Defects Involving Inadequate Bonding
Lack of fusion
Involves lack of complete melting and fusion of some portion of the weld metal in a joint. It may occur either between parent metal and weld metal or between two layers of weld metal. This can occur in fusion welding and pressure welding. The following are the reasons for incomplete fusion:
- Improper manipulation of electrodes
- Incorrect position of electrodes
- Low welding current
- Too rapid arc advance
- Improper joint fit up
- Improper edge cleaning of slag after flame cutting
- Impurities on the face of the joints in the case of pressure welding.
If lack of internal fusion is severe it is considered to be a serious defect. Incomplete fusion at the surface is of more serious consequence. As lack of fusion in the form of incomplete penetration of unbevelled or bevelled joints is not always revealed by radiography, ultrasonic examination is recommended as a supplementary test when lack of fusion is suspected.
Incomplete penetration
This is incomplete penetration of the weld through the thickness of the joint. This usually occurs at the root of the weld or between the deposits made from both sides of a joint. In double welded joints lack of penetration may occur within the wall thickness as a 'buried' defect. Incomplete penetration at the root is more severe. The reasons are:
- Incorrect joint design.
- Improper welding parameters, particularly current.
- Incorrect electrode size.
- Incorrect polarity where DC is used.
Inadequate joint penetration is not tolerated to any extent in objects which are subjected to either tension or bending stresses. It can initiate brittle fracture under either static or dynamic loads. The unfused root causes stress concentration which could lead to failure without deformation and cracks are initiated from the unfused root. Lack of fusion and incomplete penetration are not permitted by pressure vessel code. This type of defects are easily detected by ultrasonic or radiography. In case of thin ferrous welds, magnetic particle examination can be used.