I. inappropriate surface conditions

Surface pollutants such as oil stain, oxide scale and residual spray coating on the surface of workpiece often adsorb magnetic particles, which will interfere with signals and weaken or cover up some related displays. For example, in daily work, aircraft engine parts, such as gears and low-pressure turbine shafts, are often encountered. Because its working environment needs full lubrication, there are often oil stains on the surface of the workpiece to be inspected. For example, after on-site welding and manual grinding of aircraft engine parts, it is necessary to carry out magnetic particle testing, and metal oxide scale often remains on the surface of weld area. There are also some plasma sprayed workpieces that need mechanical or chemical removal before magnetic particle detection, and the coating removal is often incomplete in actual production. These oil stains, scale, debris, residual plasma spraying layer, etc. Will seriously interfere with or affect the evaluation. Therefore, before magnetic particle testing, the workpiece must be strictly cleaned to thoroughly remove the oil, metal chips and residual coating on the surface of the workpiece. A clean and smooth surface is a prerequisite for obtaining reliable test results. This seems simple, but it is often overlooked.

Second, the change of the geometric shape and interface of the workpiece.

In magnetic particle testing, the change of workpiece geometry and interface is the most common factor leading to irrelevant display. For example, the internal keyway, the near-surface threaded hole and the gap between two adjacent holes in the structure will all lead to certain magnetic leakage, thus producing a typical irrelevant display. This situation often occurs in the magnetic particle inspection of aircraft engine parts, such as parts with sharp corners or the threads of bolts. What I want to emphasize here is that it is sometimes difficult to distinguish irrelevant display from fatigue crack display due to geometric mutation and interface change. In this case, we will generally gradually reduce the magnetization current, thus reducing the size of irrelevant display of similar parts. If this display is only slightly reduced or unchanged, it can basically be judged as a crack.

Third, the magnetization current is too high.

Excessive magnetizing current will lead to leakage magnetic fields at the edges, corners and ends of the workpiece. This phenomenon is common in longitudinal magnetization. Especially when detecting the boundary lines of some changing workpieces with steep interfaces. Usually, too high magnetization current will lead to strong uncorrelated display, which is very likely to cover up some fatigue cracks and lead to missed detection. At this time, it is best to choose demagnetization, and then choose the appropriate magnetization current to re-detect. Especially some

In workpieces with different diameters, the magnetization principle should be selected from low current to high current in strict accordance with the requirements of the specification.

Fourthly, irrelevant display caused by tracks.

When two magnetized workpieces rub together, the contact surface will produce local different polarities, resulting in irrelevant display. The position and shape of this monitor are quite different from what is usually expected. When you encounter some unexplained displays, try degaussing and retest. If the previous display disappears, it can be judged that it is the influence of the magnetic track.

Irrelevant display caused by permeability change of verb (abbreviation of verb)

Due to the influence of temperature, there is local permeability change in the heat affected zone of weldment. For example, when two metals with different compositions are welded together, such as electron beam weldments, a long and narrow irrelevant display along the melting line can be found during magnetic particle inspection, which is caused by the change of magnetic permeability. When there are narrow welds such as electron beam welds, laser welds and ultrasonic welds in aircraft engine parts, special attention should be paid to distinguish between relevant and non-relevant displays.

6. Irrelevant display caused by remanence and external presence

When we use a needle or yoke to detect a workpiece or weld, there are often residual magnetic poles on its contact surface. When we detect this position in another direction, we are often confused by the display of some special shapes. At this time, the most effective evaluation technology is demagnetization and then retest.

Seven. Irrelevant display caused by metallographic structure

The change of metallographic structure will also cause irrelevant display. For example, in the heat-affected zone of weldments, due to the difference of metallographic structure, the permeability changes, which leads to irrelevant display, which has been mentioned above. Other changes in metallographic structure include plastic deformation caused by tempering structure, decarburization, cold forming, grain boundary of coarse grain structure and forging line. Most of them can only be detected in strong magnetic fields.