Porosity – ‘The good, the Bad and the Ugly’ of Radiographic Testing

”Radiographic Testing is a NDT method to detect inherent volumetric discontinuities in materials such as castings and welds” A common type of discontinuity found and most often misinterpreted and evaluated is rounded indications, whether isolated, aligned or clustered. This paper addresses the acceptance criteria of rounded indications based on various codes and standards and intends to lay at rest the confusion related to this type of discontinuity. The following are important considerations: 1. Interpretation of rounded indications based on codes and standards used in the various sectors of NDT. 2. What types of discontinuities can be regarded as rounded indications? 3. What discontinuity sizes can be regarded as relevant discontinuities? 4. What are the acceptance and rejection criteria for relevant rounded indications 5. Compiling relevant student assignments to facilitate the proper understanding of this aspect. Introduction A common type of discontinuity found and most often misinterpreted and evaluated id rounded indications, isolated, aligned or clustered. The goal of non-destructive examination for discontinuities in materials and welds are to determine whether or not the continuity and homogeneity of a component are adequate for its use. Identified discontinuities are relevant as either rejectable or nonrejectable conditions in a part. An evaluation usually is made with reference to a design basis and may include a code or rule-based criteria for acceptance and rejection. The evaluation of a discontinuity generally requires an adequate measurement of its size and location and identification of its character. Discontinuities are evaluated completely by determining their location, number, shape, size, orientation, and type. The term defect has many meanings and is used to describe almost any variation in metal structure: discontinuity, imperfection, flaw, inhomogeneity or heterogeneity. Defects may be of mechanical or metallurgical types. 1.Porosity Porosity pockets or voids usually spherical in shape – is caused by entrapment of gas evolved during weld metal solidification. Spherical gas pockets, non-spherical voids, elongated tubular gas pockets (described as worm holes or tunnelling) can occur. A large isolated gas pocket is also referred to as a blowhole and may be partially filled with slag. More info about this article: http://ndt.net/?id=19589 2 2.Elements causing porosity A major element causing or contributing to formation of porosity is hydrogen, supplied by: 2.1 The gas atmosphere surrounding the arc zone and weld deposit area; 2.2 The presence of moisture in the atmosphere, flux or electrode coating, or water in the area being welded; 2.3 Cellulose or other hydrogen-forming constituents in the flux or electrode coating; 2.4 The base metal itself. The welding arc’s intense heat dissociates the water vapour and other hydrogen-bearing constituents. In the atomic state, hydrogen diffuses readily into the molten material. As the molten metal cools, the solubility of hydrogen decreases for several common metals. As molten weld material decreases, hydrogen tends to diffuse out of the weld deposit into the atmosphere and also into the adjacent heat-affected zone of the base material. Temperature decreases, however, generally occurs so rapidly that diffusion does not take place fast enough to allow hydrogen content to remain within its solubility limits in the molten metal. The resulting super-saturation causes the combination of hydrogen and porosity formation. Being lighter the molten metal, gas bubbles tend to rise to the weld deposit’s surface. If the bubbles fail to reach the surface before solidification, they will be entrapped as internal porosity. If they reach the surface, they may appear as external or surface porosity. When weld solidification occurs as dendritic growth, the voids may also become entrapped along the grain boundaries. Other impurities may also precipitate out. Gas bubbles may also form by chemical reactions. Thus, porosity tends to form when oxides in the weld puddle are reduced by hydrogen or carbon at or slightly above their melting point. Thus, the presence of deoxidisers, when added to filler metals, may reduce the oxide reducing actions, minimizing in turn the tendency toward porosity formation. Worm holes or tunnelling can be caused primarily by turbulence in the weld pool, usually a result of excessively high welding currents. Among the welding processes, the tendency for porosity formation is greatest in manual metal arc welding with coated electrodes. This is due to the moisture in the coating and the heavier than normal surface oxidation of the core wire. The tendency for porosity is somewhat less with Mig welding. Nevertheless, porosity can form, particularly with the smaller diameter wires which have a high surface-to volume ratio, and may thus contain significant oxidation and absorbed moisture. Grease and oil lubricants on the wire may also provide the hydrogen resulting in significant porosity. The tendency for porosity formation is least with Tig welding. To minimize porosity in aluminium welds, surface treatment is particularly important to minimize the presence of oxides, moisture and other sources of hydrogen. Very rarely has porosity been associated with actual service failures. No failures have been reported in welds where the porosity was applicable code requirement. Quite likely, porosity limits two to four times those now established in codes could be tolerated in the majority of critical service applications. There is significant divergence among the major codes on the acceptability limits applicable to porosity. Porosity, even under the most liberal acceptability limits of the standards, is not likely to result in service failures so long as it is removed from the surface. Far more critical are surface conditions or notches, which can trigger mechanical or metallurgical failures. Latest results and codes should however be checked.