A new method of nondestructive testing for ferromagnetic material is developed. Based on the "Skin Effect" of current, traditional magnetic powder detection was improved by applying low-frequency current, thus the maximum depth of detectable defect in ferromagnetic material was increased to 8 mm. Moreover, there is no need to demagnetize the workpiece after detection. To indicate the shape and the position of the inside defects (including flaw etc.), a kind of indication film was invented, termed as Defect Indication Film, which is composed of transparent plastic back film, magnetic powder and white liquid. Extensive application of the method in this paper can be expected by combining the low-frequency magnetic detector (LFMD), specific probe and the portable Defect Indication Film (DIF).
Keywords: Low-frequency, Defect Indication Film, Magnetic defect detection
The nondestructive testing method (NDT) is extensively applied in industry and scientific research. In some cases, portable instrument of nondestructive testing is strongly required, for example, to detect the surface defect or inside defect in some facilities, which are difficult to move. Magnetic powder detection method is now widely used on industry. The basic principle of this method is: at first ferromagnetic work pieces are magnetized by the applied magnetic field, then magnetic lines will be deformed on the surface defect or the work pieces, magnetic powder will be attracted to the defect, thus the defect is discovered.
To magnetize work pieces, at present, the magnetizing current adopted in the portable sets of magnetic powder detection instrument are:
The advantages and disadvantages and disadvantages of these three magnetizing current are listed in Table 1.
- Alternating current (AC);
- direct current (DC) and
- commutating current (CC).
- It has high sensibility for the examination of surface defect.
- It is easy to demagnetize after detection because the magnetic field is constrained only on the surface and keeps changing.
- The direction of magnetic field is constantly changing, and this contributes to the good floating ability of the magnetic powder.
- A satisfactory detecting speed is guaranteed by the easy movement of the probe.
The detecting depth is shallow because of the skin effect, thus this method is not suitable for the detection of the defect beneath the surface. The deepest depth of magnetic field passing through steel is up to 2.5 mm with the current frequency being 50 Hz.
The magnetizing depth is deep, so is the depth of defect, which can be examined.
- The floating ability of magnetic powder is bad.
- The probe cannot travel easily and the detecting speed is slow.
- It is difficult to demagnetize after detecting.
- The magnetizing depth is deeper
- The floating ability is so-so.
- Difficult to demagnetize.
- Low detecting speed.
|Table 1 : Comparison of different magnetizing currents applied in portable nondestructive testing|
The disadvantages mentioned above can be overcome by adopting magnetizing current with low frequency. It has been theoretically proved that the relation between the penetration depth of magnetic field and current frequency is as the following equation:
d= l / Öfpml
where f is the frequency of the magnetizing current; m
and l are the magnetic permeability and electric conductivity of the detected material.
In the present paper, the application of low-frequency current increases
the detecting depth, moreover, the invention of the Defect Indication Film facilitates improves the traditional NDT method with magnetic powder.
Materials and Methods
- Portable sets of defect detecting instrument with low-frequency magnetizing current
- Specific probe
For the detection of large-scaled work piece, usually only local detection with magnetic yoke method can be applied, the detecting speed is slow. If a kind of specific cable probe is adopted, it will work as effective as a large-scaled detecting equipment of magnetic powder. The principle is as follows: welding a multicore plug and a socket at both ends of a strand cable ,for example, "1" on the plug is connected with "2" on the socket, "2" on the plug connected with "3" on the socket and so on and so forth, thus a closed magnetic field is obtained by connecting the plug with the socket. For example, wrapping a 40-core strand cable probe 10 circles on a pipe, then if the cable was supplied with 30 A current, the magnetic field obtained is 10 x 40 X 30=12000 NA, using this method is as effective as a 10000A large-scaled magnetic detecting machine.
- Defect Indication Film(DIF)
The magnetic indication film consists of a transparent plastic back film, a plastic front film with grid, magnetic powders and white liquid. The specific density of the white liquid is close to that of the magnetic powder. Then, the two components were mixed together by stirring and introducing into the plastic grid, later the back film was stuck on the front film.
During the operation of detecting, DIF was put on one side of the ferromagnetic work piece, magnetic probe can be put on both side of DIF or the opposite side of the work piece, thus the probe and the work piece will form a closed magnetic field. It is certain that a leakage flux will appear when the work piece has defect. The leakage flux around the defect will cause a trail of the magnetic powders in the DIF, the magnetic trail in DIF will remain without other magnetic field because the densities of the white liquid and the magnetic powder are almost the same. The adoption of polyester film with high strength contributes to the flexibility of DIF, this feature is especially important for some work pieces with abnormal shapes, where DIF needs to be bent or folded. Moreover, DIF can be stuck on the work pieces directly with glue according to the different shapes of the work pieces. The size of plastic grid and the fineness of magnetic powders can be determined by the requirement of the detecting sensibility.
LFMD and DIF have wide applications in industry, especially in electric power system. Some practical examples are listed as follows.
- All kinds of pipelines
For the installed joint and chamber of various pipelines which were used for years, LFMD is applicable with the help of cable probe and DIF. First the DIF was wrapped on the pipelines, then the cable probe was winded around the pipelines, finally low frequency current was supplied. So the magnetic trail will appear on the film if there are defects. By this way the peripheral defect up to 6mm under the surface can be found. If the weld overlay is polished smooth, the defect of incomplete weld and fusion can be found easily.
For axial defect the cable can be winded around the pipelines helically, and the magnetic yoke probe should be put on the pipelines directly, meanwhile the direction of magnetic field is vertical to the axis. For the uninstalled pipelines, chambers of other parts with central hole, if the thickness of their walls is less than 16mm, the cable can be put through the pipe hole, DIF should be put outside the pipelines, after that, low frequency current is supplied, if there are axial defects on the outer or inner surfaces, they will appear on the film.
- Vessels and Plates
For those vessels and metal plates whose thickness of walls is less than 8mm, DIF can be stuck on one side, on the other side the magnetic yoke probe on the same side. Once steel has defects, the leakage flux will work on the magnetic powders in the film to form magnetic trail.
For the blade without brace staying and striping, it can be wrapped by the DIF, then the inductors with low frequency was put around it. If there are transversal defects, magnetic trail will appear on the film. For the blade with brace staying and striping, the magnetic yoke probe was put on the blades, then magnetic powders were scattered, the part with defect will gather magnetic powders.
- Central hole in the axis of gas turbine
At present, for the central hole of the main axis in the gas turbine, usually ultrasonic wave detection is applied, this method does not work for the detection of the inner surface, and what's more, the technology is too complicated. With the method presented in this paper, first DIF was put on the surface of the central hole ,then the probe was put through this hole ,after the testing with LFMD, DIF can be taken out. By this way, the defect up to 10mm away from the surface vertically can be found.
Although it is well known that the detecting depth can be increased by adopting low frequency magnetizing current, it is found in our experiments that low frequency pulse current has some superiorities.
- Features of magnetic powder test with low frequency pulse current
- Deep penetration magnetic field. Experiment result revealed that man-made flaw with the size of 0.1 X 2 X 30 mm under surface up to 8mm can be detected.
- High sensibility."A"type test block can be clearly indicated.
- Good floating ability of magnetic powders. The magnetic powders can be attracted on the defects easily.
- No extra work after testing. Low frequency current can demagnetize work pieces automatically.
- Easy operation. It changes the requirement for lifting force before.
- Low requirement for the smooth finish of work pieces.
- Suitable for continual detection.
Besides the frequency of the magnetizing current, it was found in our experiments that the following factors have also influence on the detecting depth.
- The strength of the applied magnetic field. The more powerful it is, the more powerful the leakage flux strength is, and the deeper the detecting depth is.
- The size, direction and shape of defects. Leakage flux is the most powerful when cracks are vertical to the surface of work pieces. It is converse when the defects are parallel to the surface of the work piece. The higher the ratio of the thickness and width of defect is, the stronger the leakage flux is. In other words, the cracks near the surface and the incomplete weld can be found easily, but inclusion and gas hole are difficult to find.
- The superficial area of work pieces. The bigger the superficial area is, the shallower the detecting depth.
- The magnetic permeability of the material and the inclusion. The bigger the difference of the magnetic permeability of the material of work piece and the inclusion is, the deeper the detecting depth will be.
- The covering layer of the work piece. It affects the leakage flux strongly for alternating power current. the leakage flux strength is almost zero when the thickness of the paint layer is up to 1.5mm. But with the magnetic field of low frequency, the defect can still be discovered when the paint thickness is up to 5mm.
- The effect of magnetic powder. Both dry magnetic powder and magnetic liquid can be used for low frequency detection, but it would be better with the former.
- The effect of time. For low frequency detection, the result will be better with longer detecting time.
- The effect of operation method .While operating ,remove the probe away at the moment of cutting off power, it will not only reduce labor strength but also increase the detecting depth.
- The effect of lifting force. Detection with AC and DC requires high lifting force of electromagnetic yoke, but it cannot be used as a detecting index because there is no way to determine the lifting force of low-frequency magnetic detection.
- The degree of the contact between probe and work piece. For AC detecting, the work piece detected must contact well with the probe, but for low-frequency detection, defect can also be found though there is about 10mm between them.
Low current is applicable for the following magnetizing methods :
- Through current magnetization
- Through rod magnetization
- Through winding magnetization
- Through electromagnetic yoke magnetization
- Through bar magnetization
- Through induction current magnetization
- Through compound magnetization
- Advantages of DIF
At present, the dry or wet magnetic powders are mainly used in the magnetic powder detection. However, the dry magnetic powders cannot be scattered on some part of the work piece, neither is magnetic liquid. American patents (No. 3243876 and 3286346) suggest that the defect can be indicated by magnetic duplication, magnetic paint and magnetic rubber. The principle of indication with magnetic rubber is mixing magnetic powders into rubber, later after vulcanization the defect can be recorded. It takes long time to vulcanize, and this procedure is not economic.
The FIB developed in the present work has the following features:
- It can be used for flaw detection where magnetic powders can not be scattered.
- The crack trail in the film can be archived as file when there is not interference of the intensive magnetic field.
- It can be used repeatedly if necessary.
- The black magnetic powder are sealed in the indication film, for it is cleaner compared with spreading magnetic powder directly on steel.
- For the steel plate whose thickness is less than 8 mm, DIF can work equivalently as ray nondestructive testing.
- It is of low cost, simple in operation and the crack trail indicated on the film can be recognized easily.
- Low-frequency current was successfully applied in the traditional magnetic powder defect detection, according to the "Skin effect" of current , the depth of the detectable defect in work piece was obviously increased.
- A new method of defect indication was invented, Defect Indication Film overcomes the disadvantages of traditional indication methods, like rubber vulcanization. DIF is clean and portable, moreover, it can be archived and repeatedly used.
- Combining the defect detector with low-frequency current, specific probe and the Defect Indication Film, a portable sets of nondestructive instrument possess obvious advantages compared with the similar traditional methods.
- American Society For Metals, Nondestructive Inspection and Quality Control, Metals Handbook( 8th Edition), vol.11
- Henry J.Weltman, Jack D. Reynolds, John E. Halkias and William T.Kaarlela, Magnetic-Rubber Inspection, U.S.Patent
- Orlando G.Molina, Magnetic Duplication, U.S.Patent 3,243,876
- D.E. Lorenzi, Magnetic Paint, U.S.Patent 3,786,346