In a previous article about metal strengths, we looked at measuring tensile and impact strength. But there are two other very important mechanical properties that must be considered when selecting a metal for your next project. These properties are ductility and hardness.
What Is Ductility?
Ductility is the ability of a metal to deform without fracturing. Metals that can be formed or pressed into another shape without any fracturing are considered to be ductile. Metals that fracture are classified as brittle (essentially the opposite of ductile).
Ductility plays a major role in formability. Metals that are excessively brittle may not be able to be formed successfully. For example, if a piece of metal is stretched into a thin wire, it is imperative that is has some ductility. If the metal is too brittle, it will fracture as soon as the metal begins to stretch. Ductility is also a major safety consideration for structural projects. Ductility allows structures to bend and deform to some extent without rupturing when placed under heavy loads.
How to Measure Ductility
- Percentage elongation measures the length that a metal deforms as a percentage of its original length, after it is pulled to failure during a tensile test.
- Percent reduction measures the narrowest part of the cross-section of a metal specimen following a tensile test-induced rupture.
Ductility can be dependent on temperature, so the temperatures the metal will be subjected to in an application should be taken into account. Most metals have a ductile-brittle transition temperature chart which can assist.
Which Metals Are Ductile?
There are many ductile metals, including:
Metals that are considered brittle include cast iron, chromium, and tungsten. Examples of applications that require high ductility include metal cables, stampings, and structural beams.
What Is Hardness?
Hardness is a measure of the ability of a metal to resist indentation. When a metal must withstand forces without depression or denting, a metal with a high hardness should be sought.
Hardness can also give the end user an idea of the abrasion resistance of a metal. The greater the hardness a metal possesses, the better it is able to resist abrasion. This is very important for applications where the metal being used is subjected to forces and objects that may cause wear.
How To Measure Hardness
For indentations, there are several proprietary testing methods available to use to measure hardness. The most popular methods are:
In each test of hardness, a very hard object such as an industrial grade diamond is depressed into a material using a known force. The size of the depression made into the test material is then measured. This measurement can then be converted into a value such as HRB (Rockwell) or HV (Vickers).
The Mohs Hardness Test is used to measure a materials resistance to scratching. In this test, a variety of materials are that have been assigned hardness numbers are used to try and scratch the metal test specimen. The lowest numbered material to make a scratch on the specimen is assigned to that specimen, giving it a relative hardness.
What Are The Hardest Metals?
Metals that have relatively high hardness include:
- High Carbon Steel
- Cast Iron
Special processing such as heat treating or work hardening can also be used further increase the hardness of a given metal. Hardness is often inversely related to ductility, so the ductile metals mentioned above typically have relatively low hardness. Applications that require high hardness include shafts, pins, and gears.
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