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Welding imperfections and welding defects

Imperfections and defects

The terms of "defects" and "imperfections?

It is easy to understand in the above photos which discontinuities exceed limitation in the acceptance criteria (yellow line).

BS EN ISO 6520-1 definitions

Imperfections: any deviation from the ideal weld.

Defect: an unacceptable imperfection

ASME B31.3 definitions

Defects: the imperfections of a type or magnitude not acceptable by the criteria specified in para. 341.3.2 and shall be repaired, or the defective item or work shall be replaced.

Classification of welding imperfections (ASME BPVC. V)

Burn through

A collapse of the weld pool, mainly causes of an excessive welding current.

It is normally found at the center of the root pass and in RT film with variable density (darken areas).

I hope you can find out more causes of imperfection and preventive actions in CSWIP Training Source.

This imperfection is commonly found in the semi-auto / auto welding process.

A SAW process easy found at start arc, stop arc or variation arc length. That are the reasons we should make the run-in plate and run-out plate during welding.

Cracks

Crack is an imperfection produced by a local rupture in the solid state, which may arise from the effect of cooling or stresses.

ASME B31.3 grounded in the “extent of imperfection”; unacceptable regardless any size and location.

AWS D1.1 Crack prohibition, any crack shall be unacceptable, regardless of size or location.

Radiographic image: a line of darker density running lengthwise along the weld at any location in the width of the weld image

Type of cracks

• Longitudinal.
• Transverse.
• Radiating (cracks radiating from a common point).
• Crater.
• Branching (a group of connected cracks originating from a common crack).
These cracks can be situated in the: 
       • Weld metal 
       • HAZ 
       • Parent metal 

Exception: Crater cracks are found only in the weld metal.

Did you see a crack on tacking weld ?

In the right photo is a crack on the tack weld due to improperly length tack weld and not apply preheat before tack welding.

Why is preheat used when arc welding steel?

  • The reduction in hydrogen reduces the risk of cracking.
  • To slow the cooling rate of the weld and the base material

Hydrogen Cracking (cold crack)

  • Cold cracking: Cracks occur when the weld has cooled down.
  • HAZ cracking: Cracks tend to occur mainly in the HAZ.
  • Delayed cracking: Cracks may occur some time after welding has finished (possibly up to ~ 72h).
  • Underbead cracking: Cracks occur in the HAZ beneath a weld bead.

Hydrogen cracking in the HAZ of a steel occurs when four conditions exist at the same time:

Hydrogen level > 15ml/100g of weld metal deposited
Stress
> 0.5 of the yield stress
Temperature
< 300°C
Susceptible microstructure
> 400HV hardness

Solidification Cracking (hot crack)


Hot cracking: They occur at high temperatures while the weld is hot.
 
Centreline cracking: Cracks may appear down the centreline of the weld bead.

Crater cracking: Small cracks in weld craters are solidification cracks.

Lamellar Tearing is a type of cracking that occurs only in steel plate or other 
rolled products underneath a weld. 

Characteristics of lamellar tearing are: 

• Cracks only occur in the rolled products, eg plate and sections.

• Most common in C-Mn steels

• Cracks usually form close to, but just outside, the HAZ

• Cracks tend to lie parallel to the surface of the material (and the fusion boundary of the weld), having a stepped aspect.

Excessive / inadequate reinforcement

Excess weld metal is the extra metal that produces excessive convexity in fillet welds and a weld thickness greater than the parent metal plate in butt welds. This feature of a weld is regarded as an imperfection only when the height of the excess weld metal is greater than a specified limit.

Radiographic Image: in the left photo, a high contrast between the density of the image of the parent metal (darker area) and the image of the weld (light area)

Tolerance height of reinforcement or internal protrusion allowed as ASME B31.3:

Tw 
(nominal wall thickness)
Tw < 6mm
6 < Tw < 13mm
13 < Tw < 25mm
Tw > 25 mm
Height
of reinforcement allowed
<= 1.5 mm
<= 3 mm
<= 4 mm
<= 5 mm

Inclusions (slag / tungsten)

Size and distribution of rounded indications as ASME BPVC VIII-1 Appendix 4.

Rounded Indications: Indications with a maximum length of three times the width or less on the radiographic are defined as rounded indications. These indications may be circular, elliptical, conical, or irregular in shape and may have tails.

Aligned Indications: A sequence of four or more rounded indications shall be considered to be aligned when they touch a line parallel to the length of the weld drawn through the center of the two outer rounded indications.

Slag inclusions

Slag trapped during welding. The imperfection is an irregular shape and thus differs in appearance from a gas pore.

Radiographic Image: an irregular–shaped  darker density spot, usually slightly elongated and randomly spaced.

Tungsten inclusions

Random bits of tungsten fused into but not melted into the weld metal

Radiographic Image: Irregularly shaped lower density spots randomly located in the weld  image.

Note: Almost project prohibited a tungsten inclusion due to high heat input.

Incomplete Fusion

Under Fill

A continuous or intermittent depression in the surface of a weld metal running along its length, due to insufficient weld metal, indicating a thinner than normal section thickness.

Lack of Root Fusion:

Lack of fusion between the weld and parent metal at the root of a weld, one side of the root face is not melted.

Incomplete Penetration

Both fusion faces of the root are not melted.

Main causes: excessively thick root face and insufficient root gap

Misalignment

Misalignment between two welded pieces such that while their surface planes are parallel, they are not in the required same plane.

Radiographic Image: An abrupt change in film density across the width of the weld image.

Overlap

An imperfection at the toe of a weld caused by metal flowing on to the surface of the parent metal without fusing to it.

Porosity

Porosity is the presence of cavities in the weld metal caused by gas trapped during welding.

Size and distribution of rounded indications as ASME BPVC VIII-1 Appendix 4. 

Rounded Indications: Indications with a maximum length of three times the width or less on the radiographic are defined as rounded indications. These indications may be circular, elliptical, conical, or irregular in shape and may have tails.
Clustered Indications: The illustrations for clustered indications show up to four times as many indications in a local area.

Root concavity

A shallow groove that occurs due to shrinkage at the root of a butt weld.

Do not confuse with incomplete penetration or lack of root fusion that both edge of root face are melt and shrinkage in the middle of root bead. 

In case of incomplete penetration or lack of root fusion are un-melt one/both edge of root face.

Undercut

An irregular groove at the toe of a run in the parent metal or in a previously deposited weld metal due to welding. It is characterized by its depth, length and sharpness.

Undercut easy found by visual inspection, or may need support a telescopic inspection mirror/ a flashlight.

Note: Almost piping specifications are prohibited root undercut, regardless of size or location.

Radiographic image: An irregular darker density along the edge of the weld image. The density will always be darker than the density of the pieces being welded
Root undercut
Cap undercut
Almost photos share in this article for referent purpose from CSWIP Student Book
Nguyen Duy Bien
 

Enjoy learning and researching, reading book. Knowledge welding inspection painting inspection.

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welding Qc inspector

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i want to welding qc inspector level 3.2