The heat applied to the base metal or substrate to attain and maintain preheat temperature.

Preheat is the application of heat to the workpiece prior to welding, brazing or cutting. There are three basic reasons for preheating:

(1) To equalize temperature in the workpiece. Welding heats a local area of the workpiece, causing local expansion and a tendency to warp or distort. This expansion also causes local stresses which could result in rupture of a weldment. Preheating lowers the yield strength of most metals and allows stresses to be relieved or reduced. Preheating reduces the temperature difference between the base metal and the area welded.

(2) To reduce the amount of heat needed to make a weld or braze. When a cold part is to be welded or brazed, it requires a higher arc current or hotter gas torch flame to start welding than when the part is preheated to a temperature of 150 to 200°C (300 to

400°F). Obviously, the preheating should be done by some lower cost fuel than used for welding.

(3) To prevent hardening and cracking when a part is cooled too rapidly through the transformation range, particularly for cast iron and most carbon steels. Pre heating reduces the temperature differential so that heat flow from the weld area is reduced and the cooling rate is slower.

Preheating may be applied locally by oxyfuel torch, by electrical resistance techniques or by induction heating. Entire components may be heated in a furnace or oven that is large enough to hold the part.

Preheat temperature depends upon the material, the workpiece size and shape, and the welding process to be used. Cast iron requires a red heat and brass and bronze a dull red heat. Steels require a preheat of between 90 and 300°C (200 and 600°F), depending on carbon and alloy content and material thickness. Excessive preheats will remove any benefits of prior heat treatment. Aluminum should be preheated carefully, because too high a temperature will remove any prior aging or cold work benefits, or even cause partial melting. Temperature-indicating crayons are usually used to determine the temperature of the workpiece.

 

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