The direct harm of incomplete penetration to the welded structure is to reduce the bearing cross-sectional area and reduce the mechanical properties of the welded joint. The stress concentration caused by lack of penetration is far more harmful than the reduction in strength. Welded structures subjected to alternating loads, shock loads, stress corrosion or operating at low temperatures often result in brittle fractures.
The lack of fusion not only reduces the effective thickness of the welded structure, but also during the use of the workpiece, stress concentration is likely to occur at the edge of the non-fusion, and cracks are formed here, resulting in the cracking of the entire weld.
(1) Incomplete penetration Incomplete penetration is the phenomenon that the root of the welded joint is not fully penetrated. Both single-sided welding and double-sided welding may produce incomplete penetration defects, and the morphological characteristics of incomplete penetration in the weld are shown in Figure 1-10.
1) Causes of incomplete penetration defects:
Improper selection of welding parameters, such as too small welding current, too fast running speed, improper electrode angle, arc blowing, too small butt gap and improper groove angle, etc., lack of penetration has little to do with welding metallurgical factors;
Operation mistakes, such as center alignment when double-sided submerged arc welding without groove opening, etc.;
Poor groove processing, such as too thick blunt edge, or one side is thick and one side is thin, and the welding current is too small.
2) Preventive measures for incomplete penetration defects:
Welding with higher current is the basic method to prevent underpenetration defects. For fillet welds, use AC instead of DC to prevent magnetic bias blowing.
In addition, measures such as designing the groove reasonably, keeping the groove clean, and using short arc welding can also effectively prevent the occurrence of incomplete penetration defects.
(2) The part that is not fully fused between the weld metal and the base metal or the weld bead metal and the weld bead metal is called incomplete fusion.
Unfusion often occurs on the sidewall of the groove, between layers of multi-layer welding and at the root of the weld. Sometimes the gap is large, and it is indistinguishable from slag;
Sometimes the bond is tight but not welded, and microcracks tend to occur at the end of the unfused area.
1) Causes of unfused defects:
The welding surface is not cleaned, and there is oil or rust; the groove shape is unreasonable, and there are dead corners; the welding current is too small; the welding torch does not swing sufficiently: the welder increases the current without authorization to speed up the welding speed, etc.
2) Preventive measures for unfused defects:
Use a higher welding current to perform the welding operation correctly and keep the groove clean.
