Magnetic particle testing (MT) is one of the oldest and most reliable nondestructive testing technologies. But few people can fully recognize it. In the field of material testing, people usually think that it is an unattractive but essential testing method. So we usually use it to inspect the pig signaller with welded base or other products.
Although many variable factors need to be considered in the process of testing materials. As long as the qualified operator operates the testing process, using reasonable instruments and equipment. And under strict procedures, this testing method will highlight its great advantages: direct, clear, simple, and reliable results. However, because it is relatively simple, people often abuse and misuse it.
Origin of magnetic particle testing
There are many opinions about the origin of magnetic particle testing technology. One of them is that in 1868, s.h.saxoby accidentally found a crack around a magnetized barrel with a magnetic compass. However, history does not tell people that they will eventually repair or directly discard the cracked barrels.
In most records, the potential application of magnetism was first used as a non-destructive testing technology in April 1919. At that time, W.E. Hoke applied for a national patent for a precision gauge block developed by himself. Generally speaking, when we accurately grind the surface of an equivalent block, small metal chips will gather and produce tiny cracks in the grinding process.
However, as for the origin of magnetic particle testing technology, most people agree that the technology originated in the United States and developed with the joint efforts of f.b.doane, Carl Betz and Taber de forest. The early application of magnetic particle testing technology mainly includes testing some railway parts, metal castings, and ferromagnetic materials.
Working principle of magnetic particle testing
When it comes to the principle of magnetic particle testing technology, it simply means:
Magnetize the measured workpiece with appropriate magnetizing force. Then apply iron magnetic powder to the tested area of the workpiece surface. Iron magnetic particles shall exist in the form of dry powder or suspension. After magnetization, the magnetic field at the discontinuities of these materials will change. And on the surface of some magnetic leakage workpiece, it will generate a magnetic leakage field. Therefore, the magnetic particles are attracted to form an indication – magnetic particle accumulation (magnetic trace). Show the location and shape of defects under appropriate lighting conditions. Then we analyze and compare the size, shape, and distribution of these defects according to the material acceptance criteria.
The magnetic particle testing method is especially suitable for testing linear defects of materials, such as cracks, non-metallic inclusions, incomplete penetration parts, and some other defects that may cause magnetic leakage.
This method mainly detects the surface or near-surface discontinuities of ferromagnetic materials. Because the magnetic field sends out magnetic flux lines with directivity. So we must consider the direction and discontinuity of these flux lines. In general, the maximum response usually occurs when the discontinuity is 90 degrees from the direction of the flux line. Although these are basic knowledge for readers, this basic knowledge of some basic operations needs to repeat frequently. Because many people still make some mistakes when using this relatively simple non-destructive testing method.
Magnetic particle testing equipment
The equipment in magnetic particle testing technology has three categories:
1. Portable equipment: permanent magnet, AC yoke, DC contact;
2. Fixed equipment: horizontal wet system;
3. Accessories: demagnetizer box, coil, magnetic particle, exposure meter, etc.
Basic procedures for magnetic particle testing
The necessary prerequisite for effective detection is to understand the following:
(1) Testing requirements – specifications, instructions, and contract requirements;
(2) Check the materials, including the type, shape, size,e, and quantity of materials;
(3) Know the available instruments, equipment, t, and accessories in advance;
(4) Qualification of operators.
The second step is to strictly follow the procedures for testing; This should be a complete, independent, step-by-step procedure, including all requirements. Using this procedure will help to obtain meaningful, reliable, and consistent test results.
To obtain meaningful data results, magnetic particle testing of materials shall include the following steps:
(1) Evaluate the surface condition of the material.
Although this step is not as important as the penetration test, the practice shows that it is also very important to understand the surface roughness. Because these parts may confuse the test and are considered surface discontinuities. Therefore, it is best to solve the surface problem of the material before testing.
(2) Use appropriate cleaning methods to clean the surface of materials and remove all surface impurities on the surface that may interfere with the distribution of magnetic particles.
(3) The samples to be tested shall be magnetized by using relevant technologies.
(4) Evaluate the effect of magnetization treatment.
(5) In the initial test, the sample is tested with a magnetic flux line at about 90 degrees.
It should be noted that in some cases if the residual magnetic field is higher than the used magnetic field. It is necessary to demagnetize the sample before testing in the 90 ° direction.
(6) Evaluate test results.
(7) Complete the test report as required.
(8) Thoroughly clean the sample and, if necessary, apply a layer of antirust paint to the surface of the sample.
Key technologies of magnetic particle testing
When magnetic particle testing technology is used in some specific fields. It is necessary to comprehensively consider the selection of many key technologies.
(1) Continuous method vs remanence method
The continuous method (current when applying magnetic particles) will produce the strongest magnetism on the surface of the tested sample. Therefore, the maximum magnetic flux leakage will be generated at the surface discontinuity, which will help to produce more obvious magnetic traces. The application of the remote agency method is limited. Therefore, it can only detect high magnetic retention materials.
(2) Wet vs dry
Generally speaking, the ng wet process is mainly suitable for fixed equipment, such as horizontal wet systems (suspension as the dispersion medium, magnetic suspension). And it is preferentially used to detect smooth surfaces. It is also possible to pack these magnetic particles in sealed cans of portable equipment.
The dry magnetic powder has the main application for the AC yoke method and DC contact method.
(3) Visible magnetic particle vs fluorescent magnetic particle
So far, the most sensitive detection is fluorescent magnetic particles. Although the use of colored dry powder has always been able to make a sharp contrast with the test surface. However, when using fluorescent magnetic particles to observe the test surface under black light, we will find that the background is generally black or dark purple. The sharp contrast between the strong light of fluorescent magnetic particles and the black or dark purple background makes the magnetic trace more obvious and easier to observe.
(4) AC vs DC
In general, DC magnetization can detect discontinuities on the sample surface. Although this is true, it is only a general understanding. The magnetic particle testing technique shall be considered a non-destructive testing technique. Because it can detect the discontinuity of the material surface and the shallow surface under appropriate conditions. As for the discontinuity of multiple depths underground, it depends on many factors. For example, the direction, size, shape, vertical distance of the discontinuity, and the magnetism of the material. In addition, when direct current is used, there is always the possibility of arc burns where the sample is in good contact with the magnetizing equipment that is not properly maintained.
Common misunderstanding and abuse
Although magnetic particle testing technology has a role as a relatively simple non-destructive testing technology. There are still many wrong program operations and quick operations, resulting in unreliable testing results. It mainly includes the following aspects:
(1) Continuous method:
This method requires applying current (magnetizing force) to the test material and laying magnetic particles on the surface of the material. In some cases, the residual magnetic field becomes weak when placing the magnetic particle after applying the current. Especially when the magnetic retention of the material is low.
(2) Two directions:
As mentioned above, the operator must apply a magnetic field in at least two opposite 90 ° directions to ensure accurate detection of material discontinuities. In the previous case, it was only necessary to apply a magnetic field in one direction. This does not guarantee that all discontinuities in the material surface can be detected.
(3) Improving the background:
To improve the sharp contrast between the magnetic trace and the test surface, an effective method is to apply a fast-drying white background paint on the material surface before applying the magnetic particle. Although this can improve the visibility of the magnetic trace, some people think it is an additional step that wastes time, but on the whole, this extra time and the slight white paint cost are worth the effect.
(4) Terminology:
In magnetic particle testing technology, even in non-destructive testing technology, the most puzzling point is the improper use of some terms such as “defect”.
The generally accepted definition of “indication” is a response or evidence of response obtained by using non-destructive testing technology, which still needs further evaluation to determine the complete meaning of this response.
Definition of “discontinuity”: defects, flaws, and other normal structures not belonging to materials. 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 testing technology
1. The discontinuity detection results on the material surface and near-surface are very reliable and sensitive;
2. The testing equipment is easy to carry and automatic;
3. Indications (magnetic traces) are directly reflected on the surface of the tested material;
4. Coating the material surface can make a more obvious observations (such as paint);
5. Compared with the penetration test, it does not require too much surface treatment;
6. Compared with other non-destructive testing technologies, the equipment used is relatively cheap;
7. Compared with other NDT technologies, it is easier to operate and use and requires the least amount of training.
Limitations of magnetic particle testing technology
1. The operator needs to detect in at least two opposite directions;
2. Some metals, such as aluminum, magnesium, and most stainless steel cannot be detected by this method;
3. It can only detect surface and near-surface defects of materials. And it cannot detect deeply buried defects;
4. It may require a demagnetization step;
5. It may take some time according to the size, quantity, structure, and other factors of the test samples;
6. We usually inspect defects by visual method, and it requires technical experience to judge and explain magnetic traces.
Summary of magnetic particle testing in Pig Signaller with Welded Base
In a word, magnetic particle testing is a very effective nondestructive testing technology. Which is mainly to detect discontinuities on the surface and near-surface of materials. It has the advantages of fast detection, low cost, high sensitivity, and reliable results. It has a wide range of applications in the detection of various materials and structures. Among our products, pig signaller with welded bases, chemical syringes, and corrosion coupons all have welded bases. Therefore, we will test the products in this way after production. If any user needs magnetic particle testing, he can also explain it to our staff, and then we will provide corresponding certificates.
