Submerged Arc Welding

Discontinuities and Defects in Welding

Material joining processes such as welding, soldering, and brazing depend on temperature excursion from room to operating temperature and back to room temperature for completion of the joining process. During such temperature cycling, parent material and bonding zone can undergo many changes and produce microscopic and macroscopic features that may not be beneficial for joint performance. Such features can have a host of manifestations including porosity, cracks, brittle phases, phase segregation along grain boundaries, grain growth, and misplaced filler metal. Presence of such features can interrupt homogeneity of the weld, either in terms of material properties, stress fields, or physical boundaries; such an interruption is referred to as a discontinuity. A discontinuity may be considered a defect if not allowed by applicable industry standards or specified on the part drawing (keep in mind that weld requirements stated on the part drawing supersedes any industry standards), even though presence of that discontinuity may not immediately result in a weld failing to meet functional tests. For example, the weld fusion zone may have a crack that does not meet specifications but does not result in failure in leak testing.

A discontinuity is always a cause for concern for the welding engineer as he/she tries to figure out if it will have any negative influence on the weld. Unfortunately for the welding engineer, the weld/solder/braze zone proves to be a smorgasbord of discontinuities, the more you look, the more you find them. A weld section will show a range of microstructures and phases in and near the joint. Parent metal far from the joint will have the original microstructure as it is not affected by heat and should not have any discontinuities related to the welding process. Material in the vicinity of the weld which did not take part in the weld but was affected by the welding heat could have distinctly different microstructure affected by heat of the joining process; referred to as the Heat Affected Zone or the HAZ. Size of HAZ can be substantial in high-temperature arc welding processes but negligible for low-temperature soldering operations. The HAZ can have many forms of discontinuities: metallurgical and physical. The grains in HAZ may have grown in size (at the expense of neighboring grains) and would have lost some of mechanical properties in the original fine grain microstructure. HAZ may also have undergone a phase change to form more brittle phase such as martensite in some steels. The welding heat may have also resulted in formation of low melting phases that can seep along the grain boundaries resulting in cracks if there is tensile stress across the boundaries. Presence of one or more of the above discontinuities can lead to properties of the HAZ that are significantly different than the parent metal either in terms of ductility, corrosion, conductivity, or propensity for crack growth. Such a discontinuity may lead to compromised performance of the assembly over an extended period of time if the particular attribute is of concern. For example, if the weld is not likely to be flexed, either intentionally as in application of cycling loading, or unintentionally as due to thermal expansion/contraction, then the presence of the brittle phase in HAZ may not be of concern. However, if the weld will be subject to fatigue loading in use, then any discontinuities like grain growth, brittle phases, or fine cracks can result fatigue crack growth leading to failure. Figure below shows a resistance weld section with cracks in the HAZ zone.