Weld Discontinuities and Defects Quiz_CWB_Part 2/4

Weld Discontinuities and Defects in Welding Processes

• Large diameter pipes can be repaired using the same method as external undercut, while small diameter pipes with undercut defects require cutting out the section and preparing a new joint.
• Acceptable and unacceptable groove weld profiles are shown in Figure 19, with specific criteria for reinforcement, underfill, excessive weld reinforcement, undercut, and overlap.
• Concavity in welds is defined as the maximum distance from the face of a concave fillet weld to a line joining the weld toes, and excessive concavity can occur with fillet welds.
• Causes of concavity in welds include operator technique, welding parameters, and position of welding, and excessively concave welds can give a deceptive appearance of the actual weld size.
• Desirable, acceptable, and unacceptable fillet weld profiles are shown in Figure 23, with specific criteria for convexity, excessive undercut, overlap, and insufficient leg.
• Weld deficiencies due to insufficient or excessive size and poor profile may be detected by visual examination or by using suitable gauges.
• The leg and throat size of concave fillet welds can be measured with fillet weld gauges, and only the leg size of convex fillet welds can be measured with fillet weld gauges.
• Misalignment of the weld due to insufficient care in positioning automatic welding machines, incorrect bead placement, incorrect edge preparation, or inaccurate backgouging can lead to out-of-line weld beads.
• Out-of-line weld beads can result in incomplete joint penetration when complete joint penetration is required, and correction requires the removal of the weld from one side by grinding or gouging and the depositing of a new weld.
• Incomplete fusion is the failure to fuse between weld metal and fusion faces or adjoining weld beads, and it may occur at any point in the welding groove or fillet weld.
• Incomplete fusion may be caused by factors such as improper electrode selection, welding parameters, manipulation of the electrode, cleaning of material, joint design, and poor joint preparation and fit-up.
• Improper joint design, poor joint preparation, and fit-up can inhibit electrode manipulation and affect the welding process.

Weld Discontinuities and Their Causes

• Distortion in welding can be caused by factors such as improper welding parameters, weld pass sequencing, joint preparation, fit-up, and design
• Surface irregularities in welding, such as spatter, inadequately filled craters, and arc strikes, are typically the result of incorrect welding parameters or technique
• Excessive spatter poses challenges for subsequent painting operations and is labor-intensive to remove
• Tack welds left for inclusion in the completed weld may result in cracks due to rapid cooling
• Stray arc strikes may lead to quenched and embrittled conditions in the weld zone, particularly in alloy steels
• Slag inclusions, which are oxides and nonmetallic solids, can be found as elongated or multifaceted inclusions in welds
• Slag is produced during welding with covered electrodes, tubular flux cored electrodes, and submerged arc electrodes/flux, serving as scavengers of impurities in the molten metal pool
• Factors affecting the ability of slag to rise to the surface of the weld include high viscosity weld metal, rapid solidification, improper electrode manipulation, improper interpass cleaning, and weld discontinuities from a previous pass
• Rapid solidification of the weld metal can prevent slag from rising to the surface, usually due to incorrect welding parameters in high-speed applications
• Slag generally has time to float to the surface of the weld due to its lower density
• The ability of slag to rise to the surface of the weld may be affected by factors such as improper manipulation of the electrode and rapid solidification of the weld metal
• High-speed welding applications require procedures to ensure that slag has adequate time to escape

Recognizing and Preventing Porosity in Welding Processes

• Porosity in welding can be caused by moisture, chemistry and structure of the parent material, surface contaminants, shielding gas, welder technique, air contaminants, insufficient flux coverage, and slag residue.
• Moisture pick-up on flux-coated electrodes or on the surface of flux-cored wire exposed to humid conditions causes porosity.
• Proper storage of consumables is crucial to avoid moisture, with storage temperatures for SMAW above 120°C (250°F) to maintain acceptable moisture levels.
• Selecting the proper filler metal to match the chemistry and properties of the material to be welded is important to prevent porosity.
• Surface contaminants like oil, grease, paint, oxides, and rust can cause porosity, and methods of preparing materials for welding can introduce contaminants.
• Porosity associated with shielding gases can result from poor distribution, insufficient or excessive flow rates, or impurities collected in the gas through hoses and connections.
• In manual welding applications, porosity can be caused by faulty manipulation of the electrode, excessive arc voltages, incorrect electrode angle, and incorrect weave techniques.
• Welding operations in close proximity to painting operations may result in porosity problems due to air contaminants.
• Insufficient flux coverage in submerged arc welding can cause scattered surface porosity.
• Slag left on the surface of tack welds may cause porosity, and interpass cleaning operations should ensure all slag is removed before depositing the next pass.
• Porosity may be detected by visual examination or NDT methods, and relevant codes and standards should be referenced to determine acceptance limits.
• Repairing a weld with porosity that is determined to be a defect requires grinding or gouging to remove the porosity and re-welding, ensuring a desirable groove profile is achieved for the repair weld passes.