Metal Guide: Metal Properties

When it comes to picking the right tool steel for a job, there are a few factors to consider. These properties can be broadly broken down into 3 categories:

• Hardness
• Strength
• Toughness

First, let’s consider hardness. 

The simple definition of hardness is a material’s ability to resist abrasion, scratching, and penetration.

One of the most reliable ways to measure hardness is via a Rockwell hardness tester This test measures the pressure that can be applied before the steel yields by penetration of a diamond indenter. The greater the depth of penetration, the lower the hardness. Hardness is directly correlated to another property, strength. 

So let’s explore strength next. 

The simple definition of strength is a material’s capacity to withstand an applied load or stress without permanent deformation or failure. 

The strength is measured through tensile tests, to determine the elasticity or yield strength. The point at which a material fails or is permanently deformed is called the yield point. Hardness and strength tend to go hand in hand, but there is a limit. When a steel has a very high hardness, strength can be compromised and result in a brittle material. This leads us right into toughness.

So what is toughness?

The simple definition of toughness is the ability of a material to absorb energy and deform significantly before it fractures or cracks.

The tensile test is one of the main ways to determine the toughness. Soft materials can absorb significantly more strain and, therefore, have high toughness. 

What does all of this mean? 

We have to balance these three properties to create the perfect tool. Hard, strong materials often become brittle, making them prone to sudden fracture. Tough materials need some ductility (ability to deform) to dissipate stress and absorb energy, preventing a quick break. In many materials, increased hardness and strength often mean decreased toughness, and vice versa. 

When it comes to picking materials we use for our tools, a balance of both hardness (for retaining a sharp edge) and toughness (to prevent chipping) is required to deliver a dependable tool that can stand up to years of use.