Metal Hardness Testing: Methods & Challenges
While metals are often associated with hardness, this isn't always the case. Mercury, for example, is a liquid at room temperature, defying the typical perception of metals as solid substances. On the other hand, a metal like sodium can be cut into half with a kitchen knife. Since metals are used in various industries, it is important to carry out hardness testing, which further helps determine a material's ability to withstand deformation, wear, and tear.
But how do you conduct these tests, and what challenges might you face? Let’s explore the various methods of metal hardness testing, best practices, formulas, and the difficulties encountered in this blog.
What Is the Standard for the Metal Hardness Test?
The standard for metal hardness testing varies based on the material and its intended use, particularly in industries like Metal Fabrication. However, globally recognized standards are established by organizations like ASTM (American Society for Testing and Materials) and ISO (International Organization for Standardization).
The most commonly used standard is ASTM E18, which outlines the procedures for conducting Rockwell hardness tests, one of the industry’s benchmarks. For other methods, such as Vickers or Brinell hardness testing, specific ASTM or ISO standards also guide the process.
How to Do a Hardness Test on Metal?
Hardness tests measure a material’s resistance to indentation or scratching by applying a load or pressure on the metal’s surface. This is particularly important in processes like Metal Bending, where hardness plays a critical role in how the material behaves. The basic process involves the following steps:
1. Select the hardness test method:
Depending on the material and desired outcome, choose appropriate testing techniques such as Rockwell, Brinell, or Vickers. Also, it is important to note that some methods might be particularly suitable for specific materials. For example, a portable hardness tester for steel can be the Rockwell method.
2. Prepare the sample:
Ensure the material, whether using an aluminum hardness tester or another device, has a flat, smooth surface to achieve accurate measurements.
3. Apply the testing force:
Use a hardness testing machine for metal to apply force on the sample’s surface.
4. Measure the indentation:
After applying force, let’s say through a portable hardness tester for aluminum, measure the depth or size of the indentation using standard formulas or visual aids.
5. Record results:
Document the hardness value based on the method’s established scale (e.g., HRC for Rockwell, HBW for Brinell).
Metal Hardness Testing Methods
There are several metal hardness testing methods, each with unique advantages and limitations. Let’s take a look at a few of these:
Rockwell Hardness
This method involves creating two indentations on a material using two different loads. The first indentation is made with a smaller load, while the second uses a larger one. This technique is especially effective for measuring the hardness of harder materials.
|
Metal |
Hardness (Rockwell) |
|
Tungsten Carbide |
75 |
|
Tungsten |
66 |
|
Titanium |
80 |
|
Cobalt |
70 |
|
Steel |
60 |
|
Iron |
86 |
|
Brass |
55 |
|
Bronze |
42 |
|
Copper |
10 |
|
Aluminum |
20-25 |
|
Tin |
- |
|
Lead |
5 |
|
Sodium |
- |
Mohs Hardness
This method is used to determine metal hardness based on surface wear. Here, one material is scratched with another to determine the metal hardness instead of measuring through indentation.
It is important to note that this method is mostly used for minerals and gemstones but can also be used for metals.
|
Metal |
Hardness (Mohs) |
|
Tungsten Carbide |
9.0 |
|
Tungsten |
7.5 |
|
Titanium |
6.0 |
|
Cobalt |
5.0 |
|
Steel |
4.0 |
|
Iron |
4.0 |
|
Brass |
3.0 |
|
Bronze |
3.0 |
|
Copper |
3.0 |
|
Aluminum |
2.75 |
|
Tin |
1.5 |
|
Lead |
1.5 |
|
Sodium |
0.5 |
Brinell Hardness
This method involves pressing a hard steel or carbide ball into the metal surface and measuring the diameter of the indentation. This is followed by a calculation in megapascals, to determine the Brinell hardness scale.
|
Metal |
Hardness (Brinell) |
|
Diamond |
8000 |
|
Tungsten |
2000-4000 |
|
Titanium |
716-2700 |
|
Cobalt |
1265 |
|
Steel |
120 |
|
Iron |
200-1180 |
|
Brass |
|
|
Bronze |
|
|
Copper |
35 |
|
Aluminum |
15 |
|
Tin |
62 |
|
Lead |
5.0 |
|
Sodium |
0.69 |
Vickers Hardness
This method uses a diamond pyramid to indent the material and measures the size of the indentation. This method simplifies the process, as it is easier to calculate a square indentation than a circular one.
|
Metal |
Hardness (Vickers) |
|
Tungsten Carbide |
2600 |
|
Tungsten |
3430-4600 |
|
Titanium |
830-3420 |
|
Cobalt |
1043 |
|
Steel |
|
|
Iron |
608 |
|
Brass |
|
|
Bronze |
250 |
|
Copper |
343-369 |
|
Aluminum |
160-350 |
|
Tin |
|
|
Lead |
|
|
Sodium |
Formula To Test Metal Hardness
The formula for calculating hardness depends on the test method. For instance, in the Brinell Hardness test, the hardness (HBW) is calculated using the formula:
HBW= 2P / πD(D - √(D² - d²))
Where:
-
P = load applied (kg)
-
D = stands for the diameter of the ball indenter (mm)
-
d = diameter of the indentation (mm)
For Vickers, the formula is:
HV=1.854×P/d2
Where:
-
P = applied force (kg)
-
d = diagonal length of the indentation (mm)
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Challenges of Measuring Metal Hardness
Hardness is not a static property, which is why measuring it can be a little tricky. Also, measuring it is different from measuring the melting points of metals. For example, a metal like iron can appear in different forms, producing different results for different samples. Using a portable hardness tester for metals can help streamline the testing process in such cases. Plus, many metal hardness values under these different tests are neither published nor readily available.
How to Select the Best Metal Hardness Testing Methods
Choosing the best hardness testing method depends on several factors, including the microstructure – e.g. the homogeneity of the material you are testing, the type of metal, the size, and its condition. Here, using a portable hardness tester for metals can provide flexibility in testing various materials under different conditions.
In all hardness testing, the area beneath the indentation should reflect the overall microstructure of the material (unless you're specifically analyzing different components within the microstructure). As a result, if the microstructure is coarse and heterogeneous, a larger indentation is required compared to a more uniform material.
Consider these factors when selecting a method:
-
Type of material
-
Whether adherence to a specific standard is necessary
-
The estimated hardness of the material
-
The homogeneity/heterogeneity
-
The size of the component
-
Whether the sample needs to be mounted
-
The number of samples to be tested
-
The level of accuracy required for the results
Conclusion:
Understanding the various methods and challenges of metal hardness testing is crucial for ensuring the quality and durability of materials in any industrial application. By selecting the appropriate metal hardness testing method and being aware of potential challenges, you can achieve accurate and reliable results every time.
To learn more about the different methods and choose the appropriate one, call us at 440-822-6381.
Metal Hardness Testing FAQs
1. What is the most commonly used hardness test method for metals?
The Rockwell hardness test is the most commonly used method due to its versatility and ease of use for a wide range of materials.
2. Which test indicates the hardness of a metal?
The Rockwell, Brinell, and Vickers tests are the most reliable indicators of metal hardness, with Rockwell being the industry standard for many applications.
3. How to check HRC hardness?
HRC hardness is measured using the Rockwell hardness test. A diamond indenter is used, and the depth of the indentation is converted into an HRC value.
4. What is the most accurate hardness test?
The Vickers test is considered the most accurate due to its ability to test a wide range of materials, including very thin or small samples.
5. How do I choose a metal hardness test?
Select a method based on the type of metal, sample size, and desired level of precision. Consider the specific requirements of the industry or application.
6. Which is more accurate, Brinell or Rockwell?
While both are accurate, Rockwell is preferred for faster and more efficient metal hardness testing, especially for hard materials, while Brinell is better suited for softer materials.
7. Which hardness test is most acceptable by the industry?
The Rockwell test is the most widely accepted and commonly used in various industries due to its ease and versatility.
