DIFFERENT TYPES OF DISCONTINUITIES.pdf

Discontinuity is defined as an interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity could be the result of a defect, but isn't necessarily a defect. A defect, on the other hand, is a discon...

Discontinuity is defined as an interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity could be the result of a defect, but isn't necessarily a defect. A defect, on the other hand, is a discontinuity that by nature or accumulated effect (for example, total crack length) renders a part or product unable to meet minimum applicable acceptance standards or specifications. A defect results in rejection of the part or product.  For this reason, it's important for an inspector examining the weld to be able to spot a variety of weld discontinuities, including: 1.Porosity. 2.Incomplete fusion. 3.Incomplete joint penetration. 4.Unacceptable weld profiles. 5.Cracking.  Porosity - is defined as a cavity-type discontinuity formed by gas entrapment during solidification. These trapped gases in the molten weld may form bubbles or pockets as the weld solidifies. The four main reasons for the presence of gases that cause porosity are: 1.Dirty base material contaminated with hydrocarbons such as oil, grease, or paint. 2. Moisture on the joint surface or electrode in the form of water or hydrated oxides or water leaks from poorly maintained cooling systems that can introduce hydrogen into the welding process. 3. Insufficient or improper shielding caused by an inadequate shielding gas flow rate; gas that's contaminated from its source or from its delivery system; or wind or draft that prevents the gas from adequately protecting the molten weld metal. 4. Incorrect welding conditions or techniques. Porosity often is classified by its shape and distribution within the weld, such as uniformly or randomly scattered, cluster, or linear. Each of these porosity distributions may have different levels of acceptance within a welding code or standard. The most practical methods for controlling or eliminating porosity are to use clean base materials, properly store uncontaminated welding consumables, adequately maintain welding equipment, use proven welding procedures, and weld in acceptable environmental conditions. Incomplete Fusion and Incomplete Joint Penetration Incomplete fusion is a weld discontinuity in which fusion doesn't occur between the weld metal and fusion faces or adjoining weld beads. This absence of fusion can occur at any location within the weld joint and be present in fillet welds or groove welds. Incomplete fusion may result when the temperature of the base material or previously deposited weld metal is not elevated to its melting point during the welding process. Incomplete fusion often is found on one leg of a fillet weld and is caused by an incorrect welding angle, which distributes heat nonuniformly between both sides of the joint. It also may be caused by oxides or other foreign material on the surface of the base material. Incomplete joint penetration is a discontinuity in a groove weld in which the weld metal doesn't extend through the joint thickness. It's the failure of the filler metal or base metal to fill the root of the weld completely. Some common causes of incomplete joint penetration are a bad groove weld design or a fit-up that is unsuitable for the welding conditions. Incomplete joint penetration can occur if the root face dimensions are too large, the root opening is too small, or the included angle of a V-groove weld is too narrow. All of these joint design problems restrict the weld's ability to penetrate the joint's thickness. Incomplete joint penetration can be prevented with the correct joint design and fit-up in accordance with welding procedure requirements. Understanding these weld discontinuities will help welding inspectors identify them and, more important, prevent them from occurring in production. Using welding inspection as a preventive tool within the quality system is more efficient than using it only as an appraisal technique to sort bad welds from good welds. Unacceptable Weld Profiles Unacceptable weld profiles can cause a reduction in base material thickness, reduction in weld size, or stress concentrations on the weld or plate surface. These types of weld discontinuities often seriously detract from the overall performance of a welded component in service. Some weld profile discontinuities are undercut, overlap, insufficient throat, and excessive convexity. Undercut. Undercut is defined as a groove melted into the base metal adjacent the weld toe, or weld root, and left unfilled by weld metal. The term undercut describes two specific conditions. The first is the melting away of the base material at the side wall of a groove weld at the edge of a bead, which produces a sharp recess in the side wall in the area where the next bead is to be deposited. This type of undercut can entrap inclusions within the recess, which then may be covered by a subsequent weld bead. Overlap -is defined as a protrusion of weld metal beyond the weld toe, or weld root. This condition occurs in fillet welds and butt joints and produces notches at the toe of the weld that are undesirable because of their resultant stress concentration under load. This discontinuity can be caused by incorrect welding techniques or insufficient current. Insufficient Throat. -usually occurs in fillet weld and butt joint profiles that are concave. Excess concavity reduces throat thickness, which considerably reduces weld strength. This condition usually is caused by excessive welding current or arc lengths. Excessive Convexity. -can produce a notch effect in the welded area and, consequently, concentration of stress under load. For this reason, some codes and standards specify the maximum permissible convexity of a weld profile. Insufficient current or incorrect welding techniques typically cause this condition.  Cracking Cracking often is caused by stress concentration near discontinuities in welds and base metal and near mechanical notches in the weldment design. Hydrogen embrittlement, a condition that causes a loss of ductility and exists in weld metal because of hydrogen absorption, can contribute to crack formation in some materials. Hot and Cold Cracks. Cracks are classified as one of two types: hot or cold. Weld Metal Cracks. Weld metal cracks can be divided into three types: 1.Transverse, which are perpendicular to the direction of the weld. 2.Longitudinal, which travel in the same direction as the weld and often are confined to the center of the weld. This type of crack may be an extension of a crack that originally initiated at the end of a weld. 3.Crater, which can be formed by an abrupt weld termination if a crater is left unfilled with weld metal. These cracks usually are star-shaped and initially extend only to the edge of the crater. However, they can propagate into longitudinal weld cracks.