·Table of Contents
·Methods and Instrumentation
Hot Wire Inspection using Eddy CurrentJin-su Bae
Instrumentation & Control Research Group, Technical Research Laboratories, POSCO, Korea
Instrument and Space Research Division, Southwest Research Institute, U.S.A
2.1 Guiding device for hot-wire inspection
The guiding device consists of four roller guides and six guides on the guide converting system controlled with electric motor and cylinder as shown in fig 1. It has also air purge hole to remove a scale on the wire and water jacket to cool the sensor and sensor holder. To reduce the vibration of wire and keep a good fill factor, the guide device for wire of size Æ14 - Æ34 consists of five groups of guide with inner diameter of guides, 20.5 mm, 23.5 mm, 27.5 mm, 31.5 mm, and 39 mm. In the case of ECT for wire of size, 15 mm, the inner size of guide and ECT sleeve are respectively 20.5 mm and 21 mm. It is very important to select a proper guide group in in-line ECT. If we select wide guide compared to the inner size of ECT sensor, the working condition for wire production is good but the fill factor is bad. So that the guide group has to be selected in order to satisfy both the working condition and fill factor.
|Fig 1: Guide converting system|
2.2 Signal processing for ECT of hot wire
The entire wire under test is subdivided uniformly into sections of equal length and evaluated. All sections are a smallest evaluation units (SEUs) whose minimum length is related to the data processor and speed of wire. Fig 2 schematically shows the relationship between a raw signal(analog signal) measured with hot wire having various kinds of flaws and a graph in a user monitor. The bars in the user monitor are gotten from an analog signal processing for the shortest evaluation time. The detection of periodic defect using DSP is explained in experiment with a spalled roll in the following section.
|Fig 2: Relationship between an analog signal from ECT sensor and a graph in user monitor.|
2.3 User monitor for ECT of hot wire
|Fig 3: User's Monitor for ECT of Hot wire|
2.4 Configuration of hot wire inspection system for ECT
Fig 4 shows, in a schematic form, the configuration of the test system comprising the transmitter system, tester (Defectomat CS), and a very efficient computer for rapidly measured data acquisition and processing. The computer link-up between the evaluation computer and supervisory control computer having material and line information gives the more information to user. The test data processor is designed to process an analog signal for smallest evaluation units and performs the FFT technique using DSP which detects the small periodic signal without any delay during high speed detection.
|Fig 4: Block Diagram of the Overall System for Hot-Wire Testing|
The ECT system was estimated with artificial defects in wire made by spalling of roll and drilling in billet by using oxygen torch.
3.1 Spall of roll
|Fig 5: ECT results for wire rod Æ 21 of worked with a spalled roll|
3.2 ECT of artificial defects made with oxygen torch
Fig 6 shows experimental results for wire rod of Æ21 pressed with a billet of artificial defects made with oxygen torch. The test results in user monitor are represented with colored bars. Those of the previous two coils remain on the screen until the next coil will be completely tested, which makes it possible to identify quality trends by comparing the results of test.
|Fig 6: Experimental Results for wire of Æ 21 pressed with a billet of artificial defects|
There are two advantages of using developed ECT system for the surface defect detection of hot wire. The one is the guiding system having four rollers and six guides to keep straightness and minimize the vibration in case of feeding hot wire into the ECT sensor. The other is the signal processing of data, which consists of the digital signal processing card with Fast Fourier Transform to detect periodic flaws by roll or roller spall without any delay during high speed work of hot wire, in addition to assessing individual defect with an eddy current signal.
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