron
• Soft metals like aluminum and copper
• Alloys such as brass, bronze, and similar
• Hardened steels
It can be applied to a broad range of materials. However, it is not suitable for very thin surfaces or extremely hard coatings. In such cases, different test methods are used (for example, Rockwell or Vickers).
In conclusion, accurately measuring a material’s hardness is the first step in predicting how durable it will be in a given application. The Brinell method is one of the most reliable guides on this path.
Applying force to the surface alone is not enough when measuring hardness. The measured value must be meaningful and compliant with standards. In the Brinell hardness test, the unit expressing this value is called BHN, which stands for Brinell Hardness Number.
But what exactly is BHN?
The Brinell Hardness Number (BHN) is calculated by measuring the diameter of the indentation left when a steel or tungsten carbide ball of a certain diameter is pressed into the test sample with a specific force. This value reveals how much resistance the material’s surface shows against the applied force. Hard materials leave smaller indentations, while softer materials leave larger ones.
The measurement is made based on the diameter of the indentation and the BHN value is obtained using the Brinell formula.
Here, it is important to note a critical detail: the relationship between force, ball diameter, and application time is crucial for obtaining accurate and consistent results.
So, how is this relationship established?
The parameters used in the Brinell test are:
• Force (F): Generally applied in kilogram-force (kgf). Depending on the type of material, it ranges between 500 kgf and 3000 kgf.
• Ball diameter (D): Usually varies between 1 mm and 10 mm. The ball can be made of steel or tungsten carbide.
• Application time: The duration the force is applied to the material is generally kept between 10 and 30 seconds. This time can be shorter especially for softer metals.
The correct combination of these three parameters is determined according to the material type. For example:
• For soft aluminum alloys, a 10 mm diameter ball with a force of 500 kgf is used.
• For hardened steels, a force of 3000 kgf with the same 10 mm ball is preferred.
• For thin sheets or more precise measurements, combinations such as a 2.5 mm or 5 mm ball with a lower force of around 187.5 kgf can be used.
Thanks to this harmony, measurement results can be compared with international standards and quality assurance can be ensured during production processes.
In summary, the Brinell Hardness Number is not just a number. When performed with the correct parameters, it represents the material’s resistance according to standards. Therefore, the BHN value means not only the test result but also correctly interpreting the performance of that material.
There must be a justified reason why a test method has been used for such a long time. The Brinell hardness test is one of the best examples of this. Because this method has become a cornerstone of engineering and has also inspired the development of other hardness tests over time.
So, how did it all start?
The foundations of the Brinell hardness test were laid in 1900 by the Swedish engineer Johan August Brinell. At that time, the Industrial Revolution was still ongoing and metal production was rapidly increasing. However, there was a problem: the mechanical properties of the produced metals could not be standardized. Especially an important mechanical property like hardness could not be measured reliably and reproducibly.
Brinell developed his eponymous method while seeking a systematic solution to this problem.
Brinell’s approach was quite simple but revolutionary: pressing a steel ball of a certain diameter onto the material surface with a specific force and calculating the hardness of the material based on the diameter of the resulting indentation. Compared to the more primitive and observation-based methods used at the time, this idea provided much more reliable and numerical results.
Soon, this method was rapidly adopted especially in metalworking and foundry industries across Europe. Because it was easy to apply and provided repeatable results. Particularly when applied to cast iron, steel, and large metal parts, it played an important role in controlling production quality by measuring the average surface hardness.
The Brinell method was not alone in time. Engineers who followed the path it opened developed new methods to meet different needs:
• In the 1920s, the Rockwell hardness test was developed. This test was faster and caused less surface damage.
• In the 1930s, the Vickers test emerged. This method was suitable for smaller samples and precise measurements.
However, the Brinell test is still considered one of the most reliable methods for measuring the average surface hardness of thick and large metal parts. Even today, it continues to be widely used in heavy industry, automotive, and foundry fields.
When choosing a test method, not only what it measures but also how accurately, reliably, and under suitable conditions it can be applied should be considered. Although the Brinell hardness test has been used for over a century, it is still preferred in many sectors, and the reasons lie in its strong advantages. However, like every method, Brinell also has its limitations.
Let’s take a closer look at both the strengths and weaknesses of this method.
Advantages
1. Applicable to thick and large-volume materials
The Brinell test is ideal for large parts and thick surfaces. Because it allows the application of high force, it provides more realistic information about volumetric average hardness rather than just surface hardness in thick bodies. For this reason, it is especially preferred for cast parts, steel blocks, and thick plates.
2. Focuses on the average value
The width of the indentation formed on the surface allows measurement over a wider area instead of a small point. This provides a more general hardness value that is not affected by local deviations. This is an important advantage especially in regions with microstructural differences within the material.
3. Provides reliable and repeatable results
When proper test procedures are followed, the Brinell test delivers consistent results compliant with international standards. This gives confidence in quality control processes.
4. High standard compliance
Because it complies with international standards such as ISO 6506 and ASTM E10, the results are comparable and reportable on a global scale.
Choosing the right hardness test method is as important as the accuracy of the results obtained from the test. This is because each method is developed for specific applications and material types. Therefore, Brinell and Rockwell hardness tests are often compared. So, what are the differences between these two tests?
Basic Principles
In the Brinell test, a ball of a certain diameter is pressed onto the material surface with a specific force. Then, the diameter of the indentation formed is measured, and the hardness value is calculated. In other words, the key factor here is the size of the indentation on the surface.
In the Rockwell test, a completely different principle is applied. The test consists of two stages: first, a preliminary load (minor load) is applied to the material, then the main load is applied. The depth of the remaining indentation is measured, and the hardness value can be read directly. Rockwell testing machines provide the result automatically; no calculation is needed.
Applied Force and Indentation Measurement Method
The force applied in the Brinell test is high, typically ranging between 500 and 3000 kilogram-force (kgf). This high force produces larger indentations. Therefore, the Brinell test is ideal for large metal parts where surface quality is not very critical.
In the Rockwell test, lower forces are used, and instead of measuring the diameter, the depth of the indentation is measured. Hence, the indentation on the surface is usually smaller. This is why Rockwell testing is preferred for finished products or delicate parts.
Indentation Depth and Surface Effect
Indentations produced by the Brinell test are relatively large, causing permanent deformation on the surface. This may not be preferred for products where appearance is important.
In the Rockwell test, indentations are smaller, generally at the micron level, thus not damaging coating thickness. It is a less invasive method in this regard.
Comparison According to Usage Areas
• The Brinell method is suitable for bulky and dense materials such as cast irons, steel blocks, and large metal parts. It is ideal for quality control of semi-finished parts, especially in early production stages.
• The Rockwell method excels in precise parts, thin materials, heat-treated surfaces, and final product inspections.
It is important to remember:
Both methods provide reliable results when used correctly. The key is selecting the method that best suits the material type, surface characteristics, and testing purpose.
In summary, Brinell offers average hardness measurement over a larger surface area, while Rockwell provides fast, directly readable results with smaller indentations. They are not alternatives to each other but two powerful methods serving different needs.
The Brinell hardness test cannot be understood by theory alone; it requires a correct application process. Although this test appears simple, many variables must be precisely controlled. Each step must be carefully executed to ensure accurate and repeatable measurements.
First, let’s look at the equipment used.
Test Equipment
Three basic pieces of equipment are needed for the Brinell test:
• Load system: Can be hydraulic or mechanical. It must apply a constant force to the material surface for a specific duration and with precision according to standards.
• Ball: Usually a 10 mm diameter hardened steel or tungsten carbide ball is used. Different diameters may be selected depending on material hardness.
• Optical reader or microscope: Used to precisely measure the diameter of the indentation on the surface. Measuring the diameter on two perpendicular axes allows a more accurate average value calculation.
Step-by-Step Application Process
1. Sample preparation: The surface must be smooth and clean. Rough, dirty, or burr-covered surfaces directly affect the test result. Sanding may be required.
2. Positioning the test piece: The sample is fixed on the device table. The ball must be aligned perpendicular to the surface.
3. Applying the load: First, the ball contacts the surface, then a specified force (e.g., 3000 kgf) is applied for a certain duration, usually between 10 and 30 seconds. Softer metals may require shorter, harder metals longer application times.
4. Removing the load: After the load is lifted, the diameter of the indentation remaining on the surface is measured.
5. Measuring indentation diameter: Using the optical reader, the diameter is measured horizontally and vertically. The average is calculated, and the sample is ready for BHN value calculation.
There is a very important detail to pay attention to here.
Important Considerations in Sample Preparation
• The sample thickness must be sufficient; otherwise, the load may spread unevenly, causing misinterpretation of deformation.
• The surface should be homogeneous; weld seams or surface defects must not be present in the measurement area.
• The indentation must not be too close to the sample edge or another indentation, as this disturbs stress distribution and affects the result.
The Brinell hardness test provides an average value from a specific surface area. Therefore, instead of limiting results to a single indentation, taking measurements from multiple points when possible gives healthier results. This approach can make a significant difference especially if the material is not homogeneous.
In conclusion, a correctly performed Brinell test does not just provide a number; it is key to determining the reliability, durability, and suitability of that material in production.
The measurement taken during the Brinell hardness test is not meaningful on its own. It becomes significant when correctly calculated, because the diameter of the indentation on the test surface does not directly convert to the hardness number. This is where the Brinell formula comes in.
So, how is this calculation done?
Brinell Hardness Number (BHN) Formula
The hardness value obtained by the Brinell method is calculated using the following mathematical formula:
BHN = (2 × F) / (π × D × (D − √(D² − d²)))
At first glance, this formula may seem complex, but it becomes clear when you understand the parameters.
Explanation of Parameters:
• BHN: Brinell Hardness Number (the value to be found)
• F: Applied force (in kgf)
• D: Diameter of the ball (in mm)
• d: Diameter of the indentation (in mm)
• π: Pi, approximately 3.1416
The basic logic of the formula is: the smaller the indentation diameter created by the ball pressing into the surface, the harder the material is. Therefore, the calculation is inversely proportional to the indentation diameter and directly proportional to the applied force.
Let’s better understand this calculation with an example.
Example Brinell Calculation
Suppose a Brinell hardness test is performed on a steel sample with the following values:
• Applied force (F): 3000 kgf
• Ball diameter (D): 10 mm
• Indentation diameter (d): 4.0 mm
Plugging into the formula:
BHN = (2 × 3000) / (π × 10 × (10 − √(10² − 4²)))
First, calculate the square root inside the parentheses: √(100 − 16) = √84 ≈ 9.17
Next, calculate the value inside the parentheses: 10 − 9.17 = 0.83
Then, multiply the factors: π × 10 × 0.83 ≈ 3.1416 × 10 × 0.83 ≈ 26.1
Finally, calculate the result: 6000 / 26.1 ≈ 229.5 BHN
Thus, the average surface hardness of the tested material is approximately 229 BHN.
This result tells us the average surface hardness of the tested material is approximately 229 BHN.
By comparing this value to typical hardness ranges of different materials, you can determine which class the tested part is closest to. This is especially useful in quality control and material verification processes.
Points to Consider in Calculation:
• Force, ball diameter, and indentation diameter must be in the same unit system (usually mm and kgf).
• The indentation diameter must be measured as precisely as possible; small deviations can cause large differences in calculation.
• Surface smoothness directly affects calculation accuracy.
The most important thing to remember is: the Brinell hardness value is not just a mathematical result. When applied correctly, this number provides valuable information about the machinability, strength, and suitability of the material for applications.
It is not enough for a test method to simply be applicable; it must also be valid, comparable, and auditable. One of the main reasons the Brinell hardness test has become such a widespread and reliable method in industry is that it is clearly defined by international standards.
Today, the Brinell hardness test is mainly conducted according to two global standards:
• ISO 6506
• ASTM E10
Both standards are fundamentally based on the same physical principles. However, there are some differences in application details. Understanding these differences is as important as correctly performing the test and interpreting the results.
Published by the International Organization for Standardization (ISO), ISO 6506 is widely used especially in European and Asian industries. The main points of this standard are:
• Ball diameters to be used (1 mm, 2.5 mm, 5 mm, 10 mm)
• Force ranges to be applied (from 30 kgf to 3000 kgf)
• Loading time: generally 10–15 seconds, but may vary in special cases
• Methods for calculation and reporting of test results
This standard ensures global comparability of results in tests performed on steel, iron, aluminum, and similar alloys.
Published by the American Society for Testing and Materials (ASTM), E10 is primarily referenced in manufacturing and testing processes in North America.
While ASTM E10 is largely similar to ISO 6506, it includes some minor differences:
• Test duration: ASTM E10 defines loading time more flexibly, allowing shorter times especially for soft metals.
• Ball material: ASTM allows greater flexibility in some cases for using steel balls instead of tungsten carbide.
• Application conditions: Additional guidelines for ambient temperature, sample fixing methods, etc.
Though these differences may seem minor, it is mandatory in international manufacturing and quality processes to clearly specify which standard a part was tested under.
Methodological Differences and Tolerances
Common points of both standards:
• The Brinell formula is the same.
• Calculation is done based on force, ball diameter, and indentation diameter.
• Results are expressed as BHN (Brinell Hardness Number).
However, differences may occur regarding:
• Which force-ball diameter combination to use for which material
• Acceptable maximum surface roughness
• Measurement precision tolerances
Therefore, in any Brinell test, not only the result but also the standard under which it was performed must be clearly stated. Otherwise, the value may not be valid under different systems.
In brief:
• ISO 6506 is an internationally accepted standard, commonly used especially in Europe.
• ASTM E10 is preferred by U.S.-based manufacturers and laboratories.
• Both standards are reliable but have minor procedural differences.
• The standard used must always be specified in the test report.
