Cavity-type discontinuities formed by gas entrapment during solidification or in a thermal spray deposit.

Porosity reduces the strength of a weld. In fusion welds, it is caused by dissolved gases that are usually present in the molten weld metal. If the dissolved gases are present in amounts greater than their solubility limits, the excess is forced out of solution in the

form of bubble or gas pockets as the weld metal solidifies. The gases which may be present in the molten weld pool include hydrogen, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulphide, and rarely, argon, and helium. Hydrogen is the major cause of porosity in weld metal.

The welding process, procedure and base metal type directly affect the quantities and types of gases that are present in the molten weld pool. The welding process and welding procedure control the solidification rate, which in turn affects the amount of weld metal porosity. Proper welding procedures for a given combination of welding process and base metal should produce welds that are essentially free of porosity.

The common causes of porosity in fusion welds and suggested methods of preventing it are summarized in Table P-7.

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