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Why is it necessary to preheat and postheat?
Welding releases hydrogen : Hydrogen promot.es cracking. If moisture is present when the weld is made, the arc breaks down into its elements, Hydrogen and Oxygen. These elements are defect carriers and very harmful to the weld. Atomic Hydrogen produced from the moisture as the arc diffuses in the weld, goes into solution and settles in the atomic structural voids. If the metal cools rapidly, not all the hydrogen is absorbed. Some goes over the heat affected zone of the parent metal, some forms gas pockets or goes into the air, and some helps form other non-desirable impurities in the weld. Probably some light hydrogen gas becomes entrapped in the solidifying weld, forming porosity.
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