WELD RELATED DISCONTINUITIES

 

WELD RELATED Discontinuities

POROSITY

the primary causes for porosity are dust, dirt and moisture on the surface of base metal, in the welding consumables, gases and welding equipment  (for cooling).porosity differ from slag inclusion in that they contain gases rather than solid and are spherical in shape.

They can be prevented by using clean materials. In addition, avoid excessive current and arc length.

Uniformly scattered porosity – are in which void or pores are scattered more or less uniformly through out the weld metal,

Cluster porosity – pore occurs in clusters, usually when starting and stopping

Linear porosity-occurs generally in the root pass and is often regarded as inadequate joint penetration.

SLAG INCLUSION

Oxides and other non-metallic solid entrapped in weld metal or between weld metal and base metal. During deposition many chemical reaction occurs between the weld metal and electrode covering materials. Forming slag compounds soluble only aslight degree in the molten metal. Due to their lower sp.gravity, slags rise to the surface of the molten metal unless they are restrained.

It can be prevented by proper preparation of groove before the bead is deposited, using care to correct contours that are difficult to penetrate fully with in the arc.(SHRINKAGE CRACK)

TUNGSTEN INCLUSION

Non-consumable tungsten electrode to create an arc between the tungsten electrode and work piece. The occasional touching of the electrode to the work piece or to the molten weld may transfer particles of tungsten in to the weld deposit. The use superimposed high frequency current for arc starting also tends to minimize or eliminates TI at start welds. It can be minimized by throated tungsten or zirconium-tungsten electrodes with direct current, reverse polarity.

 INCOMPLETE FUSION

Caused by failure to raise the temperature of the base metal (or previously deposited weld metal) to the melting point. also by failure to dissolve, by technique as heating or fluxing, the oxides or other foreign material present on the surface to which the deposited metal must fuse. Side walls of groove in order to be certain of securing fusion and should bring the surface of the base metal to the fusion temperature to obtain metallurgical continuity of the base and weld metal.

INADEQUATE JOINT PENETRATION

Failure of filler metal and base metal to completely fill the root. Groove should be proper as per drawing and dimension.(BACK CHIPPING AND DPT)

UNDER CUT

The melting away of side wall of a welding groove at the edge of the layer or bead, thus forming a sharp recess in the side wall in the area to which the next layer or bead must fuse. With a specific electrode or, excessive current or too long arc may increase the tendency to undercut.

 CRACKS

Cracking of welded joints result from localized stress that at some point exceeds the ultimate strength of the material. After a welded joint has cooled, cracking is more likely to occur if the metal is either hard or brittle. A ductile material, by localized yielding, cause a hard or brittle material to fail.

Steps to avoid cracking

·   Change the electrode manipulation or electrical conditions, to improve the contour or composition of the deposit.

·   Decrease the travel speed. This increases the thickness of the deposit and provides more weld metal to resist the stresses.

·   Use pre heat to reduce thermal stresses

·   Use low hydrogen electrodes

·   Sequence welds to balance shrinkage stresses.

·   Avoid quenching and cooling condition

Transverse weld crack

Perpendicular to axis of weld

Longitudinal cracks

These are found mostly within the weld metal, and are seen at centre of the weld

Crater cracks

These are usually star shaped and progress only to the edge of the crater.

BASE METAL cracking Occurs with in the HAZ of the metal being welded and is most associated with hardenable materials. Hardness and low ductility in the HAZ in welded joints are metallurgical effects that result from thermal cycle of welding and are among the principle factors that tend s to cause cracking.

PRE HEAT

To remove surface contaminate and other forms of moisture that cause porosity or weld cracking

 Post weld heat-treating (PWHT)

 The two most frequent requirements involving PWHT are tempering and stress relieving. In a tempering treatment, the hard areas in the weld or adjacent metal are softened. This can involve a complete softening operation using an annealing cycle during which the metal is heated above its transformation temperature. (For steel 900 C).followed by slow cooling.PWHT of 620 to 730 C used for Carbon allow steel.

In stress relieving traetment, the residual stresses created by localized heating and cooling associated by welding are reduced by creep or plastic deformation.

HOLDING TIME –one hour per inch of thickness with a minimum of one hour appears to be safe concept when no instruction is available.