Introduction
Specially customized stainless steel welded pipes for petrochemical, shipbuilding and other industries should be 100% inspected or 20% sampled before leaving the factory according to customer requirements. So today this article will share with you the various defects that may appear on the film during the radiographic inspection of stainless steel welded pipes.
Classification of common welding defects on film
Common welding defects on film include:
Circular - pores, slag inclusions; Strip - pores, slag inclusions; Incomplete penetration; Incomplete fusion; Cracks and shape defects such as undercuts, root depressions, etc.
(1) Classification by defect morphology
1. Volume defects (also known as three-dimensional defects): such as pores, slag inclusions, incomplete penetration, undercuts, depressions, etc.
2. Planar shape defects (also known as two-dimensional defects): such as incomplete fusion, cracks, white spots, etc.
(2) Classification by density of components contained in defects
1. Defects with density greater than weld metal: such as tungsten inclusions, copper inclusions, weld bead inclusions, etc. They appear as white images on the film.
2. Defects with density less than weld metal: such as pores, slag inclusions, etc., appear as black images on the film.
Today we focus on understanding pores: Common pores in welds can be divided into spherical pores, strip pores and shrinkage pores.
Spherical pores: According to their distribution state, they can be divided into uniform pores, dense pores, chain pores, and surface pores. Spherical pores appear as small black dots on the film, with a relatively regular shape, larger black in the center, gradually fading along the edge, and the outline is clearly visible. Single scattered pores are lighter in black and have unclear outlines, mostly surface pores. Dense groups (greater than 5/cm2) are called dense pores, which are mostly located near the weld surface and are caused by nitrogen in the air entering the molten pool. Chain-like pores parallel to the weld axis are called chain-like pores, which often occur at the same time as incomplete penetration. A group of pores evenly distributed throughout the weld is called evenly distributed pores. Strip pores: According to their shape, they can be divided into strip pores, oblique needle-shaped pores (snake holes, worm holes, screw holes, etc.)
A. Strip pores: On the negative film, they are mostly parallel to the weld axis, uniformly black, and have clear contours. The starting point is mostly circular, gradually becoming thinner and sharper along the welding direction. This type of pore is mostly formed due to insufficient baking of flux or coating, and develops along the running direction of the welding rod, and mostly appears in the deposited metal of the base weld.
B. Oblique needle-shaped pores: On the negative film, they are mostly shown as various worm-like images, with one end maintaining a round or semicircular pore and the other end being conical. The width changes gradually and evenly (thin), the blackness is shallow and uniform, and the contour is still clear. This type of pores are mostly long strips along the crystal direction. Their appearance is related to the solidification mode of the weld metal and the source of gas. They are generally distributed in a herringbone shape (CO), and a few are tadpole-shaped (hydrogen pores).
Shrinkage holes: According to their causes, they can be divided into intercrystalline shrinkage holes and arc crater shrinkage holes.
A. Intercrystalline shrinkage holes: They are mainly long strip shrinkage holes formed by residual gas between crystals during the cooling process of the weld metal. This type of pores is perpendicular to the weld surface. On the negative film, they are mostly shown as large black blocks with clear outlines and irregular circular images. They appear on or near the axis of the weld and are also called pinholes.
B. Arc crater shrinkage holes: They are also called arc crater shrinkage holes. Shrinkage holes are mainly caused by the failure to fill the end of the weld and the failure to eliminate the subsequent weld bead. In the light black image of the weld pit (or arc crater) on the film, there is a block image with a blackness that is significantly greater than the surrounding blackness. The blackness is uniform, the outline is unclear, the shape is irregular, and there are shrinkage lines.
Conclusion
In conclusion, in non-destructive testing, radiographic testing is the most important part, because it can ensure that there are no other foreign matter, cracks, defects, etc. in the weld. It can ensure that the weld will not explode, crack, collapse, etc. due to the influence of defects when it is under pressure. Therefore, before purchasing welded pipes or welded parts that need to withstand pressure, please conduct non-destructive testing first to ensure the quality of the product.
