Ferrite Content Measurement in Belgium
Ferrite content measurement is a non-destructive technique used to determine the proportion of ferrite phase in austenitic stainless steel and duplex stainless steel welds and base materials. Correct ferrite levels are essential for ensuring corrosion resistance and mechanical integrity.
What is Ferrite Content Measurement?
Ferrite content measurement is a specialised non-destructive testing technique used to quantify the amount of magnetic ferrite phase present in austenitic stainless steels and duplex stainless steels. These materials derive their corrosion resistance and mechanical properties from a carefully balanced microstructure. In austenitic stainless steel welds, a controlled amount of delta ferrite (typically 3 to 15 FN — Ferrite Number) is desirable to prevent solidification cracking, but excessive ferrite can reduce corrosion resistance and toughness. In duplex stainless steels, the target microstructure is approximately 50% ferrite and 50% austenite (typically accepted within a 30-70% range), and deviations from this balance — caused by incorrect welding parameters or heat treatment — can severely compromise both corrosion resistance and mechanical properties.
The measurement is performed using a portable ferritescope, which operates on the magnetic induction principle. A probe containing a coil generates an alternating magnetic field that interacts with the ferromagnetic ferrite phase in the material. The instrument measures the magnetic permeability response, which is directly proportional to the ferrite content, and displays the result in Ferrite Number (FN) or percentage ferrite. The measurement is rapid, non-destructive and highly reproducible when performed correctly on a properly prepared surface.
LCNDTEST carries out ferrite content measurement on site and in workshops throughout Belgium, using calibrated ferritescopes traceable to international reference standards. Our technicians have extensive experience with austenitic and duplex stainless steels in the petrochemical, chemical, food processing, pharmaceutical and offshore industries, where strict ferrite content requirements are routinely specified.
Measurement Methodology
The ferrite measurement procedure begins with surface preparation of the test area. The surface must be clean, dry and free from any magnetic contamination, paint or coating that could affect the measurement. Light grinding may be performed to expose bare metal, taking care not to alter the microstructure through excessive heating. The ferritescope is calibrated before each measurement campaign using certified reference standards with known ferrite content values spanning the expected measurement range.
Measurements are taken by placing the probe perpendicular to the surface at each designated measurement point. The probe must make firm, consistent contact with the surface, and the operator must ensure that no external magnetic fields or nearby ferromagnetic objects interfere with the reading. For weld inspections, measurements are typically taken at multiple locations along the weld length, on the weld cap surface and, where accessible, on the weld root. A minimum number of readings (typically 3 to 5) is taken at each location and averaged. For duplex stainless steel welds, measurements are often required on both the weld metal and the heat-affected zone.
The results are compiled in a detailed report that identifies each measurement location (referenced to a weld map or isometric drawing), the individual readings, the average value at each location and the acceptance verdict based on the specified ferrite content requirements. Common acceptance criteria include client specifications, fabrication codes (e.g., NORSOK M-601 for duplex welds requiring 30-70% ferrite) and industry standards. Deviations from the required ferrite range may trigger additional metallurgical investigation or corrective action such as re-welding with adjusted parameters or solution annealing.
Industrial Applications
- Ferrite verification on austenitic stainless steel welds to ensure resistance to solidification cracking
- Phase balance measurement on duplex and super duplex stainless steel welds and parent materials
- Quality control of welding procedures on stainless steel piping and pressure equipment in petrochemical plants
- Verification of correct heat treatment and cooling rates on duplex stainless steel fabrications
- Incoming material inspection on stainless steel plates, pipes and fittings to confirm specified ferrite levels
- Ferrite content assessment on components for offshore, subsea and marine applications where strict requirements apply
Advantages of Ferrite Measurement
- Rapid, non-destructive and portable measurement directly on installed components and welds
- Quantitative ferrite content determination with high accuracy and reproducibility
- Essential quality assurance tool for austenitic and duplex stainless steel fabrication and welding
- Direct correlation between ferrite content and material properties (corrosion resistance, toughness, cracking susceptibility)
- Immediate on-site results enabling real-time quality decisions during fabrication and welding operations
Reference Standards
EN ISO 8249 • EN ISO 17655 • ASTM E1476 • AWS A4.2 • NORSOK M-601 • ASME Section IX
Frequently asked questions
Ferrite content directly affects the corrosion resistance, mechanical properties and cracking susceptibility of austenitic and duplex stainless steels. In austenitic stainless steel welds, a controlled amount of delta ferrite (typically 3-15 FN) is needed to prevent solidification cracking, but too much ferrite reduces corrosion resistance and toughness. In duplex stainless steels, the balance between ferrite and austenite phases must remain within specified limits (typically 30-70% ferrite) to ensure the material performs as intended in corrosive service environments.
The acceptable range depends on the material type and the applicable specification. For austenitic stainless steel welds, a ferrite content of 3 to 15 FN (Ferrite Number) is typically specified to prevent hot cracking while maintaining adequate corrosion resistance. For duplex stainless steel welds, NORSOK M-601 and similar specifications typically require 30-70% ferrite to ensure the correct phase balance. Specific client requirements or service conditions may impose tighter limits, particularly for equipment in highly corrosive or sour service environments.
Ferrite measurement is required during the qualification of welding procedures (WPQR) for austenitic and duplex stainless steels, as part of production quality control on welded joints, during incoming material inspection on stainless steel plates and fittings, and after any heat treatment that may alter the phase balance. It is particularly critical in the petrochemical, offshore, chemical, food processing and pharmaceutical industries where stainless steel components must meet strict corrosion performance requirements.