Ferrite Measurement in Duplex and Austenitic Stainless Welds

Ferrite and austenite share the structure of duplex and super duplex stainless steels, and their proportions govern how the material behaves: too little ferrite weakens the steel and lowers its resistance to stress corrosion cracking, while too much costs toughness and corrosion resistance. itmad measures ferrite non-destructively in duplex, super duplex and austenitic stainless welds and castings, to AWS A4.2 and ISO 8249, and reports in Ferrite Number (FN) or percent ferrite. itmad operates across the Middle East, Asia and Europe. Last reviewed: October 2026.

Why it is measured

The weld is the place where the proportions change, since the heat input and cooling rate of welding alter the phase balance in the weld metal and the heat-affected zone. A duplex weld cooled too fast can end up ferrite-rich and brittle, and one cooled too slowly can be austenite-rich, with lower strength and pitting resistance. In austenitic steels the aim is a controlled level of delta ferrite in the weld metal, which guards against solidification cracking; too much harms corrosion resistance and can transform into embrittling phases in elevated-temperature service.

Nothing here shows to the eye. A weld can look right, pass visual and penetrant examination, and still fail later in service, so measuring ferrite is how the balance is confirmed while it can still be corrected.

The method

A Fischer Feritscope works by magnetic induction. Because ferrite is ferromagnetic and austenite is not, the instrument's response to the magnetic character of the surface layer reads out the ferrite content directly, as FN or percent ferrite depending on the calibration standard and the specification. Certified ferrite standards are used to check the instrument before and after every job.

The measurement is made on the production component itself, with no sacrificial coupon. Metallography on a test piece tells you about a batch statistically, whereas only direct measurement tells you about the item that is being delivered. Readings are taken at several positions on each weld and on the adjacent material, since ferrite varies across a bead and from pass to pass, so one reading can pass while the weld as a whole does not.

Standards and specifications

The reference documents are AWS A4.2, on calibrating magnetic instruments for delta ferrite in austenitic and duplex weld metal, ISO 8249, on determining Ferrite Number in the same weld metal, and, where point count metallography is specified for comparison or validation, ASTM E562. The manufacturer's calibration and operating instructions also apply, as do client and project specifications, including the major operator specifications that commonly set ferrite ranges for duplex fabrication.

Acceptance criteria

Ranges come from the client specification, the applicable code and the material manufacturer, not from a universal standard. For duplex weld metal, 35 to 65 FN or 30 to 70 FN are common depending on the specification, and super duplex grades usually require tighter control. itmad reports every reading individually, states accept or reject against the range, and comments on the likely cause when a result is out of range.

Where it applies

Typical subjects are duplex and super duplex welds, base material and heat-affected zones; austenitic weld metal with specified delta ferrite limits; overlays and clad surfaces; duplex castings and forgings; pipework, vessels, valves and fittings for sour and seawater service; subsea and offshore duplex fabrication; components after heat treatment or repair welding; and duplex parts that have cracked or corroded in service.

Practical considerations

Scale, oxide, spatter and heavy grinding marks interfere with the reading and are dealt with first. Curvature, edges and thin sections change the reading too, and call for the right probe and technique.

Personnel

The work is done by technicians and engineers experienced in stainless steel metallurgy. Interpretation matters as much as the reading, because it decides whether the fault lies with welding parameters, a consumable or a heat treatment, and so what the client should do about it, and the report says which. Work is done on site, in fabrication yards, offshore or at itmad premises.

What the client receives

A report listing each item and weld measured, by number or drawing reference, with every individual reading and not an average alone. Results are in FN or percent ferrite as the specification requires, together with the instrument used and its calibration verification record, an accept or reject status against the specified range, and a comment on the likely cause where results fall outside it.

Frequently asked questions

Why is ferrite measured in stainless steel welds?

To confirm that the phase balance is within the specified range. In duplex and super duplex steel, too little ferrite reduces strength and resistance to stress corrosion cracking, and too much reduces toughness and corrosion resistance, none of which is visible on the weld.

How is ferrite measured?

By magnetic induction with a Fischer Feritscope, which reads ferrite directly because ferrite is ferromagnetic and austenite is not. It is non-destructive and is applied to the production component, with the instrument checked against certified standards before and after the job.

Which standards apply?

AWS A4.2 and ISO 8249, plus ASTM E562 where point count metallography is specified for comparison or validation. Client and operator specifications commonly set the acceptance ranges for duplex fabrication.

What ferrite range is acceptable for duplex weld metal?

It is set by the specification, the code and the material manufacturer, and 35 to 65 FN or 30 to 70 FN are common. Super duplex grades typically need tighter control, and itmad states accept or reject against the range applied.

Why are several readings taken on each weld?

Ferrite varies across a weld bead and between passes, so a single reading can pass while the weld as a whole does not. Readings are therefore taken at several positions on the weld and the adjacent base material.

What can spoil a ferrite reading?

Scale, oxide, weld spatter and heavy grinding marks, which are dealt with before measuring. Curvature, edges and thin sections also change the reading and need the correct probe and technique.