AFM with a prove made of ferric material (MFM) can also observe nano-scale magnetic field. The ductility of duplex stainless steel is well known to be decreased by thermal aging. In order to evaluate the degradation, magnetic domain structure is observed by MFM. The duplex stainless steel is used in power plants as a valve casing at around 300 0 C. Since the long ageing time is needed to evaluate the degradation at 300 0 C, the ageing temperatures are also employed at 350,400 and 450 0 C as an accelerated test. The magnetic domain structure is clearly observed in just ferrite texture for as- received material. In the materials after aging the shape of magnetic domain structure is deformed like as petals of a rose. The size of petals becomes to be smaller with increasing the ageing time. In comparison between 350 and 400 0 C, the geometries of the magnetic domain structures are very similar when these have same equivalent ageing time estimated by Larson-Miller parameter. The thermal ageing is thought to be caused by same mechanism at 350 and 400 0 C. the indistinct magnetic domain structure formed at 450 0 C is different from one formed at 350 or 400 0 C. Since the mechanism of thermal ageing might be different, the accelerated test at 450 0 C is judged to be invalid.

The nano-scale observation by MFM is useful to estimate the degradation of the duplex stainless steel . In actual plant a small material will be sampled for non-destructive evaluation.