Ferrite measurement in austenitic and duplex stainless steel castings

Practical and accurate non-destructive means for the measurement of the ferrite content of duplex stainless steel castings is a necessity from the specification and service performance consideration standpoints. The ability to determine ferrite rapidly,accurately and directly on a finished casting, in the solution annealed condition, can enhance the acceptance, save on manufacturing costs and ultimately improve service performance of duplex stainless steel cast products. If the suitability of a non-destructive ferrite determination methodology can be demonstrated for standard industrial measurement instruments, the production of cast secondary standards for calibration of these instruments is a necessity. With these concepts in mind, a series of experiments were carried out to demonstrate, in a non-destructive manner, the proper methodology for determining ferrite content. The literature was reviewed, with regard to measurement techniques and vagaries, an industrial ferrite measurement round-robin was conducted,the effects of casting surface finish, preparation of the casting surface for accurate measurement and the evaluation of suitable means for the production of cast secondary standards for calibration were systematically investigated.It was found that surface finish effects can induce significant differences in measured ferrite content. Several finishes were identified, which when applied(Feritscope® method), resulted in a significant decrease in measured ferrite content on anomaly 74 FN sample (\u3e 10 FN and well outside the 2σ variation of ± 0.5) defined fora polished surface.An interlaboratory round-robin test series revealed that cast secondary calibration standards can be produced from castings. It was found that for both Magne Gage and Feritscope®, the repeatability ferrite measurement of centrifugal castings surpassed that of statically cast materials. Reproducibility was also unaffected by ferrite measurement technique.Additional characterization of ferrite content, as a function of depth below a cast surface, revealed that the ferrite content immediately below a cast surface is not indicative of the bulk casting. At least 0.125 of material must be removed to ensure that the measured ferrite content is representative of the bulk casting. Analysis of operatorand instrument error, for the Feritscope® showed that error induced by the operator exceeds that of the instrument alone.Additional tests characterized the Feritscope® by establishing its probe interaction volume (0.050 ). Considering instrument repeatability and reproducibility,the Feritscope® was clearly identified as the superior instrument for ferrite measurement.The data obtained from this research program provides recommendations to ensure accurate, repeatable and reproducible ferrite measurement and qualifies the Feritscope® for field use on production castings