The property of a material to allow the passage of heat. The units of thermal conductivity are cal/cm2/cm/oC/s.
The three mechanisms of thermal conductivity by which heat can be transmitted from a heat source to a material that are significant to welding are conduction, convection and radiation. Conduction is most often the mechanism involved in a weldment.
The amount of heat being conducted through a body of matter is proportional to the cross-sectional area and the difference in temperature or gradient between the measuring points; and it is inversely proportional to the distance or length between the measuring points. These factors can be arranged in an equation with a proportionality constant for the thermal conductivity.
Thermal conductivity values for a number of metals and other materials used in welding are listed in Table T-3. As can be seen from the values in Table T-3, metals differ in thermal conductivity, but in the main, metals are much better heat conductors than nonmetals. Copper is an excellent conductor, which accounts for the difficulty in welding copper using a relatively low-temperature heat source, like an oxyacetylene flame. On the other hand, the good conductivity of copper explains its efficiency as a “heat sink” when employed as a hold-down fixture or as a backing bar. Iron is a relatively poor conductor compared to other metals, which partly accounts for the ease with which steel can be welded and thermally cut. Metals with high heat conductivity require more heat input during welding because the heat is conducted rapidly away from the puddle.