Weld defects are the main cause of failures and accidents in boilers and pressure vessels. Therefore, we must have a full understanding of the hazards of weld defects.
(1) Weld crater defects have adverse effects on the strength and stress level of welded joints. The weld flash not only affects the appearance of the weld, but also masks the quality of the weld toe at the weld flash, and unmelted defects often appear in this part.
(2) Undercut is a dangerous appearance defect. It not only reduces the pressure-bearing area of the weld, but also often forms a sharper gap at the root of the undercut, causing stress concentration and easily forming stress corrosion cracks and stress concentration cracks. Therefore, there are strict restrictions on undercutting.
(3) The harmful effects of volumetric defects such as pores and slag inclusions are mainly manifested in reducing the load-bearing capacity of welded joints. If the pores penetrate the weld surface. The medium accumulates in the pores. When the medium is corrosive, concentrated corrosion will form. The pores will gradually become deeper and larger, and may even cause corrosion and perforation to cause leakage. If the edge of the slag inclusion has a sharp shape, it will also form stress concentration there.
(4) Ends and gaps with defects such as incomplete fusion and incomplete penetration are places where stress is concentrated, and cracks are likely to form under alternating loads.
(5) Crack is the sharpest kind of notch, and the radius of curvature of its notch root is close to zero. There is obvious stress concentration at the sharp root. When the stress level exceeds the strength limit of the sharp root, the crack will expand and even penetrate the entire section, causing the boiler pressure vessel to fail. Especially when the welded joint is in a brittle state, the crack propagates extremely quickly, causing brittle fracture accidents. Cracks can also exacerbate fatigue damage and stress corrosion damage.
