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Weld backing is used on the back side of the joint to prevent melt-through and to ensure complete penetration of the weld joint.
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All industrial codes and standards reference the requirement for having qualified procedures. They need to be openly available and the welders need to be trained on their proper use. The welders need to be qualified on those procedures and their qualifications must be current. The supervisor needs to do this by verifying the welder's continuity report.
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When welding, the melting point of steel (approx. 27800 F) versus the solidification point of the fast freezing flux or slag (approx. 32000 F) is important to know. Also, it is important to know that the melting point varies with the carbon and alloy content of steel. Steel is magnetic until heated above the transformation temperature of approximately 1333 0 F. How fast the metal conducts heat will have a large effect of preheat and inter-pass temperature control. How much the metal expands on heating and contracts when cooling needs to be understood to control buckling and distortion.
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This is also considered the operating factor and is thought of as the percentage of a shift that a welder/welding operator is actually welding. For robotic and mechanized welding, it can be as high as 90% while for manual and semiautomatic welding it can be as low as 10-20% operating factors.
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Though the weld symbols are identical, the sequence of how the welds are made is different. The back weld is made last and the backing weld is made first.
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The shape of the output waveform from this type of power source is referred to as square wave. These types of power sources have enhanced performance on a.c. They can control the balance, amplitude, frequency, etc. to allow better control of penetration and cleaning action.
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If the weld does not combine with another weld or the base metal it is referred to as Incomplete fusion.
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A fisheye is caused by the presence of hydrogen in the weld. If this appeared on a break test of some type it may or may not be acceptable. Always verify the acceptance criteria per the code or specification.
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Undercut best describes this fashion. Some people want to call it “under fill” but it is not correct, since it is not in the weld being located next to the toe.
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This welding technique would appear in the welding instructions or in the welding procedure specification. This generally increases the heat input per weld pass, but will generally reduce the total number of weld passes required to fill the joint.
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The surrounding air around the weld pool contains oxygen, nitrogen, and hydrogen. These gases get into the molten weld pool and cause gas bubbles to form (porosity) and lead to embrittlement and weld cracking.
