10 welding defects & remedies

IV.Welding defects & Remedies

WELDING DEFECTS AND REMEDIES

INTRODUCTION:

With the correct welding conditions, techniques and material quality standards, the arc welding process will yield a very high quality weld deposit. However, weld defects can occur. Most defects encountered in welding are due to an improper welding procedure. Once the causes are determined, the operator can easily correct the problem. Defects usually encountered include incomplete penetration, incomplete fusion, undercutting, porosity, and longitudinal cracking. This paper deals with the various welding defects, their causes and corrective action that should be taken.

What is the difference between Discontinuities and Welding‐defects? A discontinuity is an objective lack of material, an interruption in the physical consistence or uniform nature of a part. Examples are cracks, seams, laps, porosity or inclusions. It may or may not be considered a defect depending if its presence endangers or not the integrity, the usefulness and the serviceability of the structure.

A defect is a rejectable discontinuity, which occurs in an amount great enough to render a particular object or structure unsuitable for its intended service based on criteria in the applicable code.

A defect is always a discontinuity but a discontinuity need not necessarily be a defect.

Welding discontinuities / defects should be judged and interpreted by qualified and trained welding inspectors by visual examination or by other non destructive testing methods like radiography, ultrasonic testing, magnetic particle testing, liquid penetrant testing, eddy current testing, etc.

Classification of Defects:

The weld defects can be broadly classified into three types:

1. Planar defects / two dimensional defects: Planar defects such as cracks, lack of fusion, lack of penetration, are critical in nature and are not tolerated to any extent.

2. Voluminar defects / three dimensional defects: Voluminar defects such as slag inclusion, cavities, porosities, etc are tolerated to a certain extent depending on the product class and applicable code.

3. Geometric defects: Geometric defects such as excess reinforcement, underfill, root suckback, and distortion are also permitted to a certain extent. If they form sharp notches, they are smoothened out wherever accessible to avoid stress concentration.


TYPES OF WELD DEFECTS:

Incomplete Penetration

This type of defect is found in any of the three ways:

• When the weld bead does not penetrate the entire thickness of the base plate.

• When two opposing weld beads do not interpenetrate.

• When the weld bead does not penetrate the toe of a fillet weld but only bridges across it.

Welding current has the greatest effect on penetration. Incomplete penetration is usually caused by the use of too low a welding current and can be eliminated by simply increasing the amperage. Other causes can be the use of too slow a travel speed and an incorrect torch angle. Both will allow the molten weld metal to roll in front of the arc, acting as a cushion to prevent penetration. The arc must be kept on the leading edge of the weld puddle.

Figure 1 ‐ Examples of Lack of Penetration

Lack of fusion

Lack of fusion occurs when there is no fusion between the weld metal and the surfaces of the base plate. This defect can be seen in Figure 2. The most common cause of lack of fusion is a poor welding technique. Either the weld puddle is too large (travel speed too slow) and/or the weld metal has been permitted to roll in front of the arc. Again, the arc must be kept on the leading edge of the puddle. When this is done, the weld puddle will not get too large and cannot cushion the arc. Another cause is the use of a very wide weld joint. If the arc is directed down the center of the joint, the molten weld metal will only flow and cast against the side walls of the base plate without melting them. The heat of the arc must be used to melt the base plate. This is accomplished by making the joint narrower or by directing the arc towards the side wall of the base plate. When multipass welding thick material, a split bead technique should be used whenever possible after the root passes. Large weld beads bridging the entire gap must be avoided. Lack of fusion can also occur in the form of a rolled over bead crown. Again, it is generally caused by a very low travel speed and attempting to make too large a weld in a single pass. However, it is also very often


caused by too low welding voltage. As a result, the wetting of the bead will be poor. When welding aluminum, the common cause of this type of defect is the presence of aluminum oxide. This oxide is a refractory with a melting point of approximately 35000F (19270C). It is also insoluble in molten aluminum. If this oxide is present on the surfaces to be welded, fusion with the weld metal will be hampered.

The best safeguard against this is to remove all oxide as soon before welding as possible. Although iron oxide (rust, mill scale) can be welded over in mild steel, an excessive amount can cause lack of fusion.

Figure 2 – Example Lack of Fusion

Undercutting

As shown in Figure 3, undercutting is a defect that appears as a groove in the parent metal directly along the edges of the weld. It is most common in lap fillet welds, but can also be encountered in fillet and butt joints. This type of defect is most commonly caused by improper welding parameters; particularly the travel speed and arc voltage. When the travel speed is too high, the weld bead will be very peaked because of its extremely fast solidification. The forces of surface tension have drawn the molten metal along the edges of the weld bead and piled it up along the center. Melted portions of the base plate are affected in the same way. The undercut groove is where melted base material has been drawn into the weld and not allowed to wet back properly because of the rapid solidification. Decreasing the arc travel speed will gradually reduce the size of the undercut and eventually eliminate it. When only small or intermittent undercuts are present, raising the arc voltage and using a leading torch angle, are also corrective actions. In both cases, the weld bead will become flatter and wetting will improve.

Figure 3 – Examples of Undercutting

However, as the arc voltage is raised to excessive levels, undercutting may again appear. This is particularly true in spray arc welding. When the arc becomes very long, it also becomes too wide. This


results in an increased amount of base material being melted. However, the heat transfer of a long arc is relatively poor, so actually the arc is supplying no more total heat to the weld zone. The outermost areas are very quickly cooled and again proper wetting is prevented. The arc length should be kept short, not only to avoid undercutting but to increase penetration and weld soundness. Excessive welding currents can also cause undercutting. The arc force, arc heat and penetration are so great that the base plate under the arc is actually”blown” away. Again, the outermost areas of the base material are melted but solidify quickly. Puddle turbulence and surface tension prevent the puddle from wetting properly. It is always advisable to remain within the current ranges specified for each wire size.

Porosity

Porosity is gas pores found in the solidified weld bead. As seen in Figure 4, these pores may vary in size and are generally distributed in a random manner. However, it is possible that porosity can only be found at the weld center. Pores can occur either under or on the weld surface. The most common causes of porosity are atmosphere contamination, excessively oxidized work piece surfaces, inadequate deoxidizing alloys in the wire and the presence of foreign matter. Atmospheric contamination can be caused by: 1) Inadequate shielding gas flow. 2) Excessive shielding gas flow. This can cause aspiration of air into the gas stream. 3) Severely clogged gas nozzle or damaged gas supply system (leaking hoses, fittings, etc.) 4) an excessive wind in the welding area. This can blow away the gas shield.

Figure 4 – Examples of Porosity

The atmospheric gases that are primarily responsible for porosity in steel are nitrogen and excessive oxygen. However, considerable oxygen can be tolerated without porosity in the absence of nitrogen. Oxygen in the atmosphere can cause severe problems with aluminum because of its rapid oxide formation. The gas supply should be inspected at regular intervals to insure freedom from leakage. In addition, excessive moisture in the atmosphere can cause porosity in steel and particularly aluminum. Care should be exercised in humid climates. For example, a continuous coolant flow in water cooled torches can cause condensation during periods of high humidity and consequent contamination of the shielding gas. Excessive oxidation of the work pieces is an obvious source of oxygen as well as entrapped moisture. Again, this is particularly true for aluminum where a hydrated oxide may exist. Anodized coatings on aluminum must be removed prior to welding because they contain water as well as being an insulator. Porosity can be caused by inadequate wire de‐oxidation when welding semi‐killed or rimmed


steels. The oxygen in the steel can cause CO porosity if the proper deoxidizing elements are not present. Foreign matter can be a source of porosity. An example is excessive lubricant on the welding wire. These hydrocarbons are sources of hydrogen, which is particularly harmful for aluminum. Other causes of porosity may be extremely fast weld solidification rates and erratic arc characteristics. When solidification rates are extremely rapid, any gas that would normally escape is trapped. Extremely high travel speeds and low welding current levels should be avoided. Erratic arc can be caused by poor welding conditions (voltage too low or high, poor metal transfer) and fluctuation in the wire feed speed. All these occurrences cause severe weld puddle turbulence. This turbulence will tend to break up the shielding gas envelope and cause the molten weld metal to be contaminated by the atmosphere.

Longitudinal cracking

Longitudinal or centerline cracking, of the weld bead is not often encountered in MIG welding. However, that which does occur can be one of two types: hot cracks and cold cracks. Typical hot cracks are shown in Figure 5. Hot cracks are those that occur while the weld bead is between the Liquidus (melting) and Solidus (solidifying) temperatures. In this temperature range the weld bead is”mushy”. Hot cracks usually result from the use of an incorrect wire electrode (particularly in aluminum and stainless steel alloys). The chemistry of the base plate can also promote this defect (an example would be any high carbon stainless steel casting). Any combination of the joint design, welding conditions and welding techniques that results in a weld bead with an excessively concave surface can promote cracking. One form of this defect, which may often be encountered, particularly with any 5000 series aluminum, is called a crater crack. These are small cracks, which appear, at the end of the weld where the arc has been broken. Although small, these cracks are troublesome since they can propagate into the weld bead. A crater crack is shown in Figure 6. The major reason for this defect is the incorrect technique for ending the weld. To properly end a weld, the crater should be filled. This is done by reversing the arc travel direction before breaking the arc. This technique is depicted in Figure 7. In addition, if the welding control is designed to supply gas for a short time after the arc is broken, the crater should be shielded until it is completely solidified.

Figure 5 –Longitudinal Cracking Figure 6 ‐ Example of Crater Cracking

Those cracks that occur after the weld bead has completely solidified are called cold cracks. These defects occur only when the weld is too small to withstand the service stresses involved.


Figure 7 – Crater Filling Technique

Slag inclusions

These can occur when several runs are made along a V groove when joining thick plates using flux cored or flux coated rods and the slag covering a run is not totally removed after every run before the following run.These are formed due to entrapment of oxides and non‐metallic solid material in the weld deposit or between the weld metal and base metal. Slag inclusions as shown in figure 8 usually appear as a linear continuous or interrupted band

.

Proper preparation of the groove before depositing further layers, preheating the base metal to have control on the rate of solidification and hence release of slag from molten metal, avoiding larger size electrodes for root pass welding, thus preventing the slag flowing down into the root opening, are some of the ways of preventing slag inclusion.

Tungsten inclusions

These are particles deposited in the weld metal from a tungsten electrode in TIG process, by the occasional touching of the electrode to the job of molten metal. All tungsten inclusions are not generally considered harmful unless their size and number becomes excessive. Replacement of electrodes at the correct time will ensure the removal of this defect. In Radiography they are revealed as white spots due to higher density of tungsten compared to the weld metal. To avoid this defect thoriated or zirconiated tungsten electrodes are used in place of pure tungsten electrodes.

Figure 8 ‐ Radiograph of a butt weld showing two slag lines in the weld root


Figure 9: Tungsten Inclusions

Burn through

It is the icicles or graphs found at the root of the weld. Sometimes it creates an opening at the root. This occurs because of the usage of very high current and low travel speed. Use of consumable inserts restricts burn through.

Figure 10 – Burn through

ARC STRIKES

Arc strikes result when the arc is initiated on the base metal surface away from the weld joint either intentionally or accidentally. They represent any localized HAZ caused by an arc. By giving proper instructions to the welder and by good housekeeping this defect can be avoided. By dressing up the area, the ill effects are removed.

SPATTER

Spatters are metal particles expelled during fusion welding that do not form a part of the weld. These are mostly attached to the base metal adjacent to the weld. Spatters may mask the defects and are to be removed by grinding or chipping. Spatters occur due to high welding currents which can cause excessive


turbulence in the weld zone.The use of argon mixtures will reduce the amount of spatter compared to the amount produced when straight CO2 shielding gas is used.

Figure 11 – Spatter

Mismatch

Mismatch is a term associated with a condition where two pieces being welded together are not properly aligned. The radiographic image shows a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by the failure of the weld metal to fuse with the land area.

Figure 12 – Mismatch

For quick reference, Table1 lists all possible welding defects, their causes and corrective action

FAULT OR DEFECT CAUSE AND / OR CORRECTIVE ACTION

1) POROSITY A. Oil, heavy rust, scale, etc. on plateB. Wire – may need higher in Mn & Si C. Shielding problem; wind, clogged or small nozzle, damaged


gas hose, excessive gas flow, etc.D. Welding over slag from covered electrode

LACK OF PENETRATION A. Weld joint too narrowB. Welding current too low C. Electrode stick out weld puddle rolling in front of the arc

LACK OF FUSION A. Welding voltage and/or current too low B. Wrong polarity, should be DCRP C. Travel speed too low D. Welding over convex bead E. Torch oscillation too wide or too narrow F. Excessive oxide on plate

UNDERCUTTING A. Travel speed too highB. Welding voltage too high C. Excessive welding currents D. Insufficient dwell at edge of weld bead

CRACKING A. Incorrect wire chemistryB. Weld bead too small C. Poor quality of material being welded

UNSTABLE ARC A. Check gas shielding.B. Check wire feed system

POOR WELD STARTS OR WIRE STUBBING

A. Welding voltage too lowB. Inductance or slope too high C. Wire extension too long D. Clean glass or oxide from plate

EXCESSIVE SPATTER A. Use Ar‐CO2 or Ar‐O2 instead of CO2

B. Arc voltage too low C. Raise inductance and/or slope

BURNTHROUGH A. Welding current too highB. Travel speed too low C. Decrease width of root opening D. Use Ar‐CO2 or Ar‐O2 instead of CO2

CONVEX BEAD A. Welding voltage and/or current too low B. Excessive electrode extension C. Increase inductance D. Wrong polarity, should be DCRP E. Weld joint too narrow

Welding distortion

Welding distortion or deformation or warping of weldment during welding is a natural outcome of intrinsic non‐uniform heating and cooling of the joint. Distortion is the result of the action of internal stresses, which are produced while welding, and remain in the part after heating is removed. It is thus undesirable change in original shape due to high heat input and mechanical forces.

Distortion if not controlled within the permissible specified a limit becomes a geometric defect.


Stresses are due to volume changes with heating and to decreasing yield strength at elevated temperature. Metal subject to thermal expansion while heating tends to be compressed by the surrounding cool structure. The heated volume has now lower yield strength at high temperature, and then it is easily upset to shorter dimensions. Upon cooling the same material tends to contract in all directions and is now stressed in tension by the attached cool structure, which did not move appreciably in the process. By now the yield strength is again higher, at lower temperature; so that the upset material cannot regain its original dimensions.The result is the development of internal tension stresses in the weld. These residual stresses are the cause of deformation.

Distortion of Arc‐welded components is generally caused by two factors: shrinkage of the cooling weld metal and local expansion and contraction of the plate. Longitudinal shrinkage shortens the weld, transverse shrinkage decreases the width, and angular distortion causes rotation of the plates. Apart from these simple effects of shrinkage, longitudinal contraction of a weld may cause components to bow in a direction depending on the location of the weld in relation to the neutral axis of the component. The middle of a length of weld will bow towards the neutral axis.

Some values for shrinkage quoted in TWI's booklet CONTROL OF DISTORTION IN WELDED FABRICATIONS (available from Woodhead Publishing) are as follows:

Transverse shrinkage Fillet welds: 0.8mm per weld where the leg length of the weld does not exceed 0.75 x the plate thickness. Butt welds: 1.5 ‐ 3mm per weld for 60° V joints, depending on the number of runs per weld.

Longitudinal shrinkage

• Fillet Welds: 0.8mm/3m of weld

• Butt Welds: 3mm/3m of weld. The above allowances for distortion apply to welded joints that are free to move; in practice, the restraint built up during the fabrication will determine the distortion.

How to decrease Distortion?

Sometimes to decrease the amount of deformation it is sufficient to place the elements at an angle before welding in order to counter the movement by a certain opposite displacement to be introduced before welding Or (for butt welds) to weld a short length at one end and then start again from the other end, while the first end is already rigidly welded. Otherwise one can distribute short stretches of welding at distant places, by introducing sequences aimed at avoiding local concentration of heat input.

One can provide very rigid fixturing that will not allow any movement while welding is performed. Movement can be prevented: residual stresses cannot. One can easily prove that the part is now highly stressed (presenting a high level of residual stresses).

After taking the part out from the fixture if one removes some of the weld by partial asymmetric cutting or grinding, the remaining portion will deform considerably to rearrange the remaining internal stresses.


This deformation is an indirect proof of the high level of stresses which were present in the as welded part. Material removal is at the base of certain methods of residual stresses measurement. Occasionally when the residual stresses exceed material strength, cracks may appear in the weld at high temperature or while cooling near room temperature.

It is good practice to relieve the residual stresses of a constrained welded assembly. The reason is that these stresses can sum up with external stresses in service and exceed the material strength, producing failure, or further deformations.

The most common method of stress relieving is performed by heating the welded assembly in a suitable furnace. By heating the welded structure uniformly in a furnace at elevated temperature, the remaining maximum stress will be reduced to the lower yield strength value which the material exhibits at that temperature.

Another method uses mechanical peening of the weld either by hammering or by shot peening equipment which is done sometimes on tool steels immediately after shielded metal arc welding and before cool down

The general approach would be to reduce the causes of Welding‐distortion by providing suitable pre‐heating, if possible, so that there will be less expansion difference, between material at weld temperature and surrounding structure.

Rectification of distortion is possible by the use of mechanical force or judiciously applied heating, but the cost of correction is generally at least ten times that of making the job to the required dimensional tolerances in the first place.


SUMMARY

Imperfections may exist in the weld and / or base metal; they are generally described as discontinuities. If a certain discontinuity is of sufficient size, it may render a structure unfit for its intended service. Codes dictate the permissible limits for discontinuities. Those with values greater than these limits are termed defects. Defects are discontinuities, which require some corrective action. Judgment of defects should be done by properly trained, experienced welding inspectors. By knowing how these defects can form, the welding inspector may be successful at spotting the causes and prevent them from occurring.

Rules for minimizing distortion during welding

1. Design fabrications so that welds are balanced on each side of the neutral axis.

11. Use high speed welding processes where possible, e.g. iron powder MMA electrodes, MIG welding or Mechanised welding.

2. Do not over specify fillet weld sizes. 12. Use frequent tacking.

3. Use double sided welds rather than single sided and minimum bevel angles, to reduce the amount of weld metal.

13. Balance welding on each side of the neutral axis, i.e. do not weld all one side before starting the other.

4. Use minimum gap sizes. 14. Weld fabrications clamped back to back and preset if possible; alternatively stress relieve before releasing from the clamps.

5. In non‐fatigue sensitive areas use intermittent fillet welds where possible.

15. Use block welding to prevent movement.

6. Use double fillet welds where possible, rather than full penetration T butt welds.

16. When block welding thick plate, butter the sides of the preparation and build up the buttering progressively towards the centre of the joint, so that most of the joint can contract transversely before the joint is bridged.

7. Use clamps strong backs, jigs or fixtures.

17. Weld first the joints that cause the most contraction.

8. Use welding petitioners so that welding can be carried out in the flat or horizontal‐vertical positions with high deposition rates.

18. Make use of sub‐assemblies.

9. Deposit a few weld runs alternately on each side of the joint in double V butt welds.

19. Make frequent dimensional checks during welding, and if distortion is evident change the welding sequence or the clamping arrangements accordingly.

10. Weld a large construction from the center outwards.

20. Use proper welding sequence with appropriate Interpass temperature.

10 welding defects & remedies

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