Training Programme for Coe Instructors

D N SHARMA – TRAINING OFFICER

Training Programme for CoE Instructors -

FOREMEN TRAINING INSTITUTE, DGE & T , Ministry of Labour, Govt. of India, Bangalore

HAND BOOK ON WELD DEFECTS

Any process results in some deviations in the final product normally due to several variations in the process parameters-minor or major. These deviations results in discontinuities in the final product / service. When these discontinuities are not affecting the intended application / sections, then these discontinuities are acceptable. If they are unacceptable, then these discontinuities are treated as defects. These defects can be repaired; there by the product can be made fit to the intended job. Then it is said that the part is salvaged. Sometimes if the defect is to severe the part can be rejected out rightly.

Welding process is no exception. Hence discontinuities / defects do arise during welding. In this booklet, various weld defects and their origin is described at length. As the presence of imperfections in a welded joint may not render the component defective in the sense of being unsuitable for the intended application, the preferred term is imperfection rather than defect. For this reason, production quality for a component is defined in terms of a quality level in which the limits for the imperfections are clearly defined, for example Level B, C or D in accordance with the requirements of BS EN ISO 5817. For the American standards ASME X1 and AWS D1.1, the acceptance levels are contained in the standards.

The application code will specify the quality levels which must be achieved for the various joints.

Imperfections can be broadly classified into those produced on fabrication of the component or structure and those formed as result of adverse conditions during service. The principal types of imperfections are:

Fabrication:

• lack of fusion
• cracks
• porosity
• inclusions
• incorrect weld shape and size

Service:

• brittle fracture
• stress corrosion cracking
• fatigue failure

Welding procedure and welder technique will have a direct effect on fabrication imperfections. Incorrect procedure or poor technique may produce imperfections leading to premature failure in service.

Welding Codes are documents establishing legal obligations spelled out through laws and rules to be complied with whenever engaging in the manufacture of certain constructions regulated by the appointed Authority. In other words Welding-codes are laws covering minimum mandatory requirements essential to guarantee public safety and reliability of large structures.

Some of the regulated items, object of law enforcement by means of Welding-codes, are grouped in two types:

1. for pressure containment: Unfired Pressure Vessels, Power Boilers, Pipelines, etc.
2. for structures: Bridges, High Rise Structures, Ships, Lifts and lifting equipment etc.

Welding-codes describe the types of construction they are concerned with.

Welding Specifications cover requirements for voluntary adoption and no enforcement is meant unless they are agreed upon between procuring agency and contractor as spelled out in a Purchase Order. One could say that Welding-codes are essentially enforced Specifications.

Welding Standards cover industry agreed upon conventions like definitions of terms and of symbols as used in the industry for conveying information.

Welding Recommended Practices reflect the recommendations of some of the most knowledgeable professionals in the field with the purpose of helping people improve the practical implementation of different techniques and procedures and obtain best results.

One should always inquire which Authority is in charge of safety in the particular place, and which are the latest binding Welding-codes applicable to the construction considered. The contractor should understand Welding-codes as they represent legal obligations between the manufacturer and the purchaser (or owner).

Welding-codes meeting is needed for Quality implementation.

When built into the product, Quality is not an expenseit is a powerful asset contributing to the economic success of any enterprise.

Can you assess the Quality of your welded Production?

Implementing Quality is NOT scrapping defective parts!

Of the three essential parameters for measuring the success of an industrial operation

• Price,
• On-time-Delivery and
• Quality

The last one is possibly the most important.

Welding Defects

1. Introduction

Common weld defects include:

i. Lack of fusion (LOF)

ii. Lack of penetration (LOP) or excess penetration

iii. Porosity

iv. Inclusions

v. Cracking

vi. Undercut

vii. Lamellar tearing

Any of these defects are potentially disastrous as they can all give rise to high stress intensities which may result in sudden unexpected failure below the design load or in the case of cyclic loading, failure after fewer load cycles than predicted.

2. Types of Defectsi and ii. - To achieve a good quality join it is essential that the fusion zone extends the full thickness of the sheets being joined. Thin sheet material can be joined with a single pass and a clean square edge will be a satisfactory basis for a join. However thicker material will normally need edges cut at a V angle and may need several passes to fill the V with weld metal. Where both sides are accessible one or more passes may be made along the reverse side to ensure the joint extends the full thickness of the metal.
Lack of fusion results from too little heat input and / or too rapid traverse of the welding torch (gas or electric).Excess penetration arises from to high a heat input and / or too slow transverse of the welding torch (gas or electric). Excess penetration - burning through - is more of a problem with thin sheet as a higher level of skill is needed to balance heat input and torch traverse when welding thin metal.

Best practice in prevention

The following techniques can be used to prevent lack of root fusion:

• In TIG welding, do not use too large a root face and ensure the welding current is sufficient for the weld pool to penetrate fully the root
• In MMA welding, use the correct current level and not too large an electrode size for the root
• In MIG welding, use a sufficiently high welding current level but adjust the arc voltage to keep a short arc length
• When using a joint configuration with a joint gap, make sure it is of adequate size and does not close up during welding
• Do not use too high a current level causing the weld pool to bridge the gap without fully penetrating the root.

Acceptance standards

The limits for lack of penetration are specified in BS EN ISO 5817 for the three quality levels.

Lack of root penetration is not permitted for Quality Level B (stringent) and Level C (intermediate). For Quality Level (moderate) short lack of penetration imperfections are permitted.

Incomplete root penetration is not permitted in the manufacture of pressure vessels but is allowable in the manufacture of pipework depending on material and wall thickness.

Remedial actions

If the root cannot be directly inspected, for example using a penetrant or magnetic particle inspection technique, detection is by radiography or ultrasonic inspection. Remedial action will normally require removal by gouging or grinding to sound metal, followed by re-welding in conformity with the original procedure.

Relevant standards

BS EN ISO 5817:2003 Welding - fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) - Quality levels for imperfections.

BS EN ISO 10042:2005 Welding - Arc welded joints in aluminium and its alloys - Quality levels for imperfections.

Magnetic arc blow

When welding ferromagnetic steels lack of fusion imperfections can be caused through uncontrolled deflection of the arc, usually termed arc blow. Arc deflection can be caused by distortion of the magnetic field produced by the arc current through:

• residual magnetism in the material through using magnets for handling
• earth's magnetic field, for example in pipeline welding
• position of the current return

The effect of welding past the current return cable which is bolted to the centre of the place. The interaction of the magnetic field surrounding the arc and that generated by the current flow in the plate to the current return cable is sufficient to deflect the weld bead. Distortion of the arc current magnetic field can be minimised by positioning the current return so that welding is always towards or away from the clamp and, in MMA welding, by using AC instead of DC. Often the only effective means is to demagnetise the steel before welding.

Best practice in prevention

The following fabrication techniques can be used to prevent formation of lack of sidewall fusion imperfections:

• use a sufficiently wide joint preparation
• select welding parameters (high current level, short arc length, not too high a welding speed) to promote penetration into the joint side wall without causing flooding
• ensure the electrode/gun angle and manipulation technique will give adequate side wall fusion
• use weaving and dwell to improve side wall fusion providing there are no heat input restrictions
• if arc blow occurs, reposition the current return, use AC (in MMA welding) or demagnetise the steel

Acceptance standards

The limits for incomplete fusion imperfections in arc welded joints in steel are specified in BS EN ISO 5817 for the three quality levels (see Table). These types of imperfection are not permitted for Quality Level B (stringent) and C (intermediate). For Quality level D (moderate) they are only permitted providing they are intermittent and not surface breaking.

For arc welded joints in aluminium, long imperfections are not permitted for all three quality levels. However, for quality levels C and D, short imperfections are permitted but the total length of the imperfections is limited depending on the butt weld or the fillet weld throat thickness.

Acceptance limits for specific codes and application standards

Detection and remedial action

If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub-surface imperfections, detection is by radiography or ultrasonic inspection. Ultrasonic inspection is normally more effective than radiography in detecting lack of inter-run fusion imperfections.

Remedial action will normally require their removal by localised gouging, or grinding, followed by re-welding as specified in the agreed procedure.

If lack of fusion is a persistent problem, and is not caused by magnetic arc blow, the welding procedures should be amended or the welders retrained

ii. Porosity - This occurs when gases are trapped in the solidifying weld metal. These may arise from damp consumables or metal or, from dirt, particularly oil or grease, on the metal in the vicinity of the weld. This can be avoided by ensuring all consumables are stored in dry conditions and work is carefully cleaned and degreased prior to welding.

porosity

The characteristic features and principal causes of porosity imperfections are described. Best practice guidelines are given so welders can minimise porosity risk during fabrication.

Identification

Porosity is the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can take several forms:

• distributed
• surface breaking pores
• wormhole
• crater pipes

Cause and prevention

Distributed porosity and surface pores

Distributed porosity is normally found as fine pores throughout the weld bead. Surface breaking pores usually indicate a large amount of distributed porosity

Fig. Uniformly distributed porosity

Fig. Surface breaking pores (T fillet weld in primed plate)

Cause
Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal.

Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.

Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.

Surface coatings like primer paints and surface treatments such as zinc coatings, may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides Special mention should be made of the so-called weldable (low zinc) primers. It should not be necessary to remove the primers but if the primer thickness exceeds the manufacturer's recommendation, porosity is likely to result especially when using welding processes other than MMA.

Prevention

The gas source should be identified and removed as follows:

Air entrainment

- seal any air leak

- avoid weld pool turbulence

- use filler with adequate level of deoxidants

- reduce excessively high gas flow

- avoid draughts

Hydrogen

- dry the electrode and flux

- clean and degrease the workpiece surface

Surface coatings

- clean the joint edges immediately before welding

- check that the weldable primer is below the recommended maximum thickness

Wormholes

Elongated pores or wormholes

Characteristically, wormholes are elongated pores which produce a herring bone appearance on the radiograph.

Cause
Wormholes are indicative of a large amount of gas being formed which is then trapped in the solidifying weld metal. Excessive gas will be formed from gross surface contamination or very thick paint or primer coatings. Entrapment is more likely in crevices such as the gap beneath the vertical member of a horizontal-vertical, T joint which is fillet welded on both sides.

When welding T joints in primed plates it is essential that the coating thickness on the edge of the vertical member is not above the manufacturer's recommended maximum, typically 20µm, through over-spraying.

Prevention

Eliminating the gas and cavities prevents wormholes.

Gas generation

- clean the workpiece surfaces

- remove any coatings from the joint area

- check the primer thickness is below the manufacturer's maximum

Joint geometry

- avoid a joint geometry which creates a cavity

Crater pipe

A crater pipe forms during the final solidified weld pool and is often associated with some gas porosity.

Cause
This imperfection results from shrinkage on weld pool solidification. Consequently, conditions which exaggerate the liquid to solid volume change will promote its formation. Switching off the welding current will result in the rapid solidification of a large weld pool.

In TIG welding, autogenous techniques, or stopping the wire before switching off the welding current, will cause crater formation and the pipe imperfection.

Prevention

Crater pipe imperfection can be prevented by removing the stop or by welder technique.

Removal of stop

- use run-off tag in butt joints

- grind out the stop before continuing with the next electrode or depositing the subsequent weld run

Welder technique

- progressively reduce the welding current to reduce the weld pool size

- add filler (TIG) to compensate for the weld pool shrinkage

Porosity susceptibility of materials

Gases likely to cause porosity in the commonly used range of materials are listed in the Table.

Principal gases causing porosity and recommended cleaning methods

Detection and remedial action

If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub surface imperfections, detection is by radiography or ultrasonic inspection. Radiography is normally more effective in detecting and characterising porosity imperfections. However, detection of small pores is difficult especially in thick sections.

Remedial action normally needs removal by localised gouging or grinding but if the porosity is widespread, the entire weld should be removed. The joint should be re-prepared and re-welded as specified in the agreed procedure.

iv. Inclusions - These can occur when several runs are made along a V join when joining thick plate using flux cored or flux coated rods and the slag covering a run is not totally removed after every run before the following run.

As slag is the residue of the flux coating in MMA welding, it is principally a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs.