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 in order to be effective. These cutting tools are usually made of high-speed steel. Knife blades also use high hardness steel to keep a sharp edge of the blade. Bearings must have a very hard surface that will withstand continued stresses.
Strengthening of Metals
The strength of metals and alloys can be modified through various combinations of cold working, alloying, and heat treating. As discussed in the previous section, the ability of a crystalline material to deform largely plastically depends on the ability for dislocation to move within a material. Therefore, impeding the movement of dislocations will result in the strengthening of the material. For example, a microstructure with finer grains typically results in higher strength and superior toughness than the same alloy with physically larger grains. In the case of grain size, there may also be a tradeoff between strength and creep characteristics. Other strengthening mechanisms are achieved at the expense of lower ductility and toughness. There are many strengthening mechanisms, which include:
- Solid Solution Strengthening (alloying)
- Work Hardening (Cold Working)
- Precipitation Hardening
- Grain Refinement
- Transformation 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.