Thursday, 23 March 2017

Welding Defects : Types, Causes, Testing and Remedies

Today we will learn about welding defects, its types, causes, testing, and remedies. Defects are common phenomenon in any manufacturing process. This is due to some process limitation and some human behavior. It is impossible to form a defect free welding joint but it can be reduce to some extent by taking some precautions. Today we will learn about all types of welding defects.

Types of Welding Defects:

After welding some defects are found which make the strength of joint weak. Such defects are called welding defects. Some welding defects are given below.

Porosity:

It is common type. In this defect, air bubbles or gases are present in the weld zone. The distribution of air bubbles in weld zone is random. Porosity caused by gases release during melting of the weld area but trapped during solidification, chemical reaction during welding or by contaminants. This defect can be minimized by the proper selection or electrode, filler material, improve welding Technics, more attention to weld area during welding preparation and slower speed to allow gases time to escape. The effect of porosity on performance depends on quality, size and orientation to stresses.

Welding Defects : Types, Causes, Testing and Remedies

Spatter:

Metal drop expelled from the weld that stick to surrounding surface is known as spatter. Spatter can be minimize by correcting the welding condition and should be eliminated by grinding.

Welding Defects : Types, Causes, Testing and Remedies

Causes:
  • Welding current too high.
  • Arc is too long.
  • Incorrect polarity.
  • Insufficient gas shielded.


Remedies:
  • Reduce welding current and arc length.
  • Use correct polarity according to the welding condition.
  • Increase torch to plate angle and use correct gas shielding.


Slag inclusions:

Slag inclusions are compound such as oxides, fluxes and electrode contains material that is trapped in the weld zone. These defect are commonly associated with undercut, incomplete penetration and lack of fusion in weld. Insufficient cleaning between multi-pass welds and incorrect electrode and current can leave slag and unfused section along the weld joint. Slag inclusion not only reduces cross section area strength of joint but also may serve as initiation point for serious cracking. This defect can only be repaired by grinding down or gouging out and re-welding.

Welding Defects : Types, Causes, Testing and Remedies

Incomplete fusion:

In this types of welding defect gap is not totally filled by molten metal. It is due to inaccuracy of the welder so pre solidification of welding metal.


Welding Defects : Types, Causes, Testing and Remedies


Causes:
  • Heat input is too low.
  • Weld pool is too large and running ahead of the arc.
  • Joint included angle is too low.
  • Electrode and torch angle is incorrect.
  • Unfavorable bead position.


Remedies:
  • Increase welding current and decrease the travel speed.
  • Reduce deposition rate.
  • Increase joint include angle.
  • Position electrode or plate angle such a way so the plate edges will melt.
  • Position bead in such a way that the sharp edges with other bead or plate are avoided.


Incomplete Penetration:

It is occurs when the depth of the welded joint is insufficient.

Under cutting:

This defect occurs when base of metal melts away from the weld zone and the consequent generation of a groove is in the shape of sharp recess or notch. It reduces the fatigue strength of the joint.

Welding Defects : Types, Causes, Testing and Remedies

Causes:
  • Arc voltage too high or arc too long.
  • Incorrect electrode use or incorrect electrode angle.
  • The electrode is too large.
  • High electrode speed.

Remedies:
  • Lower arc voltage or reduce arc length.
  • Apply electrode angle form 30 degree to 45 degree with the standing leg.
  • Use a smaller diameter electrode.
  • Reduce travel speed.


Under filling:

Under filling occurs when the joint is not filled with the proper amount of molten metal.
Welding Defects : Types, Causes, Testing and Remedies

Inclusion:

It is a defect in which several passes are made along a V-joint when joining thick plate using flux cored or flux coated rods and the slag covering a run is not totally removed every pass before following pass.

Lamellar tears:

This is mainly a problem with low quality steel. It occurs I plate that has a low ductility in the thickness direction which causes by non-metallic inclusions like sulphides, oxides that have been elongated during the rolling process. Lamellar tearing can occur in both fillet and butt welds but the most affected joints are T and corner joints where the fusion boundary is parallel to the rolling plane.

Cracks:

Cracks may occur in various location and direction in the weld area. The typical types of cracks are longitudinal, transverse, crater, under bead and toe cracks. When joint is at elevated temperature then a crack occurs and it is known as hot crack. When it occurs after solidifying weld metal, it is known as cold crack.

Causes:  
  • It is due to the welding ended far too adruptly. The cracks begins at a void in welding crater, caused by the solidification shrinkage.

Remedies:
  • When finishing move back the electrode to fill up the crack.
  • With root pass welding, quickly move the arc form welding pool to the plate edge.
  • Increase crater fill time by power source.


Arc blow:

Causes:
  • Arc deflection as a result of magnetic effects into the opposite direction of the earth lead clamp.
  • Arc deflection as a result of magnetic effects in the direction of heavy part of work piece especially at corner and edges.

Remedies:
  • Use AC electrode where possible.
  • Try welding away from earth clamp connection. Try splitting the earth clamp and correct to both side of the joint.
  • Keep arc as short as possible.

Types of weld testing:

The majority of weld testing and inspection can be separated into two categories.

Destructive testing:

Destructive testing is usually a cheaper method of inspection. It lends its self to mass produced parts, where sacrificing one or two component for testing is acceptable. It is quite useful for setting up welding equipment. It is also a good learning tool during training courses, as it allows the students to do lots of testing at minimum cost and is often easier to understand than some of the NDT method. The two most common tests used in training are Macro Etch testing and Root and Face bend Testing.

Non destructing testing:

There are various methods to find weld defect by NDT testing, some are reasonably simple, but others require specialist operators and expensive equipment, such as X-ray testing.
There are also more simple methods such as dye penetrant testing, that can be carried out with minimum equipment, and in most workshops. Florescent penetration testing is advance method of dye penetration testing. There are other common NDT testing methods are Ultrasonic testing, Megnatic particle testing, X-ray testing etc.


This is all about welding defects types and testing. If you have any query regarding this article ask by commenting below. If you like this article, don’t forget to share it on social networks.  

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