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Underwater Services Company Lome, Piraeus, Globally BEVALDIA

WELDING FAULTS BY NON – CERTIFIED DIVERS

Source: http://www.mpigroup.co.uk

BY Brendan Fitzsimons  and Trevor Parry co-authors of a chapter entitled “Paint and Coating Failures and Defects” in the new edition of “Corrosion by L Shreir” ON DECEMBER, 2009

uw welding

Welds

Where a protective coating system is to be applied over a weld, the application of a well brushed-in stripe coat to the prepared surface is strongly recommended to ensure adequate protection and to prevent bridging by the coating system. In addition, any welding faults of the types described below should be appropriately rectified before any coating is applied.

Welding Faults

The following section describes various faults in fusion welds in construction steels. This is not a comprehensive compilation of all likely welding faults, only those which can present problems with the application of coatings unless the faults are rectified prior to the appropriate surface preparation and application of the protective coating system.

All comments are for guidance only.

Cracks

Description: Crack visible at the surface.

Probable Causes: Excessively deep or wide bead. High current and/or welding speed. Large root gap.

Repair: Cut out defective weld length plus 5 mm beyond visible end of crack and reweld.

Comment: Protective coatings can mask cracks but rarely successfully bridge cracks. Unless rectified prior to the application of the coating, such cracks can result in premature failure of the coating system, irrespective of any danger that might be caused to the structure by their presence.

Crater Pipes

Description: Crater resulting from shrinkage at the end of a weld run.

Probable Causes: Incorrect manipulative technique or current decay to allow for crater shrinkage.

Repair: All such porosities must be appropriately filled prior to application of a protective coating system.

Comment: Coatings rarely bridge pores in a metal substrate and if they do, this can lead to premature coating failure.

Excess Penetration Bead

Description: Weld metal normally extends below the parent metal surface but this is only a problem when excessive and exceeds the specification.

Probable Causes: Incorrect edge preparation providing insufficient support at the root, and/or incorrect welding parameters.

Repair: Where sharp changes in contour occur these need to be smoothed to prevent bridging of the protective coating system.

Comment: Thorough brushing in of a stripe coat can be effective but some form of filling may be necessary to prevent bridging of the coating. Any undercuts must be appropriately treated to prevent bridging by the coating system

Root Concavity

Description: Shrinkage at the weld root resulting in a deep pit along the weld.

Probable Causes: Shrinkage of molten pool at the weld root, resulting from incorrect weld preparation or insufficient heat input. Also from incorrect welding technique.

Repair: Ensure the edges are smooth with no sharp changes in contour.

Comment: All sharp edges must be ground smooth in accordance with the specification, prior to surface preparation and application of the coating system.

Spatter

Description: Globules of metal which adhere to the metal surface, often some distance from the weld.

Probable Causes: Incorrect welding conditions, such as too high a welding current and/or contaminated consumables or preparations giving rise to explosions within the arc and weld pool.

Repair: Grind or needle gun off all globules to leave a smooth surface, prior to full surface preparation.

Comment: Weld spatter can rarely be successfully coated and can result in premature coating breakdown.

Stray Arcing

Description: Small hard spots, craters or rough surface features.

Probable Causes: Accidental contact of electrode or welding torch with plate surface remote from the weld.

Repair: Grind smooth, blast clean or use other appropriate treatment to ensure the surface is sufficiently prepared for adequate adhesion of the protective coating.

Comment: The application of a protective coating to rough, insufficiently prepared areas can lead to premature coating failure.

Surface Porosity

Description: Pores visible at the surface of a weld.

Probable Causes: Excessive contamination from grease, dampness or atmospheric entrainment. Occasionally caused by excessive sulphur in consumables or parent metal.

Repair: All such porosities must be appropriately filled prior to application of a protective coating system.

Comment: Coatings rarely bridge pores in a metal substrate and if they do, this can lead to premature coating failure.

Undercut

Description: Undercut cavity which results from washing away of the edge preparation when molten.

Probable Causes: Poor welding technique and/or unbalanced welding conditions.

Repair: All such undercuts must be appropriately prepared, rewelded or filled prior to application of the coating system.

Comment: Coatings rarely bridge undercuts in a metal substrate and if they do, this can lead to premature coating failure.

 

WELDING FAULTS BY NON – CERTIFIED DIVERS

Source: http://www.tech.plym.ac.uk

BY David J Grieve ON SEPTEMBER 18, 2003

Welding Defects

  1. Introduction

Common weld defects include:

  • i. Lack of fusion
  • ii. Lack of penetration or excess penetration
  • iii. Porosity
  • iv. Inclusions
  • v. Cracking
  • vi. Undercut
  • vii. Lamellar tearing

Any of these defects are potentially disastorous 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.

  1. Types of Defects
    i 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.
  2. 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.
  3. 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.
  4. Cracking – This can occur due just to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage.
    In the case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking.
    Where alloy steels or steels with a carbon content greater than about 0.2% are being welded, self cooling may be rapid enough to cause some (brittle) martensite to form. This will easily develop cracks.
    To prevent these problems a process of pre-heating in stages may be needed and after welding a slow controlled post cooling in stages will be required. This can greatly increase the cost of welded joins, but for high strength steels, such as those used in petrochemical plant and piping, there may well be no alternative.

Solidification Cracking
This is also called centreline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulphur and phosphorus, are more likely to occur in higher carbon steels.
Solidification cracks are normally distinguishable from other types of cracks by the following features:

  • they occur only in the weld metal – although the parent metal is almost always the source of the low melting point contaminants associated with the cracking
  • they normally appear in straight lines along the centreline of the weld bead, but may occasionally appear as transverse cracking
  • solidification cracks in the final crater may have a branching appearance
  • as the cracks are ‘open’ they are visible to the naked eye

On breaking open the weld the crack surface may have a blue appearance, showing the cracks formed while the metal was still hot. The cracks form at the solidification boundaries and are characteristically inter dendritic. There may be evidence of segregation associated with the solidification boundary.
The main cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:

  • insufficient weld bead size or inappropriate shape
  • welding under excessive restraint
  • material properties – such as a high impurity content or a relatively large shrinkage on solidification

Joint design can have an influence on the level of residual stresses. Large gaps between components will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Hence weld beads with a small depth to width ratio, such as is formed when bridging a large wide gap with a thin bead, will be more susceptible to solidification cracking.

In steels, cracking is associated with impurities, particularly sulphur and phosphorus and is promoted by carbon, whereas manganese and sulphur can help to reduce the risk. To minimize the risk of cracking, fillers with low carbon and impurity levels and relatively high manganese content are preferred. As a general rule, for carbon manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%. However when welding a highly restrained joint using high strength steels, a combined level below 0.03% might be needed.

Weld metal composition is dominated by the filler and as this is usually cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Parent metal composition becomes more important with autogenous welding techniques, such as TIG with no filler.

Avoiding Solidification Cracking
Apart from choice of material and filler, the main techniques for avoiding solidification cracking are:

  • control the joint fit up to reduce the gaps
  • clean off all contaminants before welding
  • ensure that the welding sequence will not lead to a buildup of thermally induced stresses
  • choose welding parameters to produce a weld bead with adequate depth to width ratio or with sufficient throat thickness (fillet weld) to ensure the bead has sufficient resistance to solidificatiuon stresses. Recommended minimum depth to width ratio is 0.5:1
  • avoid producing too large a depth to width ratio which will encourage segregation and excessive transverse strains. As a rule, weld beads with a depth to width ratio exceeds 2:1 will be prone to solidification cracking
  • avoid high welding speeds (at high current levels) which increase segregation and stress levels across the weld bead
  • at the run stop, ensure adequate filling of the crater to avoid an unfavorable concave shape

Hydrogen induced cracking (HIC) – also referred to as hydrogen cracking or hydrogen assisted cracking, can occur in steels during manufacture, during fabrication or during service. When HIC occurs as a result of welding, the cracks are in the heat affected zone (HAZ) or in the weld metal itself.

Four requirements for HIC to occur are:

  • a) Hydrogen be present, this may come from moisture in any flux or from other sources. It is absorbed by the weld pool and diffuses into the HAZ.
  • b) A HAZ microstructure susceptible to hydrogen cracking.
  • c) Tensile stresses act on the weld
  • d) The assembly has cooled to close to ambient – less than 150oC

HIC in the HAZ is often at the weld toe, but can be under the weld bead or at the weld root. In fillet welds cracks are normally parallel to the weld run but in butt welds cracks can be transverse to the welding direction.

vi Undercutting – In this case the thickness of one (or both) of the sheets is reduced at the toe of the weld. This is due to incorrect settings / procedure. There is already a stress concentration at the toe of the weld and any undercut will reduce the strength of the join.

vii Lamellar tearing – This is mainly a problem with low quality steels. It occurs in plate that has a low ductility in the through thickness direction, which is caused by nonmetallic inclusions, such as suphides and oxides that have been elongated during the rolling process. These inclusions mean that the plate cannot tolerate the contraction stresses in the short transverse direction.
Lamellar tearing can occur in both fillet and butt welds, but the most vulnerable joints are ‘T’ and corner joints, where the fusion boundary is parallel to the rolling plane.
These problems can be overcome by using better quality steel, ‘buttering’ the weld area with a ductile material and possibly by redesigning the joint.

  1. Detection
    Visual Inspection
    Prior to any welding, the materials should be visually inspected to see that they are clean, aligned correctly, machine settings, filler selection checked, etc.
    As a first stage of inspection of all completed welds, visual inspected under good lighting should be carried out. A magnifying glass and straight edge may be used as a part of this process.

Undercutting can be detected with the naked eye and (provided there is access to the reverse side) excess penetration can often be visually detected.

Liquid Penetrant Inspection
Serious cases of surface cracking can be detected by the naked eye but for most cases some type of aid is needed and the use of dye penetrant methods are quite efficient when used by a trained operator.
This procedure is as follows:

  • Clean the surface of the weld and the weld vicinity
  • Spray the surface with a liquid dye that has good penetrating properties
  • Carefully wipe all the die off the surface
  • Spray the surface with a white powder
  • Any cracks will have trapped some die which will weep out and discolour the white coating and be clearly visible

X – Ray Inspection
Sub-surface cracks and inclusions can be detected ‘X’ ray examination. This is expensive, but for safety critical joints – eg in submarines and nuclear power plants – 100% ‘X’ ray examination of welded joints will normally be carried out.

Ultrasonic Inspection
Surface and sub-surface defects can also be detected by ultrasonic inspection. This involves directing a high frequency sound beam through the base metal and weld on a predictable path. When the beam strikes a discontinuity some of it is reflected beck. This reflected beam is received and amplified and processed and from the time delay, the location of a flaw estimated.
Porosity, however, in the form of numerous gas bubbles causes a lot of low amplitude reflections which are difficult to separate from the background noise.
Results from any ultrasonic inspection require skilled interpretation.

Magnetic Particle Inspection
This process can be used to detect surface and slightly sub-surface cracks in ferro-magnetic materials (it can not therefore be used with austenitic stainless steels).
The process involves placing a probe on each side of the area to be inspected and passing a high current between them. This produces a magnetic flux at right angles to the flow of the current. When these lines of force meet a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more than elsewhere, indicating the location of any discontinuities.

welding faults
This process may be carried out wet or dry, the wet process is more sensitive as finer particles may be used which can detect very small defects. Fluorescent powders can also be used to enhance sensitivity when used in conjunction with ultra violet illumination.

  1. Repair
    Any detected cracks must be ground out and the area re-welded to give the required profile and then the joint must be inspected again.