A very poor weld toe blend angle
An improved weld toe blend angle
The excess weld metal height is within limits but the toe blend angle is unacceptable
Generally specifications tend to state that “The weld toes shall blend smoothly” This statement can cause many problems as it is not a quantitative instruction, and therefore very much open to individual interpretation. To help in the assessment of the acceptance of the toe blend it should be noted that the higher the angle at the toe then the higher is the concentration of stresses. When the toe angle reaches 30° - 40° the stress concentration ratio at the weld toe becomes > 2:1
A poor toe blend will always be present when the excess weld metal height is excessive or the weld profile is excessively bulbous, however it may be possible that the height is within the given limits, yet the toe blend is not smooth, and is therefore a defect, and unacceptable. It should also be remembered, that a poor toe blend in the root of the weld has the same effect. It can be clearly seen that any rapid change in the section will induce stress concentration and therefore the use of the term reinforcement to describe any amount of excess weld metal is very misleading and inaccurate, though this term is very often used in many application standards.
6 mm
80°
3 mm
30°
3 mm
90°
Welding Inspection of Steels WIS 5 Section 03 Welding Imperfections
Rev 09-09-08 Copyright 2009 TWI Middle East
3:8 C:
An Irregular Bead Width is a surface imperfection, which is often referenced in application standards as. “The weld bead should be regular along its length”
Undercut
Undercut can be defined as a depression or grove at the toe of a weld in a previous deposited weld or base metal caused by welding. Undercut is principally caused by an incorrect welding technique, including a high a welding current, or slow a travel speed in conjunction with the welding position i.e. 2F/2G or PB/PC. It is often found in the top toe of fillet welds when attempting to produce a leg length >9mm in one run. Undercut can be considered a serious imperfection, particularly if sharp as again it causes high stress concentrations. It is thus gauged in its severity by length, depth and sharpness.
Undercut (Base metal, “Top toe”) Undercut (Base metal)
THE WELDING INSTITUTE
Welding Inspection of Steels WIS 5 Section 03 Welding Imperfections
Rev 09-09-08 Copyright 2009 TWI Middle East
3:9 Shrinkage Grooves
Shrinkage grooves occur on both sides of the root base metal caused by contraction forces of the shrinking weld pulling on the hot plastic base metal. They are often wrongly identified as root undercut which may occur in the root but is caused mainly by gravity i.e. G2/PC though being grooves they are all evaluated in length, depth and sharpness.
Undercut (Weld Metal)
Undercut (Root Run or “Hot Pass”)
Shrinkage Grooves
Welding Inspection of Steels WIS 5 Section 03 Welding Imperfections
Rev 09-09-08 Copyright 2009 TWI Middle East
3:10 Root Concavity. (Suck Back in USA)
This may be caused when using too high a gas backing pressure in purging. It may also be produced when welding with too large a root gap and depositing too thin a root bead, or too large a hot pass which may pull back the root bead through contractional stresses.
Excess Root Penetration
May be caused by using too high a welding current, and/or, too slow travel speed, too large a root gap, and/or too small root face. It is often accompanied by burn through, or a local collapse of the weld puddle causing a hole in the weld root bead. Penetration is only excessive when it exceeds the allowable limit, as given in the application standard.
Root Oxidation
Root oxidation may take place when welding re-active metals such as Stainless Steels or Titanium etc. with either contaminated or an inadequate purging gas flow.
Incompletely Fused Tack Welds and Stop/Starts
It is often a procedural requirement for tack welds or for the end of root run welds to be feathered (Lightly ground and blended) prior to welding/re-striking. This requirement is very dependent upon the class of work. Feathering should enable tack welds or previous welds to be more easily blended and any failure to achieve this correctly may result in a degree of lack of root fusion/penetration and/or irregularities occurring in the weld root.
Root concavity
Pipe Plate
Un-feathered
start
of run Un-featheredroot tack
Incomplete Penetration Irregular Root Bead
Un-feathered
end
of runIrregular Root Bead
THE WELDING INSTITUTE
Welding Inspection of Steels WIS 5 Section 03 Welding Imperfections
Rev 09-09-08 Copyright 2009 TWI Middle East
3:11
A Burn Through may be caused by a severely excessive root penetration bead followed by local collapse of the weld root in the effected area.
It may be generally caused by a combination of the following factors:
a) > welding current b) > root gap
c) < root face d) < speed of travel
Its occurrence is also very dependent upon the welding position and the effect of gravity.
Excess Root Penetration (Beyond the specified limit) Root Oxidation
(In Stainless Steel)
This may lead to a Burn Through (A local collapse of the weld pool leaving a hole in the root area)
Burn Through
Welding Inspection of Steels WIS 5 Section 03 Welding Imperfections
Rev 09-09-08 Copyright 2009 TWI Middle East
3:12
To summarise, surface/profile welding imperfections are as follows:
1) Incompletely Filled Groove/Lack of Sidewall/Root Fusion 2) Cold Laps/Overlap
3) Spatter
4) Arc Strikes. (Stray arcs) 5) Incomplete root penetration 6) Bulbous, or Irregular Contour 7) Poor Toe Blend
8) Irregular Bead Width
9) Undercut. (Weld and/or Base metal)
10) Root Concavity. Root Shrinkage Grooves/Root Undercut 11) Excess Penetration. Burn Through
(Comparatively measured as radiographic density in some line pipe standards) 12) Root Oxidation
Surface and profile imperfections are mainly caused by a lack of applied welding skill.
6) Mechanical/Surface damage
Mechanical/Surface damage
This can be defined as any material surface damage caused during the manufacturing or handling process, or in-service conditions. This can include damage caused by:
1) Grinding 2) Chipping
3) Hammering 4) Removal of welded attachments by hammering 5) Chiselling 6) Using needle guns to compress weld capping runs 7) Corrosion (Not caused through welding, but is considered during inspection) As with arc strikes the above imperfections are detrimental to quality as they reduce the plate or wall thickness through the affected area. They may also cause local stress concentrations and corrosion sites and should thus be repaired prior to acceptance.
Chisel Marks Pitting Corrosion Grinding Marks Surface Scale
THE WELDING INSTITUTE
Welding Inspection of Steels WIS 5 Section 03 Welding Imperfections
Rev 09-09-08 Copyright 2009 TWI Middle East
3:13
7) Misalignment
There are 2 main forms of misalignment in plate materials, which are termed:
1) Linear Misalignment 2) Angular Misalignment or Distortion
Linear Misalignment: can be controlled by the correct use/control of the weld set up technique i.e. tacking, bridging, clamping etc. Excess Weld Metal Height and the Root Penetration Bead must always be measured from Lowest Plate to the Highest Point of the weld metal, as shown below.
Angular Misalignment: may be controlled by the correct application of distortion control techniques, i.e. balanced welding, offsetting, or use of jigs, fixtures, clamps, etc.
Hi-Lo is a term that is generally used to describe the unevenness across the root faces between pipes found during set up and prior to welding. This unevenness is often caused by an un-matching and/or irregular wall thickness, or between pipes having any degree of ovality.
Angular Misalignment/Distortion measured in degrees
15
Hi-Lo
Linear Misalignment measured in mm
3 mm Excess Weld Metal Height
Welding Inspection of Steels WIS 5 Section 03 Welding Imperfections
Rev 09-09-08 Copyright 2009 TWI Middle East
3:14