Metal hardness is a characteristic of metals the describes how well the material resists local plastic deformation.
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It is easy to assume that all metals are hard, but there are some oddballs like mercury &#; which is liquid at room temperature and doesn&#;t even have a hardness value &#; as well as sodium metal, which you can cut with a kitchen knife.
There are several different scales and systems that measure metal hardness. In this article, we will provide a general overview of these measuring systems.
When comes to hardness, most people are familiar with the Mohs hardness scale. This system determines the hardness based upon surface wear &#; scratching one material with other materials. If material A can scratch material B, then material A is harder than B.
This scale is almost exclusively used for minerals and gemstones but can also be applied to metals.
Metal Hardness (Mohs) Sodium 0.5 Lead 1.5 Tin 1.5 Aluminum 2.75 Copper 3.0 Bronze 3.0 Brass 3.0 Iron 4.0 Steel 4.0 Cobalt 5.0 Titanium 6.0 Tungsten 7.5 Tungsten carbide 9.0The Rockwell hardness method compares two indentations made in a material. One made with a small load, and the other with a large load.
A unique feature of the Rockwell scale is that it correlates linearly with material tensile strength; Rockwell hardness is generally reserved for harder materials.
Metal Hardness (Rockwell) Sodium &#; Lead 5 Tin &#; Aluminum 20 &#; 25 Copper 10 Bronze 42 Brass 55 Iron 86 Steel 60 Cobalt 70 Titanium 80 Tungsten 66 Tungsten carbide 75
The Brinell hardness scale is a widely accepted measure of hardness in materials. It involves pressing a ball of steel (or tungsten carbide for harder materials) into the test piece at a constant and known force. The softer the material, the deeper the ball will penetrate and vice versa.
The next step is to take a measurement of the diameter of the resulting impression, followed by a calculation, typically in megapascals, to determine the Brinell hardness scale.
Typical Brinell hardness values for a few popular materials and metals are as follows:
Material Hardness (Brinell) Sodium 0.69 Lead 5.0 Tin 62 Aluminum 15 Copper 35 Bronze Brass Iron 200- Steel 120 Cobalt Titanium 716- Tungsten - DiamondVickers hardness
The Vickers hardness scale uses a square-based pyramid shaped diamond to impress into the material. The impression is then measured; the size of the impression determines how far it was pushed into the material. A formula is then applied to determine the hardness of the material.
One benefit here is that the width of a square impression is much easier to measure than a circle. This means that you can use the same formula (no matter the size of the indenter) as well as the same indenter for all material types, unlike many other methods.
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Metal Hardness (Vickers) Sodium Lead Tin Aluminum 160-350 Copper 343-369 Bronze 250 Brass Iron 608 Steel Cobalt Titanium 830- Tungsten - Tungsten carbideHardness is not an intrinsic material property.
What does this mean? Unlike melting points, it can change from one material sample to another. This is especially true for a metal like iron, which can appear in many different forms.
As such, different methods will produce different results for the same material. It&#;s not uncommon to see a disparity in the values of metal hardness on the internet; many values for metal hardness under these different tests aren&#;t always published or even available.
We should note that there are conversion tables available that can be used to convert between one hardness scale method to another.
Other prominent hardness tests not specifically outlined here include the Knoop hardness and Shore hardness scales.
Hardness testing is one of the quickest and easiest ways to check product quality or process results, with very little sample prep needed and simple tests that can be taught in a matter of minutes (you can read more about metallographic neophytes learning the AMH55 in an earlier blog post), but one critical step in the testing process is checking the calibration of the instrument. To verify the hardness test results are, in fact, accurate, the hardness tester must be proved to be accurate. Various standards for hardness testing have various frequencies for checking calibration, but all of them require it up front. Checking the calibration of a hardness tester is as simple as using a test block. This is a block with a certified hardness number at a certain force for a certain type of hardness test. Figuring out what those levels are, however, can get confusing.
ASTM E18 is a standard that goes into detail about Rockwell hardness testing, and it is helpfully straightforward in its block selection. The instrument must be verified daily with a test block within 15 Rockwell points of the test specimen. This means if your sample should be 40 on the Rockwell C scale, your test block needs to be between 25 and 55 on that same scale. If you do not have an appropriate test block, you can use two test blocks, one higher and one lower, to verify the calibration.
While Rockwell testing is the most common type of hardness testing, other test methods such as Brinell, Knoop, or Vickers may also be used. The standards associated with these tests are not nearly so straightforward in their test block selection. Various standards, such as ASTM E10 (Brinell) and ASTM E384 and ASTM E92 (Knoop and Vickers) specifically call for a test block at &#;approximately the same hardness value as the material to be measured,&#; which is not very specific at all. While ASTM E384 does include a table with hardness ranges, these ranges have hundreds of levels of variation. What, therefore, does &#;approximately&#; mean?
The most important thing to remember when trying to pick an appropriate test block is that you are actually verifying the calibration of the instrument and not the material you are testing. A test block that matches your sample exactly is not as important as being able to confirm that your instrument is functioning appropriately in the range you are testing at. This is why LECO suggests keeping a set of test blocks covering the range of your common materials tested, and when you have concerns about a test block being &#;approximately&#; close enough, test two blocks instead, one above and one below your sample, to confirm the calibration over the entire range.
Starting with a good standard test block is critical to confidence in your results, and that is why LECO offers a wide variety of test blocks to match all your laboratory&#;s needs. We can accommodate nearly any request for hardness test blocks for type, indenter, hardness, and load. Our commitment to quality extends to all LECO-branded consumables, standards, and reference materials.
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