Saturday, 5 March, 2011

CAUSES AND REMEDIES FOR FUSION WELD DISCONTINUTIES


CAUSES CORRECTIVE ACTION
A. POROSITY
1. Contamination of work piece Clean joint area
2. Excessive moisture pickup in electrode covering Follow manufacturers recommended rebaking procedure.
3. Moisture on work surfaces Use preheating/warm up work piece.
4. High Sulphur content  Use basic coated base metal electrodes.
5. a) A long arc length
b) Excessive current
c)Higher travel speed
Change welding parameters and technique.
6. High solidification rate Use preheat
Increase heat input.
B. INCLUSIONS
1. Improper cleaning procedure Clean work surfaces and each weld run throughly. Whereever necessary use power wire brush, grinders, chisel to ensure through removal of slag.
2. Improper welding technique
a) Long arc length
b) High travel speed
c) Slag flooding ahead of welding arc.

Improve welding technique.
Reposition work to prevent loss of slag control wherever possible
Restrict weaving to minimum
3. Narrow, inaccessible joints Increase groove angle.
C. INCOMPLETE FUSION
1. Improper joint design Increase includes angle of groove joint.
Change the groove design to 'J' or a 'U' type
2. Presence of slag or oxide film Clean weld surfaces prior to welding.
3. Incorrect electrode position and operating current Maintain proper electrode position and current.
4. Improper manipulation of arc Use correct manipulation techniques to melt the joint faces properly.
D. INADEQUATE PENETRATION
1. Improper joint preparation.
a) Excessively thick root face.
b) Insufficient root opening
c) Bridging of root opening
Use proper joint geometry.
Reduce root face height.
Use wider root opening.
2. Electrode diameter too large Use smaller electrode in root.
Increase root opening.
3. Inadequate current Follow correct welding current and technique.
E. CRACKS
1. High rigidity of joint Use preheating
Relieve residual stresses minimises shrinkage stresses, using backstep or block welding sequences.
2. Poor joint fit up Adjust root opening all alignment.
3. Higher carbon content of weld metal and/or hardenable base material Use proper electrode.
Use buttering layers wherever necessary.
4. Too small a weld bead Decrease travel speed to increase cross section of bead.
Increase electrode size.
5. High sulphur content in base level of sulphur of weldmetal element like Mn. Use filler with high level of sulphur fixing element like Min.
6. Hot cracking Reduce the heat input.
Minimum joint restraints.
7. Cracking at the crater Filling up the crater before withdrawing the electrode.
Use taper poer control device.
Use back step welding technique.
8. High hardenablity Preheat the job.
Post weld heat treatment without cooling to room temperature.
9. Hydrogen induced cracking/Delayed cracking Use low hydrogen welding electrode.
Use suitable preheat and post weldheat treatment.
10. Presence of brittle phases in the micro-structure of the base material Soften the material before welding
11. Low ductility of the base material Use preheat
Anneal the base metal.
Use ductile weldmetal.
12. High residual stresses Redesign the weldmetal and reduce restraints.
Change welding sequence.
Use intermediate stress-relief heat treatment.
13. Excessive dilution Change welding current.
Use buttering technique wherever possible.