1 Shielding Gases Introduction Air in the weld zone is displaced by a shielding gas in order to prevent contamination of the molten weld puddle. This contamination is caused mainly by nitrogen, oxygen and water vapor present in the atmosphere. As an example, nitrogen in solidified steel reduces the ductility and impact strength of the weld and can cause cracking. In large amounts, nitrogen can also cause weld porosity. Excess oxygen in steel combines with carbon to form carbon monoxide (CO). This gas can be trapped in the metal, causing porosity. In addition, excess oxygen can combine with other elements in steel and form com- pounds that produce inclusions in the weld metal. When hydrogen, present in water vapor and oil, combines with either iron or aluminum, porosity will result and ”underbead” weld metal cracking may occur. To avoid these problems associated with contamination of the weld puddle, three main gases are used for shielding. These are argon, helium and carbon dioxide. In addition, small amounts of oxygen, nitrogen and hydrogen have proven beneficial for some applications. Of these gases, only argon and helium are inert gases. Compensation for the oxidizing tendencies of other gases is made by special wire electrode formulations. Argon, helium and carbon dioxide can be used alone, in combinations or mixed with others to provide defect free welds in a variety of weld applications and weld processes.
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