Metal hardness is a characteristic that determines the surface wear and abrasive resistance. The ability of a material to resist denting from impact is related to hardness as well as a material's ductility. Various degrees of hardness may be achieved in many metals by tempering, a heat treatment process used in cold rolled and cold worked metals.
As the grain structure of the metal undergoes cold forming, the grains are stretched and altered. The surface becomes harder, resisting deformation from contact. Tempering heats the worked metal to temperatures at which the grains begin to dissolve.
There are series of standard tempers available. These tempers and their availability in a particular alloy vary, depending on the nature of the grains as they recrystallize. The temper designation is actually determined by this grain size, rather than the yield strength of the metal.
Metal Hardness Chart
The below chart is useful for determining which metals will be impervious to scratching and dinging, as it relates to the Rockwell scale and ductility. The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload.
Processes to Increase Metal Hardness
There are a number of ways to harden architectural metal, through the mill, or during the fabrication process. Each of the hardening mechanisms are introducing crystal lattice irregularities into the metal crystal structure, causing dislocation of the metal's structure to become more difficult. The result is a harder, less ductile metal surface.
Work hardening refers to the straining or cold-hardening of a metal surface. As metal is bent or strained repeated, the plasticity of the metal reduces, becoming work-hardened and less ductile. Usually refers to strain-hardening behavior of the metal as it is worked at room temperature. Certain metals alloys such as nickel-titanium do not undergo strain hardening but actually has a characteristic of strain relieving as it returns to the original shape.
Solid Solution Strengthening refers to a metal in the alloying process, in which an alloying constituent is inserted into a solid material. One or more elemental constituents are able to enter into a heated but solid solution. The metal is then rapidly quenched to capture the element in solid solution.
Age hardening is a process which occurs rapidly in the first few days after casting, then much slower over the next several weeks. This process is often referred to as "natural age-hardening". Another artificial version of this process can be used by heating the metal for a short period of time at a high temperature. The result is that it will stabilize the properties, further strengthening the alloy. This process is known as "artificial age-hardening," or precipitation hardening.
Anodization, a process specific to aluminum, has a hardening effect. The final step in creating anodized aluminum is to harden and seal the surface by use of deionized boiling water or metal salt sealers. Sealing is required to close the pores of the oxide film and provide uniformity to the exception of the alloying constituents.
Case hardening refers to a surface heat treatment process used to produce a hard, wear-resistant surface on metal. Methods of case-hardening include carburization, cyaniding, nitriding, flame hardening, and electroinduction hardening.
Tempering is a heat treatment process used in cold rolled and cold worked metals. As the grain structure of a metal undergoes cold forming, the grains are stretched and altered. The surface becomes harder, resisting deformation from contact. Tempering heats the cold worked metal to temperatures at which the grains begin to dissolve into one another. There are series of standard tempers available. These tempers and their availability in a particular alloy vary, depending on the nature of the grains as they recrystallize. The temper designation is actually determined by this grain size, rather than the yield strength of the metal.
Back-blasting a metal surface is a way of flattening metal, which also tends to greatly increases the surface hardness. It is advised to back-blast a material after forming operations, because the material will become harder to work and form after blasting the surface.
- References: Zahner, L. William. Architectural Metal Surfaces. New York: John Wiley, 2004.
- Zahner, L. William. Architectural Metals: A Guide to the Selection, Specification and Performance. New York: John Wiley, 1995.