What Is Ferrite Testing?
Ferrite testing measures the volume fraction of delta ferrite in austenitic stainless steel weld deposits or the ferrite-austenite phase balance in duplex stainless steel base metal and welds. The ferrite content directly affects corrosion resistance, mechanical properties, and susceptibility to cracking. Too little ferrite in austenitic weld metal causes hot cracking during solidification. Too much or too little ferrite in duplex stainless steel degrades pitting resistance and toughness.
Ferrite measurement is specified by ASME, NACE, NORSOK, and most major oil and gas operator specifications for stainless steel and duplex piping systems.
Why Ferrite Matters
| Material | Ferrite Role | Risk if Too Low | Risk if Too High |
|---|---|---|---|
| Austenitic SS weld (308L, 316L) | Prevents solidification cracking | Hot cracking during welding | Reduced corrosion resistance; sigma phase risk at >800 deg C |
| Duplex SS (2205, 2507) | Phase balance = corrosion + strength | Pitting corrosion, SCC susceptibility | Reduced toughness, hydrogen embrittlement |
| Super duplex weld | Same as duplex, tighter control | Accelerated pitting in chloride service | Impact toughness failure at low temp |
Measurement Methods
| Method | Standard | Measurement | Application |
|---|---|---|---|
| Magnetic instrument (ferrite meter/ferritescope) | AWS A4.2, EN ISO 17655 | Ferrite Number (FN) or %Fe | Non-destructive; field and shop measurement on finished welds |
| Metallographic (point count) | ASTM E562 | Volume % ferrite | Destructive; lab analysis of cross-section (most accurate for duplex base metal) |
| Image analysis | ASTM E1245 | Area % ferrite | Destructive; automated microscope image processing |
| WRC-1992 diagram | Prediction from composition | FN predicted from Cr-eq / Ni-eq | Used during WPS development to select filler metal |
| Schaeffler/DeLong diagram | Prediction from composition | FN predicted | Older method; WRC-1992 preferred |
The magnetic instrument (ferritescope) is the standard field method. It measures the magnetic response of the ferrite phase in a non-magnetic austenitic or mixed-phase matrix. Calibration is performed on certified reference standards per AWS A4.2.
Ferrite Acceptance Ranges
| Material / Application | Ferrite Requirement | Source |
|---|---|---|
| 308L/316L weld deposit | 3-20 FN (typical project spec) | ASME III, owner specs |
| 308L/316L weld deposit (sour service) | 3-10 FN | NACE MR0175/ISO 15156 |
| Duplex 2205 base metal | 40-60% ferrite | ASTM A790, NORSOK M-601 |
| Duplex 2205 weld (root) | 30-70% ferrite (35-65% common) | NORSOK M-601, owner specs |
| Duplex 2205 HAZ | 30-70% ferrite | NORSOK M-601 |
| Super duplex 2507 base metal | 40-60% ferrite | ASTM A790, NORSOK M-601 |
| Super duplex 2507 weld | 35-65% ferrite | NORSOK M-601 |
Note: FN (Ferrite Number) is not identical to volume percent ferrite, but the two are approximately equal below 10 FN. Above 10 FN, FN reads higher than actual volume percent.
Factors Affecting Ferrite Content
| Factor | Effect on Ferrite |
|---|---|
| Chemical composition (Cr-eq/Ni-eq ratio) | Higher Cr-eq/Ni-eq = more ferrite |
| Welding heat input | Higher heat input = slower cooling = lower ferrite in duplex |
| Interpass temperature | Higher interpass temp = longer at high temp = lower ferrite |
| PWHT/solution annealing | Restores phase balance if done correctly |
| Nitrogen content (duplex) | Higher N2 stabilizes austenite = lower ferrite |
| Shielding gas (duplex) | N2-added gas promotes austenite at root = prevents excessive ferrite |
| Number of weld passes | Multi-pass reheating transforms ferrite to austenite |
Ferrite measurements are documented on the welding inspection report and procedure qualification record (PQR), alongside hardness and impact test results. Records are verified during pipe inspections and referenced on mill test certificates for duplex base material.
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