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STANDARDS PRESS OF CHINASteel tubes-The inspection method on eddy current flaw test GB/T 7735-1995 (eqv ISO 9304:1989)Substitute of GB 7735-871. ScopeThe standard prescribe the automatic flaw testing principle, method, contrast testing samples, detector apparatus, flaw testing condition & procedure, and how to decide the flaw testing result on eddy current of the seamless steel tube and weld steel tube (except the hidden arc weld steel tube).The standard apply to the eddy current test of the steel tube not less than outside diameter 4mm.The check and accept of the standard can be divided to grade A and grade B (see Table 1 and Table 2). Grade A can be regarded as the substitute of the steel tube hydraulic pressure testing way.2. Referenced StandardThe standard contains the following clauses, the clauses referenced by this standard will compose the clauses of this standard. After this standard printed, the referenced edition will be valid. Each edition will be modified, each side which use this standard should discuss the possibility of turning the most advanced edition into realities. YB 4083-92 Steel Tubes Automatic Inspection System Integrated Testing Method 3. The Inspection principle on eddy current test3.1 Eddy Current Inspection made by the principle of the electromagnetic induction. When the steel tube passes the AC coil, the eddy current will be changed the surface or near the surface of the steel tube, which leads to the change of the coils impedance or the induction voltage, then get the signal of the flaw, from the signals range and place etc, can judge the flaw.3.2 To judge the testing result of the standard, is that the signals range compared with the signal of the artificial flaw testing and natural flaw testing, that is the equivalent contrast. Contrast testing was used to set and calibrate the steel tube inspection eddy current testing apparatus.4. The method on eddy current flaw testing4.1In order to test the whole circumference of the seamless steel tube and the weld steel tube, use the technology of passing the coil eddy current testing, or the technology of the rotation-flat coil eddy current testing. See Picture 1 and Picture 2.Note: With the short distance between the two sides of the steel tube, the testing may cant be made.Note: Picture 1 is one of the diagrams to multi-coil, the multi-coil can be divided into march-past or initial coil, double differential coil etc.Picture 1 The diagram of passing the coil type eddy current flaw testing (a) Rotary flat coil technology (b) Rotary steel tube technology(The steel tube straight move opposite (The flat coil straight move To the accessories of flat coil) along with the length of the steel tube) Note: The coil in Picture 2 (a) and (b) can be made in different models. Such as Single coil, multi-coil etc, depends on the actual apparatus and the other factors.Picture 2 The diagram of the rotary steel tube-flat coil eddy current flaw testing (spiral scan)4.1.1 When take the technology of passing the coil eddy current method to test the seamless tube and the weld steel tube, the outside diameter of the steel tube which being tested should be less than 180 mm.Note: One point should be stressed that when take this flaw testing technology, the precision rate will the best when the surface of the steel tube near the coil, the precision rate will be lower according to the thickness increased, see Attachment A (Referenced Attachment)4.1.2 When take the technology of the rotary steel tube/flat coil eddy current method to test the seamless steel tube and the weld steel tube, the steel tube which being tested and the coil should be moved face to face, in order to scan the whole surface of the steel tube, the outside diameter has no limitation under this technology.Note: One point should be stressed that the crack only in the appearance of the steel tube can be found out, please pay attention to it.4.2 To test the seam of the weld steel tube, can take fan shape coil eddy current flaw testing technology, showed in Picture 3. The coil and the weld seam should be in the same straight line, in order to scan the whole seam. Note1 One point should be stressed that when take this flaw testing technology, the precision rate will the best when the surface of the steel tube near the coil, the precision rate will be fell along with the wall thickness being increased (see Attachment A).2 With the short distance between the two sides of the steel tube, the flaw testing may not being made.Picture 3 The diagram of the fan shape coil eddy current flaw testing5. Contrast testing samples5.1 Material5.1.1 The steel tube which was manufactured for contrast testing samples should have the same dimension, chemical ingredient, surface state, heat treatment state as the steel tube being tested, along with the similar electromagnetic character.5.1.2 The contrast testing samples should be level, free of dirt, no flaw which will influence the calibration.5.1.3 Contrast testing applied for setting and calibrating the not destroyed eddy current flaw testing apparatus. The artificial flaw of the contrast testing samples dimension should not be explained that the minimum dimension which the testing apparatus can test.5.2 LengthThe steel tube for contrast testing samples should meet the requirements of the automatic eddy current flaw testing apparatus, when comprehensive testing, it should be applicable to the clauses of the standard YB 4083.5.3 Artificial flaw5.3.1 ShapeThe artificial flaw shape of different methods of eddy current inspection should be applicable to the following clauses:a) Under the method of passing coil model eddy current flaw testing, the artificial flaw shape should be hollow holes.b) Under the method of rotary steel tube-flat coil model eddy current, the artificial flaw shape should be drill a hole or notch.c) Under the method of fan shape coil testing seam, the artificial flaw shape should be hollow holes.5.3.2 Place5.3.2.1 Under the method of passing coil model eddy current flaw testing, the contrast testing samples should have five hollow holes, three of them locate in the central part of the contrast testing samples, distributed along with the circumference, almost mutual angle is 120, and mutual axial distance should be less than 200mm. The weld steel tube should have one hole which being drilled in the seam. The other method is to drill one hole at the central part of the contrast testing samples. During the setting and calibrating, the place of the contrast testing samples hole should be in 0, 90, 180, and 270passing the testing apparatus respectively. And at the place of less than 200mm far from the contrast testing samples both ends should be drilled a hole each side, for testing the end reaction. The holes should be drilled through the whole wall.5.3.2.2 Under the method of rotary steel tube-flat coil model eddy current, the contrast testing samples can be drilled one hole, drilled along radial, drilling through the whole wall thickness, weld steel tube should be drilled in the seam, or add a vertical cutting slot at the outer surface of the contrast testing samples.5.3.2.3 Under the method of the fan shape coil testing the weld steel tube seam, to drill a hole at the seam of the contrast testing samples, along radial to drill through the whole wall thickness.5.3.3 Dimension5.3.3.1 Drilled holeWhen the drilled hole was adopted in the contrast testing, the dimension of the drilled hole can be divided into acceptance Grade A and Grade B, the drilled hole diameter dimension showed in Table 1. Grade A can be regarded as the substitute of the steel tube hydraulic pressure testing method. Acceptance Grade B will be indicated in the contract negotiated by the supplier and the buyer.Stainless steel tube flaw testing or regarded as one of the methods of the hydraulic pressure testing way, its drilled hole diameter is 1.2mm (but the outside diameter is less than 51mm, the drilled hole diameter will be 1.6mm), when the wall thickness of the steel tube is more than 3mm, the drilled hole diameter will be 1.6mm (but the outside diameter is less than 51mm, the drilled hole diameter will be 2.0mm).Table 1 The bit diameter of acceptance Grade A and Grade BAcceptance Grade AAcceptance Grade BOutside diameten,DmmDnilled hole diametermmOutside diameten,DmmDnilled hole diametermmD271.20D60.5027D481.706D190.6548D642.2019D250.8064D1142.7025D320.90114D1403.2032D421.10140D1803.7042D601.40D180Contract made by both sides60D761.8076D1142.20114D1522.70152D1803.20D180Contract made by both sides5.3.3.2 SlotWhen the slot was adopted as contrast testing samples, vertical slot, its shape like the Diagram 4. The slot should parallel with the main axis, the two sides of the slot should mutual parallel, slot bottom and slot edges should mutual vertical. Its dimension be divided into acceptance Grade A and Grade B, showed in Table 2. Grade A can be regarded as the substitute of the steel tube hydraulic pressure testing method. Acceptance Grade B will be negotiated on the contract made by the supplier and the buyer.Table 4 N shape slotTable 2 The dimension of the slot for Acceptance Grade A and Grade BGrade AGrade BSlot depth, h refers to the percentage of the wall thicknessSlot lengthSlot width, bSlot depth, h refers to the percentage of the wall thicknessSlot lengthSlot width, b12.5%, the min depth 0.5mm, the max depth 1.5mmNot less than 50mm or not less than two times length of the tested coilNot more than the slot depth5%, the min depth 0.30mm, the max depth 1.30mmNot less than 50mm or not less than two times length of the tested coilNot more than the slot depth5.3.4 The fabrication of the contrast testing samples 5.3.4.1 Dilled holeKeep the bit stable, and avoid to make the part overheated and bring the burr on the surface. When the bit diameter is less than 1.2mm, the drilled hole diameter should be not more than the set value 0.1mm, when the bit diameter is not less than 1.1mm, the drilled hole diameter should be not more than the set value 0.2mm.5.3.4.2 SlotThe slot was made by the machining, electric discharge, and the other ways, the slot bottom and the slot base angle can be made into circular. The permitted tolerance should be +15% of the slot depth, or +0.05mm, to select the bigger one.5.3.4.3 VerifyingThe dimension and shape of the slot or the contrast testing samples drilled hole, to verify them under the applicable methods.6 Flaw testing apparatus The flaw testing apparatus was made of the testing unit, the transporting tube unit, automatic giving a alarm signal unit or record unit etc.6.1 Testing unitThe testing unit was made of the oscillator, coil, signal treatment unit, the record of flaw signal unit etc., it should have the super inspection character. 6.2 Transporting tube unit, magnetic saturation unit and record unitThey should have the super character when use these units for flaw testing and judging the result.7 Inspection condition and procedure7.1 Inspection condition7.1.1 The steel tube eddy current inspection usually being tested after all the process of the steel tube being finished.7.1.2 The steel tube eddy current flaw testing was made by the person who got the eddy current flaw testing certificate authorized by the related department, and the operator will be supervised by the person who got the skill rank certificate or above appointed by the supplier. Before the flaw testing was verified by the third party, it should be negotiated by the seller and the buyer. 7.1.3 The steel tube being tested should be flat and straight enough to guarantee the correctness of the testing. And the steel tube being tested should be clean from the impurities, or it will influence the correctness of the result. 7.2 Inspection procedure7.2.1 After the flaw testing apparatus power on, the system should have to empty work not less than 10 min.7.2.2 Under the method of passing coil eddy current flaw testing, when the contrast testing samples passing the inspection apparatus, the flaw testing apparatus should be adjusted to that can offer the stable and clean distinguished signal. The signal was used for setting the trigger-alarm electric-limitation of the flaw testing apparatus.To drill multi holes in the contrast testing samples, the value of the minimum signal from the drilling was used to set the trigger-alarm electric-limitation of the inspection apparatus. To drill one hole at the contrast testing samples, the place of the contrast testing samples holes are 0, 90, 180and 270passing the inspection apparatus respectively by turn, the value of the minimum signal from the drilling was used to set the trigger-alarm electric-limitation of the flaw testing apparatus.Under the method of the rotary steel tube-flat coil eddy current flaw testing, the contrast testing samples were for the slot and the drilled hole, the examined signal and the value are used to set the trigger-alarm electric-limitation of the flaw testing apparatus.7.2.3 During the setting time, the mutual move speed between the contrast testing samples and the coil, it should be the same as the mutual move speed of the steel tube being tested. And adopt the same set value, such as the frequency, sensitivity, facies distinguishing, the rate of wave filter, magnetic saturation etc. In order to increase the efficiency of the testing, the sensitivity can be increased to more decibels.7.2.4 During the flaw testing of the steel tube with the same diameter, wall thickness, and the same trademark, it should be checked and verified the set value of the apparatus regularly, the method is to make the contrast testing samples pass the flaw testing apparatus. The frequency of checking and verifying the setting value is at least one time every four hours, and verifying one time respectively when the apparatus operator was changed or at the start and end of the flaw testing.At random system was adjusted or the outside diameter, wall thickness, and trademark of the testing steel tube being changed, it should be set and verified the inspection apparatus again.7.2.5 During the continuous inspection testing, it should be set and verified the value when doubt the precision rate of the apparatus any time. If the sensitivity of the apparatus dropped, enhancing 3 decibels being permitted, in case it dont give the alarm when testing every artificial flaws of the contrast testing samples, followed by the following clauses:a) To calibrate the apparatus again, then retest the steel tube which being tested from last calibration.b) Even though the sensitivity dropped 3dB from last setting, the record of every tube is clean and easy to distinguish, and can accurately distinguish which one is qualified steel tube or the questionable steel tube, retest the steel tube is not compulsory. Then reset the apparatus, continuous testing. 7.2.6 Under the method of passing the coil eddy current flaw testing, the comprehensive testing should be in accordance with the clauses of YB 4083.8 To judge the flaw testing result8.1 Qualified steel tubeAt random steel tube, passing the apparatus of eddy current flaw testing, its signal lower than the trigger-alarm electric-limitation, it should be judged as passed the eddy current inspection testing.8.2 Questionable steel tube At random steel tube, passing the apparatus of eddy current flaw testing, its signal equal or more than the trigger-alarm electric-limitation, it should be judged as the questionable steel tube.When the questionable being tested again, its signal not equal or more than the trigger-alarm electric-limitation, it should be judged as passed the eddy current inspection testing.8.3 Treatment to the questionable steel tubeTo the questionable steel tube, depends on the clauses of the product standard, one or several methods of the following treatment can be chosen:a) After the questionable part being checked out, then polish, the wall thickness of the after-polished should be within the range of the permitted tolerance. Then retest the eddy current flaw testing of the steel tube by the clauses of the standard. If its signal not equal and more than the trigger-alarm electric-limitation, it should be judged as passed the eddy current inspection testing.If the questionable part being tested with the other methods of non-destructive, the methods of the flaw testing and acceptance of the check and accept should be negotiated by the seller and the buyer:b) Cut the questionable part of the steel tube away, then to test the eddy current flaw testing again, if its signal not equal and more than the trigger-alarm electric-limitation, it should be judged as passed the eddy current inspection testing.c) The questionable steel tube was judged as the rejected steel tube of not passing the eddy current inspection.9 Inspection testing reportIf required by the buyer, the seller should offer the testing report signed by the person who got the Skill Rank certificate or above authorized by related department. Its content should contain the following clauses:a) The trademark, stove lot size No., specification, weight (or pieces ) and the standard No. of the being tested steel tube.b) This standard No., the shape of the contrast testing samples artificial flaw and the grade of acceptance.c) The length of the non-detect area at the ends.d) The result being judged.e) Operator and signer as well as its technical title.f) Inspection date.g) Organization name of flaw testing being made.
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