An Experimental Comparative Review of Ferrite Measurement Techniques Used in Duplex Stainless Steel (UNS S32205) Welds

Abstract

Duplex stainless steels (DSS) have proven to be very promising engineering materials as substitutes to conventional austenitic stainless steels for structure of off-shore platforms, parts and equipment in petrochemicals and refinery industries, predominantly for sour service applications where corrosion resistance including stress corrosion cracking (SCC) resistance is required in aggressive chloride and/or sulphide environments. Resistance to SCC in chloride containing environment depends on the available ferrite content in the carefully welded duplex (ferritic-austenitic) stainless steel structure. There is, of course, a degree of variation in weld metal ferrite content and in reproducibility of measurement which deserve a review and comparative study on different weld metal ferrite measurement techniques being widely used. So, present work has been carried out by collaborating with a reputed filler metal manufacturer who deliberately produced special batches of standard DSS compositions matching SMAW electrodes with nickel varied content in the range of 9-9.5 %, 9.5-10.5, 10.5-11.5 and 11.5 to 12.5 % to obtain weld deposits in varied ferrite content range15% to 40%, or 20 FN to 50 FN to be experimentally investigated by (1) Feritscope® instrument method, (2) ASTM E 562-11 volume fraction measurement by systematic manual point count method and (3) theoretically by WRC-1992 Diagram. The purpose of this paper is to give a comparative overview of above methods. According to the findings of the study, readings on the top surface of weld metal were consistent with those on the cross section, this indicates proper guidelines can be given for selecting location of testing Feritscope® instrument measurement. FN estimates in predictive methods like image analysis/manual point count methods depends on proper placement of the points on ferrite grains morphology, fineness, discreteness and its irregularity.