Measurement of ferrite content of stainless steel sheet welds using a new Ferrite Density Number scale

The retained δ-ferrite content in austenitic stainless steel welds is a crucial parameter to control, since several properties of the weld depend on this. It is conventionally measured using a Ferrite Number (FN) scale, which has been defined in the internationally accepted standards such as ISO 8249 and AWS A4.2 M. A FeritScope® is commonly used to measure the FN of the welds, which takes advantage of the fact that the retained δ-ferrite phase at room temperature is magnetic whereas austenite is not. The readings obtained from the FeritScope® are known to be influenced by several constraints such as the sheet thickness, weld clad thickness, curvature of the surface, etc. Some correction factors have been introduced by the FeritScope® manufacturer to account for these influential conditions. However, when the corrected FNs of the welds made on thin 304 L stainless steel sheets (≤ 2.4 mm thick) were plotted against the cooling rates, the trends obtained contradicted those available in the literature. The FN was found to increase with an increase in the cooling rate, whereas it was expected to decrease. Moreover, addition of nitrogen as an alloying element, which is known to be a strong austenite promoting element, was not found to influence the measured FN of the welds, again contradicting the literature. On further investigation it was found that some additional geometrical features, apart from those mentioned in the FeritScope® user manual must be considered while measuring the FN. This lead to the definition of a new scale – Ferrite Density Number (FDN) – which was found to be a better indicator of the amount of retained δ-ferrite in the welds as against the conventionally used FN scale. Using the FDN scale could eliminate the contradictions to the literature and explain the strong austenite-promoting behaviour of nitrogen.