How Does Twice-Tempered Steel Work?

Engineers in a variety of fields, including nuclear science, construction or even home goods, will benefit from understanding the tempering process, applicable to a variety of materials, from chocolate to steel. Materials such as steel that are twice-tempered, or subjected to the tempering process twice in a row, are often stronger.

Tempering Process

Tempering is a way of strengthening a metal’s internal structure through heating and is often used to improve the durability and load-bearing capacity of steel. The tempering process involves heating the metal to a very high point without allowing it to melt, then allowing it to cool down, sometimes in air, sometimes in oil or other liquids. This reduces the internal stresses of the metal and reduces its brittleness, or tendency to break, under pressure. Depending on the type of steel and its intended purpose, tempering takes place at a range of temperatures.

Twice-Tempering Benefits

Twice-tempering is simply a process whereby the steel is heated and then cooled twice in succession, not necessarily at the same temperature each time. A study published in the Journal of Nuclear Materials entitled “Effect of Twice Quenching and Tempering on the Mechanical Properties and Microstructures of SCRAM Steel for Fusion Application” found that twice-tempered steel’s strength depended mostly on the temperature of the second tempering, and that a decrease in temperature from 1,033 to 1,013 degrees Kelvin between the first and second tempering processes did increase the steel’s overall strength.

Ductile-Brittle Transition Temperature

The study also found that the ductile-brittle transition temperature was reduced in steel that had been twice tempered. The ductile-brittle transition temperature means the point at which a material goes from ductile, or able to bend and absorb impact, to brittle, or liable to break on impact. This is important for steel, as it is used in many crucial load-bearing construction projects. By lowering the temperature at which steel will break instead of absorbing an impact, it is stronger and can withstand more cold-imposed stress.

Twice-Tempered Steel Uses

Steel is used in the fabrication of large construction materials, smaller tools or industrial applications, or simple mechanisms such as bedsprings. Many mattress companies brag that their mattress coils are composed of twice-tempered steel, making them stronger and less liable to sag over the years. The steel used for construction of tools in industrial metal, composite or ceramics production industries also benefits from double tempering, though at lower temperatures than required to temper it for nuclear applications, usually only between 450 and 540 degrees Celsius.


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