Can PT be used in place of MT?
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A batch of equipment handling highly hazardous media, made of carbon steel, requires 100% MT testing per design specifications. Now, manufacturers require PT to be used instead of MT for butt welds with DN < 250. 1. Can it be used as a substitute? 2. What is the basis for this?
Other manufacturing units have raised this issue before; in order to simplify things, those units want to change the original MT to PT, but it is recommended not to make any changes. Clause 3.2.10.2.1(4) of TSG 21-2016: Surface inspection of welded joints in pressure vessels made of ferromagnetic materials should preferably be carried out using magnetic particle testing ； Clause 10.1.3 of GB/T 150.4-2011: Magnetic particle testing shall be preferentially used for the surfaces of welded joints in containers made of ferromagnetic materials. NB/T 47015.1-2015, Clause 5.1.7: For ferromagnetic materials, magnetic particle testing should be preferred for detecting surface or near-surface defects; other non-destructive testing methods may be used only when magnetic particle testing is not feasible due to reasons such as the shape of the structure. PT can only detect surface openings in materials; it cannot show the depth of these defects, nor their internal depth and size. The equipment required is simple, the operation is easy and fast, yet it is difficult to achieve quantitative control over the inspection process. The quality of the inspections depends on the experience, sense of responsibility, and attitude of the inspectors. MT can detect surface openings and near-surface defects in magnetic materials; the detection depth is 1–2 mm. It allows for an accurate visualization of the shape, location, and size of these defects, and it is possible to roughly determine their nature. It has high sensitivity, enabling the detection of defects with a minimum width of 1 micron; however, it is not possible to determine the depth and height of these defects.
For the surface inspection of welded joints in pressure vessels made of ferromagnetic materials, magnetic particle testing should be given priority. In other words, for those with a diameter of DN65 or more, MT testing must be used; whereas for those with a diameter of DN50 or less, where MT testing is not applicable, PT testing can be used.
For surface inspection of welded joints in ferromagnetic materials, magnetic particle testing should be given priority; MT should be chosen whenever possible. When it cannot be used, select PT detection.
Substitution is not allowed; PT is used to detect surface defects, while MT is used to detect near-surface defects. The 2nd floor offers authoritative explanations.