Ferrite content analysis provides critical data pertaining to delta ferrite content in austenitic stainless steel and duplex materials. The delta ferrite percentage or number allows a technical assessment of material corrosion susceptibility, mechanical properties, service suitability, and reliability.
Ferrite testing is performed on austenitic stainless steel and duplex items, including (but not limited to) weldments, castings, weld overlays, wrought materials, and forgings. Ferrite content analysis can be obtained on butt/fillet welds, Category A-D welds, and stainless weld overlays on non-ferrous interfaces, cast components, and pre- and post-weld heat-treated components.
Ferrite testing, utilizing portable digital technology with variable calibration in both Ferrite Number (FN) and % Ferrite (FN) using AWS standards, allows for rapid and accurate analysis. Both in-service and in-construction components are tested.
To perform proper ferrite testing, both a minimum material thickness and a minimum specimen size are required. The shape of the specimen may have a negative effect on the results obtained, although correction calculations can be performed in some instances.
Surface preparation is very important to ensure result accuracy. Ferrite testing is not recommended where the material is at temperatures greater than approximately 125 F. Test results are interpreted in accordance with current specifications and/or customer requirements. Reports issued are accompanied when necessary by drawings to identify locations tested.
Effects of Ferrite Content:
Fully austenitic stainless steel weld deposits have a tendency to develop small fissures even under conditions of minimal restraint. These small fissures tend to be located transverse to the weld fusion line in weld passes and base metal that were reheated to near the melting point of the material by subsequent weld passes. Cracks are clearly injurious defects and cannot be tolerated. On the other hand, the effect of fissures on weldment performance is less clear, since the very tough austenitic matrix quickly blurts these micro-fissures. Fissured weld deposits have performed satisfactorily under very severe conditions. However, a tendency to form fissures generally goes hand-in-hand with a tendency for larger cracking, so it is often desirable to avoid fissure-sensitive weld metals.
The presence of a small fraction of the magnetic delta ferrite phase in an otherwise austenitic (nonmagnetic) weld deposit has an influence in the prevention of both centerline cracking and fissuring. The amount of delta ferrite in as-welded material is largely controlled by a balance in the weld metal composition between the ferrite-promoting elements (chromium, silicon, molybdenum, and columbium are the most common) and the austenite-promoting elements (nickel, manganese, carbon, and nitrogen are the most common). Excessive delta ferrite, however, can have adverse effects on weld metal properties. The greater the amount of delta ferrite, the lower will be the weld metal ductility and toughness. Delta ferrite is also preferentially attacked in a few corrosive environments, such as urea. In extended exposure to temperatures in the range of 900 to 1700 F (482 to 927 C), ferrite tends to transform in part to a brittle intermetallic compound that severely embrittles the weldment.
Portable ferrite indicators are designed for on-site use. Ferrite content of the weld deposit may be indicated in percent ferrite and may be bracketed between two values. This provides sufficient control in most applications where minimum ferrite content or a ferrite range is specified.