Weld Quality

As the welded joints are finding applications in critical components where the failure results into a catastrophy, the inspection methods and acceptance standards are increasing. Acceptance standards represent the minimum weld quality and are based upon test of welded specimens containing some discontinuities, usually a safety factor is added to yield the final acceptance standard. A good research effort is being directed to correlate the discontinuities with the performance.

In the present discussion we shall study the weld discontinuities commonly observed in the welds, their causes, remedies and their significance. Small imperfections, which cause some variation in the normal average properties of the weld-metal are called discontinuities. When the discontinuity is large enough to effect the function of the joint it is termed a defect.

Standard codes do permit limited level of defects based on fracture mechanics principles, taking consideration the service conditions of the fabrication. Inspite of all this, the fabricator

must strive to prevent the occurrence of weld defects in the first instance and to rectify them if they do occur. There are many types of defects which have been classified in various documents (e. g., BS499 part I, 1965). For our purpose we shall be discussing the most important ones shown in Fig. 9.1. These are undercuts, cracks, porosity, slag inclusions, lack of fusion and lack of penetration.

9.1 UNDERCUTS

The term is used to describe a groove melted into the base metal adjacent to the toe of a weld and left unfilled by the weld metal. It also describes the melting away of the sidewall of a welding groove at the edge of a layer or bead. This melting away of the groove forms a sharp recess in the sidewall in the area in which the next layer or bead must fuse. (Slag may be “keyed” into this undercut which, if not removed prior to subsequent passes, may become trapped in the weld.) An undercut, therefore, is a groove that may vary in depth, with, and sharpness at its root.

9.2 CRACKS

Cracks are linear ruptures of metal-under stress. Although sometimes wide, they are often very narrow separations in the weld or adjascent base metal. Usually little deformation is apparent. Three major classes of cracks are generally recognised: hot cracks, cold cracks, and macrofissures. All types can occur in the weld or base metal.

Fig. 9.2 illustrates a variety of cracks including underbead cracks, toe cracks, crater cracks, longitudinal cracks, and transverse cracks. The underbead crack, limited mainly to steel, is base metal crack usually associated with hydrogen. Toe cracks in steel can be of simi­lar origin. In other metals (including stainless steel), cracks at the toe are often termed edge of weld cracks, attributable to hot cracking in near the fusion line. Crater cracks are shrinkage cracks which result from stopping the arc suddenly.

9.3 POROSITY

Porosity is the presence of a group of gas pores in a weld caused by the entrapment of gas during solidification (when solidification is too rapid). They are small spherical cavities, scat­tered or clustered locally. Sometimes, the entrapped gas may form a single large cavity which is termed as a blow hole.

Causes:

1. Lack of deoxidisers

2. Base metal sulphur content being high

3. Presence of oil, grease, moisture or mill scale on the joint surface

4. Excessive moisture in flux

5. Inadequate gas shielding

6. Low current or long arc

7. Rapid solidification of weld deposit

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