Ferrite Testing Guide: Measuring Ferrite Content in Steel and Welds
Representative Infinita Engineering Visual explaining the four-step workflow for Ferrite Testing Guide.What Is Ferrite Testing?
Ferrite testing measures the delta ferrite content in austenitic and duplex stainless steel welds, castings, and wrought materials. Ferrite content directly affects corrosion resistance, hot-cracking susceptibility, and mechanical properties — too little ferrite in an austenitic weld increases hot-cracking (solidification cracking) risk, while too much ferrite in a duplex weld reduces toughness and corrosion resistance.
Ferritescope (Magnetic Induction) Method
The most common non-destructive method uses a portable ferritescope operating on the magnetic induction (permeability) principle: a probe generates an alternating magnetic field, and the magnetically permeable ferrite phase alters the inductance in proportion to its volume fraction.
- Ensure the test surface is clean and, ideally, at ambient temperature — readings taken above roughly 40C can show artificially reduced apparent ferrite content
- Place the probe on the test surface, forming a closed magnetic circuit
- Record the ferrite reading, expressed as either a Ferrite Number (FN) or ferrite percentage
- Take multiple readings across the weld or component per the applicable specification
Metallographic Point Counting (ASTM E562)
The reference method for disputed or high-stakes ferrite determinations: a cross-section is cut through the weld, polished and etched to reveal phase contrast between ferrite and austenite, and the phases are counted under a calibrated optical microscope using a grid overlay — typically at 500x to 1000x magnification for stainless and duplex materials. This method is more accurate but far more time-consuming than ferritescope testing, and is generally reserved for procedure qualification and arbitration rather than routine production testing.
Typical Acceptance Ranges
For duplex stainless steel, 35-65% ferrite is generally preferred, with roughly 50% considered optimal for balancing corrosion resistance, toughness, and strength.
Industry Specifications Referencing Ferrite Testing
- ASTM A800/A800M: estimating ferrite content of stainless steel castings
- ASTM E562: manual point count metallographic method
- AWS A4.2, EN ISO 8249: welding-specific ferrite measurement standards
- API 938C, NACE MR0175/MR0103: industry specifications referencing ferrite content requirements
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Conclusion
Ferritescope testing is the practical default for routine production QA because it’s fast, portable, and non-destructive — metallographic point counting stays in reserve as the higher-cost, higher-accuracy method for settling disputes or qualifying new welding procedures.
Why is ferrite content important in stainless-steel welds? A controlled amount of ferrite can reduce the risk of solidification cracking in austenitic stainless-steel welds. In duplex stainless steels, an unsuitable ferrite–austenite balance may affect toughness, strength, and corrosion resistance.
What is Ferrite Number? Ferrite Number, or FN, is a standardized value describing the magnetic response of ferrite in stainless-steel weld metal. FN is not always directly equivalent to ferrite percentage and should be reported using the specified measurement procedure.
How is ferrite content measured? Common methods include portable magnetic measurement, metallographic point counting, image analysis, and estimates based on chemical composition. The appropriate method depends on the material, test location, accuracy requirement, and applicable specification.
What is metallographic ferrite testing? Metallographic testing involves preparing, polishing, and etching a cross-section so the ferrite and austenite phases can be distinguished under a microscope. ASTM E562 provides a systematic point-count method for estimating phase volume fraction.
What factors can affect ferrite readings? Results may be influenced by surface roughness, material thickness, curvature, probe position, calibration, weld geometry, and measurement location. Multiple readings are usually taken to account for local variations.
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