Defect In Welding

Defects in welds are an unpleasant phenomenon since to some extent they are the cause of extremely cost-intensive weld dressing. In many cases defects and damage may be prevented by taking simple precautions. Measures such as these may be implemented at various stages of planning and fabrication. They may range from choosing the best possible filler metal to mastery of the welding procedure and regular maintenance of the power source.

This subsection does not provide a complete overview of possible welding defects but limits itself
to types of defects that can be prevented by relatively simple measures. In addition to a description of the defects and its causes, the following tables also contain possible countermeasures that may have a favourable effect. Refer to standards, welding guidelines and other literature for more detailed information.

Occurrence and Prevention of Defects
The following list provides explanations for possible unfavourable phenomena when welding steels and presents measures to prevent these defects. Basically speaking most of the phenomena listed can be reduced by optimising the chemical composition of steel and filler metal. The list is not ranked according to the importance of the defects described.

1. HYDROGEN-INDUCED CRACKS

During welding atomic hydrogen diffuses into the weld metal and into the heat-affected zone. During and after cooling this may lead to crack formation especially in areas of higher internal stresses and high dislocation density (e.g. at grain boundaries in martensite).

COUNTERMEASURE

• Use filler metals that lead to a very low hydrogen content in the weld metal.
• Re-dry the filler metal.
• Prevent the application of high stress.
• Preheat the joint.
• Use low-hydrogen postweld annealing.
• Reduce hardness increases in the weld
• metal and the heat-affected zone.

2. DUCTILITY TOO LOW
Generally speaking the ductility of specific parts of the heat-affected zone compared with the base metal is adversely affected by coarse grain formation or hardness increase.

COUNTERMEASURE
Select the optimum welding temperature cycle.

3. SOLIDIFICATION CRACKS

Solidification cracks are mainly linked with trace elements such as sulphur and phosphorous.
These may be precipitated in the centre of the bead during solidification. They are the result of
the formation of low fusion films around the grain boundaries. These films reduce the deformability of the weld metal and longitudinal cracks may form due to shrinkage stresses on solidification of the weld metal.

COUNTERMEASURE

• Modify the welding parameters so that the individual
• beads become broader and flatter, i.e.
• reduce the depth to width ratio of a bead.
• Reduce dilution with the base metal.
• Reduce the welding speed.
• Note: solidification cracks rarely occur in steels
• with low proportions of sulphur and phosphorous.