Magnetic particle testing or penetrant testing can effectively find surface cracks, folds, double skin, hairline, pinholes and other surface defects. For ferromagnetic materials, magnetic particle inspection method should be preferred, because of its high detection sensitivity; For non-ferromagnetic material, such as stainless steel seamless mild steel pipe, then a penetration assay.
Magnetic detection is done by leakage magnetic field in the vicinity of the defect in order to detect the accumulation of ferromagnetic material at or near the surface of a nondestructive defect detection method. Is simply the magnetic material by magnetic steel, iron, cobalt, nickel, etc., detection of surface microcracks. For example, engines, bearings, steel rapid detection of surface cracks. The object to be measured is placed in a strong magnetic field or by a large current to the magnetization, if the surface or near the surface defects (cracks, folds, inclusions, etc.) exist, because they are non-ferromagnetic, the magnetic field lines through the resistance large magnetic field lines in the vicinity of these defects will produce magnetic flux leakage. When the good magnetic permeability (usually magnetic iron oxide powder) is applied to the object, the defect will suck nearby magnetic leakage field, piled a visible magnetic track marks, thus bringing the defect appears. Magnetic particle inspection is defective or near the surface of the workpiece is magnetized, when the defect orientation direction angle of the magnetic field, due to the defect permeability changes, the magnetic field lines to escape the surface, a leakage magnetic field, forming a magnetic powder adsorption marks.
Reserved resection margin when both ends is small, causes the detection apparatus, both ends of the head may not be effectively detected, and ends there is most likely the site of cracks or other defects. If there is a potential end tendency to crack, the installation of the weld heat affected also possible to make the potential crack. Therefore, it should be noted that certain areas of the welded steel pipe detection, to detect defects in steel tip extensions.
Use the online austenitic seamless insulation is damaged or when rain may have infiltrated the site, should pay attention to the infiltration testing to find stress corrosion cracking or pitting corrosion and other defects. However, magnetic particle or penetrant testing can only detect the outer surface of the steel pipe, internal and surface defects are powerless. Inner surface of the test, especially crack class defect detection through ultrasonic testing must be carried out.
