What is ferrite content in duplex stainless steel?

Ferrite content in duplex stainless steels should be close to 50% ideally, and within the range of 35% to 60%, in order to optimise their mechanical and physical properties.

Duplex and super duplex stainless steels achieve their desirable combination of properties from their ‘duplex’ austenite:ferrite microstructure. The ferrite phase provides high strength and resistance to chloride stress corrosion cracking, whilst the austenite phase provides corrosion resistance and impact strength toughness. Consequently, too much ferrite reduces toughness, whilst too little ferrite reduces the resistance to chloride stress corrosion cracking.

Ferrite can be detrimental to corrosion resistance in some environments plus there are applications where magnetic characteristics interfere with the performance of the end product.

The ferrite content of the cast alloy can be controlled through alloy composition. There is a number of different methods to predict the ferrite content but one of the most common is the Delong diagram. Ferrite reduces the tendency for the steel to solidification crack during cooling.

Ferrite content can be measured in two main ways, either by metallography or magnetic measurement. Metallography is a destructive test, performed on a test piece cut from the metal product. After etching the surface to reveal the microstructure, it can be examined under a high-power optical microscope. Based upon this inspection, the ferrite content is most commonly reported as a volume fraction (% ferrite). Where the removal of a test sample is more problematic, then an in-situ measure of the metals magnetic permeability can be undertaken. The ‘Ferritescope’ associates the physical properties with the approximate proportion of magnetic ferrite present, as austenite is non-magnetic. Results from this technique are typically reported as a ‘ferrite number’. This technique is often used as a quality control test for duplex stainless steel welds.

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