1 Scope
This standard specifies the testing principles, testing equipments, testing modes, reference standards and cutting dimension, testing conditions, testing procedures, testing evaluation and testing report for ferromagnetic seamless and welded (except submerged arc-welded) steel tubes (hereinafter referred to as steel tubes).
This standard is applicable to longitudinal and transverse magnetic flux leakage testing (hereinafter referred to as testing) on internal and external surfaces of steel tubes with external diameter greater than or equal to 9 mm.
2 Definitions
This standard adopts the following definitions.
2.1 Magnetic Flux Leakage
Leakage magnetic flux leaked from the imperfect area of tested steel tubes.
2.2 Survey Element Component
The element components converting imperfect magnetic flux leakage to electrical signal, such as survey coil, hall element, magneto diode and flux gate element component.
2.3 Reference Standards
A segment of steel tubes (called test tubes) meeting the product standard and with artificial imperfections; it is used to set and calibrate testing equipment.
3 Testing Principles
3.1 Where the ferromagnetic steel tubes are fully magnetized, the magnetic line in tube wall is obstructed by imperfections on or near its surface, distortion occurs to the magnetic line at imperfect area and magnetic leakage field is formed on the internal and external surface of steel tubes with partial magnetic line leaked out of them. The magnetic flux leakage testing refers an electromagnetic testing method where survey element component is used to inspect imperfections of magnetic leakage field. Where magnetic leakage field is picked up by survey element component located on steel tubes surface and conducting relative movement with steel tubes and is converted into imperfect electrical signal, the signal on imperfection may be reflected through searching unit so as to evaluate the imperfections. The external surface of steel tubes has the highest testing sensitivity, which reduces with the increase of wall thickness from the external to the internal surface.
Where the imperfection direction is vertical to the magnetic line direction, the magnetic leakage field at imperfect area has the most strength as well as the highest testing sensitivity. As the imperfection direction deflects and the strength of magnetic leakage field reduces gradually until both have the same direction, the strength of magnetic leakage field is approximately zero. Therefore, where longitudinal and transverse testing equipments are adopted, blind angular zone will be formed easily due to insensitive reaction to oblique imperfections.
3.2 Due to different equipments, there must be a segment of inaccessible zone on both ends of steel tubes during testing.
GB/T 12606-1999 The following standards are cited：
