Welding Imperfections

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Contents

Introduction

There are six different grouping of welding imperfections:

(i) Cracks

(ii) Cavities

(iii) Solid inclusions

(iv) Lack of fusion and penetration

(v) Imperfect shape and dimensions

(vi) Miscellaneous imperfections

Cracks

Welding-cracks.jpg
An imperfection produced by a local rupture in the solid state, which may arise from the effect of cooling or stresses. Cracks are the most critical among the imperfections, as their geometry produces a very large stress concentration at the crack tip, eventually leading to fracture.

Types of Cracks (Location)

  • Longitudinal crack - can occur in weld metal, HAZ (heat affected zone), or parent metal
  • Transverse crack - can occur in weld metal, HAZ (heat affected zone), or parent metal
  • Radiating crack - cracks radiating from a common point. It occur in weld metal, HAZ (heat affected zone), or parent metal
  • Crater crack - can occur only in in weld metal
  • Branching crack - group of connected cracks originating from a common crack. It can occur in weld metal, HAZ (heat affected zone), or parent metal

Types of Cracks (Nature)

Cracks can also be sub-divided based on their nature:

  • Hot cracks
  • Precipitation induced cracks
  • Cold cracks
  • Lamellar tearing

Hot cracks

This type of cracking occurs when the weld is starting to solidify, in the pasty state, as it posses very little strength and therefore any residual loading is likely to cause it to break before it has fully solidified. The problem can be compounded by impurities that are forced out of the solidifying weld, becoming trapped in the centre of the weld during final solidification. Hot cracking can occur where their is a high degree of restraint in the structure of the fabrication or where the structure moves slightly as the weld solidifies.

Depending on their location and mode of occurrence, hot cracks can be:

  • Solidification crack
  • Liquation crack

Solidification crack

Solidification-crack.jpg
It occur as a result of solidification process. It is in weld metal usually along the centerline of the weld. It normally occurs when: weld metal has high carbon or impurity, depth-to-width ratio of the wed bead is large, or when the disruption of the heat flow conditions occurs.

The impurities like sulphur and phosphorus are a major factor since these elements segregate during solidification. Hence it is important to remove the oil or grease contamination from the weld area before welding. Also metals like copper, tin, lead, and zinc, with low melting point should be avoided.
These cracks can be wide and open to the surface like shrinkage voids or in sub-surface and narrow.
Solidification cracks are normally readily distinguished from other types of cracks due to the following characteristic factors:

  • they occur only in the weld metal
  • they normally appear as straight lines along the centreline of the weld bead, as shown above, but may occasionally appear as transverse cracking depending on the solidification structure
  • solidification cracks in the final crater may have a branching appearance
  • as the cracks are often 'open', they can be visible to the naked eye

Liquation crack

These occurs in the coarse grain HAZ, in the new vicinity of the fusion line as a result of heating the material to an elevated temperature, high enough to produce liquation of the low melting point constituents placed on grain boundaries.

Precipitation induced cracks

These are reheat cracks present in crack resisting steels.

Cold Cracks (Hydrogen induced cracks)

Types of cold cracks
Cold cracks or Hydrogen Induced Cracking occurs when diffusible Hydrogen ions become trapped in the welded structure. As the weld cools, the Hydrogen ions attempt to escape the highly stressed areas in which they are trapped by migrating away from the weld, attempting to reunite with other Hydrogen ions to become hydrogen gas (H2) again. The Hydrogen reunion invariably occurs in the grain-coarse region of the heat affected zone (HAZ) and manifests itself as a crack in the toe of the weld.

Cold cracks can only occur if all of the following conditions are present:

  • Presence of H+ (can be from moisture, organic dirt, oil, grease, paint, etc) - hydrogen level greater than 15 ml/100 g of weld metal deposited
  • Susceptible Micro-structure (higher Carbon Equivalent materials are more susceptible) - greater than 400 HV hardness
  • Residual Tensile Stresses (present with restrained joints, thick material, no pre-heat, etc) - greater than 0.5 of the yield stress
  • Moderate Temperatures (-150 to 100 deg C)

Cold cracking can be prevented thorough the following methodologies:

  • Apply preheat to slow down the cooling rate and avoid the formation of susceptible microstructures
  • Maintain a specific interpass temperature
  • Post head on completion of welding
  • Use shielding gas to reduce hydrogen content
  • Reduce residual stress
  • Clean rust from moisture to prevent formation of hydrogen from water

Lamellar tearing

in a T weld
Lamellar tearing is a type of welding defect that occurs in rolled steel plates. Its main distinguishing feature is that the cracking has a terraced appearance. It has rarely been an issue since the 1970s because steel produced since then has less sulfur.
appearance of fracture face
The principal distinguishing feature of lamellar tearing is that it occurs in T-butt and fillet welds normally observed in the parent metal parallel to the weld fusion boundary and the plate surface. The cracks can appear at the toe or root of the weld but are always associated with points of high stress concentration.

There is a combination of causes: non-metallic inclusions, too much hydrogen in the material, and shrinkage forces perpendicular to the face of the plates. The main factor among these reasons is the non-metal inclusions, of which the sulfur is the main problem. Lamellar tearing is no longer a problem anymore because sulfur levels are typical kept below 0.005%.

Some things that are done to overcome lamellar tearing are: reducing amount of sulfur in the material or adding alloying elements that control the shape of sulfide inclusions, such as rare earth elements, zirconium, or calcium. A more drastic option is change the workpieces to castings or forgings because this type of defect does not occur in those workpieces.

It is generally recognised that there are three conditions which must be satisfied for lamellar tearing to occur:

  • Transverse strain - the shrinkage strains on welding must act in the short direction of the plate ie through the plate thickness
  • Weld orientation - the fusion boundary will be roughly parallel to the plane of the inclusions
  • Material susceptibility - the plate must have poor ductility in the through-thickness direction

Thus, the risk of lamellar tearing will be greater if the stresses generated on welding act in the through-thickness direction. The risk will also increase the higher the level of weld metal hydrogen

Cavities

Cavity may be either gas cavity due to entrapment of gas or due to shrinkage caused by shrinkage during solidification. The types of cavities that are formed by entrapment of gas are:

  • Gas pore
  • Worm hole
  • Surface pore

Gas pore

Porosity is a condition in which gas pockets or voids occur in a metal as a result of contamination or poor protection of the molten solidifying metal. It is generally accepted that porosity in weld metals is formed by the entrapment of evolved gases in the solidifying weld metal. The discontinuity formed is generally spherical but may be cylindrical.

It has been suggested that bubbles are first formed at the solid liquid interface and remain there until they have grown to a sufficient size to float up through the molten weld metal and escape to the atmosphere.

If the rate of detachment and flotation is less than the rate of advance of the solidification front, the bubbles ate trapped and the resultant weld is porous. In carbon steel welds porosity can result from the following reactions that occur either singly or in combination.

Hydrogen, oxygen and nitrogen are gases dissolved in the metal. Pores in weld metal are, in general, spherical in shape, but pores of other shapes can occur weld metal porosity (black) and heat affected zone (dark portion around the weld bead).

A gas cavity of essentially spherical shape trapped within the weld metal. It can be of various forms:

  • Isolated
  • Uniformly scattered porosity - is porosity uniformly distributed throughout a single pass weld or through several passes of multiple pass weld. It is usually faulty welding technique or materials that is the cause.
  • Clustered porosity - is a localized grouping of pores that results from improper initiation or termination of the welding arc.
  • Linear porosity - is porosity aligned along a joint boundary, the root of the weld, or an inter-bead boundary.

Causes:

  • Damp flux / corroded electrodes (MMA)
  • Presence of grease, hydrocarbon, water on prepared surface
  • Air entrapment in gas shield (MIG/MAG, TIG)
  • Incorrect deoxidant in electrode, filler or parent metal
  • Too high arc voltage or length
  • Gas evolution from primer
  • Too high shielding gas flow rate causing turbulence (MIG/MAG, TIG)

Worm hole

A form of pores, which is most prevalent, apart from spheres, is a type known as worm hole porosity. This pore is roughly elliptical in shape and has its major axis normal to the advancing solidification wave. Where irregular shaped voids are present or where cavities occur in groups, these are perhaps due to mechanically entrapped gas caused by arc instability. Worm holes in fillet welds extends from the root of the weld toward the surface of the weld. Most of the worm holes found in welds do not extend to the surface.

Causes:

  • Gross contamination of prepared surface
  • Laminated work surface
  • Crevices in work surface due to joint geometry

Surface porosity

A gas pore that breaks the surface of the weld.

The types of cavities that are formed by shrinkage during solidification are:

  • Interdentrite shrinkage
  • Crater pipe
  • Microshrinkage

Crater pipe

A shrinkage cavity at the end of the weld run. The main cause is the shrinkage during solidification.

Solid Inclusions

Solid foreign substances entrapped in the weld metal is called solid inclusion. These can be the following types:

  • Slag inclusion
  • Flux inclusion
  • Oxide inclusion
  • Metallic inclusion

Depending on the type, the first three may also be of the following:

  • Linear
  • Isolated
  • Clustered

Slag Inclusions

Slag inclusion

It is caused by non-metallic solid material or slag trapped during welding. The imperfection is irregular shape and is found either in the weld metal or between weld metal and the base metal. In general, slag inclusions result from faulty welding techniques and failure of the designer to provide proper access for welding within the joint.

Elongated cavities usually parallel to the axis of the weld, which contain slag or other foreign matter are called Slag lines.

Flux Inclusions

It is caused by flux trapped during welding. The imperfection is of an irregular shape and thus differs in appearance from gas port.

Oxide Inclusions

It is caused by oxides trapped during welding.

Metal Inclusions

There can be three different types depending on the kind of metal - Tungsten, Copper or Other metal.

1. Tungsten Inclusion

Tungsten Inclusion

Particles of tungsten can become embedded during TIG welding. This imperfection appears as a light area on radiographs due to the fact that tungsten is denser than the surrounding metal and absorbs more X-ray/gamma radiation.

Lack of Fusion and Penetration

Lack of Fusion

Lack of fusion is the result of improper welding techniques, improper penetration materials for welding or improper joint design. Deficiencies causing incomplete fusion include insufficient welding heat or lack of access to all boundaries of the weld joint that are to be fused during welding, or both.

1. Lack of sidewall fusion
2. Lack of inter-run fusion
3. Lack of root fusion

Lack of Penetration

1. Incomplete Penetration

2. Incomplete Root Penetration

Imperfect shape and dimensions

Undercut

Undercut is generally associated with either improper welding techniques or excessive welding currents, or both. It is generally located at the junction of weld and base metal (at the toe or root). Undercut discontinuities create a mechanical notch at the weld fusion boundary.

Excess Weld Metal

Excess Penetration

Overlap

Misalignment

Incompletely filled groove

Irregular Width

Root Concavity

Burn-through

Miscellaneous imperfections

Stray Arc

Local damage to the surface of the parent metal adjacent to the weld resulting from arcing or striking the arc outside the weld groove. This results in random areas of fused metal where the electrode, holder, or current return clamp have accidentally ouched the work.

Some of the causes of this are:

  • Poor access to the work piece
  • Missing insulation on electrode holder or torch

Spatter

Torn Surface

Grinding Damage

Chipping Damage

Under Flushing

References

  • American Welding Society - Welding Inspection Guide
  • CSWIP - Welding Inspection Guide