Acceptance range, measurement methods, and significance of the ferrite-austenite balance in super duplex stainless steel
The defining feature of duplex and super duplex stainless steels is the balanced two-phase microstructure of approximately 50% ferrite and 50% austenite. Ferralium 255 (UNS S32550) is qualified for service when the ferrite content falls within the 35 to 55% range in the parent metal and 35 to 65% in the weld metal, per NORSOK M-630 MDS D55. The two phases play complementary roles: austenite delivers toughness and ductility, ferrite carries the high yield strength and chloride-pitting resistance. Outside the acceptance band the alloy loses one or the other property: too much ferrite (above 65%) and the toughness collapses (Charpy below the qualifying value), too little ferrite (below 30%) and the yield strength drops below the 550 MPa minimum and the chloride-pitting resistance is reduced.
| Region | Acceptance band | Standard |
|---|---|---|
| Parent metal (bar, plate, pipe, forging) | 35 to 55% | NORSOK M-630, ASTM A479, A815 |
| Weld metal | 35 to 65% | NORSOK M-630, ASME IX with M-630 supplementary |
| Heat-affected zone (HAZ) | 30 to 65% | NORSOK M-630 |
| Welding consumable as-deposited | 22Cr matching: 25 to 45%; 25Cr matching: 30 to 65% | AWS A5.4 / A5.9 / NORSOK M-630 |
| Method | Standard | Use case |
|---|---|---|
| Point counting (metallographic) | ASTM E562 | Reference method, accepted in MTC and QTR |
| Image analysis (digital) | ASTM E1245 | Automated digital alternative to point counting |
| Magnetic permeability (Feritscope) | ANSI / AWS A4.2 (Ferrite Number, FN) | Field / on-site weld QC; FN value, not vol% |
| X-ray diffraction | ASTM E975 | Research method; absolute phase fraction |
The Feritscope (magnetic permeability) reads in Ferrite Number (FN) and is the standard on-site weld-QC method, but FN is not the same as volume percent ferrite. The conversion FN to vol% is non-linear and grade-dependent; for Ferralium 255 weld metal a typical FN reading of 50 to 70 corresponds to approximately 45 to 60 vol% ferrite. The MTC and QTR use the metallographic point count (ASTM E562) as the reference, with the Feritscope as a production-floor screening method only.
