How to Measure Ductility and Hardness – Part 2

Measuring Metal Strength (Part 2)

In a previous article about metal strengths, we looked at measuring tensile and impact strength. While these properties are critical for understanding a material’s load-bearing capacity and resistance to sudden forces, they do not provide a complete picture of a metal’s overall performance. 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.

Ductility refers to a metal’s ability to stretch or deform without breaking, while hardness measures a material’s resistance to surface deformation, including scratches, dents, and wear. Understanding the hardness and ductility relationship is important since increasing one often leads to a decrease in the other.

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 it 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. The ductility of steel and other materials allows structures to bend and deform to some extent without rupturing when placed under heavy loads.

How to Measure Ductility?

How is ductility measured? Engineers and manufacturers rely on specific tests to quantify ductility and ensure that materials meet the necessary mechanical performance requirements. There are two main ways 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 that can assist with this.

H2 Which Metals Are Ductile?

There are many ductile metals, including:

• Aluminum
• Brass
• Copper
• Low carbon steel
• Gold
• Silver
• Tin
• Lead

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 testing is commonly performed to ensure that metals can withstand mechanical stress without experiencing permanent deformation.

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. Heat treatment processes such as quenching and tempering can also enhance hardness while maintaining an optimal balance between strength and ductility, ensuring that the material remains both tough and resistant to surface damage.

How to Measure Hardness

For indentations, there are several proprietary testing methods available to use to measure hardness. The choice of test method depends on factors such as the type of material being tested, its thickness, and the required precision of the measurement. The most popular methods are:

• Vickers
• Brinell
• Rockwell

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 Vickers hardness test is often used for very thin materials or coatings due to its precise indentation measurement, while the Brinell test is preferred for softer metals like aluminum and copper.

The Mohs Hardness Test is used to measure a material’s 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. While the Mohs test does not provide precise numerical values like Vickers or Rockwell, it remains an effective way to compare hardness differences between materials.

What are the Hardest Metals?

Metals that have relatively high hardness include:

• Tungsten
• High Carbon Steel
• Cast Iron

Special processing, such as heat treating or work hardening, can also be used to 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.

What is the Relationship Between Hardness and Ductility?

The hardness and ductility relationship is generally inverse, meaning that as hardness increases, ductility tends to decrease, and vice versa. When a material undergoes processes such as heat treatment or work hardening, its hardness typically increases, making it more resistant to indentation but also reducing its ability to elongate before fracturing.

Temperature also plays a role in this relationship. Some metals exhibit a ductile-to-brittle transition at lower temperatures, meaning that they may be ductile at room temperature but become brittle when exposed to colder conditions. Furthermore, while higher hardness often correlates with increased tensile strength, it does not necessarily mean improved toughness. A material can be strong, resisting breakage under tension, but still brittle if it cannot absorb energy before fracturing.