Weld Discontinuities and defects

Causes & Remedies

Definition

Influence

Types

Causes

Remedies

Discontinuity & Defect

Discontinuity refers to any interruption or in-homogeneity in the material that is otherwise homogenous

Defect refers to an interruption having dimensions/features beyond acceptable limits.

Defects are otherwise called as Flaw or Imperfections that are to be repaired so that the material is made more homogeneous as assumed in the design

Weld joint is itself is a Discontinuity

The Microstructure is different in the different regions of weld zones of weld joint.

So the mechanical properties also vary from zone to zone causing heterogeneity

PMWM

HAZHAZ

Common Surface Discontinuities

Surface Porosity

Spatter

Wire sticking

Crater pin hole

Crater crack

Surface cracks

Common surface Discontinuities

Un-equal weld size

Under sized weld

Weld offset ( from seam)

Non-uniform weld size (along the length)

Oversized weld

Excess Penetration

Weld crater

Common surface Discontinuities

Improper Weld bead Profile

Coarse weld Ripples

Excess Convexity

Root concavity

Excess Reinforcement

Overlap

Undercut

Misalignment

Lack of Penetration

Common Internal Discontinuities

Porosity

Blow Holes / Aligned / distributed / Cluster / worm hole Porosities

Lack of fusion/ Lack of Penetration

Lack of side wall fusion / Inter run fusion / lack of fusion at root

Cracking

Under-bead cracking / delayed cracking / Delayed cracking /Hydrogen induced cracking

Hot cracking, shrinkage cracks, Fatigue cracking

Porosity

Incomplete joint penetration

Lack of penetration

Arc Strike

• Definition: A localized coalescence outside the weld zone.

• Cause: Carelessness

• Prevention: In difficult areas, adjacent areas can be protected using fire blankets.

• Repair: Where applicable, arc strikes must be sanded smooth and tested for cracks. If found, they must be remove and repaired using a qualified repair procedure and inspected as any other weld.

Spatter

• Definition: Small particles of weld metal expelled from the welding operation which adhere to the base metal surface.

• Cause: Long arc length, severe electrode angles, high amperages.

• Prevention: Correct the cause. Base metal can be protected with coverings or hi-temp paints.

• Repair: Remove by grinding or sanding. Sometimes must be tested as if it were a weld.

Arc strikes & Spatter

Excess Penetration

Proper Penetration

Excess Penetration P

Excess Penetration

Misalignment (hi-lo)

• Definition: Amount a joint is out of alignment at the root

• Cause: Carelessness. Also due to joining different thicknesses (transition thickness)

• Prevention: Workmanship. Transition angles not to exceed 2.5 to 1.

• Repair: Grinding. Careful on surface finish and direction of grind marks. Inside of Pipe /Tube difficult.

Mis-alignment

Porosity - Cause /Source

Moisture in

Plate edges

Non-baked/dried Electrodes

Filler wire

Wet / Non-dried flux

Pneumatic fixture

In the atmosphere

Inadequate Gas shielding due to

• Improper setting of gas flow rates

• Improper Torch angles

• Improper setting of welding Parameters

Cavities / voids formed due to entrapment of gases when the molten metal solidifies from liquid state when more gases get dissolved into molten metal

Surface Porosity

10 mm

Internal Porosity

• Definition: A crack in the weld that occurs during solidification.

• Cause: Micro stresses from weld metal shrinkage pulling apart weld metal as it cools from liquid to solid temp.

• Prevention: Preheat or use a low tensil filler material.

• Repair:

Hot Crack

10 mm

Hot Cracking

Types of defects

Types of Cracking

Undercut• Definition: A groove cut at the

toe of the weld and left unfilled.

• Cause: High amperage, electrode angle, long arc length, rust

• Prevention: Set machine on scrap metal. Clean metal before welding.

• Repair: Weld with smaller electrode, sometimes must be low hydrogen with preheat. Sometimes must gouge first.

Undercut - Causes

Undercut(cont......)

Undercut typically has an allowable limit. Different codes and standards vary greatly in the allowable amount.

Plate - the lesser of 1/32” or 5% (typ.)

Excessive Concavity or Convexity

• Definition: Concavity or convexity of a fillet weld which exceeds the specified allowable limits

• Cause: Amperage and travel speed

• Prevention: Observe proper parameters and techniques.

• Repair: Grind off or weld on. Must blend smoothly into the base metal.

Concavity

Convexity

Weld bead Profile

Fillet Weld with straight bead

Fillet Weld with Convex bead

Fillet Weld with Concave bead

Excess Penetration

Proper Penetration

Excess Penetration P

Excess Penetration

1-Excessive Penetration

2-slag Inclusion

Gas Pores

Incomplete fusion -Causes

• Improper manipulation of electrodes

• Incorrect position of electrodes

• Impurities on the weld surface

Weld Strength

100% Joint Efficiency is obtained when

The weld is carried out on both sides

Back gouging is done after welding the first side

LPI or MPT is carried out after Back gouging

100% RT / UT is carried out after second side welding

Weld Strength

The Electrode or the filler wire should be selected such that it

Matches with the strength of the base material

Has fairly low carbon & controlled impurities to avoid cracking

Moisture level is controlled and is of low H2 type

Is usable in all positions