Ferrite Number in Duplex and Austenitic Stainless Steel Welds: Measurement and Control

The ferrite number, or FN, quantifies how much ferrite a stainless steel weld contains, and welders control it because it drives weld performance. For duplex and super duplex welds the target ferrite content is generally about 30 to 70 FN, matching a balanced two-phase structure, while austenitic welds usually aim for a small but nonzero ferrite content of roughly 3 to 10 FN to prevent hot cracking. Measured most often with a calibrated ferritescope and predicted with the WRC-1992 diagram, FN is a practical control parameter that ties welding procedure to corrosion resistance and toughness.

What Ferrite Number Represents
Ferrite number is a standardized magnetic index, not a direct volume percentage. It came about because early measurements of "percent ferrite" varied widely between laboratories, so the industry adopted a reproducible scale defined by magnetic response and traceable to secondary standards. At low levels FN and volume percent ferrite run close, but they diverge as ferrite increases, which is why duplex specifications state acceptance in FN or in a converted percentage with a defined method. The number matters because ferrite and austenite behave differently: ferrite gives strength and resistance to chloride stress corrosion cracking, austenite gives toughness and ductility. A weld that solidifies with the wrong balance trades one property away to get the other.

Why Austenitic and Duplex Welds Differ
In austenitic grades such as 304L and 316L the base metal is essentially fully austenitic, but the weld metal is deliberately formulated to solidify with a few percent delta ferrite. That small ferrite fraction breaks up continuous austenite grain boundaries and dissolves impurities such as sulfur and phosphorus, heading off solidification hot cracking. Too little ferrite, below about 3 FN, risks cracking; too much, above roughly 10 to 12 FN, can lower toughness and, after service exposure, promote embrittlement. Duplex grades work the other way: the base metal is already a balanced mix, and the goal is to reproduce that balance in the weld. Excess ferrite in a duplex weld, which happens when the weld cools too fast for austenite to re-form, degrades toughness and corrosion resistance, while excess austenite lowers strength and can locally cut pitting resistance.

Predicting FN with the WRC-1992 Diagram
The WRC-1992 diagram is the modern tool for predicting weld ferrite from composition. It plots a chromium equivalent, which sums ferrite-forming elements, against a nickel equivalent, which sums austenite-forming elements, and reads off an expected FN. The chromium equivalent is Cr + Mo + 0.7 x Nb, and the nickel equivalent is Ni + 35 x C + 20 x N + 0.25 x Cu. The heavy coefficient on nitrogen shows why nitrogen additions push a weld toward austenite, and why they are used deliberately in duplex filler and shielding gas. By comparing the predicted FN of a base metal, a filler, and their dilution, a welding engineer can pick consumables and dilution that land inside the target window before cutting a single test coupon.

Table: Typical Ferrite Targets

Measuring Ferrite in Practice
The ferritescope is the workhorse on the shop floor. It applies a magnetic field to the weld and measures the response, which scales with ferrite content, giving a direct FN reading in seconds once it's calibrated against certified standards. It's non-destructive, portable, and well suited to production checks on both root and cap passes. For arbitration or research, metallographic point counting on an etched cross-section gives a volume percentage, and both methods are covered by recognized standards. Take multiple readings across the weld and heat-affected zone rather than a single spot, because dilution and cooling vary along and across the joint. Remember too that a ferritescope reads only the surface region it touches, so grinding, cold work, and surface contamination can bias the result, and readings on curved small-diameter pipe may need a geometry correction. In an equipped laboratory, ferrite measurement is paired with impact testing and corrosion testing to confirm that a good number actually translates into acceptable mechanical and corrosion performance.

Controlling Ferrite Through Procedure
Two process levers dominate ferrite control: heat input and nitrogen. Heat input governs cooling rate, and cooling rate governs how much austenite can re-form from the ferrite that solidifies first. In duplex welding, too little heat input produces a fast quench that freezes in too much ferrite and starves austenite, hurting toughness; too much heat input slows cooling so far that intermetallic phases can form. A controlled range, often with an interpass temperature ceiling near 150 degrees Celsius, keeps the structure balanced. Nitrogen is the chemical lever: adding a few percent nitrogen to the argon shielding gas and choosing an over-alloyed, nickel- and nitrogen-rich filler promotes austenite re-formation and raises corrosion resistance in the weld. The autogenous case deserves extra caution, because a weld made without filler and without nitrogen in the gas relies entirely on base-metal chemistry and drifts toward excess ferrite, which is why duplex root passes are almost always made with a nitrogen-bearing backing gas. Get it wrong and the consequences are concrete. A ferrite-heavy duplex weld shows low Charpy toughness and can crack under chloride stress; a ferrite-lean weld loses strength and pitting resistance; and an austenitic weld with too little ferrite can crack during solidification. Specify an FN window, predict it with WRC-1992, control heat input and nitrogen, and verify with a ferritescope, and weld quality becomes a controlled, documented outcome rather than a hope.

Frequently Asked Questions
Q: What ferrite number should a 2205 duplex weld have?
A: A balanced 2205 weld typically targets roughly 30 to 60 FN; too much ferrite reduces toughness and corrosion resistance, while too little lowers strength.
Q: How is ferrite number measured on finished welds?
A: Most commonly with a calibrated ferritescope, which reads FN non-destructively in seconds; metallographic point counting is used for arbitration and gives a volume percentage.

For duplex and austenitic pipe welded and tested to controlled ferrite targets, contact Zhejiang Wenqiang Stainless Steel Co., Ltd. Tel +86 577 8922 2595 / https://www.chinawqsteel.com/