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How to identify weld defects and non-destructive testing methods

Weld defects:

1. Incomplete penetration: the blunt edges in the middle (X groove) or root (V, U groove) of the parent metal joint are not completely fused together, leaving a partial lack of fusion. Incomplete penetration reduces the mechanical strength of the welded joint, and stress concentration points will be formed at the notches and ends of the incomplete penetration, which will easily lead to cracks when the welded parts are subjected to load.

2. Unfused: the solid metal and the filler metal (between the weld bead and the base metal), or between the filler metal (between the weld bead or the weld layer in the case of multi-pass welding) partially fused and bonded, or In spot welding (resistance welding), the base metal and the base metal are not completely fused together, and sometimes slag inclusions are often present.

3. Porosity: During the fusion welding process, the gas in the weld metal or the gas intruding from the outside can overflow before the molten pool metal cools and solidify, and the cavities or pores formed in the interior or surface of the weld metal remain in the weld metal. The morphology can be divided into single pores, chain pores, dense pores (including honeycomb pores), etc., especially in arc welding, because the metallurgical process takes a short time, the molten pool metal solidifies quickly, and the gas generated during the metallurgical process, The gas absorbed by the liquid metal, or the flux of the welding rod is damp and decomposes to produce gas at high temperature, even if the humidity in the welding environment is too high, it will decompose gas at high temperature, etc. These gases will form pore defects when they are too late to precipitate. Although pores have less stress concentration tendency than other defects, they destroy the compactness of the weld metal, reduce the effective cross-sectional area of ​the weld metal, and reduce the strength of the weld.

Non-destructive testing:

Non-destructive testing is a testing method for inspecting the surface and internal quality of the inspected parts without damaging the working state of the workpiece or raw materials.

Commonly used non-destructive testing methods:

• Ultrasonic flaw detection: A method of inspecting parts defects by using ultrasonic energy to penetrate into the depths of metal materials and from one section to another, reflecting the characteristics of the edge of the interface to inspect the defects of the part. When the ultrasonic beam passes from the surface of the part to the probe Inside the metal, when it encounters a defect and the bottom surface of the part, a reflected wave is generated separately, forming a pulse waveform on the phosphor screen, and the position and size of the defect are judged based on these pulse waveforms.

• Radiographic inspection (X-ray, γ-ray): A flaw detection method that uses rays to penetrate the object to find the internal defects of the object.

• Magnetic particle inspection: It is a detection method used to detect surface and near-surface defects of ferromagnetic materials. When the workpiece is magnetized, if there is a defect on the surface of the workpiece, magnetic flux leakage will occur due to the increase of the magnetic resistance at the defect, and a local magnetic field will be formed. The magnetic powder will show the shape and location of the defect here to determine the existence of the defect.