Surface Treatments
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- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex Ferrite Content (NORSOK M-630)
Ferrite content in super duplex 2507 (UNS S32750, EN 1.4410) is the single most-cited microstructural acceptance parameter on a mill test certificate or weld procedure qualification. The required range is 35 to 55 percent in base material and 35 to 65 percent in weld metal per NORSOK M-630. Out-of-range ferrite is a non-conformance that signals either incorrect chemistry, an inadequate solution-anneal quench rate, or excessive welding heat input. Two measurement methods are accepted: image-analysis point counting per ASTM E562 on a polished and etched cross-section (the reference method, used for base material), and magnetic Feritscope (per the principles of ANSI / AWS A4.2M) on the surface (the production-floor method, calibrated against image analysis).
Why a Balanced Ferrite-Austenite Microstructure
Super duplex 2507 derives its strength from ferrite and its toughness from austenite. The 50 to 50 ferrite-austenite microstructure is the engineering optimum: high yield strength (550 MPa minimum), high tensile strength (800 MPa minimum), high elongation (25 percent or more), and high pitting resistance (PREN above 41) all depend on both phases being present in roughly equal amounts. Skew the balance, and the alloy loses one or more of these properties.
| Ferrite Content | Consequence |
|---|---|
| Below 30 percent (austenite-rich) | Yield strength drops below specification; chloride-stress-corrosion-cracking resistance degrades toward austenitic-grade levels |
| 30 to 35 percent | Below NORSOK M-630 minimum; non-conformance |
| 35 to 55 percent (acceptable, base material) | Engineering optimum; balanced strength, toughness, corrosion resistance |
| 55 to 65 percent (acceptable, weld metal only) | Slight strength bias, slight toughness loss; allowed in weld metal because of the rapid solidification cycle |
| Above 65 percent (ferrite-rich) | Charpy toughness drops sharply, especially at low temperature; 475 deg C embrittlement risk in long-term service |
Image Analysis per ASTM E562
The reference method for base-material ferrite measurement is manual or automated point counting on a polished and etched cross-section, per ASTM E562 (Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count). Steps are:
- Section the bar, plate, or forging at mid-thickness (or at the location specified in the project document).
- Mount and polish the cross-section to a 1 micron diamond finish.
- Etch using a duplex-selective etchant. Common choices are modified Beraha (potassium metabisulfite plus hydrochloric acid) or 40 percent NaOH electrolytic at 2 to 3 V for 5 to 10 seconds. The etchant darkens ferrite and leaves austenite light, producing high contrast under bright-field optical microscopy.
- Photograph at 200x to 500x magnification across at least 10 fields uniformly distributed over the cross-section.
- Point count: overlay a 100-point grid (or use automated image-analysis software with calibrated phase thresholds) and count the points falling on dark (ferrite) regions. Mean ferrite percentage is the average across all fields.
- Report mean and standard deviation; per NORSOK M-630, mean must fall within 35 to 55 percent and no individual field below 30 percent or above 60 percent.
Magnetic Feritscope Measurement
A Feritscope is a hand-held instrument that measures the magnetic permeability of the surface and converts the reading to ferrite percentage via a calibration curve. It is the standard production-floor method for in-process and weld-metal ferrite checks because it is non-destructive, portable, and provides results in seconds. Reading principles:
- Surface preparation: ferrite measurement is sensitive to surface roughness, cold work, and oxide layers. The measurement surface should be ground to at least 240 grit, free of scale, and at ambient temperature.
- Calibration: instruments are calibrated against secondary standard blocks at three points covering the expected range (typically 30, 50, 70 percent). Calibration is verified before each shift and after any instrument disturbance.
- Number of readings: minimum 10 readings on a base-material location, 6 readings on a weld bead. Report mean, minimum, and maximum.
- Cross-correlation: Feritscope readings on super duplex 2507 are normally biased 2 to 5 percent low compared with ASTM E562 image analysis on the same cross-section. The correlation is linear and the offset is normally documented by the WPS qualification record so that Feritscope acceptance limits are adjusted accordingly.
Ferrite Control During Welding
Weld-metal ferrite content is more sensitive to process parameters than base-material ferrite. The principal control variables are filler metal chemistry, heat input, interpass temperature, and shielding-gas nitrogen content.
| Variable | Effect on Weld-Metal Ferrite | Acceptable Range |
|---|---|---|
| Filler metal (ER2594 vs ER2553) | Both formulated to balance ferrite within 35 to 65 percent under nominal conditions | Per welding-filler page |
| Heat input (kJ per mm) | Higher heat input slows cooling and shifts microstructure toward austenite (lower ferrite) | 0.5 to 2.5 kJ per mm |
| Interpass temperature | High interpass slows cooling and lowers ferrite | 150 deg C maximum |
| Shielding-gas nitrogen (Ar + 2 to 5 percent N2) | Nitrogen pickup raises austenite formation and lowers ferrite, particularly in the GTAW root pass | 2 to 5 percent N2 in Ar for GTAW root |
| Heat-affected zone (HAZ) | HAZ ferrite tends higher than weld metal because of rapid cooling without filler dilution | Acceptance per project; typically 35 to 65 percent |
Consequences of Out-of-Range Ferrite
A super duplex 2507 component with ferrite outside the 35 to 55 percent base-material range or 35 to 65 percent weld-metal range is non-compliant with NORSOK M-630. The disposition normally depends on which way the result has drifted and by how much:
- Ferrite below 30 percent: yield strength is likely below 550 MPa minimum and chloride-stress-corrosion-cracking resistance is degraded. The material is normally scrapped or re-solution-annealed at the upper end of the soak window (1100 deg C) to drive the equilibrium toward higher ferrite.
- Ferrite between 30 and 35 percent: borderline; concession may be granted if other tests (yield strength, ASTM G48 pitting) pass. Project specification approval is required.
- Ferrite between 55 and 65 percent (base material): borderline; toughness is at risk. Charpy V-notch at minus 46 deg C must pass 45 J average.
- Ferrite above 65 percent: significant toughness loss; 475 deg C embrittlement risk in long-term service. Material is normally scrapped or re-solution-annealed at the lower end of the soak window (1040 deg C) to favour austenite formation.
Related Super Duplex 2507 References
- UNS S32750 reference page
- Heat treatment
- Sigma-phase embrittlement
- Welding procedure
- Welding filler (ER2594, ER2553)
- NORSOK M-630 super duplex MDS
- Mechanical properties
Super Duplex Ferrite Content FAQ
What ferrite range is required for super duplex 2507?
35 to 55 percent in base material and 35 to 65 percent in weld metal, per NORSOK M-630. The wider weld-metal range accommodates the rapid solidification cycle, which biases the microstructure slightly toward ferrite even with filler metals formulated for balanced solidification.
How is ferrite content measured?
Two methods are accepted. The reference method is image-analysis point counting per ASTM E562 on a polished and etched cross-section. The production-floor method is magnetic Feritscope, calibrated against image analysis. Image analysis is destructive and slow but precise; Feritscope is non-destructive, portable, and biased 2 to 5 percent low against image analysis on super duplex 2507.
Why is the weld-metal range wider than the base-material range?
Weld metal solidifies and cools far faster than base material because the heat sink is the surrounding cold parent metal. The rapid cooling biases the microstructure toward ferrite (austenite has insufficient time to nucleate fully from the high-temperature ferrite phase). The 35 to 65 percent weld-metal range allows for this cooling effect while still providing adequate strength, toughness, and corrosion resistance.
What happens if ferrite is below 35 percent?
Yield strength drops below the 550 MPa minimum, and chloride-stress-corrosion-cracking resistance degrades toward austenitic-grade levels. The component is normally non-conforming and either scrapped or re-solution-annealed at the upper end of the soak window (1100 deg C) to drive the equilibrium toward higher ferrite before re-quenching.
What happens if ferrite is above 55 percent in base material?
Charpy V-notch toughness drops, particularly at low temperature, and 475 deg C embrittlement risk increases in long-term service. The material is normally non-conforming. Disposition depends on the specific result: ferrite between 55 and 60 percent may pass with concession if Charpy toughness still meets 45 J average at minus 46 deg C; ferrite above 65 percent normally requires re-solution annealing or scrap.
Can a Feritscope replace ASTM E562 image analysis?
For production-floor screening and weld-quality verification, yes; for first-of-class qualification and acceptance per NORSOK M-630, no. The standard requires image-analysis ASTM E562 as the reference method. Feritscope readings are normally cross-correlated to image analysis during WPS qualification, after which Feritscope can be used for production verification with the established offset built into the acceptance limits.
How does the heat-treatment quench rate affect ferrite content?
A faster quench freezes the high-temperature equilibrium, which is biased toward ferrite. A slower quench gives austenite more time to nucleate from ferrite, lowering ferrite content. Within the acceptable solution-anneal window, the standard agitated water quench achieves ferrite in the 40 to 50 percent range. Excessive cooling rate (cold quench tank, very thin section) can push ferrite above 55 percent; slow quench (heavy section, weak agitation) can push ferrite below 35 percent and also risk sigma-phase formation.
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