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One element critical to a weld quality system is the final examination of the weld. Fabricators rely on radiography or ultrasound to check weld integrity. However, deficient welds can often be identified through visual inspection.

Thereby, surface porosity can be detected by eye. Gas bubbles trapped in the weld material cause this porosity. A cross-section of a weld, when examined under a microscope, is distinguished by its “Swiss-cheese” appearance. The gas bubbles can come from low quality or gassy metals or from interaction among weld materials. However, contaminant’s oil or rust on the weld surface more commonly causes it. Minor internal porosity does not significantly affect weld performance but surface porosity is a serious condition. Welds with surface porosity should always be replaced.

Like porosity, slag inclusions replace the weld material with a non-metallic component. These slag inclusions resemble black shards of glass on the top of the fillet. Lighter than the weld material, they tend to float to the top of the molten metal. If the slag cannot escape they become trapped in the weld. The number, size and distribution directly determine the weld strength. Electrode, flux debris and non-metallic inclusions also produce slag and reduce the strength of the weld, reducing tensile strength and tensile ductility. The risk of slag inclusions can be reduced by properly preparing the weld surfaces. Time should be allowed for the slag to rise to the surface of the molten metal. Rapid solidification is most likely to trap the inclusions. Flux has a significant influence on the type of foreign materials that may form in the weld. A more fluid-like flux will enable the slag to reach the surface before the weld has solidified. Slag inclusions weaken the weld because of a lack of metal homogeneity and lack of fusion. Lack of fusion occurs when the base metal fails to melt or mix with the weld material. Surface oxides, such as rust or scale often cause this. Unless these oxides are removed, lack of fusion becomes likely. In some cases, these discontinuities can be identified by a small gap between the fillet and the base metal or along the toe edge.

Weld cracks are serious defects that have little margin of safety. The two types of cracking most likely to occur are hot cracking (just after the weld has solidified) and cold cracking (which occurs near room temperature after the weld has cooled). Most weld cracks are caused by hot cracking, with the weld being pulled apart during cool-down. If the configuration of the part does not allow the weld area to contract as it cools, then hot cracking becomes likely. Hot cracking is common when high phosphorus, sulfur or lead content is present in the base metal. Chromium steels are particularly sensitive to microcracking, but this can be avoided with appropriate preheating procedures. Methods to control or eliminate hot cracking are usually detailed in work instructions or weld procedures. Variables such as the type of alloy or type of weld can affect the weld sequence and the part’s susceptibility to cracking.