What are the magnetic particle inspection methods? Scope of application of magnetic particle inspection?

Date：2022-11-09 15:07:18Views：534

Magnetic particle testing is a method of observing defects with magnetic particle as the display medium. According to the type of magnetic powder medium applied during magnetization, the testing methods are divided into wet method and dry method; According to the time of applying magnetic powder to the workpiece, the inspection methods are divided into continuous method and remanence method.

After the ferromagnetic material workpiece is magnetized, due to the existence of discontinuity, the magnetic line of force on the surface and near the surface of the workpiece will be partially distorted to produce a magnetic leakage field. The magnetic powder applied on the surface of the workpiece will be adsorbed to form a visible magnetic mark under appropriate light, thus showing the location, size, shape and severity of the discontinuity. Also known as magnetic particle inspection or magnetic particle flaw detection, it belongs to one of the five conventional nondestructive testing methods.

Magnetic particle testing can only be used to detect defects on or near the surface of ferromagnetic materials. Because discontinuous magnetic marks are stacked on the surface to be tested, the shape, position and size of discontinuities can be visually displayed, and their properties can be roughly determined.

The sensitivity of magnetic particle testing can detect discontinuities with a width of 0.1 μ m。 Using multiple magnetization methods, magnetic particle testing is almost independent of the size and geometric shape of the workpiece, and can detect defects in all directions of the workpiece.

Magnetic particle testing method

1. Wet method and dry method

The suspension of magnetic particles in oil, water or other liquid media is called wet method. It is to evenly distribute the magnetic suspension on the surface of the workpiece during the testing process, and show the shape and size of defects by using the flow of the carrier liquid and the attraction of magnetic leakage field to the magnetic particles. In the wet testing, due to the dispersion and suspension performance of the magnetic suspension, the magnetic particle particles can be smaller. Therefore, it has high detection sensitivity. It is especially suitable for detecting surface micro defects, such as fatigue cracks, grinding cracks, etc. Wet process is often used with fixed equipment, as well as mobile and portable equipment. Magnetic suspension for wet process can be recycled.

The dry method is called dry powder method. In some special cases, when the wet method cannot be used for testing, the special dry magnetic powder is directly applied to the magnetized workpiece according to the procedure, and the defects of the workpiece will show magnetic marks. Dry method inspection is mainly used for local area inspection of large castings, forgings blanks, large structural parts and weldments, and is usually used with portable equipment.

2. Continuous method and remanence method

(1) The continuous method, also known as the accessory magnetic field method or the current magnetic field method, is to apply magnetic powder or magnetic suspension to the workpiece for magnetic particle flaw detection under the action of an external magnetic field. The observation and evaluation of the workpiece can be carried out under the action of external magnetic field or after the magnetic field is interrupted.

(2) Residual magnetism method The residual magnetism method is to magnetize the workpiece first, then pour the immersion suspension on the workpiece, and observe after the magnetic powder is gathered. This is a method to detect the residual magnetism of materials, so it is called the residual magnetism method.

Magnetic particle inspection: magnetic particle inspection is one of the most commonly used methods because of its intuitive display, simple operation and people's willingness to use.

Magnetic flux leakage flaw detection: magnetic flux leakage flaw detection, which does not use magnetic particle display, is commonly called magnetic flux leakage flaw detection. It often reflects defects by means of induction coils, magnetic sensors, Hall elements, etc. It is more hygienic than magnetic particle flaw detection, but not as intuitive as the former. Because magnetic flaw detection mainly uses magnetic powder to show defects, people sometimes call magnetic flaw detection directly, and its equipment is called magnetic flaw detection equipment.

Applicable scope of magnetic particle testing method

1. It is applicable to detecting surface and near surface defects of ferromagnetic materials, such as cracks with very narrow gaps between the surface and near surface and other defects that are difficult to see visually. It is not suitable for detecting deep buried internal defects.

2. It is applicable to testing iron nickel based ferromagnetic materials, such as martensitic stainless steel and precipitation hardening stainless steel, but not applicable to testing non-magnetic materials, such as austenitic stainless steel.

3. It is applicable to testing raw materials (such as steel billets) not processed, semi-finished products and finished products processed, and workpieces in service and used.

4. It is applicable to testing pipes, bars, plates, shapes, forgings, steel castings and weldments.

5. It is applicable to the detection of defects where the extension direction of the workpiece surface and near surface is as perpendicular as possible to the direction of the magnetic line of force, but it is not applicable to the detection of defects where the angle between the extension direction and the direction of the magnetic line of force is less than 20 degrees.

6. It is suitable for detecting small defects on the surface and near surface of the workpiece, not suitable for detecting shallow and wide defects.

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