The increase in the dimensions of metals caused by heat. See COEFFICIENTOF LINEAR EXPANSION.

When metals are heated they expand in every direction. The expansion in length is linear expansion; the increase in volume is cubical expansion. Conversely, a decrease in temperature causes the metal to contract, decreasing the cubical and linear dimensions.

Each metal is susceptible to this change in volume, and each metal expands a specific amount in relation to a specific rise in temperature. In many cases, the only practical concern is the increase in length. The amount of linear expansion can be calculated by measuring the unit length of a specimen rod of the metal when it is raised through one degree of temperature. This amount is the coeficient of linear expansion.

The increase in length produced by a rise in temperature is equal to the original length, multiplied by the coefficient of expansion, multiplied by the rise in temperature in “E The cubical expansion is calculated at three times the linear expansion.

Assuming that the arc is producing a given amount of heat, the amount of expansion at a given point in the structure being welded will depend on the length of

time the arc is operating at that point. The contraction will be equal in amount to the expansion, assuming that no internal strain is left. Therefore, it is evident that total expansion or contraction tending to deform the workpieces will be less if 105 kT/h (100 BTUh) is applied than if heat is applied at the rate of 1050 kJh (1000 BTUh). The total expansion or contraction depends on the amount of heat applied; in general, the

lower the amount of heat applied to a welded seam, the smaller the expansion and contraction effect will be.

A bar of iron expands as it is heated and contracts when cooling; both actions are attended by great force. Expansion in length, width, and thickness is governed by a rise in temperature; contraction is regulated by a fall in temperature. Expansion rates of various metals differ. A copper bar one foot long expands 0.1 in. per 1000°F.A gray iron bar of the same size would expand a little over 0.1 in. when heated to 1500″F, an expansion rate of 0.067 in. per ft per 1000°Frise in temperature. The expansion of aluminum is 0.148 in. per ft for each 1000°F.

Expansion in Welding

One of the greatest challenges in welding is adapting welding conditions to control the expansion and contraction brought about by differences in temperature of different parts of the workpiece. When welding ductile metals like iron and steel, allowances must be made for expansion and contraction, because warping, distortion or buckling will inevitably take place. In non-ductile materials, such as cast iron, aluminum

alloys and copper, the strains produced by heat may cause the metal to crack or fracture, because the strength of these materials is lower when near the melting point. Although in many cases distortion or fracture have not taken place, the expansion and contraction effects have produced serious internal strains, which require only a slight additional strain to produce failure by exceeding the strength of the metal. It is for this reason that failure can occur in articles which appear to have been successfully welded.

The metalworker cannot restrain by force the expansion of metals caused by raising the material to a high temperature, and the contraction to approximate original dimensions caused when the heat diminishes and disappears. The forces of expansion and contraction are irresistible, and if attempts are made to control them by force, using clamps, jigs, or other means, distortion, serious internal strains, or failures will result.

 

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