The deformation-induced martensite variant selection in a supermartensitic stainless steel (SMSS) has been examined in the temperature range from -60°C to 150°C, using in-situ tensile testing in combination with electron backscatter diffraction (EBSD) analyses in the scanning electron microscope (SEM). In the as-received (i.e., intercritically annealed) condition, the base material contains about 40 vol pct of retained austenite. At each testing temperature, this austenite transforms back to martensite during plastic deformation at a rate which is controlled by the accumulated plastic strain in the material. On the other hand, the applied strain rate and crystallographic orientations of the prior austenite grains do not affect the overall transformation rate. Moreover, the subsequent Schmid factor analysis reveals that the martensite variant selection is independent of the local slip activity within the austenite. Therefore, no new martensite variants, besides those already present in the parent steel, develop during the phase transformation. At the same time, their individual intensities remain approximately constant within each prior austenite grain. This means that the deformation-induced martensite variants nucleate from the same sites as those that are operative in the intercritically-annealed base material. Thus, the observed variant selection is another example of the inherent reversible nature of the martensite transformation.
During Statoil’s recent riser replacement project more than 30 used risers have been dissected onshore [1]. The main objective has been to reveal the root cause of carcass axial tearing failures. However, this also gave the opportunity to investigate details of other carcass damages, armor corrosion, annulus environment, external sheath breaches and polymer ageing as described in [5], [6] and [7]. This paper discusses some of the new knowledge obtained from the dissections.
New tools and methods have been developed for carcass pitch measurements in order to determine axial movements, maximum load level seen by the carcass and determine the utilization against tearing.
Offshore free-volume annulus testing and gas sampling is compared to liquid sampling and actual liquid found when dissecting, with significant deviations observed. Expected annulus environment is assessed in light of actual observed corrosion, with generally less corrosion than expected, however at some few selected areas significant corrosion attacks are found. Residual stresses and condition of polymer layers is quantified, with generally more degeneration and larger changes than anticipated before riser recovery.
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