Ferrite Measurement

What Is Ferrite Measurement?

Ferrite measurement is a measurement in which the amount of delta ferrite present in austenitic and duplex stainless steel and in weld metal is determined by a magnetic method (ferrite number, FN) or by metallographic point counting (volume percentage).

Purpose of Ferrite Measurement

The purpose is to verify that the ferrite proportion in weld metal is within the range that provides hot-cracking resistance balanced with corrosion resistance and toughness.

Which Products Is Ferrite Measurement Applied To?

Austenitic stainless-steel welds
Duplex stainless steels
Stainless-steel castings
Pressure-vessel welds

In Which Sectors Is Ferrite Measurement Used?

MetalSteelMiningMachineryOil & Gas

Standards Related to Ferrite Measurement

ASTM E562
AWS A4.2
ISO 8249
ASTM A800

Ferrite Measurement Conditions

Magnetic method: Ferrite number (FN) measurement with a calibrated ferritescope
Metallographic method: Volume percentage via ASTM E562 systematic point counting
Calibration: With primary or secondary standards per AWS A4.2
Number of measurements: At a defined number of locations; averaged

Equipment for Ferrite Measurement

Ferritescope (magnetic ferrite meter)
AWS A4.2 calibration standards
Metallurgical microscope
Specimen-preparation line

Sample Requirements for Ferrite Measurement

The measurement surface must be flat, clean and free of weld slag
If the weld surface has been ground, the grinding depth must be reported
Specimen thickness and geometry affect the measurement
Measurement locations must be documented

Duration of Ferrite Measurement

Magnetic measurement takes hours; the metallographic method takes 1–2 days.

Ferrite Measurement Procedure

1The ferritescope is calibrated with AWS A4.2 standards.
2The measurement surface is prepared.
3FN is measured at the defined locations.
4The average and scatter are calculated.
5It is confirmed by metallographic point counting if required.

Ferrite Measurement Acceptance Criteria

The acceptance criterion is given in the welding specification or product standard as an FN or volume-percentage range.

Evaluation of Results

Ferrite number (FN) and volume percentage are not the same quantity and cannot be directly converted between each other; which one the specification requires must be clarified.

Ferrite Measurement Report

Standard and method applied
Calibration records
Measurement positions
FN or volume-percentage values
Average and spread

Accreditation

EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.

Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.

Scope and authorisations

Related Regulations

Pressure-vessel and welding specifications

Metal and Metallurgy Testing — What Does This Testing Field Cover?

The section below describes the field that the Ferrite Measurement test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.

Metal and metallurgical testing determines the load a metal or a welded joint can carry, how it fails and what its internal structure looks like. The output of this family is often not about the product itself but about the identity of the MATERIAL: is the steel really of the declared grade, does the weld seam deliver the strength the design assumes, does the casting contain an unacceptable internal defect?

The practical importance of the field comes from the fact that a metal part rarely fails gradually. A plastic part deforms, ages, cracks and usually gives warning. A weld seam or a fastener, by contrast, often breaks without warning — especially where fatigue and brittle fracture are involved. That is why in fields such as pressure equipment, lifting gear, structural steel and pipelines, material testing is a requirement of REGULATION rather than of design.

The family brings together three groups of methods, and each answers a different question. Mechanical tests quantify how the material behaves under load. Non-destructive examination looks for defects without damaging the part. Metallography and chemical analysis explain why the material behaves the way it does — through grain structure, phase distribution, inclusion content and alloy composition.

The acceptance criterion for these tests almost never comes from the test standard, and this is the key to understanding the field. The test standard says how the measurement is to be made; the acceptable values come from the product standard, the material grade or the fabrication code. ISO 6892-1 tells you how to run a tensile test; EN 10025 tells you what yield strength that steel must deliver. When reading a report, the product standard it was assessed against is essential.

Sampling is an inseparable part of the result in this field. Two specimens taken from the same heat may give different values depending on the direction and location they were taken from; in rolled products, specimens parallel and perpendicular to the rolling direction produce different results. Product standards therefore also govern sampling location and orientation, and the report must document this.

Method groups in this field

Mechanical tests
Tensile, compression, bending, notch impact (Charpy), hardness (Brinell, Vickers, Rockwell), fatigue, creep and fracture toughness. These give the behaviour of the material under load in numerical form.
Non-destructive examination
Radiography, ultrasonic examination, penetrant, magnetic particle and visual examination. It looks for surface and internal defects without damaging the part and is an indispensable step in welded fabrication.
Metallography
Specimen preparation, etching and microscopic examination. Grain size, phase distribution, inclusion content, case depth and the weld heat-affected zone are assessed.
Chemical analysis
Alloy composition is verified by optical emission spectrometry and wet chemistry; confirmation of the material grade runs through this step.
Corrosion behaviour
Salt spray, pitting and crevice corrosion and intergranular corrosion tests. In stainless steels these can indicate the quality of the heat treatment.

Commonly confused in this field

A material certificate (such as EN 10204 3.1) is not a test report; it is the manufacturer's declaration. In critical applications independent verification is carried out, and the two must not be confused.
ASTM and ISO tensile tests measure the same quantities but use different specimen geometries; the results are not treated as directly equivalent, and whichever the specification calls for is the one applied.
Conversion tables between hardness and strength are APPROXIMATE. Calculating yield strength from hardness is not sufficient evidence for material acceptance.
In non-destructive examination, "no defect found" does not mean there is no defect; defects below the detection limit of the method may be present. The report must state the method applied and the sensitivity level.
A weld seam passing does not show that the welder or the procedure is qualified; these are separate processes.

Which decisions these results support

Verifying the material certificate and accepting incoming material
Approving welded fabrication; welder and procedure qualification
Manufacturing control of pressure equipment and structural components
Failure analysis and determining the root cause of field damage
Verifying heat treatment and surface treatment processes

Related Tests

Frequently Asked Questions About Ferrite Measurement

Are FN and ferrite volume percentage the same?

No. The ferrite number is a scale based on magnetic response, while volume percentage is a metallographic area fraction. The two values may be close but are not equivalent, and the specification must clearly state which one is required.