Handbook-Expansion & Contraction

9 Thermal Conductivity Some metals are much better conductors of heat than others. The thermal conductivity of copper is eight times that of carbon steel; some stainless steels have only one-fourth the conductivity of carbon steel. In practical terms, this means that if you weld copper, the metal four inches away from the weld zone will get much hotter than it would if the material were carbon steel. If you weld stainless steel, the metal four inches from the weld zone will not get as hot as it would if the material were carbon steel. Since the degree of expansion, or internal stress created when expansion is restrained, depends on temperature rise, thermal conductivity has a very real bearing on the ”expansion-contraction” problem. The differences between steel, copper, and aluminum need not concern the oxy- acetylene welder greatly, since he is likely to weld carbon steels most of the time, and all carbon steels have about the same coefficient of expansion and about the same rate of heat conductivity. However, the variation between different metals with respect to these two temperature-related properties, and the substantial differences between metals in their strength at elevated temperatures, are matters of considerable significance to the welding engineer, especially in the design of fixtures for mechanized welding. Fig. 11-7. Here is a simple jig which can be constructed from pieces of steel plate and several C-clamps to permit making either a butt weld or a flange weld in sheet steel without undesirable distortion. Note the groove in the bottom plate, provided to eliminate the possibility of welding the sheet metal to the jig, With a jig like this, there is no need to make tack welds, or to provide extra space between the parts at the finishing end of the seam, both of which must be done when unjigged pieces are to be welded (as directed in Chapter 14, which follows.)