Hardening of Metals

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 same procedures.

High strength of materials is useful in many applications. A primary application of strengthened materials is for construction. In order to have stronger buildings and bridges, one must have a strong frame that can support 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 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 uses a high hardness steels to keep a sharp edge of blade. 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, resistance to abrasion, resistance to 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 a pure metal, we can use different ways, which include:

Hardness and Tensile Strength

Besides the correlation between different hardness numbers, there are also some correlations possible with other material properties. For example, for heat-treated plain carbon steels and medium alloy steels, another convenient conversion is that of Brinell hardness to ultimate tensile strength. 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.

References:
Materials Science:

U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
William D. Callister, David G. Rethwisch. Materials Science and Engineering: An Introduction 9th Edition, Wiley; 9 edition (December 4, 2013), ISBN-13: 978-1118324578.
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Gaskell, David R. (1995). Introduction to the Thermodynamics of Materials (4th ed.). Taylor and Francis Publishing. ISBN 978-1-56032-992-3.
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J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

See above:
Metalworking