Chapter # 4                                                                                       PROCESSES

                                                                           Welding Defects

4.2.5   Types Of Welded Joints

          Depending upon the profile of the job the following types of welded joints the are most common.

Types of welded joints (Figure – 4.5)

          Before the commencement of the welding process, the mating surfaces have to be shaped and thoroughly cleaned.  The operations are called edge preparation and weld preparation.  While there are general guidelines for such preparations, each manufacturer may follow his practices depending upon the specific requirement of the job and the available infrastructure.

4.2.6   Quality of Weld

          Defects in the weld are caused by several factors including, among others, incorrect settings on the welding machine, poor quality of welding consumables, variations in metal composition, improper edge preparation, unfavourable ambient conditions, and not following procedures & method statements.  The skill of the welder is one of the most important factors in achieving good weld.  Most of the defects can be identified by visual inspection and non-destructive testing methods.  A weld should have a reasonably smooth, uniform, and consistent appearance.

          Acceptance norms vary for different industries.  The nuclear industry and pressure vessel manufacturers have stringent acceptance norms for different reasons.  Welds in the Bus industry need not be subjected to such standards.  It needs to establish acceptance criteria that are fit for purpose.

          Defects in welding can be classified into external and internal defects.

4.2.6.1   External Defects

·         Surface Crack – It is the most serious of all defects that can occur.  It can further be classified as hot or cold cracks.  It can be observed on the surface or may manifest itself within the weld.  It is caused by the concentration of residual stresses for several reasons.

·         Overlap - The weld pool overflows on the surface of the weld metal. The molten metal does not fuse with the base metal.

·         Undercut - Results in thinning of the parent metal at the edge due to inadequate feed.

·         Underfill - When the weld surface remains below the adjacent surface of the base metal.

·         Excess Penetration - When the melt flows beyond the root gap and solidifies.

·         Spatter - Condensing on the surface, along the weld, of metal droplets that are expelled.

·         Surface Porosity - Caused probably by the entrapment of the gas during the welding process.

·         Distortion - It can be caused by uneven heat distribution, shrinkages, and inadequate clamping, among other reasons.  The distortion can take place in the axial, perpendicular, or torsional direction.

·         Profile - A weld run must be uniform with a repetitive pattern.  Depending upon the thickness and edge preparation, the weld may look like a string of beads on a woven ribbon.  A weld may be required to be flat, concave, or convex for specific applications.

·         Crater - Formed when arc gets extinguished allowing the edges to cool faster than the pool.

·         Excessive Convexity - Caused by large deposition due to low speed of welding, larger size electrodes among other reasons.

·         Oxidation - Caused by inadequate shielding and impurities in the gas.  This can lead to other defects when multiple runs are required for completing a joint.

 

External welding defects (Figure – 4.6 A)

4.2.6.2  Internal Defects 

·         Slag inclusions – These are compounds that are generated during the welding process and get trapped in the weld or on the surface of the weld.  Insufficient cleaning can leave slag on the surface that may get embedded during subsequent runs.

·         Sub Surface Crack – Caused by a concentration of residual stresses.

·         Internal Porosity – Caused by entrapment of gases

·         Incomplete Fusion – This phenomenon takes place when the molten pool is not of a continuous and homogeneous composition.  Sometimes fusion may not occur in certain localized spots.  The likely locations of the lack of fusion are at the root of the weld, sidewall, and inter-run.  This may be due to unmelted oxide and non-metallic inclusions.

·         Incomplete Penetration - The depth of penetration should ideally be equal to the thickness of the plate.  Incomplete or excessive penetrations are harmful.

Internal welding defects (Figure – 4.6 B)

Continued..........