The application of Leakage magnetic test in tank

Leakage magnetic testing can find surface and close to appearance crack flaw,biggest advantage is it can discover internal corrosion pit and other flaw from external tank，and does not need to stop polish treatment on its surface，suitable for tank online testing,and tank online test is currently urgent by customer,it can reduce unnecessary lockout, reduce testing cost。In addition,leakage magnetic testing can detect some features and dimensions of flaws,such as depth and length。Leakage magnetic test applied to tank mostly are online test,use leakage magnetic technique of outer wall of the container ,dissect container wall internal flaw condition，especially internal external surface corrosion severity and distributing status。Moreover,leakage magnetic test can also applied to flaw detection of mobile pressure container。Currently ,about device that no way to stop internal inspection ,there is no other else testing method but  endoscopic view,besides due to internal external surface medium has no reason to calculate, so maybe can not stop effective observation on internal and external surface。Leakage magnetic test can used to scanning test on surface with coat of paint，and can detect corrosion pit size and depth of wall from external so as to reduce testing cost due to unnecessary park and polish,at the same time,Reduce labor intensity。Online leakage magnetic testing can be understood as tank internal and external surface status and flaw distributing status,for leakage magnetic testing result of major device and important parts,can be saved as  regular check ,to forecast tank wall thickness reduction speed,make sure device can run to next detecting circle safely。

Eddy current as one of five common nondestructive testing method，widely used in steel industry，including metal bar、wire inspection、structure crack flaw detection、material ingredient and impurity content identification、heat treatment condition identification、mixed material sorting、measuring metal thin plate thickness and so on。Recently years,along with deeper in eddy current testing technique  and computer 、instrument and development of digital signal dealing technique,eddy current nondestructive technique has achieved certain breakthrough in steel industry，for past testing limitation or”no possible” problems,discovered solutions and thinking。Such as at present ,some people come up with morethan1100℃ high temperature continuous casting slab surface flaw analog online test，make traditional eddy current test target temperature improved hundred degree,but a company in Sweden has researched eddy current testing device that test 1000℃ high temperature and other metal panel。In addition ,application of eddy current test extends to stainless steel capillaries 、wire that diameter lower than 1mm and mold liquid level test and so on。

The principle of generating eddy current

 Eddy current test is take advantage of electromagnetic induction,by testing eddy current change in workpiece to assess conductive material and its some properties of workpiece, or discover flaw nondestructive testing method。When coil crosses high frequency alternating current ,it will produce alternating magnetic field,if the magnetic field is close to metal workpiece surface,then it will induce current,simply called eddy current。The strength of eddy current is connected to conductivity 、magnetic permeability 、geometric dimension and its flaw form of metal material。Eddy current itself is also produce magnetic field， its intensity is depend on strength of eddy current, ,its direction is opposite with coil current magnetic field, It overlays coil magnetic field and then produce alternating current impedance。The change of Eddy current magnetic field will cause change of coil impedance，measure amplitude of impedance change and phase then can indirectly measure workpiece surface and close to surface  material abnormal or flaw dimension。

Applicable summary 

1  1000℃ high temperature continuous casting slab surface flaw inspection

 Limitation of Eddy current testes high temperature material is mainly depend on bearing temperature of probe，traditional eddy current testing technique can up to 550 ℃ testing temperature under high temperature condition， if adopts  water cooling probe detection,the temperature can also be improved.using wind cooling and water cooling combined method,make internal temperature of sensor maintained below 40℃,can long time bear intensive high temperature amplitude。Experiment shows ,Utilizing the high temperature probe enable to online detect depth is 1.5mm,width is 0.3mm,length is 100 surface flaw of casting that more than 1100℃ 。The technique can effectively restrain noise effect that casting surface generates,and based on computer signal dealing technique,realizing positioning 、quantitate analysis 、and printing record on thermal stat casting blank surface flaw  ，providing technical base for achieving continuous casting online nondestructive test。

According to information introduction，a company in Sweden according to eddy current technique，designing and manufacturing device that able to detect 1000℃ steel and other metal panel surface flaw。The device can make sure steel surface is scanned completely。Utilizing analyzing machine consist of computer ，divided input signal into serious flaw,nondestructive flaw and not recognize ,and enable to find any flaw position。 The device is able to accurately measure 0.5mm notch position on black surface 。

2. Detection of Stainless steel capillary 

For extremely small pipe diameter such as stainless steel capillary offline or online nondestructive flaw detection，adopting magnetic eddy current testing method is fine,but it must equip with special probe to get satisfied result。Due to pipe diameter of capillary is extremely small,at present,technology level can not manufacture internal probe。And can not use spot type probe to test，only by using external probe to test,after calculating and preferring width,thicknessof coil ,distance between coils,interval between probe and capillary,wire diameter, equipped with tailor made super special probe，when detecting frequency is 666khz,detect on stainless capillary with Φ1mm and Φ0.45mm all get good results。

3  online detection of steel wire 

Wire online testing generally use two methods:one is rotating probe type,eddy current probe around the steel wire at high speed,This method is mainly used to detect the crack along the longitudinal extension wire, scratch and drawing scratches。Relative to the movement of the wire，The probe trajectory is spiral。Use multiple detectors for high speed rotation can up to detection，But its surface flaw detection sensitivity is limited。Not easy to maintain a constant distance between the probe and the wire,Clearance is increased sensitivity is decreased,If the wire eccentric,Gap will change,adopting high speed processor can automatically sense interval,and keep compensating，improving sensitivity of the system 。The other is a round coil, wire through from the toroid,Transducer to effectively check the distribution of the eddy current in a profile,and compare to a profile,suitable testing point form flaw and circumferential crack,For transverse crack, crack, inclusion, v-shaped pits and folding has a high sensitivity. Inspection speed, wide in diameter。

Driving current of Circumferential coil is higher than rotating type, have better depth penetration。System stability is good，Is not affected by temperature change and other factors,when ferromagnetic material is below 800℃，Magnetic saturation after the signal is restrained，But you can avoid magnetic saturation by adjusting the magnetic field strength，improve sensitivity,Now mostly use surround coil ,above two methods can be used in combination,eddy current is widely used in production of wiredrawing，Oil tempering production line,cold-heading wire or spring steel wire,

Water around the coil for testing steel wire rod with temperature over 1100 ℃,its testing speed is more than  500km/h.

4  .liquidlevelmeasurement

The accurate detection of the crystallizer liquid level in the process of continuous casting production the key to realize the liquid level automatic control ,Eddy current type liquid steel level gauge has quick response speed, measuring precision is high,no other special security,arrange and maintain are easy。Practical process is fast, this measuring system adopts eddy current sensor to measure liquid steel level。The 50 KHZ high frequency signal is produced by the oscillator supply sensor of the primary coil (excitation coil), due to affected by eddy current internal liquid steel，producing alternating magnetic field by primary coil is changeable with liquid level。Producing voltage V V2

By second coil is  in direct proportion to coil magnetic field intensity,and then

Different voltage is changeable ,by amplifying V1-V2,phase,frequency ,amplitude analysis and  linearization, sent to 16 set high properties single chip 80C196KCto deal, and get liquid level altitude measuring signal。Transform into 4-20mA signal by controlling machine and sent to crystallizer liquid level controlling system PLC。.this controlling machine measuring range is 0-150mm,resolution is 0.1mm,it shows this liquid level controlling system property is stable and reliable,using precision is ±3mm,not only reduce casting surface crack,improve product quality ,but also economic。