GB/T 32547-2016 English PDF (GBT32547-2016)
GB/T 32547-2016 English PDF (GBT32547-2016)
GB/T 32547-2016: Method for magnetic flux leakage testing of round steel
NATIONAL STANDARD OF THE
PEOPLE REPUBLIC OF CHINA
Method for magnetic flux leakage
testing of round steel
ISSUED ON: FEBRUARY 24, 2016
IMPLEMENTED ON: NOVEMBER 01, 2016
Issued by: General Administration of Quality Supervision, Inspection and Quarantine;
Standardization Administration of the People's Republic of
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Normative references ... 4
3 Terms and definitions ... 4
4 Testing principle ... 4
5 Testing method ... 5
6 Comparative sample ... 6
7 Testing equipment ... 8
8 Testing conditions and steps ... 10
9 Result determination ... 11
10 Testing report ... 12
This Standard was drafted in accordance with the rules given in GB/T 1.1-2009. This Standard was proposed by China Iron and Steel Association.
This Standard shall be under the jurisdiction of National Technical Committee on Steels of Standardization Administration of China (SAC/TC 183).
The drafting organizations of this Standard: Iron and Steel Research Institute, Metallurgical Industry Information Standards Institute, Steel Yannak Testing Technology Co., Ltd.
Main drafters of this Standard: Fan Hong, Zhang Jianwei, Dong Li, Xu Lei, Zhang Ke, Shen Haihong, Jia Huiming.
Method for magnetic flux leakage
testing of round steel
This Standard specifies terms and definitions, testing principle, testing method, comparative sample, testing equipment, testing conditions and steps, result determination and testing report for magnetic flux leakage testing of round steel. This Standard is applicable to magnetic flux leakage testing on surface and near surface of round steel of which diameter is 10mm ~ 350mm (including steel wire). Other specifications shall refer to this Standard for implementation. 2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 9445, Non-destructive testing - Qualification and certification of personnel
GB/T 12604.5, Non-destructive testing - Terminology - Terms used in
magnetic particle testing
YB/T 145, Die Casting and Size Measurement Method of Artificial Defects on the Reference Sample Pipes
YB/T 4289, Measurement Method of Comprehensive Properties for
Automatic Magnetic Flux Leakage Flaw Detection System for Steel Tubes
3 Terms and definitions
For the purposes of this document, the terms and definitions defined in GB/T 12604.5 apply.
4 Testing principle
After ferromagnetic round steel is magnetized, the existence of discontinuity on surface or near surface of round steel makes magnetic line on surface or near surface of round steel distorted so as to generate a leakage magnetic field. Use a testing element (such as Hall element, magneto-sensitive diode or induction coil) that is sensitive to magnetic field to move on surface of magnetized round steel (or round steel is moving, testing element is fixed). When passing through defect, testing element interacts with leakage magnetic field to generate electrical signal, which shall be used to determine that defect exists. The deeper defect is buried below round steel surface, the lower sensitivity it shall be tested out.
When defect orientation is perpendicular to direction of magnetic line, leakage magnetic field strength is the largest at the defect, and testing sensitivity is also the highest. As defect orientation is skewed, leakage magnetic field strength gradually decreases. When two orientations are consistent, leakage magnetic field strength is close to zero. Therefore, when longitudinal magnetization testing equipment is used, it is insensitive to oblique defect reaction, then it is easy to form a blind corner area.
5 Testing method
5.1 Testing mode
In order to detect vertical defect on surface and near surface of round steel, it needs to use circumferential magnetization testing mode, as shown in Figure 1. According to difference between probe and movement mode of round steel, vertical testing is divided into two categories: rotating-probe type and fixed- probe type. Rotating-probe type refers to that probe rotates and tested round steel moves in straight line to complete scanning of entire surface, as shown in Figure 1a). Fixed-probe type refers to that probe is fixed and tested round steel moves forward spirally to complete scanning of entire surface, as shown in Figure 1b).
Figure 1 -- Vertical testing schematic
Rotating direction of probe
steel Round steel
Rotating direction of probe
a) Rotating-probe type b) Fixed-probe type
5.2 Magnetization mode
According to magnetization mode of round steel, testing method is divided into DC magnetization and AC magnetization.
a) DC magnetization
DC current excites electromagnet to generate a magnetic field. Use this magnetic field to magnetize tested round steel. DC magnetization
efficiency is low. Magnetization intensity can be adjusted by controlling current size.
b) AC magnetization
AC current excites electromagnet to generate a magnetic field. Use this magnetic field to magnetize tested round steel. It is easy for AC magnetic field to generate skin effect and eddy current in tested round steel; and as current frequency increases, depth of magnetization decreases.
Therefore, only defect on surface or near surface of tested round steel can be tested. AC magnetization efficiency is high. Magnetization intensity can be adjusted by controlling current size.
6 Comparative sample
Comparative sample is used to test sensitivity of magnetic flux leakage testing equipment, to measure overall performance of testing equipment as well as to calibrate equipment during testing process. Size of artificial defect on comparative sample shall not be interpreted as the minimum size of defect that magnetic flux leakage testing equipment may detect.
Nominal specification of comparative sample shall be same with that of tested round steel. Surface conditions and electromagnetic properties are same or similar. On comparative sample, there shall be no discontinuity that affects normal indication of artificial defect.
6.3 Length and straightness
Length and straightness of comparative sample shall meet requirements of testing method and testing equipment.
6.4 Artificial defect
Artificial defect shall be processed into rectangular, U-shaped, V-shaped longitudinal grooves. Longitudinal groove is parallel to round steel axis. Groove centerline shall pass through round steel axis.
188.8.131.52 Straight line transmission testing of comparative sample
Five longitudinal grooves of same size are processed on surface of comparative sample, see Figure 2. Three artificial defects are in the middle of sample. Circumferential interval is 120?? to each other. Axial distance shall be such that the artificial defect display signal can be clearly distinguished and not more than 200 mm. The rest two artificial defects are respectively at places not more than 200 mm from the ends of sample. For straight-line transmission testing of round steel, it may also refer to 184.108.40.206 to make comparative sample. But when comparative sample is used to test sensitivity of magnetic flux leakage testing equipment, to measure overall performance of testing equipment as well as to calibrate equipment during testing process, it shall make artificial defect on comparative sample respectively at 0??, 120??, 240?? for testing. Artificial defect cannot be added to both ends of coil round steel wire sample.
Figure 2 -- Schematic of comparative sample
220.127.116.11 Spiral transmission testing of comparative sample
Three longitudinal grooves of same size are processed on surface of
comparative sample. One artificial defect is located in the middle of sample. The rest two are respectively at places not greater than 200 mm far from sample end.
Artificial defect size (groove depth, groove width, groove length) can, according to round steel product standard, select sizes in Table 1. Or agreed by the supplier and the purchaser in contract, use groove depth that is half of diameter tolerance specified according to round steel product standard as testing criteria. For size of longitudinal groove, use h to represent depth, use b to represent mm
width and use L to represent length. Groove depth at least cannot be less than half of product tolerance.
Table 1 -- Size and quality level of artificial defect
Size of artificial defect
Groove depth h
Groove width b Groove length L
6.4.4 Production and measurement
Size and allowable deviation of longitudinal groove shall meet requirements of Table 1. Processing-production method is recommended to use spark erosion machining method, machining method or corrosion method. Measurement
method is performed according to YB/T 145. It may use complex method or other methods.
7 Testing equipment
Testing equipment usually consists of rotary scanning device, feed drive mechanism, magnetization device, magnetic flux leakage sensor, electronic instrument, mechanical transmission.
7.1 Rotary scanning device
Rotary scanning device leads magnetization device and magnetic flux leakage sensor to surround round steel that is passing through in straight line, rotating at a constant speed to form spiral scanning on round steel surface. In order to reliably test defect?€?s leakage magnetic field, polar boots of magnetization device rotate around round steel at a constant speed. Magnetic flux leakage sensor sticks on round steel surface or keeps a constant spacing from round steel surface.
7.2 Feed drive mechanism
Feed drive mechanism ensures that round steel concentrically passes through rotary scanning device, making polar boots of magnetization device and
magnetic flux leakage sensor remain a constant clearance with round steel. 7.3 Magnetization device
Magnetization device applies magnetic field to round steel testing area that acts on magnetic flux leakage sensor, so as to make it near saturation.
Magnetization device can be permanent magnet, direct current electromagnet or alternating current electromagnet. When permanent magnet is used, in order to achieve a suitable level of magnetization, there shall be means to adjust magnetic pole gap. Excitation current of direct current electromagnet or alternating current electromagnet shall be continuously adjustable; and there shall be current intensity display device.
7.4 Magnetic flux leakage sensor
Sensor is used to test defective magnetic field in round steel. Magnetic flux leakage sensor consists of one or more detection elements. It is used to test magnetic flux change. Detection element of magnetic flux leakage sensor can be induction coil, Hall element, magnetic sensitive diode, magnetic control switch or magnetic resistance. To ensure testing sensitivity and signal-to-noise ratio, the maximum axial width of each detection element in sensor is 30 mm. In magnetic flux leakage sensor, there shall be a sufficient number of detection elements (i.e. sensor length) to ensure that when scanning round steel surface at a required speed, it shall achieve 100% scanning.
Depth and orientation of surface defect affect amplitude of receiving signal. During testing, as spacing between magnetic flux leakage sensor and round steel increases, sensitivity shall be significantly reduced. Therefore, sensor shall be remained clean and stably contact with round steel surface. If there is gap, it shall ensure that sensor remains a constant distance from round steel surface.
7.5 Magnetic flux leakage testing instrument
Magnetic flux leakage testing instrument is used to amplify and process signals from sensor and implement an alarm. Each channel shall be able to respectively control respective sensitivity and threshold, so as to independently set threshold to determine as waste. Magnetic flux leakage testing instrument shall have marking function to record testing signal and control detected defect. Magnetic flux leakage testing instrument shall especially pay attention to prevent noise interference.
7.6 Mechanical transmission
Mechanical transmission makes tested round steel concentrically pass through testing host (rotary scanning device) at a constant speed. According to different testing mode, mechanical transmission can drive round steel spirally forward or straightly forward.
8 Testing conditions and steps
8.1 Testing conditions
8.1.1 Testing is usually carried out after round steel is processed.
8.1.2 The ratio of surface roughness of tested round steel to specified artificial defect size is not greater than 1:3. There shall be no iron filings, no burrs at the end. Straightness meets the requirements of testing equipment.
8.1.3 Overall performances of testing system such as signal-to-noise ratio, circumferential sensitivity difference, false negative rate, false positive rate, end dead zone and stability shall meet requirements of YB/T 4289.
8.1.4 Magnetic flux leakage testing equipment shall be calibrated regularly (calibration period is generally less than 1 year). It shall be used within validity period.
8.1.5 Testing equipment shall be operated by testing personnel who have obtained technical qualification of Level 1 and above identified by relevant departments in accordance with the requirements of GB/T 9445. When it is tested by a third party, it must be approved by both parties. The person who issues testing report shall obtain technical qualification certificate of Level 2 and above identified by relevant department in accordance with the requirements of GB/T 9445.
8.2 Testing steps
8.2.1 Power on, pre-heating of instrument
Testing instrument shall be powered on, preheated before debugging, so as to ensure stable performance during instrument use.
8.2.2 Equipment adjustment
18.104.22.168 Perform adjustment of scanning device, adjustment of testing sensitivity for equipment.
22.214.171.124 Adjustment of scanning device: according to round steel size and testing requirements, adjust probe scanning speed and round steel running speed, so as to endure probe coverage. Relative to round steel, probe is fed spirally. It shall ensure probe performs 100% scanning to round steel surface. At the same time, there shall be a repeated coverage that is no less than 10%.
126.96.36.199 Adjustment of testing sensitivity: in order to fully display artificial defect on comparative sample, adjust variable parameters such as magnetization current, gain and filtering.
8.2.3 Tested sensitivity
Before testing, based on the above adjustments, use selected testing speed to continuously run comparative sample. Ensure it passes at least 3 times. All artificial defects on comparative sample each time shall be able to alarm reliably. Use as tested sensitivity.
Perform testing after the above debugging is completed.
8.2.5 Equipment calibration
When equipment is continuously used, it shall at least calibrate once every 4h according to steps in 8.2.3. If requirements are met, it shall continue testing; otherwise conduct re-debugging according to 8.2.2~8.2.3. Re-test round steel that has been calibrated by the previous equipment.
8.2.6 End blind area testing
End blind area of round steel may use other testing methods to ensure its quality (excluding coil round steel wire).
9 Result determination
9.1 Quality level
Artificial defect size (groove depth, groove width, groove length) can, according to round steel product standard, select sizes in Table 1. Or agreed by the supplier and the purchaser in contract, quality level is carried out according to Table 1. Selection of quality level shall consider factors such as surface roughness, straightness and machining status of tested round steel.
9.2 Accepted product
For round steel that has been tested as above, if there is no super alarm threshold defect signal, it shall be determined that it has passed magnetic flux leakage testing.
9.3 Suspicious product
During round steel testing (excluding coil round steel wire), if there is super alarm threshold defect signal, it shall be determined as suspicious product of magnetic flux leakage testing. At this moment, re-perform magnetic flux leakage testing according to the method specified based on this Standard. During re- testing of magnetic flux leakage, if there is no super alarm threshold defect signal, this round steel shall be determined that it has passed magnetic flux leakage testing. If super alarm threshold defect signal appears again, it shall be processed as suspicious product.
9.4 Disposal of suspicious product
For suspicious product, according to product standard?€?s requirements, if there are no requirements for product standard, it may adopt one or more measures listed below:
a) After suspicious area of suspicious round steel is detected, it can be grinded. Check that specification, size of round steel which has been
grinded shall be within allowable deviation. Then re-conduct magnetic flux leakage testing for this round steel according to method specified in this Standard. If there is no super alarm threshold defect signal, this round steel can be determined that it passes magnetic flux leakage testing.
b) Cut out suspicious area of suspicious round steel. Then re-conduct
magnetic flux leakage testing for this round steel according to method
specified in this Standard. If there is no super alarm threshold defect signal, this round steel can be determined that it passes magnetic flux leakage testing.
c) Suspicious area of suspicious round steel can be checked by other non- destructive testing techniques and other methods. It shall use method
agreed by the supplier and the purchaser as well as acceptance standard. d) Suspicious round steel is determined as rejected round steel via magnetic flux leakage testing.
9.5 Disposal of steel wire defect
Alarm defect of steel wire shall be accurately marked. Record number of defects in testing report.
10 Testing report
After testing of entire batch of round steels ends, fill in testing report. Testing report shall contain the following information:
a) designation, furnace number, lot number, specification, number of piece or (steel wire) length, technical conditions of tested round steel;
b) testing mode;
c) testing type;
d) quality level;
e) testing instrument model;
f) testing device model;
g) number of acceptance and amount of rejection;
h) testing personnel and report issuing personnel;
i) testing date.
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