How to Determine Ferrite Content in Austenitic Stainless Steel: The Tool Most Buyers Misjudge

How to Determine Ferrite Content in Austenitic Stainless Steel: The Tool Most Buyers Misjudge

Austenitic stainless steel combines corrosion resistance, heat resistance, low-temperature toughness, easy formability, and excellent weldability; it accounts for roughly 70% of all stainless steel output. Valve bodies, bonnets, and discs are almost exclusively made of it — mostly ASTM A351 CF-class castings and ASTM A182 F304 and F316 forgings, i.e., the 18-8 and 18-12 types (the numbers indicating approximate Cr and Ni contents). A note on designations before we begin: the 18-8 / 18-12 families correspond to AISI 304 and 316 (UNS S30400 / S31600); CF8 and CF8M castings in China follow GB/T 12230, and F304 / F316 forgings map to the GB/T 1220 equivalents. Nuclear-grade controlled-nitrogen grades referenced in this article — 304NG and 316NG — correspond to AISI 304LN / 316LN and the European X2CrNiMoN17-13-3 (1.4429).

What most buyers do not know is that “austenitic” does not mean 100% austenite. A magnet is often held against a valve in acceptance testing; weak magnetism is routinely taken as proof of a quality problem. That is a misunderstanding. Austenitic stainless steel normally contains a controlled amount of delta ferrite — the bcc, magnetic phase that forms during solidification and is retained at room temperature — and ferrite is both a tool and a risk, depending on the window you specify. Nuclear-safety valves, defense valves, and important SHA-class chemical-plant valves now all specify ferrite limits for base metal and weld metal, so knowing how to measure and calculate ferrite content is no longer optional. This article, adapted from the Chinese heat-treatment and equipment material press, explains the role, the formation mechanism, the three measurement methods, and the acceptance windows actually used in nuclear and chemical service. Related standards and designations are cross-referenced below.

1. The Role of Ferrite

For valves, the most important effect of ferrite is on weldability, followed by its effects on corrosion resistance, mechanical properties, and workability.

Content. Austenitic stainless steel usually contains a certain amount of ferrite. According to the Metals Handbook, CF-class castings typically contain 5–25% ferrite — which is why ASTM A351 covers “austenitic-F-ferritic (duplex) cast steel for pressure-containing parts.” Ferrite is therefore a design variable, not an impurity.

Weldability. Delta ferrite of about 4–12% in the weld metal prevents solidification (hot) cracking: ferrite dissolves more S, P, Si, and Nb, preventing their segregation and the formation of low-melting eutectics, and it blocks low-ductility behavior along secondary grain boundaries. Too much is harmful: welds that will be heat-treated above 600 °C, or operated long-term at 600–850 °C, precipitate the Cr-rich sigma phase from delta ferrite, which depletes surrounding chromium and embrittles the weld; for such service the ferrite window should be narrowed to 3–8%, or the component re-solution-treated.

Corrosion resistance. Ferrite dispersed as small islands between austenite grains breaks up the continuity of the austenite columnar and dendritic structure and interrupts the network precipitation of chromium carbides on austenite grain boundaries, so it helps prevent intergranular corrosion. Because ferrite is insensitive to stress-corrosion cracking, ferrite-bearing austenitic welds resist stress corrosion better than near-ferrite-free welds of the same composition. In special media, however — urea and acetic acid, for example — ferrite is selectively attacked and must be limited.

Mechanical and processing properties. Ferrite raises strength while lowering ductility and impact toughness, as Table 1 shows. Too much ferrite damages forgeability: for high-ratio forgings the stock ferrite is usually limited to 3–8%, and for cold-working applications — deep drawing, cold heading, cold drawing, cold extrusion — to below 5%. In castings, a controlled ferrite content also prevents casting cracks and segregation and raises mechanical properties.

2. Ferrite Formation Mechanism

All stainless steels are iron-based alloys with more than 12% chromium. Above about 800 °C the base structure is fcc austenite; at room temperature it wants to transform to bcc ferrite (or martensite). Adding more than about 7% nickel, or other austenite formers such as C, N, and Mn, stabilizes the austenite at room temperature. If the total austenite-forming strength — the nickel equivalent — is insufficient, part of the structure remains ferrite. The microstructure is therefore decided by the balance between two families of elements: ferrite formers (the chromium-equivalent elements: Cr, Mo, Si, Nb, Ti) and austenite formers (the nickel-equivalent elements: Ni, Mn, C, N).

Table 2 summarizes the roles of the main alloying elements. Two points matter for ferrite control in practice. Molybdenum is a ferrite former with a chromium equivalent of 1, so Mo-bearing grades must add more nickel to stay balanced — CF3M, for example, combines 2.0–3.0% Mo with 9.0–13.0% Ni. Nitrogen is a strong austenite former with a nickel equivalent of 30 and is the element behind the modern controlled-nitrogen grades (AISI 304N / 304LN at 0.10–0.16% N, and the nuclear grades 304NG, 316NG, and the French X2CND18-12 at 0.06–0.10% N) that solved BWR IGSCC problems.

3. Three Ways to Measure or Calculate Ferrite

Delta ferrite in austenitic stainless steel is measured by three methods: magnetic instruments, metallography, and calculation.

Magnetic method. Delta ferrite content is proportional to the ferromagnetism of the steel, so a dedicated ferrite meter reads it directly. Castings and welds can be measured directly on the natural solidified surface. Wrought products — forgings, bars, plates, electrodes, wire — have heavily deformed ferrite with altered magnetic response, so samples must be prepared per code (e.g., ASME Section III): an autogenous remelt, usually by gas-tungsten arc welding without filler, is made and at least six readings are averaged. Note that foreign instruments are usually calibrated in Ferrite Number (FN) per the US WRC convention; FN and volume percent are nearly equivalent.

Metallographic method. Because delta ferrite is distributed as small, discontinuous islands, its area fraction is rated in a microscope against standard charts (the Chinese national standard practice). The same sampling rule applies: wrought material must be autogenously remelted into a solidified block before rating.

Calculation method. The fastest and most accessible route: from the chemical analysis, compute the chromium and nickel equivalents and read the ferrite content from the intersection in a constitution diagram. The choice of diagram and of the equivalent formulas is the key.

Schaeffler diagram. The earliest and most widely used constitution diagram, applicable to castings, forgings, wrought products, and naturally solidified welds. Its equivalents are:

Cr eq = %Cr + %Mo + 1.5 × %Si + 0.5 × %Nb
Ni eq = %Ni + 30 × %C + 0.5 × %Mn

The Schaeffler diagram ignores nitrogen, so its accuracy is ±4% ferrite — yet it remains the standard basis for valve base-metal (casting and forging) evaluation, as in RCC-M MC1000 for nuclear-safety pressure castings, which explicitly does not consider N.

DeLong diagram. An improvement on Schaeffler that adds nitrogen, suited to nitrogen-bearing and controlled-nitrogen steels and gas-shielded welds, with accuracy of ±2%:

Cr eq = %Cr + %Mo + 1.5 × %Si + 0.5 × %Nb
Ni eq = %Ni + 30 × %C + 30 × %N + 0.5 × %Mn

ASME’s DeLong diagram gives both percent ferrite and FN and specifies how to treat nitrogen: use the measured value if available; otherwise assume 0.08% N for GMAW weld metal, 0.12% for self-shielded flux-cored GMAW, and 0.01% for other processes. RCC-M provides a similar diagram without the FN concept. The WRC-1992 diagram extends FN to 100 and is intended mainly for duplex stainless steels, whose ferrite and austenite each approach 50%.

4. Acceptance Windows in Practice

China has no unified national limit for ferrite in austenitic stainless steel and its weld metal; limits come from nuclear, defense, and important chemical applications. Table 3 compiles the typical requirements from standards, control ranges, and experience.

Weld metal and filler metal above exclude valve seat hard-facing deposits.

5. Conclusions

Austenitic stainless steel normally contains 5–15% ferrite; CF castings commonly 5–25%. That ferrite is double-edged: in base and filler metal it prevents weld hot cracking and improves resistance to intergranular and stress corrosion, and in castings it prevents casting cracks and raises mechanical properties — but in high-temperature, ultra-low-temperature, and selective-corrosion service its adverse effects must be contained by specification.

The practical lesson for buyers and inspectors: do not judge austenitic stainless steel with a magnet. Weak magnetism is usually normal ferrite, not a defect. Specify the ferrite window that matches the service — 3–8% for forging stock, 4–12% in welds, ≥ 5 FN under ASME, 12–25% for RCC-M pressure castings — and verify it with the right tool: a ferrite meter or metallography for castings and welds, a prepared autogenous sample for wrought products, or a Schaeffler / DeLong / WRC-1992 calculation from the mill certificate when speed and equipment matter more than ±2% accuracy. Ferrite is not a contaminant to be feared; it is a variable to be specified, measured, and controlled.