Ferrite Measurement

Ferrite measurement determines the proportion of the delta ferrite phase in austenitic and duplex stainless steel weld metal. The result is given as a ferrite number (FN) or as a percentage by volume; measurement uses portable instruments based on magnetic permeability, or a metallographic method.

The aim is to verify that ferrite content lies within the target range. Too little ferrite makes the weld prone to hot cracking; too much brings loss of toughness and reduced corrosion resistance. The range is narrow, which is why it must be measured.

• Austenitic stainless-steel welds
• Duplex and super-duplex welds
• Chemical-process equipment
• Food and pharmaceutical industry piping
• Marine environment and chloride-service components

NDT has no interval of its own; how often it is applied follows the inspection programme it serves. In manufacturing the governing factor is the coverage required by the product standard and the specification (e.g. a given percentage of welds); in operation it is the asset integrity or risk-based inspection programme.

• Ferrite number (FN) or volume percent in the weld metal
• Conformity of the measurement with the target range
• Ferrite distribution in root, fill and cap passes
• Assessment of the heat-affected zone
• Correlation with heat-input and interpass-temperature records

• ISO/IEC 17020
• ISO 8249
• ASTM E562
• ASTM A800
• EN ISO 17655
• ISO 9712

• Ferrite meter (based on magnetic permeability)
• Certified ferrite calibration standards
• Metallographic preparation and microscope (for the point-counting method)
• Surface preparation equipment

1. 1Selection of Method and TechniqueThe type and location of the discontinuity sought and the material properties are assessed to select a suitable method and technique; a written procedure is prepared.
2. 2Verification of Personnel and EquipmentThe level and industrial-sector qualification of the operator under EN ISO 9712 and the calibration records of the equipment are checked.
3. 3Surface PreparationThe inspection surface is prepared as the method requires; coating, rust and contamination directly affect the sensitivity of the method.
4. 4Calibration and Sensitivity VerificationA reference block or control specimen is used to demonstrate that the system can detect the discontinuity sought.
5. 5Performing the InspectionScanning is carried out according to the procedure; indications are recorded and located.
6. 6AssessmentIndications are evaluated against the applicable acceptance standard to determine whether they are defects; they are sized.
7. 7ReportingMethod, technique, equipment, calibration, findings and acceptance criteria are reported together.

The measurement result is compared with the ferrite range prescribed by the specification or relevant standard. In duplex steels the typical target range is narrower, and both the lower and upper limits are critical; a result outside the range requires the welding parameters to be reviewed.

• The inspection body and the personnel's EN ISO 9712 level and sector
• Method and technique applied and the written procedure number
• Part and weld examined, with a location sketch
• Equipment, consumables and calibration records
• Surface condition and inspection conditions
• Indications recorded, with their location and size
• Acceptance standard applied and the result of the evaluation
• Accept / reject decision

Non-destructive testing is carried out as an accredited inspection activity under ISO/IEC 17020. The second decisive factor in NDT is personnel qualification: the operator must be certified under EN ISO 9712 for the method, level and industrial sector concerned. An examination performed with a method outside the scope of that certificate is not valid. EUROLAB holds internationally recognised accreditation under ISO/IEC 17020, granted by accreditation bodies that are signatories to the ILAC mutual recognition arrangement.

• VT Visual
• PT Penetrant
• MT Magnetic Particle
• UT Ultrasonic
• RT Radiographic
• ET Eddy Current
• PAUT
• TOFD