Method for automated measurement of the residual magnetic field strength of magnetized ferromagnetic workpieces

A method for measurement of a residual field strength of a magnetized ferromagnetic pipe, comprising the steps of:

• moving a measuring probe in relation to the pipe, or the pipe in relation to the measuring probe, at least in a region of a pipe end and beyond the pipe end, said measuring probe constructed to measure at least one magnetic field component on an exterior surface of the pipe,
• measuring with a magnetic measuring probe at least one of an axial and a radial magnetic field component at a plurality of defined positions located radially outwardly from the exterior surface of the pipe at least in a region of the end of the workpiece, and
• determining from the measured at least one magnetic field component on the exterior surface of the pipe the residual magnetic field strength present at the end face of the pipe based on a previously determined correlation curve stored in a database between the residual magnetic field strength measured directly at an end face of a test pipe and the at least one magnetic field component measured on the exterior surface of the test pipe, wherein the previously determined correlation curve was determined by:
• providing an elongated ferromagnetic test piece having a residual magnetic field strength, said test piece defined by a pipe diameter and a material characteristic,
• placing a magnetic measuring sensor in direct contact with an end face of the test piece and measuring in a first measurement the residual magnetic field strength directly on the end face, and further separately measuring in a second measurement an axial component, a radial component and a circumferential component of the residual magnetic field strength with the magnetic measuring sensor or with another magnetic measuring sensor at a plurality of defined positions located radially outwardly from an exterior surface of the test piece distal from the end face,
• determining a correlation curve for the test piece by correlating the residual magnetic field strength of the first measurement to the axial, radial and circumferential components of the residual magnetic field strength of the second measurement at the plurality of defined positions,
• repeating the first and second measurements for additional test pieces having different pipe diameters and material characteristics and storing additional correlation curves determined therefor in the database.