Austenitic stainless steel looks straightforward to weld — no preheat, no post-weld heat treatment in most structural applications, and readily available filler metal. But the metallurgy hides a specific cracking mechanism that standard WPS documentation often misses: hot cracking in the weld metal caused by insufficient ferrite content. Understanding Ferrite Number and how AWS D1.6 addresses it is not optional for shops welding structural stainless.

What Is Ferrite Number and Why Does It Matter?

Ferrite Number (FN) is a dimensionless quantity that represents the magnetic ferrite content in austenitic stainless steel weld metal. It is measured magnetically using a calibrated instrument on the weld surface or predicted from weld metal composition using diagrams such as the WRC-1992.

The significance: austenitic stainless steel welds solidify in one of two modes depending on chemistry — austenitic or ferritic-austenitic. Welds that solidify in the austenitic mode (very low FN, approaching 0) are highly susceptible to solidification hot cracking. Liquid films form at grain boundaries during solidification; when the weld is under restraint, those films tear rather than fuse, producing cracks that run through the center of weld beads.

Welds with at least 3–5 FN solidify partially as delta ferrite first, which disrupts the continuous grain boundary liquid film network and dramatically reduces hot cracking susceptibility. This is why minimum FN requirements exist — not as a material property goal for the finished joint, but as an insurance mechanism against solidification cracking.

Above approximately 10–15 FN (varying by alloy), ferrite content itself begins to reduce corrosion resistance (ferrite is anodic relative to austenite in many service environments) and can cause sigma phase embrittlement over time at elevated temperatures. The practical target is a narrow window: enough ferrite to prevent cracking, not so much that service properties suffer.

AWS D1.6 Ferrite Control Requirements

AWS D1.6:2017 Structural Welding Code — Stainless Steel governs welding of structural stainless steel components. Unlike AWS D1.1, which focuses entirely on carbon and low-alloy steel, D1.6 must address the unique solidification and corrosion behavior of austenitic alloys.

AWS D1.6 addresses ferrite control primarily through filler metal selection requirements. The code requires that:

  1. Filler metal selection must account for dilution effects. The weld metal FN is not simply the FN of the filler rod or wire at face value — it is a function of base metal dilution. Higher dilution (deeper joint, hotter process settings) shifts the weld metal composition toward the base metal chemistry, which may raise or lower the resulting FN depending on the alloy pair.

  2. Certified test reports (CTRs) must document weld metal composition. The electrode or wire manufacturer's certification should include actual weld metal chemistry (not just electrode chemistry), from which FN can be predicted using the WRC-1992 diagram if the shop requires verification before welding.

  3. Prequalified filler metals for structural grades. D1.6 Table 3.1 lists prequalified filler metal classifications for common austenitic base metals (304/304L, 316/316L, 321, 347, and duplex grades). For joints within those combinations, using a listed filler metal with documented conformance satisfies the filler selection requirement without additional FN testing on every lot.

See AWS D1.6 stainless structural welding procedure overview for the broader qualification framework and the full list of essential variables under D1.6.

The WRC-1992 Diagram in Practice

Before a PQR is run, the WPS writer can predict whether the selected filler metal will achieve adequate FN by using the WRC-1992 (Welding Research Council) diagram — the current industry standard for FN prediction. The diagram maps Cr-equivalent (Cr_eq) and Ni-equivalent (Ni_eq) to predicted FN:

Cr_eq = %Cr + %Mo + 0.7×%Nb
Ni_eq = %Ni + 35×%C + 20×%N + 0.25×%Cu

These values come from the electrode or wire certified test report (filler metal composition) and, when accounting for dilution, a weighted blend of filler metal and base metal chemistries.

For a typical 316L base metal welded with ER316L filler at 25–30% dilution, the resulting weld metal Cr_eq and Ni_eq can be plotted on the WRC-1992 diagram to confirm that FN falls in the 3–10 range before the first arc is struck. If the plot falls in the hot-cracking region (FN < 3), the filler selection must be reconsidered — either a higher-FN wire designation or reduced dilution parameters.

This is a pre-qualification screening step, not a guarantee. The actual PQR coupon weld metal should still be measured with a ferritescope on the deposited weld for confirmation.

What the WPS Must Document

A well-prepared AWS D1.6 WPS for austenitic stainless welding must explicitly address ferrite:

Filler metal specification and classification. The AWS A5.9 (bare wire/rod), A5.22 (flux-cored), or A5.4 (covered electrodes) classification must appear on the WPS. The FN prediction is only as good as the filler metal traceability.

Weld metal FN range or minimum. State the required FN on the WPS: Weld metal FN: 3 minimum (WRC-1992 predicted from CTR chemistry; verify by ferritescope measurement on PQR coupon). This makes the requirement visible to QC and the inspector without digging through the PQR.

Dilution control parameters. Travel speed range, voltage range, and amperage range all affect dilution. Documenting these as ranges (not single values) controls the dilution corridor and keeps the resulting FN prediction valid across the parameter range.

Heat input limits. Excessive heat input on stainless not only reduces corrosion resistance in heat-affected zones (sensitization risk for non-stabilized grades) but also affects solidification mode. The WPS should state a maximum heat input per the joint's service requirements.

PQR Testing for FN Confirmation

The PQR for a stainless structural WPS should include ferritescope readings as supplemental test data even where not explicitly required by D1.6 or the owner's specification. The procedure is straightforward:

  1. Grind or polish a cross-section of the weld to remove surface oxides (ferritescope reads through a thin oxide, but heavy scale skews readings).
  2. Take three to five ferritescope readings across the weld deposit, avoiding the fusion line (diluted zones read differently than the bulk weld metal).
  3. Record the average and range of readings.
  4. Compare to the WRC-1992 prediction. Agreement within ±2–3 FN is typical for a well-characterized filler metal lot.

If the PQR ferritescope readings fall below 3 FN despite a prediction above 3, investigate the filler metal lot — verify CTR chemistry against the lot actually used, and consider whether the PQR dilution was higher than the WPS range allows.

See PQR test laboratory selection and documentation under AWS D1.1 for general PQR testing protocols. While that article focuses on D1.1, the PQR record-keeping principles are the same across codes.

Duplex Stainless Steel: A Different FN Target

Duplex stainless steel (2205, 2507, and similar grades) is increasingly used in structural applications for its higher strength-to-weight ratio and improved corrosion resistance. Duplex welds require a higher FN than austenitic welds — typically 30–60 FN in the weld metal — because the target microstructure is intentionally austenitic-ferritic. Too much austenite (low FN) produces a corrosion and strength-degraded weld.

For duplex WPS documentation:

  • Use the WRC-1992 diagram to predict FN from filler metal CTR chemistry at the expected dilution level.
  • Target weld metal FN is typically 30–60 FN per the filler metal manufacturer's specification and the owner's corrosion engineering requirements.
  • PWHT (solution annealing) may be required by the owner for critical service, which eliminates the as-welded ferrite-austenite ratio — confirm with the design engineer.

AWS D1.6 does not have dedicated prequalified procedure provisions for duplex stainless in the same way it does for standard austenitic grades. All duplex WPSs require a tested qualification PQR, and ferrite measurement is part of the standard test record.

Service Environment and FN Upper Limits

The upper FN limit depends on service conditions:

Cryogenic service. Above approximately 10 FN, the weld metal becomes increasingly susceptible to low-temperature embrittlement. Cryogenic stainless weld procedures typically target 3–8 FN with a documented maximum, and impact testing at the design service temperature is required.

High-temperature service (above ~700°F / 370°C). Delta ferrite transforms to sigma phase over time in the 800–1650°F (427–900°C) range. Sigma phase is brittle and reduces corrosion resistance. Long-term service in this temperature range may warrant a lower FN target with the owner's confirmation.

Aqueous corrosion service. Ferrite is anodic to austenite in most chloride and oxidizing environments. High-FN weld metal is preferentially attacked. For chemical process piping supports and coastal structural stainless, lower FN targets (3–7 FN) with a verified maximum are often specified.

For structural stainless in general building applications (handrails, architectural columns, exposed connections), the 3–10 FN range covers most cases without special upper-limit restrictions.

When to Use a Third-Party FN Testing Lab

In most structural stainless WPS qualification situations, the shop's own ferritescope is adequate for PQR coupon testing. When a third-party measurement is warranted:

  • Owner or contract specification requires independent verification. Some pharmaceutical, food processing, or marine structural specifications require an ISO 17655 or AWS A4.2 compliant FN measurement by a third-party laboratory.
  • Filler metal lot has known CTR variability. If a filler metal lot certificate shows chemistry near the hot-cracking boundary, independent FN measurement adds defensible documentation.
  • Dispute resolution. If a weld is rejected on hot cracking grounds, independent FN measurement of a cross-section establishes the factual record.

For procurement-level filler metal traceability, see welding consumable certifications and traceability for the documentation chain from mill certificate to weld-as-run.

Managing stainless WPS qualification records — including FN targets, PQR test data, filler metal lot traceability, and CTR documents — is more involved than carbon steel WPS management. Purpose-built welding procedure software keeps FN documentation tied to specific filler metal lots and flags when a new lot CTR shows chemistry that would change the FN prediction. See WPS Pro pricing to evaluate whether integrated qualification management fits your shop's stainless work volume.

Rule library based on AWS D1.1:2025; AWS D1.6:2017 for stainless structural welding. Verify against your governing edition and applicable construction code.