Welding Defects - Causes and Remedies

DEFECTS IN WELDING (CAUSES)

1. Improper welding parameter and wrong welding procedure introduce the welding defects in welded Joints.
2. A defective welding Joint may fall under service condition and can cause damage to property and human lives.
3. Welding defects can be broadly classified into three categories as:

(a) Warpage: It is unintentional and undesirable deformation of welded Joint. It occurs due to different rates of solidification.

(b) Overlapping: An overlap occurs when the molten metal from the electrode flows over the parent metal surface and remains, without getting correctly fused.

• It may occur due to:
• Lower arc current.
• Slower arc travel speed.
• Longer arcs.
• Incorrect electrode diameter.

(c) Porosity: Porosity is a group of small voids, blowholes or gas pockets. They occur mainly due to entrapped gases.

The parent metal melted under the arc tends to absorb gases like carbon monoxide, nitrogen and oxygen If they are present around the molten weld metal pool. It occurs due to:

• The improper coating on an electrode.
• Fast speed of arc travel.
• Too high or too low current.
• Damp electrode.
• Scale, rust, oil, grease, moisture etc.
• Incorrect welding technique

(d) Inclusions: Inclusions may be in the form of slag or any other foreign material. This foreign material does not get a chance to float on the surface of solidifying weld metal and gets trapped inside the same. Inclusions lower the strength of salt and make it weak. Factors promoting Inclusion are:

• Slag from previous runs not being cleaned away.
• Insufficient cleaning and preparation of the base metal before welding commences.

 (e) Incomplete penetration: Penetration is the distance from the base plate top surface to the maximum extent of the weld. Various causes of incomplete penetration are:

• Improper joints.
• Gap too small.
• Less arc current
• Faster arc travel speed.
• Too large electrode diameter.
• Incorrect polarity.

(f) Cracks: Cracks may be on a microscopic scale or macroscopic scale depending on their size. The break may appear In the base metal, base metal-weld metal boundary or the weld metal. The crack may be on the weld surface or under the weld bead. A break may occur in the creator or the root.

Leading causes of crack formation are:

• Rigidity In joint a joint member not free to expand or contract.
• Unsuitable parent metals used in the weld
• Hardenability
• Fast arc travel speed.
• Electrode with high H2 content.

(g) Incomplete fusion: Sometimes, the molten metal deposited by the electrode does not fuse properly with the cold base metal, and two metals do not fuse completely. Fusion may be lacking at the root, sides or between two runs in the multi-run weld. Factors promoting Incomplete fusion are:

• Lower arc current.
• Faster arc travel speed.
• Improper welding technique.
• Presence of oxides, rust, scale and other Impurities.
• Incorrect joint preparation.

(h) Spatter: Sometimes, the molten metal splash with small droplets or sprinkles around the melted metal is a common defect known as spatter. It is due to the gap between workpiece and electrode, speed of welding etc.

(i) Undercut: Undercut is the groove melted into the base metal adjacent to the toe of the weld. It is due -

• the non-uniform feed of the filler.
• Too fast a travel
• Bad welding technique
• Too high a heat build-up

Defect remedies:

1. Improve core venting, provide venting channels, and ensure core prints are free of dressing.
2. Reduce amounts of gas. Use slow-reacting binder. Reduce the quantity of binder. Use coarser sand if necessary.
3. Apply dressing to cores, thus slowing down the rate of heating and reducing gas pressure.
4. Dry out cores and store dry, thus reducing the absorption of water and reducing gas pressure.
5. Reduce the moisture content of sand. Improve conditioning of the sand. Reduce inert dust content.
6. Improve gas permeability. Endeavour to use coarser sand.