The increase in dimensions of an unrestrained material caused by being heated. As a corollary, thermal coefficients of contraction measure the shrinkage of a material when cooled; generally expressed per degree Celsius or degree Fahrenheit change in temperature.

The thermal expansion of various metals differ. For example, copper expands about 0.1 in. per ft when the temperature is raised 1000°F. A gray iron casting expands much less than copper: 0.067 in. per ft per 1000°F. When this rate of expansion is applied to a cast iron bar 10ft long, the expansion will be approximately 0.6875 in. when the temperature is raised 1000°F. Under similar conditions, a 10-ft aluminum bar will expand nearly 1.5 in.; a bronze bar, 0.781 in.; and a steel bar slightly more than 0.75 in. See Table C-3.

Distortion

Differences in the amount of thermal expansion due to changes in temperature are responsible for the distortion commonly found in completed welds. When the hot regions in the vicinity of the weld are restrained from expanding by stiff, colder metal around them, they can deform plastically. These dimensional changes are reflected as distortion after the weldment cools. Stresses due to thermal contraction can be sufficiently severe to cause brittle metals

such as cast irons to crack.

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