Arc welding discontinuities
Welding discontinuities can affect product performance and longevity. Thoroughly understanding the various defects, their causes, severity, and remedies can help ensure high-quality and superior performance. This article presents an overview of welding defects and discusses design strategies to help prevent them.
|Common discontinuities include incomplete penetration, undercut, misalignment, and root undercut. |
Photo courtesy of AlcoTec Wire Corporation, Traverse City, MI.
Quality by Design
A weldment's service performance is determined largely at the design stage. This is the time to decide which materials to use based on accepted material specifications and considering the mechanical properties to be guaranteed in the finished construction.
This stage also is the designer's opportunity to select joint types that will transmit the service stresses successfully, provided the inevitable imperfections introduced by the welding processes are at an acceptable level and under control.
The designer must establish precise criteria for any possible imperfection to be found in manufacturing to indicate clearly which and what size discontinuities must be repaired or rejected.
Economy and manufacturing costs, including the weld dimensions, the amount of filler metal to be deposited, and the suitability of the process employed, are decided, knowingly or unwittingly, at the drawing board.
Joint design and weld details have a major effect on the fatigue life of structural elements. Fatigue is the formation and propagation of cracks, leading to fracture, under fluctuating stresses of limited intensity that would not be harmful if they were static.
The expert designer knows how to avoid discontinuities and sudden geometric changes that can cause stresses to rise locally and initiate cracks. The exact location and the magnitude of stress concentration in welded joints depend on the joint's design and load direction.
Welding Joints and Discontinuities
Common weld types are butt, T, corner, lap, and edge. These may be defined further as square, bevel or special groove, fillet, and simple (if they are welded from one side only) or double, from both sides.
An occasional improper condition can render a manufactured welded structure imperfect. A lack of local physical material continuity at or near the weld is called a discontinuity or flaw. Such a feature has to be detected and evaluated to determine if it is a harmless imperfection or an intolerable defect that must be removed because of the service the structure will sustain in operation.
Possible rare unintentional load or temperature overrun caused by unrelated equipment malfunctions also must be taken into account.
When found, discontinuities always should be assessed. Detection techniques need be sensitive enough to detect harmful or rejectable indications only. An oversensitive nondestructive inspection technique is not an advantage, however. In fact, it may be dangerous because the presence of too many irrelevant indications can mask those few but harmful discontinuities that have to be evaluated and eliminated.
A defect is nothing more than an excessive discontinuity. It is determined by design acceptance/rejection requirements based on past experience, or on more modern criteria of fitness for service and calculated using the rules of fracture mechanics.
Fitness for service is a concept that allows individual flaws to be assessed and permits the designer or the operator to decide whether or not to repair.
Fracture mechanics is a method of quantitative analysis, a numerical computation procedure for evaluating structural behavior (fracture instability) in terms of applied stress, crack size and shape, and specimen or structure component geometry.
Different approaches exist for calculating maximum acceptable flaw dimension according to fitness for service principles. These approaches are formally presented in codes, some of which already have been used widely.
Discontinuities commonly are grouped according to geometric characteristics. Cracks and planar discontinuities are the most dangerous, especially if fatigue loading conditions (i.e., successively increasing and decreasing) are present in service. Their shape extends mainly in two dimensions and constitutes stress raisers. In visual inspection, only a linear indication may be visible.
Different types of cracks are described. Usually none are tolerated (at the prescribed detection level), and they must be removed by careful grinding (if superficial) or repaired by welding. The most insidious cracks are those not open to the surface that may require specialized techniques for detection and evaluation.
Globular, volumetric, 3-D discontinuities, porosity, or inclusions usually are found deep inside the weld.
Porosity is a collective name describing cavities or pores caused by gas entrapment in molten metal during solidification. Contaminants, moisture, or inadequate shielding may be the cause. Hydrogen can diffuse in molten aluminum and is rejected upon solidification, which causes porosity. Relatively large bubbles or diffused clusters of small pores or pinholes, spherical or elongated, can appear. Shrinkage voids also can show a similar aspect.
The effects of porosity on performance depend on quantity, size, alignment, and orientation. When clustered at the weld's center, porosity is not considered a dangerous fatigue promoter, nor detrimental to fatigue resistance, although it may reduce the static stress carrying capacity of the weldment.
Inclusions are generated by extraneous material and disrupt the base metal continuity. They can be slag, tungsten, sulfide or oxide inclusions.
Incomplete fusion or penetration is an internal planar discontinuity that is difficult to detect and evaluate, but it is most dangerous, especially if low-impact strength and elevated ductile-to-brittle-transition temperature (DBTT) are determined for the material considered, and if cold weather may occur in service to promote low toughness and brittle fracture.
Geometric imperfections refer to weld characteristics such as incorrect fit-up, misalignment, and poor bead shape (undercut, underfill, overlap, melt-through, and distortion) as determined by visual inspection. They are an indication of poor workmanship and may be cause for concern if they exceed requirement limits.
Craters are visually inspectable depressions that indicate improper weld terminations, usually with the presence of radial cracks. They should be avoided or eliminated through improved welding skills or repaired if present.
Spatter—metal drops expelled from the weld that stick to surrounding surfaces—can be minimized by correcting the welding conditions and should be eliminated by grinding when present.
Arc strikes appear as localized, remelted metal from inadvertent or careless arc manipulation. They must be avoided, and any traces removed because small cracks and their localized heat-affected zone can become the origin of dangerous fatigue failures.
Lamellar tearing is a dangerous planar defect that can occur when certain plate materials with laminations are welded to a perpendicular element. A special joint design could be selected to minimize the defect, but the best precaution is to specify material of adequate quality and test it at the receiving inspection.
More information about weld discontinuities can be found in the following resources:
A complete treatise on weld quality is offered in Chapter 13 of the AWS Welding Handbook, 9th edition, Volume 1, pages 534 - 577. Joint discontinuities are shown and discussed in-depth. Readers involved in quality are urged to obtain the book.
The following document provides a useful reference: ISO 5817, Revision: 2ND, Chg: TC1, Date: 02/15/06, Welding—Fusion-welded Joints in Steel, Nickel, Titanium and Their Alloys (beam welding excluded)—Quality Levels for Imperfections.