Strength and hardness are different material properties. Strength is the ability of a material to resist deformation, while hardness is the ability to withstand surface indentation and scratching. These properties are not interchangeable, yet their improvements are based on similar but not the same procedures.
The high strength of materials is useful in many applications. A primary application of strengthened materials is for construction. To have stronger buildings and bridges, one must have a strong frame that can support the high tensile or compressive load and resist plastic deformation. Tools are also based on high-strength materials (e.g., tool steel or beryllium copper).
High hardness of materials is required for other applications. A primary application of hardened materials is for machine cutting tools (drill bits, taps, lathe tools), which need to be much harder than the material they are operating on to be effective. These cutting tools are usually made of high-speed steel. Knife blades also use high hardness steel to keep the blade’s sharp edge. Bearings must have a very hard surface that will withstand continued stresses.
Hardening of Metals
In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, abrasion, indentation, or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.
Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. To improve the hardness of the pure metal, we can use different ways, which include:
- Hall-Petch Method
- Solid Solution Hardening (alloying)
- Work Hardening (Cold Working)
- Precipitation Hardening
- Transformation hardening
- Dispersion Hardening
- Surface Hardening
Hardness and Tensile Strength
Besides the correlation between different hardness numbers, some correlations are possible with other material properties. For example, another convenient conversion is that of Brinell hardness to ultimate tensile strength for heat-treated plain carbon steels and medium alloy steels. In this case, the ultimate tensile strength (in psi) approximately equals the Brinell Hardness Number multiplied by 500. Generally, a high hardness will indicate a relatively high strength and low ductility in the material.
In industry, hardness tests on metals are used mainly as a check on the quality and uniformity of metals, especially during heat treatment operations. The tests can generally be applied to the finished product without significant damage.
