Advanced Micromagnetic NDE for the Assessment and Early Detection of Hydrogen Attack in X 20 Cr 13 Steel and Fatigue Damage in Stabilized Austenitic Stainless Steel

In the oil, gas and petroleum industry hydrogen attack is one reason for sometimes catastrophic components failures, i.e. in high strength martensitic or bainitic steels for pipes. The hydrogen is delivered by the medium stored or pumped and diffuses into the material at local positions of higher compressive residual stresses which are dislocations, segregated grain boundaries, non-metallic inclusions, etc. The recombination from the atoms to the molecules locally enhances the residual compressive stress and vice versa the tensile residuals stress in the ferrite matrix of these steels. The contribution reports to research results obtained together with the BASF Inc. as co-operator at pipe segments of X 20 Cr 13-steel, NACE-solution exposed at the id-surface and under local mechanical stamp load from the od-surface. Barkhausen noise as well as the higher harmonic analysis of the tangential magnetic field clearly reveal the potential for early detection of the damage before first cracks occur. Austenitic stainless steels are in wide-spread application in the chemical as well as nuclear industry, mainly because of their high toughness and insensitivity against corrosion attack. However under static as well as fatigue load the material has the tendency to response with localized phase transformations from the non-magnetic g- to the martensitic and ferromagnetic a'-phase. The process starts localized at positions of higher stress intensity, i.e. at microstructure inhomogeneities like non-metallic inclusions, etc. Magnetic sensitive sensors like SQUID or GMR are suitable to already sense small contents of the martensitic phase in the bulk volume. The report is to the Ti-stabilized austenitic stainless steel X6 CrNiTi 18 10 (1.4551) fatigued under HCF-conditions, at ambient temperature and 320°C. The magnetic results show a clear early indication of damage.