Magnetic particle testing (MT) is one of the oldest and most reliable NDT techniques, but it has not been fully recognized. In the field of materials testing, it is often seen as an unattractive but essential test.

Although many variables need to be considered in the process of testing materials, as long as the testing process is carried out by a qualified operator, using reasonable equipment and according to strict procedures, this testing method is not enough. It will highlight its great advantages: straightforward, simple, reliable results. However, due to its relative simplicity, abuse and misuse often occur.

The origin of magnetic particle testing

There are many theories about the origin of magnetic particle inspection technology. One of them is that in 1868, S.H.Saxby accidentally took a magnetic compass and passed around a magnetized gun barrel and found a crack in the barrel, but history does not tell people that the cracked barrel was finally repaired. or was directly discarded.

The earliest recorded potential application of magnetism as a non-destructive testing technique was in April 1919, when W.E. Hoke applied for a national patent for a precision gauge block he had developed. It is generally believed that when the surface of the weight block is precision ground, small metal shavings collect in the tiny cracks created by the grinding process.

However, regarding the origin of magnetic particle inspection technology, most people agree that the technology really originated in the United States and was developed under the joint efforts of F.B. Doane, Carl Betz and Taber de Forest. The early application of magnetic particle inspection technology mainly includes the detection of some railway parts, metal castings and some ferromagnetic materials.

How Magnetic Particle Testing Works

When it comes to the principle of magnetic particle testing technology, it is simply: use a suitable magnetizing force to magnetize the workpiece to be tested, and then apply ferromagnetic powder (in the form of dry powder or suspension) on the surface of the workpiece to be tested. After magnetization, the magnetic field at the discontinuities of these materials will change drastically, and a leakage magnetic field will be generated on the surface of the workpiece where part of the magnetic flux leaks, thereby attracting magnetic particles to form a sign indicating that magnetic powder accumulation (magnetic traces), under appropriate lighting conditions, reveal the location and shape of the defects, and then analyze and compare the size, shape and distribution of these defects according to the material acceptance criteria.

The magnetic particle inspection method is especially suitable for detecting linear defects in materials, such as cracks, non-metallic inclusions, incomplete penetration and some other defects that may cause magnetic leakage phenomenon.

This method is mainly used to detect discontinuities on or near the surface of ferromagnetic materials. Since the flux lines from the magnetic field are directional, the orientation of these flux lines and discontinuities must be considered. In general, the maximum response usually occurs when the discontinuity is 90 degrees from the direction of the magnetic flux lines. Although these are basic knowledge for the reader, this basic knowledge and some basic operations need to be repeated frequently, because many people still make some mistakes when adopting this relatively simple NDT method.

Magnetic particle testing equipment

The equipment used in magnetic particle testing technology can be roughly divided into three categories:

Portable equipment: permanent magnets, AC yokes, DC contacts;

Fixed equipment: horizontal wet system;

Accessories: demagnetizer box, coil, magnetic powder, exposure meter, etc.

Permanent Magnet Kit

Basic Procedures for Magnetic Particle Testing

The necessary prerequisites for effective detection are the following:

(1) Testing requirements – specifications, instructions and contract requirements;
(2) Check the material to be tested, including the type, shape, size and quantity of the material;
(3) Know in advance the available equipment and accessories;
(4) Qualifications of operators.

In the second step, the test should be strictly followed; this should be a complete, self-contained, step-by-step procedure, including all requirements, which will help to obtain meaningful, reliable, and consistent testing results.

In order to obtain meaningful data results, magnetic particle testing of materials should include the following steps:

(1) Evaluate the surface condition of the material.

Although this step is not as critical as it is in penetration testing, it has been found in practice that it is also important to understand surface roughness, as these may cause confusion during the inspection and be seen as surface discontinuities. Therefore, it is best to address the surface of the material before testing.

(2) Use an appropriate cleaning method to clean the surface of the material to remove all surface impurities that will interfere with the distribution of magnetic powder on the surface.

(3) Use the relevant technology to magnetize the sample to be tested.

(4) Evaluate the effect of magnetization treatment.

(5) In the initial test, the sample is detected at approximately 90 degrees using magnetic flux lines.

It should be noted that in some cases, if the residual magnetic field is higher than the applied magnetic field, it is necessary to degauss the sample before detection in the 90-degree direction.

(6) Evaluate the test results.

(7) Complete the test report as required.

(8) Thoroughly clean the sample and, if necessary, apply a layer of anti-rust paint to the surface of the sample.

Key Technologies of Magnetic Particle Testing

When magnetic particle inspection technology is used in some specific fields, it is necessary to comprehensively consider the selection of many key technologies.

(1) Continuous method VS residual magnetism method

The continuous method (current flowing while the magnetic powder is applied) produces the strongest magnetism on the surface of the test sample, and therefore produces the greatest flux leakage at the surface discontinuities, contributing to more pronounced magnetic marks.

The scope of application of the remanence method is limited, and it is only suitable for detecting those materials with high coercivity.

(2) Wet method VS dry method

In general, wet methods (with suspension as a dispersion medium, magnetic suspensions) are mainly suitable for stationary equipment, such as horizontal wet systems, and are preferred for inspection of smooth surfaces. These magnetic powders can also be packaged in airtight canisters for use in portable devices.

Dry magnetic powder is mainly used for AC yoke method and DC contact method.

(3) Visible magnetic powder VS fluorescent magnetic powder

So far, the most sensitive detection is fluorescent magnetic powder. Although the use of colored dry powder has always been used to form a sharp contrast with the detection surface, when using fluorescent magnetic powder to observe the detection surface under the illumination of a black light, it will be found that the background is generally black or dark purple, and the fluorescent magnetic powder is strongly The sharp contrast of the light against the black or dark purple background makes the magnetic marks more visible and easier to observe.

Wet Fluorescent Magnetic Particle Testing Using Black Light

(4) AC VS DC

It is widely believed that the use of direct current to magnetize the sample enables the detection of discontinuities on the subsurface of the sample. While this is true, this is only a general understanding and magnetic particle inspection technology should be considered as a non-destructive testing technique capable of detecting surface and shallow subsurface discontinuities of materials under suitable conditions. As for how deep the discontinuity of the subsurface can be reliably and effectively detected, it depends on many factors, mainly the orientation, size, shape, vertical distance of the discontinuity, and the magnetic properties of the material to be detected. Also, with direct current, there is always the potential for arc burns where the test sample is in good contact with magnetizing equipment that is not properly maintained.

Articulated AC yoke

Common Mistakes and Abuses

Although magnetic particle testing technology has always been regarded as a relatively simple non-destructive testing technology, there are still many wrong program operations and quick operations, resulting in unreliable testing results, mainly including the following aspects:

(1) Continuous method: This method requires applying a current (magnetizing force) to the test material while laying magnetic powder on the surface of the material; This problem is exacerbated when the material has low coercivity.

(2) Two directions: As mentioned above, the magnetic field must be applied in at least two opposite 90-degree directions to ensure accurate detection of material discontinuities. There have been previous cases where the magnetic field was only applied in one direction, which did not guarantee that all discontinuities on the surface of the material would be detected.

(3) Improve the background: In order to improve the sharp contrast between the magnetic marks and the test surface, an effective method is to apply a fast-drying white background paint on the surface of the material before applying the magnetic powder. While this improves the visibility of the magnetic marks, it is considered a time-wasting additional step, but overall the extra time and slight white paint cost are worth it for the effect that can be achieved.

(4) Terminology: One of the most confusing aspects of magnetic particle testing technology, or even non-destructive testing technology, is the inappropriate use of terms like “defect”.

The generally accepted definition of “indication” is: a response or evidence of a response using non-destructive testing techniques that still requires further evaluation to determine the full meaning of the response.

“Discontinuities” are generally defined as imperfections, imperfections, and other things that are not part of the normal structure of a material.

It is very important to use these terms correctly.

Several common reference standards:

(1) ASTM E709: Standard Guide for Magnetic Particle Testing;
(2) ASTM E1444: Standard Practice for Magnetic Particle Testing;
(3) A275-15: Standard Practice for Magnetic Particle Testing of Steel Forgings;
(4) ASTM A966/A966M-15: Standard Practice for Magnetic Particle Testing.

Advantages of Magnetic Particle Inspection Technology

1 It is very reliable and sensitive for discontinuity detection results on the material surface and near-surface;
2 The testing equipment is easy to carry and can be automated;
3 indications (magnetic marks) are directly reflected on the surface of the test material;
4 can be more clearly observed by applying a coating (such as paint) to the surface of the material;
5 Compared with penetration testing, it does not require too much surface treatment;
6 Compared with other non-destructive testing techniques, the equipment used is relatively cheap;
7 Compared with other non-destructive testing techniques, it is easier to operate and requires the least amount of training.

Limitations of Magnetic Particle Inspection Technology

1 Requires detection in at least two opposite directions;
2 Certain metals, such as aluminum, magnesium, and most stainless steel cannot be detected by this method;
3 It can only be used to detect surface and near-surface defects of materials, and cannot detect deeply buried defects;
4 A demagnetization step may be required;
5 It may take a certain amount of time depending on the size, quantity, structure and other factors of the test sample;
6 Visual inspection is usually used for defects, and technical experience is required for the judgment and interpretation of magnetic traces.

Summary of Magnetic Particle Testing

In conclusion, magnetic particle inspection is a very effective non-destructive testing technique for detecting discontinuities on and near the surface of materials. It has fast detection, low cost, high sensitivity and reliable results, and is widely used in the detection of material structures such as various new buildings and buildings in service.

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