🔩 Ferrite Number FN (delta ferrite content) in weld metal of austenitic and duplex Cr-Ni stainless steels — detachment force of a standard magnet, calibration with copper coating thickness standards; PN-EN ISO 8249

Materials and NDT PN-EN ISO 8249

Overview

WHAT THE STANDARD COVERS. ISO 8249:2018 (clause 1) specifies the method and apparatus for: measuring the delta ferrite content, expressed as the Ferrite Number (FN), in weld metal of predominantly austenitic and duplex ferritic-austenitic stainless steels, by the attractive force between a weld metal specimen and a standard permanent magnet; making and measuring standard pads from manual metal arc covered electrodes (the general method is also recommended for production welds and weld metal from other processes — TIG, MIG/MAG, submerged arc — in which case the way of making the pad has to be defined); calibrating other instruments for measuring FN. The method is intended for weld metal in the as-welded condition and after heat treatment that transforms ferrite completely or partly into a non-magnetic phase; austenitizing treatment that changes the size and shape of the ferrite changes the magnetic response. The method is NOT intended for measuring ferrite in castings, forgings and wrought products of austenitic and duplex steels. The standard was prepared by the International Institute of Welding (IIW), Commission II; the third edition (2018) replaced ISO 8249:2000 (corrections in Table 2 — formerly 1 — and editorial changes). The "Scope" field of the PKN card of PN-EN ISO 8249:2018-11, in full (our translation): "This document specifies the method and apparatus for: — the measurement of the delta ferrite content, expressed as the Ferrite Number (FN), in weld metal of predominantly austenitic or duplex ferritic-austenitic stainless steels, by the attractive force between a weld metal specimen and a standard permanent magnet; — the preparation and measurement of standard inserts for manual covered electrodes. In general, the method is also recommended for the measurement of ferrite in production welds and for weld metals from other processes, such as gas tungsten arc welding, gas metal arc welding and submerged arc welding (in this case the way of producing the insert should be defined); — the calibration of other instruments for measuring FN. The method specified in this document is intended for use on weld metals in the as-welded condition and on weld metals after heat treatment causing complete or partial transformation of ferrite into any non-magnetic phase. Austenitizing heat treatments that change the size and shape of the ferrite will change the magnetic response of the ferrite. The method is not intended for measuring the ferrite content of castings, forgings or wrought specimens of austenitic steels or specimens of duplex ferritic-austenitic steels." The "inserts" (wkładki) and "insert" (wkład) on the card are, in the text of the standard, the standard pads of clause 6.

WHY A "FERRITE NUMBER" AND NOT A PERCENTAGE (introduction). There is no agreement on which method gives an absolute measurement of the amount of ferrite in weld metal, which is why the concept of the Ferrite Number was introduced — a description of the ferrite content determined by a standardized procedure. At low FN it was considered approximately equal to the ferrite percentage; more recent data indicate that at higher FN the Ferrite Number may overstate the volume fraction of ferrite by a factor of 1.3–1.5, depending on the alloy composition. The ferrite content determined by this method is conventional and not necessarily true, which is why the result is given as FN, not as "per cent ferrite". Principle (clause 4): the attractive force between a two-phase specimen and a magnet increases with the fraction of the ferromagnetic phase; in austenitic and duplex weld metal ferrite is magnetic, whereas austenite, carbides, sigma phase and inclusions are not.

CALIBRATION (ISO 8249:2018, clause 5 — text read). Coating thickness standards: non-magnetic copper on an unalloyed steel substrate 30 mm × 30 mm, substrate thickness not less than the experimentally determined thickness above which the attractive force no longer increases; coating thickness known to ± 5 % or better; composition of the steel (Table 1, % by mass): C 0.08–0.13, Si up to 0.10, Mn 0.30–0.60, P up to 0.040, S up to 0.050; the copper may have a thin chromium layer. NIST standards (formerly NBS) may be used. Standard magnet: cylindrical permanent magnet 2 mm in diameter and about 50 mm long, one end hemispherical with a radius of 1 mm, polished (example: magnetic steel with 36 % cobalt, 48.45 ± 0.05 mm, saturated and demagnetized to 85 %); detachment force from the standards within ± 10 % of the relationship of Figure 1 (excluding the mass of the magnet), which corresponds to 5.0 ± 0.5 FN/g. The instrument applies an increasing force to the magnet perpendicular to the surface until detachment and measures it accurately; reading in FN, grams-force or other units (then a calibration curve). Calibration curve: detachment force from several standards, coating thickness converted to FN according to Table 2 or formula (1): FN = exp{1.805 9 − 1.118 86 [ln(t)] − 0.177 40 [ln(t)]² − 0.035 02 [ln(t)]³ − 0.003 67 [ln(t)]⁴}, t in mm (e.g. t = 0.100 mm → 43.2 FN; 0.500 mm → 12.3 FN; 1.000 mm → 6.1 FN according to Table 2). For the range 0 – approx. 30 FN (austenitic weld metal) at least eight standards with a copper thickness from approx. 0.17 to approx. 2 mm are recommended; for extension to 100 FN (duplex) — at least five standards of 0.03–0.17 mm. NOTE to 5.4: this calibration procedure may give MISLEADING results in instruments that measure ferrite otherwise than by the attractive force or with a magnet other than the standard one — such instruments are calibrated according to clause 8.

PADS FROM COVERED ELECTRODES (clause 6). The pad is deposited between two copper bars 70 × 25 × 25 mm on a base plate, layer upon layer, to a height of at least 12.5 mm (with a base plate of austenitic Cr-Ni steel X2CrNi18-9 or X5CrNi18-9 — at least 13 mm; with carbon-manganese steel — at least 18 mm); with electrodes ≥ 4 mm each layer in a single bead; the arc as short as possible, the arc must not touch the copper; the welding direction is reversed after each bead; water cooling between beads not earlier than 20 s after the bead, interpass temperature not more than 100 °C, beads of the last layer air-cooled below 425 °C before quenching in water; each bead cleaned; the top layer in a single bead with a weave of not more than three times the core wire diameter. Current and approximate dimensions of the measuring area (Table 3, width × length): electrodes 1.6 and 2 mm — 35–45 and 45–55 A, 12.5 × 30 mm; 2.5, 3.2 and 4 mm — 65–75, 90–100 and 120–140 A, 12.5 × 40 mm; 5 mm — 165–185 A, 15 × 40 mm; 6.3 mm — 240–250 A, 18 × 40 mm (current or 90 % of the maximum value recommended by the electrode manufacturer). Surface preparation: austenitic weld metal (< 30 FN) — smooth and flat, with a 350 mm flat file drawn along the weld, with its axis perpendicular to the weld, without cross-filing and without heavy cold working; duplex (> 30 FN) — grinding with progressively finer abrasive to grit 600 or finer, without excessive pressure, polishing or overheating; the surface continuous over the measured length, at least 5 mm wide, without traces of scale.

STATUS (state of the PKN cards on 10.10.2026). The card of PN-EN ISO 8249:2018-11 (English version) carries no withdrawal label: title "Spawanie -- Określanie liczby ferrytu (FN) w stopiwie nierdzewnych chromowo-niklowych stali austenitycznych i ferrytyczno-austenitycznych dupleks" (Welding — Determination of Ferrite Number (FN) in austenitic and duplex ferritic-austenitic Cr-Ni stainless steel weld metals; our translation), publication 15-11-2018, 40 pages, Sector of Metallurgy, KT 165 Welding and Allied Processes, "Replaces: PN-EN ISO 8249:2005 - Polish version", "Introduces: EN ISO 8249:2018 [IDT], ISO 8249:2018 [IDT]", ICS 25.160.40. The card of PN-EN ISO 8249:2005 (Polish version, introduced EN ISO 8249:2000 and ISO 8249:2000): withdrawn on 15-11-2018 and replaced by PN-EN ISO 8249:2018-11. The current edition is therefore available in English only. In the PKN search engine (query "ISO 8249", 10.10.2026) there is also the card PN-EN ISO 18249:2026-05 — a different standard. In the iTeh catalogue (read on 10.10.2026) ISO 8249:2018 has the status "Published", and the last periodic review ended with confirmation of the standard on 29.08.2026.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database of 10.10.2026 and current PCA documents read on 10.10.2026). The number 8249 appears in 9 records at 7 laboratories: AB 001 (2 records), AB 608, AB 1141, AB 1646, AB 1700 (2), AB 1888 and AB 1898; the standard is present in all 7 current PCA documents, no entry is suspended. Forms: "PN-EN ISO 8249:2018-11" (5 laboratories), "PN-EN ISO 8249" without a year (AB 608) and "EN ISO 8249:2018-11" without PN (AB 1141). The "Laboratories" tab ("PN-EN ISO 8249") shows 6 laboratories; AB 1141 is outside it because of the entry without "PN-".

WHAT THE LABORATORIES TEST (copy and PCA documents). Delta ferrite content (AB 1700 also "Ferrite Number (FN)") in welds and weld metal of austenitic stainless steels, in austenitic clad layers (AB 001), in the material and weld metal of chromium-nickel steels (AB 1898), in welds of austenitic and duplex steels (AB 1141). All seven laboratories describe the technique as the "magnetic induction method".

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — the principle of the instrument and calibration: the method of the standard is the detachment force of a standard magnet; an instrument working differently (e.g. on the magnetic induction principle, as all laboratories describe their test in their scopes) calibrated directly with coating thickness standards may, according to the note to 5.4, give misleading results — such an instrument is calibrated according to clause 8 of the standard. Second — FN is not a percentage: at high FN the number may overstate the volume fraction of ferrite by a factor of 1.3–1.5; the result is given in FN. Third — a subject outside the scope: castings, forgings and wrought products are excluded by the standard; the parent material (AB 1898: "material and weld metal") is not weld metal. Fourth — surface: cross-filing or heavy cold working of an austenitic pad and overheating when grinding duplex change the reading. Fifth — heat treatment: austenitizing treatment changes the magnetic response of ferrite.

WHAT WE DO NOT GIVE. The read sample of the standard ends at the heading 6.3.2 (individual measurements). We do not give the number and distribution of readings on the pad, the way the result is determined, clause 7 or 8 (calibration of other instruments, including those working on a different principle), the test report, or the full Table 2 (we have given selected rows).

Method principle

Delta ferrite in austenitic and duplex weld metal is the only ferromagnetic phase, so the attractive force between the specimen and a magnet increases with its fraction. The instrument applies an increasing force perpendicular to the surface until the standard magnet (Ø 2 mm, hemispherical end R = 1 mm) detaches and measures the detachment force; a calibration curve from thickness standards of a non-magnetic copper coating on unalloyed steel (Table 2 or formula (1)) converts the force into the Ferrite Number FN (ISO 8249:2018, clauses 4 and 5).

Applications

• Weld metal of predominantly austenitic and duplex stainless steels — as welded and after heat treatment (ISO 8249:2018, clause 1)
• Standard pads from covered electrodes (classification and control of electrodes); also recommended for production welds and weld metal from TIG, MIG/MAG and submerged arc welding
• Calibration of other instruments for measuring FN (clause 8 — we did not read its content)
• In the accreditation scopes: 7 laboratories; all describe the technique as the "magnetic induction method"

Key parameters

Standard

Standard number

PN-EN ISO 8249:2018-11 (wersja angielska; wprowadza EN ISO 8249:2018 [IDT] oraz ISO 8249:2018 [IDT])

Title (PL)

Spawanie — Określanie liczby ferrytu (FN) w stopiwie nierdzewnych chromowo-niklowych stali austenitycznych i ferrytyczno-austenitycznych dupleks

Title (EN)

Welding — Determination of Ferrite Number (FN) in austenitic and duplex ferritic-austenitic Cr-Ni stainless steel weld metals

Step-by-step procedure

1. Calibration

   Determine the detachment force of the standard magnet from several coating thickness standards; convert the thickness to FN (Table 2 or formula (1)) and plot the calibration curve. Instruments of a different principle — according to clause 8 (we DO NOT GIVE its content).

2. Pad (covered electrodes)

   Between Cu bars on the base plate, layer upon layer to at least 12.5 mm (13/18 mm depending on the base plate); current according to Table 3; interpass temperature ≤ 100 °C; reverse the direction after each bead.

3. Surface preparation

   < 30 FN: with a file along the weld, without cold working; > 30 FN: grinding to grit 600, without overheating; area at least 5 mm wide, without traces of scale.

4. Measurement

   Measurement of the detachment force on the prepared surface; we DO NOT GIVE the number and distribution of readings or the calculation of the result (6.3.2 onwards — outside the read sample).

5. Result

   Report as the Ferrite Number FN, not as a ferrite percentage; with an instrument of a different principle state the calibration method. PN-EN ISO 8249:2018-11 current, PN-EN ISO 8249:2005 withdrawn on 15-11-2018 (state of the PKN cards on 10.10.2026).

Required equipment and apparatus

Health and safety (OHS)

• Welding the pads — welding fumes (Cr-Ni steels), arc radiation, burns; quenching a hot pad in water — risk of steam scalding
• Grinding and filing — eye and respiratory protection
• NOTE ON SUBSTANCE: calibration with coating thickness standards applies to instruments measuring the detachment force of the standard magnet

Frequently asked questions