BS ISO 24497-1-2020 Non-destructive testing一Metal magnetic memory Part 1: Vocabulary and general requirements.
4.2 The MMM technique enables the detection of SFIs and gives recommendations for additional nondestructive testing of vessels, pipelines, equipment (e.g. steam generators, turbines, heat exchangers, rails), and construction welded joints. ISO 24497-2 shall be applied for testing of welded joints.
NOTE SFIs of lOs are conditioned by the fabrication technology (fusion, forging, rolling, turning, press forming, thermal treatment, etc.).
4.3 Under certain conditions the MMM technique can be used on non-magnetic lOs, particularly if a ferromagnetic phase is present (e.g. metastable austenitic steels, mill scale, coatings).
NOTE Metastable austenitic steels can be inspected if their microstructure Is sensitive to y – a phase transformationll8l. The evaluation of SFs is restricted to the ferromagnetic phase.
4.4 The temperature range during MMM testing shall be within the normal and safe working range for the operator (NDT inspector).
5 Requirements for the inspected object
5.1 Equipment and structures (lOs) should be inspected by MMM in in-service state (under load) as well as in the maintenance state (after removal of operating loads). If possible, the initial magnetic service state of the 10 should be determined.
5.2 Surface dressing and preparation are not required. It is recommended to remove insulation to reduce sensor to surface lift-off to gain reliability and avoid SF1 from the insulation. In particular cases, non-magnetic insulation can be allowed during inspection. Any permissible insulation layer shall be verified experimentally. The results shall be attached to the test report.
5.3 Limiting factors for the application of MMM testing are the following:
— dc-magnetization and intentional magnetization of the 10;
— foreign external (electro-)magnetic fields near to the Inspected object, near the inspected region of interest;
— temperature changes can influence the test results (e.g. at Curie temperature);
— Sensor to 10 surface distance (lift-off) and its changes during the measurement.
5.4 Strong temperature changes in the 10 cause changes of the thermoremanent magnetization and should be taken into account during processing of the inspection results.
5.5 Sources of SF1 along the 10 are the following:
— shape and geometry of the 10 (geometry changes and the edges of the 10) are sources of SF and have to be considered, because surface geometry are sources of strong local stray and demagnetizing fields[Z41[Z5l146il0l;
— high mechanical stress gradients;
— boundaries of heterogeneous plastic deformation;
— changes in the microstructure;
— external magnetic fields, e.g. (welding) electric current flow at the inspected object, strong and heterogeneous magnetic fields close to the tested area;BS ISO 24497-1 pdf download.