Welding Defect

Welding Defects

Prepared by : Ahmed K. Sakr
Alexandria University
Faculty of Engineering
Production Engineering Department


















Weld Defects
Porosity may be considered to be gas bubbles trapped in the weld metal. The word "trapped" is appropriate since the "bubbles" due to their buoyancy would normally escape, time permitting, but if this is prevented by rapid weld metal solidification it is accurate to say "trapped." Gas bubbles may sometimes consist of trapped shielding gas argon or helium but usually the gas represents the vaporization of surface contaminants such oil, grease, soaps, inks, lubricants or drawing compounds. Low temperature materials such as sulfur, zinc, cadmium and bismuth in the base material can contribute to the formation of porosity. Moisture provides hydrogen a major source of porosity. Shop dust consisting of a wide range of materials produced in manufacturing operations can contribute. Formed parts produced on dies fabricated from low temperature materials as well as die lubricants are also sources. Adhesives, tapes or marking crayons are offenders. It is evident that parts must be carefully cleaned and protected after cleaning

Pores or porosity are usually spherical inasmuch as the pressure of the gas they contain is multi-axial but they can be elongated by local restraint or by the joining of pores. The porosity or pores may be singular and randomly dispersed or they may be clustered and sometimes aligned as a stringer depending on the characteristics of their source. Improper weld techniques can cause weld porosity. Contaminated or leaky hoses provide another source. Occasionally, even following the most stringent cleaning and post cleaning protection porosity may still occur. This may be the result of pores, voids or inclusions in the parent metal. The parent metal may have contaminants introduced in the original melt. Some porosity producing materials like sulfur are deliberately added to improve machining characteristics. Others may be added to improve heat treatment, etc. For these reasons, porosity either internal or at the surface may be unavoidable. If so then vacuum melted materials must be used. Porosity depending on several conditions can have negative effects on weld performance. For example, depending on the size of the voids, their proximity when in clusters and number of voids, the dimension of the weld cross-section is lowered and thus its strength. If porosity is located close to or at the surface, fatigue strength may be effected. Depending to some extent on the shape of the voids and the chemistry of their walls they may initiate cracking. If located near the toe of fillets or at the edge of weld reinforcements which are stress concentration areas they may initiate cracking. We have mentioned the importance of cleaning but welding techniques may also be involved if the weld is allowed to solidify before gas can escape. Weld speed may have to be reduced or techniques involving oscillation may be needed. Keyhole welding utilizing either electron beam or laser are prone to porosity entrapment due to their very narrow welds though vapor pressure probably expels some porosity. Chill fixtures may accelerate weld solidification and trap gas bubbles.
Most welds probably have some porosity and may be acceptable provided it is within the weld or product specification limits.
Cracks which occur in the weld and heat affected zones are the major threat to the survival of the weldment. Cracks form stress concentration areas at their extremities causing them to propagate. Cracking is rarely acceptable.
Cracks are the result of stresses usually generated by shrinkage forces concentrated in localized areas where brittleness (C)1997, 1996 Zane Publishing, Inc. All rights reserved or where low strength grain boundary precipitants exist and the material responds to the stress by cracking rather than yielding or elongating. The stresses must exceed the strength of the metal in that area. Rapid chilling of the weld can cause cracking by the metallurgical formation of martensite a brittle phase and the accelerated rate of shrinkage caused by the rapid chill. Materials may endure a slow increase in their loading but cannot survive a sudden increase. Multiaxial stresses are particularly severe. Longitudinal cracking in the solidifying weld metal can also be caused by rapid chilling since the longitudinal center of the weld may be the last to solidify and must absorb the stress of the accumulated shrinkage. From the foregoing the need for preheat and the avoidance of too rapid a chill is apparent. This is especially true of materials which are basically brittle, have low ductility as well as those materials which have been strengthened or hardened by heat treating.
Cracking can also result in materials which may have low melting constituents for example sulfur. These constituents are the last to solidify, they may precipitate and gather along the periphery of grains or between dendrites and as such are local areas of weakness and have the potential for initiating hot cracking defined as such since it occurs at high temperature.
The cracking discussed thus far is metallurgical cracking inasmuch as material chemistry, temperature and time are fundamental. Cracks however can be caused not necessarily by metallurgical effects but the mechanical effects of joint design. Joint designs and weld requirement which eliminate stress concentration areas are important. Stress concentration areas in partially penetrated weld joints or overlapping weld melt at the toe of welds. Undercutting, incomplete fusion provide area of stress concentration. The crater and burn off at beginning and the end of welds may provide stress concentration locations. Cracks which originally occurred metallurgically may propagate in cold metal by mechanically produced and concentrated stress forces at the ends of previously created narrow cracks. Residual stresses due to shrinkage restraint may respond as delayed cracking when the stress load is increased during the ultimate assembly function. Stress relieving is important.

There are other conditions and discontinuities which can promote cracking in the weld metal. Porosity patterns, voids, laminations, tunneling and piping all have the potential for initiating cracks. Hydrogen available from poorly shielded welds or moisture from inadequate cleaning of hydrogen bearing contaminants provide a significant source of cracking.
Lack of fusion is a serious defect. It can occur when the molten weld metal can not wet and bond with the surface of the weld joint. This may be caused by oxidation of the weld joint surfaces by lengthy exposure to air or by inadequate cleaning of surface contaminants. The need for effective preweld cleaning and shielding of the weld joint and weld pool during welding is evident. It may in addition to the torch shielding gas be necessary to shield the underbead.
Inadequate penetration can be the basis for weld joint failure due to its stress concentration effect and reduced material section. Weld joints for which less than 100% penetration is specified should if possible include radiused edges or other criteria at the root of the weld joint to distribute rather than concentrate stresses.
Undercutting along the edge of the weld is usually the result of poor welding technique or the use of unsuitable parameters. The undercut may have a notch effect and a stress riser and promote cracking. In addition undercutting may significantly lower the joint cross section and therefore lower its strength.
Inclusions are solid, foreign particles in the weld melt. They may be derived from the high melting temperature tungsten electrode which might have momentarily touched the molten weld pool. Ceramic and refractory materials, aluminum oxide for example from a grinding or abrading operation or certain constituents of shop dust may be absorbed or immersed in the weld pool. The effect of inclusions can be similar to that of porosity.
Mismatch of the weld joint edges depending on its severity creates an area of stress concentration when the joint is in tension. Fatigue strength may be reduced by flexing. Stresses manifested as tension can be concentrated on one side of the mismatched joint.
Suck back and drop through can be considered to be related defects. Suck back is caused by the shrinkage of the deposited filler metal because it has not been adequately bonded or fused to the faying surfaces of the weld joint. Therefore as the melt shrinks and by surface tension it is pulled up from the root. This implies improper or inadequate heating and or cleaning. Residual oxides or other inhibiting contaminants on the weld joint surfaces can prevent wetting and bonding.
Drop through can occur if the the weight of the weld melt exceeds the ability of the surface tension to hold the melt in suspension within the weld joint while it fuses and solidifies. For example, a deep penetration weld involving a considerable volume and weight of molten weld metal. This condition is also influenced by an excess width of the weld joint gap, thickness of the details, the time at melting temperature and weld joint surface cleanliness. Lack of wetting fusion resulting from inadequate cleaning can contribute to "drop through."
Residual stresses are those forces generated by the restraint of the solidifying and shrinking weld melt and adjacent heated base metal. Residual stress can occur in local areas of the weld or be a general condition. Micro cracking is evidence of localized stress. In all cases stress involves the restraint of the natural shrinkage movement. It may be caused by fixturing or the assembly configuration. If cooling of the weld melt does not progress uniformly throughout the weldment, then local restraint can be generated within the solidified weld. Unrelieved stress forces are retained and not necessarily be visually apparent. They exist as molecular tension. With time and those circumstances which may be involved in the assembly function i.e. thermal cycling and mechanical forces. These forces can demonstrate their existence in weld or base metal cracking and distortion. The magnitude of the residual stresses is dependent on several conditions including the extent and level of heating, strength of the base material, its yield temperature, melt volume, degree of restraint and cooling rate. When residual stress is anticipated the weld and it's involved adjacent areas should be stress relieved using the appropriate combination of time and temperature.
Metallurgical changes can occur in the heat affected zone close to the weld when its temperature is within the transformation range of the base metal. These changes may manifest themselves as losses in corrosion resistance, ductility, hardness and strength. The heat sensitive characteristics of the material to be welded should be understood and accordingly preweld, welding and postweld techniques and appropriate treatments adopted.
Distortion results from the expansion and contraction of the weld details during heating and cooling. During the solidification of the weld melt it shrinks transversely and longitudinally and as a result tends to draw the details together resulting in buckling or other visual evidence of distortion. Other heated base metal areas adjacent to the weld though not at melting temperature expand and may distort. On cooling this distortion may reverse itself and return to its original configuration unless it's yield strength, which has been lowered by the elevated temperature, has been exceeded and the material permanently stretched. Clamping, cooling fixtures and weld techniques may be used to prevent distortion, however the restraint imposed by fixturing can produce residual stress which can result in time delayed distortion.