Ferrite Measurement and Control in Cast Duplex Stainless Steels

Increased interest has been focused on the ability to predict and control the ferrite content of cast stainless steels with duplex austenitic/ferritic structures. To this end the interrelationship has been examined between the ferrite content in cast CF-type (19Cr-9Ni) stainless steels as estimated by optical microscopy, its magnetic response measured by the Magne-Gage and the Ferritescope, and its chemical composition. The range of ferrite contents examined extended from zero to 48 volume percent in casting sections measuring 27 mm (1 1/16 in.) in diameter to a maximum thickness of 152 mm (6 in.) in a step block casting. Based upon these tests, statistical relationships were developed to compute the ferrite content from the chemical composition. In addition, calibration procedures were developed for measuring the ferrite content by means of the Magne-Gage and Ferritescope with traceability to the National Bureau of Standards Coating Thickness Standards. The effect of section size on ferrite content is discussed.