A new technology of magnetic particle detection -- low-frequency magnetic flaw detection
關鍵詞：低頻、脈沖、充電Key words: low frequency, pulse, charging
※ Improve the inspection depth, detect deeper defects on the surface and near the surface.
※ There is no need to demagnetize after inspection, and the magnetic powder has good disturbance.
※ It is used in the environment without external power supply.
1
Skin effect：
Skin effect is also called skin effect, when the alternating current through the conductor, the current will be concentrated on the surface of the conductor flow, this phenomenon is called skin effect. The depth of magnetization of the workpiece is inversely proportional to the frequency of the current.
※ AC can only effectively detect defects within 1 mm near the surface, DC and half-wave rectification inspection depth is large, but the inspection speed is slow, and demagnetization is required after the inspection.
※ Low frequency current is basically close to the detection depth of direct current, and has the characteristics of alternating current, no demagnetization after detection.
2
Pulse current：
※ The pulse current can make the row of magnetic powder slightly vibrate, which helps to arrange the magnetic powder and form magnetic marks.
※ Rechargeable battery DC inverter high voltage frequency conversion numerical control technology, using high energy lithium, through the full effect inverter voltage boost, as the pulse source.
※ DC pulse current phase modulation frequency conversion principle, instant start impulse magnetization current is large, different magnetization frequency produces different pulse width.
The low-frequency magnetic flaw detector adopts the principle of DC pulse current phase modulation and frequency conversion to magnetize the magnetic yoke. Because it is driven by DC pulse current, the impact magnetization current of instant start is large, and the magnetic flux generated is ten times that of the traditional AC magnetic field flaw detector with power frequency 50 (Hz). Different pulse widths are generated with different magnetization frequencies to make the equipment consume the lowest energy when the detection sensitivity is highest.
※ Low voltage DC inverter technology, lightweight, small volume design using high energy lithium, through the full wave efficient inverter boost, as a pulse source. The efficiency of this circuit is more than 93%, and the conversion efficiency of the same type of products is less than 50% by half-wave mode.
※ Low voltage DC inverter technology, lightweight, small volume design using high energy lithium, through the full wave efficient inverter boost, as a pulse source. The efficiency of this circuit is more than 93%, and the conversion efficiency of the same type of products is less than 50% by half-wave mode.
※ CNC high voltage pulse controller, pulse frequency controllable, according to different test requirements set different working frequency, can effectively test the measured workpiece defects;
※ Frequency conversion control multi-frequency combination, dual frequency in line with the application, can simultaneously measure the workpiece surface defects and sub-layer defects, surface defects with high frequency current, near surface defects with low frequency pulse current;
※ The application of high permeability magnetic materials can effectively improve the electromagnetic conversion rate, small magnetic loss, high permeability, low power consumption, which is conducive to miniaturization design and energy saving and environmental protection;
※ Low frequency pulse energy saving, especially suitable for batteries, the use of lithium battery DC more than three times the time, a full charge can work continuously for more than 8 hours.
※ Low-frequency magnetic flaw detector DC magnetization lifting force up to 235 N, AC magnetization lifting force up to 98 N. Due to the greatly increased lifting force of the yoke, it is easier to find the display of high sensitivity test pieces (A1-15/100, D-7/50 or Type C (linear)8/50).
※ It can detect the test block of 8 mm under the strip layer, and the technical index is significantly higher than the standard requirements, which can greatly reduce the leakage efficiency of magnetic particle detection to ensure the safe operation of products and equipment.
※ Low-frequency magnetizing current is a new magnetic flaw detection method, combining the advantages of DC, AC, pulse current and other magnetic flaw detectors, with the function of detecting buried defects, it breaks through the current magnetic particle detection can only detect metal surface defects limitations, and can play the same effect as X-ray detection for steel within 8 mm vertical depth from the surface. It makes up for the shortcomings of ultrasonic inspection of thin steel plate. High sensitivity, no need to grind the surface of the workpiece can be directly detected, no need to demagnetize after detection, fast detection speed, small size, light weight, energy saving.
※ Changed the past requirements for lifting force, low labor intensity; The probe design has special probes such as blades, pipes and containers according to different workpiece shapes. With a special probe, it can detect parts and workpieces that are difficult to detect, such as blades, pipes and containers, in a large area.
est results of pressure vessel testing station of China Aerodynamic Research and Development Center:
Engineering site inspection:
The yoke type low-frequency magnetic flaw detector was used for 100% magnetic particle detection of butt weld on inner surface and fillet weld on outer surface of 2000 vertical medium pressure steel spherical storage tank. Magnetization with 20/10/3 Hz excitation current was used, and 5 cracks were found to exceed the standard.
A rotating magnetic field type low-frequency magnetic flaw detector was used to detect 100% magnetic particle of the butt weld on the inner surface of the 2000 vertical medium pressure steel spherical storage tank during periodic inspection. Magnetized by 40/10 Hz excitation current, 3 cracks were found to exceed the standard.
· Application of new technology of low frequency magnetic detection ·
※ The probe can be self-priming on the surface of the workpiece, saving time and labor, high efficiency
※ The contact head can be rotated, and the effective contact surface of the probe is large, the sensitivity is high, and the lifting force is large
※ Suitable for vertical and suspended surface detection of large workpieces
※ Instrument used: self-priming rotating magnetic field flaw detector
※ The probe is adsorbed on the surface of the workpiece, so that the contact head with a rotating Angle can move freely on the workpiece, and after the low frequency current is passed, the defect is displayed with a display film
※ Automatic crawling to the container, pipe detection position
※ Automatic control of magnetic spray suspension and automatic magnetization
※ Video inspection and video recording
※ White and ultraviolet lighting
※ Permanent magnet adsorption, 8-wheel drive, steering
※ Can detect the base material and weld of the container pipe, especially the longitudinal weld and girth weld of the pipe about 300 mm
1
Pressure vessel：
For containers and plates with a wall thickness of less than 8 mm, the display film can be attached to one side, the magnetic field applied with a yoke probe on the other side, or the probe and the display film can be placed on the same side. When the steel has defects, the resulting leakage magnetic field adsorbs the magnets in the display film, forming magnetic marks.
2
Forgings - Blades：
Without tension and girding blade, the display film can be wrapped on the blade, the low-frequency current wire is snared on the blade, if there is a transverse defect, there is a magnetic trace in the display film.
For a blade with a rib and a band, place the yoke probe on the blade, and then sprinkle the magnetic powder, and the defective part will accumulate magnetic powder. Low-frequency magnetic flaw detector for blade detection, the magnetic field can penetrate the cross section of the blade.
3
Forgings - turbine journal：
At present, ultrasonic detection is generally used in the center hole of turbine main shaft, but the detection effect of inner surface is not ideal and the technology is complicated. This problem can be solved by using display film and low frequency magnetic flaw detector. The specific usage is: the display film is placed into the inner surface of the center hole, and then the cable is pierced into the center hole, and the high-power low-frequency magnetic flaw detector is energized to test, and the results are shown after the film is removed. This method can detect defects at a vertical depth of 10 mm from the surface.
Trace the elimination process
(Q1 grinding length 135mm, depth 11mm)
In the process of magnetic particle inspection, it was found that there were cracks in many fillet welds.
In the process of magnetic particle inspection of boiler equipment, cracks were found in many fillet welds.
Lower cylinder crack Defect (15 mm) Cracks in the hole of the lower cylinder bolt
After magnetic particle inspection, cracks were found on the outer surface of the lower cylinder, the length of 8 ~ 25 mm.
In the process of magnetic particle inspection of the fillet weld of the header (ceiling, economizer, water wall header) in the thermal power plant, the fillet weld of the pipe seat has some defects such as cracks, bad forming and pipe damage.
In the process of magnetic particle inspection of the fillet weld of the header (ceiling, economizer, water wall header) in the thermal power plant, the fillet weld of the pipe seat has some defects such as cracks, bad forming and pipe damage。
In the process of magnetic particle inspection of the fillet weld of the header (ceiling, economizer, water wall header) in the thermal power plant, the fillet weld of the pipe seat has some defects such as cracks, bad forming and pipe damage。
Q2-2（35 mm） Q3（40 mm）
1#Middle partition sleeve upper half Q1-1（50 mm）
Q1-3（140 mm）其他位置裂紋缺陷（30 mm）
crack（50 mm） crack（60 mm）
crack（250 mm）
Crack defect（145 mm）
