Tunnel pores represent one more type of extended pores. Usually tunnel pores with formation of contraction cavity represent the most dangerous defect in the case, if main designation of welded seam is in retaining gas or liquid; since at that exists the big possibility of leakage path formation.
The reason for pores formation is usually existence of contaminants or moisture in welding zone, which transform into gas as a result of decaying under conditions of high temperature. Electrode, base metal, shielding gas or surrounding atmosphere can besources for appearance of contaminants or moisture. However, changing the welding method can also be a reason for these pores formation. Using to long arc during welding by metallic coated electrode of hydrogen-free type can be an example. Another example – to high traveling speed of heat source when executing hidden arc welding, and tunnel pores form as a result. Consequently pores formation indicates the fact that welding works in any aspect are executed unduly.
On radiographic image pores look like clearly delineated shadow zone, since their existence signifies significant decreasing of material density. Usually this zone has round shape, except for tunnel pores. Loop with typical rounding will be visible for this type of pores.
The example of radiographic image of pores agglomeration is shown on fig. 1.