Surface Treatments
Certifications
- ISO 9001:2015
- NORSOK M-650
- NACE MR0175
- ISO 15156-3
- PED 2014/68/EC
- EN 10204 type 3.2
- AD 2000 W2/W7/W10
Ferralium 255 Ferrite Content (UNS S32550)
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.
Acceptance Bands
| 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 |
Measurement Methods
| 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.
Process Controls That Drive Ferrite Content
- Solution-anneal temperature: higher temperature (≥ 1100 °C) increases ferrite; lower temperature (≤ 1050 °C) decreases ferrite. Hold at 1080 °C ± 20 to land in band
- Quench rate: faster quench preserves ferrite from the anneal temperature; slow cooling allows ferrite to transform to austenite
- Nitrogen content of the heat: nitrogen is the strongest austenite stabiliser; a heat with N at the upper end of the spec (0.20 to 0.25%) tends to lower ferrite
- Welding heat input: high heat input slows the cooling rate through the 1000 to 800 °C window, allowing more ferrite-to-austenite transformation; low heat input preserves more ferrite. Target heat input 0.5 to 2.5 kJ/mm
- Filler-metal alloying: super duplex filler is over-alloyed in nickel relative to the parent to compensate for fast weld-metal cooling that would otherwise leave too much ferrite
Related Pages
- Ferralium 255 heat treatment
- Ferralium 255 sigma phase
- Ferralium 255 mechanical properties
- Ferralium 255 welding procedure (weld ferrite control)
- Ferralium 255 welding filler
- NORSOK M-650 QTR (ferrite measurement in QTR)
Need Ferralium 255 stainless steel? Email info@torqbolt.com or WhatsApp +91-22-66157017 with form, dimensions, quantity, ASTM/EN spec, MTC level (3.1 / 3.2). NACE MR0175 sour-service stamp on every heat. NORSOK M-650 QTR on request. Per-heat EN 10204 type 3.2 MTC included with every consignment.
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