Ferrite content in duplex stainless steels should ideally be close to 50%, and within the range of 35% to 60%, to optimise their mechanical and physical properties. Duplex and super duplex grades take their desirable combination of strength and corrosion resistance from this balanced austenite-ferrite microstructure.
The ferrite phase provides high strength and resistance to chloride stress corrosion cracking, while the austenite phase provides corrosion resistance and impact toughness. Too much ferrite reduces toughness; too little reduces resistance to chloride stress corrosion cracking.
Ferrite can also be detrimental to corrosion resistance in some environments, and there are applications where its magnetic character interferes with the performance of the end product.
Predicting ferrite content
The ferrite content of the cast alloy can be controlled through alloy composition. Several methods exist to predict it, one of the most common being the DeLong diagram. Ferrite also reduces the tendency for the steel to solidification crack during cooling.
Measuring ferrite content
Ferrite content is measured in two main ways: by metallography or by magnetic measurement.
- Metallography is a destructive test, performed on a test piece cut from the product. After etching the surface to reveal the microstructure, it is examined under a high-power optical microscope. The ferrite content is most commonly reported as a volume fraction (% ferrite).
- Magnetic measurement is used where removing a test sample is problematic. The Ferritescope relates the metal's magnetic permeability to the approximate proportion of magnetic ferrite present, since austenite is non-magnetic. Results are typically reported as a ferrite number, and this technique is often used as a quality control test for duplex stainless steel welds.