Tube mill welding is a widely used method in various industries, including fluid transportation, construction components, and hardware furniture production. The quality of welded tubes is crucial, as they should be free from defects such as cracks, incomplete fusion, and surface imperfections. In order to ensure the quality of tube mill welding, an effective inspection method is required. One such method is high-frequency welding combined with online eddy current testing, which offers fast detection and quality control capabilities.
The Significance of Online Eddy Current Testing for High-Frequency Welded Tubes
Quality Control in Tube Mill Production
High-frequency welding of steel tubes is conducted on production lines, allowing for continuous and rapid manufacturing. However, ensuring the quality of welded tubes solely through manual post-inspection is challenging. This is where eddy current testing comes into play, offering advantages such as high-speed detection, non-contact scanning, and superior sensitivity. It is well-suited for quality control and inspection in tube mill production.
Application of Eddy Current Testing in Online Inspection
Online eddy current testing refers to synchronous testing performed during the production process, primarily for quality control purposes. The inspection of welded steel tubes follows the standard outlined in GB/T7735-2004, “Nondestructive Testing of Steel Tubes by Eddy Current Testing.” The amplitude of the signals displayed by artificial defects in the reference samples is compared with that of natural defects, ensuring that the reference sample’s nominal size, chemical composition, surface condition, and heat treatment state are similar to those of the test sample.
Real-Time Detection and Reporting with Online Inspection Systems
Online inspection systems enable real-time defect detection, recording, alerting, and delayed marking functions. The inspection report data can be stored long-term on computer hard drives and printed when necessary.
Selection of Inspection Methods
Choosing the Right Testing Method
For high-frequency welded tubes produced on manufacturing lines, localized magnetization testing is recommended for inspecting longitudinal welds. This method involves using flat weld seam probes for scanning along the weld seam. Localized testing is suitable for various tube mill units and has improved reliability and versatility, allowing for weld seam deviations up to ±45°.
III. Selection of Eddy Current Testing Instruments When it comes to online inspection of welded tubes, both single-channel and dual-channel eddy current testing instruments can meet the requirements of GB7735 standards. The ZM-26A dual-channel eddy current instrument, combined with the localized testing method, offers advantages. By using absolute and differential dual-channel probes in a flat or sector scan configuration, it can effectively detect short, abrupt defects with the differential channel while providing some capability to detect gradual defects with the absolute channel. Investing in a dual-frequency or dual-channel testing instrument, if budget permits, allows for better surface and subsurface defect detection by employing different excitation frequencies.
Installation Considerations for Eddy Current Testing
Ideal installation positions for eddy current testing systems are located behind the water tank, with a recommended longitudinal length of 1 meter. It is advisable to have a set of pressure rollers in front of the testing area to ensure tube stability during inspection.
Principles of Eddy Current Testing
Eddy current testing relies on electromagnetic induction principles. A high-frequency sinusoidal current is applied to the probe coil, generating an alternating magnetic field around the coil. When the probe approaches the metal surface, induced currents, known as eddy currents, circulate within the metal. These eddy currents produce a counteracting magnetic field, which affects the impedance of the coil. As the probe moves across the metal surface and encounters defects or material variations, changes in impedance are measured by the eddy current testing instrument, allowing for the detection of surface defects and physical property changes.
Introduction to System Accessories
Probe Selection
Flat weld seam probes are used for scanning the weld area of steel tubes. These probes have smaller coil volumes, providing high sensitivity but limited coverage. The optimal distance between the probe and tube surface is within 0.6mm, and a single probe can cover multiple tube specifications.
Marking Controller
During the inspection process, if marking is required for defect locations, a delay is necessary due to the distance between the probe and marking device. The delay time, ranging from 0-64 seconds with a resolution of 10 milliseconds, allows for precise marking. Inspection reports can store up to 500 detected defects. The delay signal triggers the marking control unit, which activates the marker at the specified defect location. This delay signal can also be utilized to control flying saws or other external mechanisms, facilitating automated production, inspection, and sorting.
Audio-Visual Alarm
The audio-visual alarm is connected to the real-time alarm output of the eddy current instrument. When the probe detects a defect and displays it on the screen, the instrument sends a signal to the audio-visual alarm, activating the relay and generating an audible and visual alert.
Magnetic Saturation Device Selection
For ferromagnetic materials, uneven magnetization resulting from processing (such as cold drawing, heat treatment, rotary swaging, and welding) creates noise signals that often exceed the response signals of defects, making defect detection challenging. Additionally, the relative permeability of ferromagnetic materials is typically much higher than 1, limiting the penetration depth of eddy currents due to skin effect. To overcome these challenges when inspecting ferromagnetic steel tubes, it is recommended to use magnetic saturation devices. Localized and through-type saturation devices are commonly employed based on different inspection methods. The ideal installation position for the saturation device is before water cooling and sizing, ensuring sufficient distance from the flying saw and high-frequency unit.
Conclusion
In conclusion, tube mill welding plays a vital role in various industries, and maintaining high-quality standards is crucial. High-frequency welding combined with online eddy current testing provides an effective solution for real-time defect detection, quality control, and inspection during production processes. By selecting the appropriate inspection methods, instruments, and system accessories, manufacturers can ensure the reliable and efficient production of welded tubes while meeting stringent quality requirements.
