Characteristics of welding process

WELDS can be characterized according to a number of criteria, including the welding process used, size, shape, mechanical properties, chemical composition, and a number of others. The appropriate methods of characterization depend on the weld's function and the particular set of properties required for the application. In some instances, the ability of a weld to function successfully can be addressed by characterizing the size or shape of the weld. An example of this is where factors related to the welding procedure, such as inadequate weld size, convexity of the bead, or lack of penetration, may cause a weld to fail. In other cases, it is important to characterize metallurgical factors such as weld metal composition and microstructure. Examples might include welds for which the goal is to avoid failures due to inadequate strength, ductility, toughness, or corrosion resistance. In general, the goals of weld characterization are to assess the ability of a weld to successfully perform its function, to thoroughly document a weld and welding procedure that have been demonstrated to be adequate, or to determine why a weld failed. In the first part of this article, characterization of welds will be treated as a sequence of procedures, each more involved than the last and concerned with a finer scale of detail. Initially, non-destructive characterization procedures will be the focus. The first level of characterization involves information that may be obtained by direct visual inspection and measurement of the weld. A discussion of nondestructive evaluation follows. This encompasses techniques used to characterize the locations and structure of internal and surface defects, including radiography, ultrasonic testing, and liquid penetrant inspection. The next group of characterization procedures discussed are destructive, requiring the removal of specimens from the weld. The first of these is macrostructural characterization of a sectioned weld, including features such as number of passes; weld bead size, shape, and homogeneity; and the orientation of beads in a multipass weld. Macroscopic characterization is followed by microstructural analysis, including microsegregation, grain size and structure, the phase makeup of the weld, and compositional analysis. The third component of weld characterization is the measurement of mechanical and corrosion properties. The goal of any weld is to create a structure that can meet all the demands of its service environment. In many cases, the best way of assessing the performance of a weld is to establish its mechanical properties. In addition to a number of standard material tests, many mechanical tests are directed specifically at determining a weld's capabilities. Examples of mechanical properties typically characterized for welds include yield and tensile strength, ductility, hardness, and impact or fracture toughness. Corrosion testing is often employed in situations where a welding operation is performed on a corrosion-resistant material, or in a structure exposed to a hostile environment. Although absolute corrosion performance is important, a major concern is to ensure that a weld and its heat-affected zone (HAZ) are cathodic to the surrounding metal. Following the discussion of the characterization procedures, the second part of this article will give examples of how two particular welds were characterized according to these procedures.