Welded structures subjected to shock loads, alternating loads, low temperature operation or stress corrosion can lead to brittle fracture. Unfusion reduces the effective thickness of the welded structure and tends to cause stress concentrations at the edges of the unfused workpiece during use, which spreads outwards to form cracks that can lead to cracking of the entire weld.
1, the phenomenon that the root of the welded joint is not completely welded through is called unwelded. Unwelded defects can occur in both single-sided and double-sided welding.
The reasons for unwelded defects: improper selection of welding parameters, such as welding current is too small, improper angle of the electrode, running speed is too fast, butt gap is too small, arc blowing bias, improper bevel angle, etc.. Operational errors, such as no bevel double-sided submerged arc welding centre misalignment; poor bevel processing, such as one side too thick and one side too thin, or blunt edge too thick, welding current is too small.
Preventive measures for defects of incomplete penetration: higher current welding is the basic method to prevent defects of incomplete penetration. For fillet welds, to prevent magnetic bias blowing, AC can be used instead of DC. In addition, reasonable design of the bevel and keep the bevel clean, short arc welding can also prevent the generation of imperfect weld through defects.
2, incomplete fusion between weld metal and base metal or weld metal and weld metal is called unfusion. Bevel sidewalls, multi-layer welding between layers and the root of the weld often appear unfused. Sometimes, if the bond is tight but not welded, microcracks usually appear at the end of the unfused zone.
The causes of unfused defects: welding surface oil or rust, not cleaned up; welding current is too small; torch swing is not enough; bevel unreasonable, there is dead space; welders unauthorised increase in current to speed up the welding speed, etc..
Measures to prevent non-fusion: the correct welding operation, the use of higher welding current, keep the bevel clean.
