Steel is an alloy of iron. Carbon is the primary alloying element, but different grades of steel will include varying amounts of carbon, manganese, chromium, vanadium, and tungsten. The thermal properties of steel vary based on the amount and types of alloying elements used.
In general, the thermal properties of a compound reflect how that compound interacts with heat. The two primary terms used to describe thermal properties are thermal conductivity and thermal transmittance. A third term, thermal resistivity, is the inverse of thermal conductivity.
The American Heritage Science Dictionary describes thermal conductivity as a measure of the ability of a material to transfer heat. Given two surfaces on either side of the material with a temperature difference between them, the thermal conductivity is the heat energy transferred per unit time and per unit surface area, divided by the temperature difference. As such, thermal conductivity is usually reported in Watts per Kelvin per meter, or W/(K)(m).
The thermal properties of steel will vary based on the ratio of alloying elements in the steel. For example, The Engineering Toolbox lists thermal conductivities for steel containing 0.5 per cent carbon at 18.23W/(K)(m), while steel containing 1.5 per cent carbon is listed as having a thermal conductivity of 12.35W/(K)(m). Moreover, the thermal conductivity of stainless steel varies from 12.11 to 45.0W/(K)(m).
Thermal resistivity is the inverse of thermal conductivity. The higher the thermal resistivity, the lower the thermal conductivity. Thus, thermal resistivity also describes the rate at which heat travels through a compound but is most commonly used to describe materials which insulate against heat.
The thermal resistivity of 0.5 per cent carbon is 0.055 (K)(m)/W, while steel containing 1.5 per cent carbon is has a thermal resistivity of 0.081 (K)(m)/W, and the thermal resistivity of stainless steel will vary from 0.083 to 0.022 (K)(m)/W.
Thermal transmittance measurements take into account the thermal conductivity of the steel as well as heat in the surrounding environment. It is commonly reported as the U-value for a particular area of material. The U-value for steel will vary by alloyed elements, area, thickness and the surrounding environment.