The family of duplex stainless steels includes lean duplex, duplex, and superduplex grades. For ranking the pitting resistance equivalent (PRE) number is used. PRE is based on the chemical composition of the steel and can be calculated as following:

PRE=%Cr+3.3(%Mo+%W)+16(%N)).

Increasing some of the parameters, like in this case, the amounts of Cr and N, leads to the development of stainless steel with ultrahigh corrosion resistance - hyperduplex steel. It contains about 27%Cr, 7%Ni, 4.5%Mo, and 0.4%N, so that PRE is equal to 49. Pitting potential at a fixed temperature and critical pitting temperature (CPT) both increase with the PRE value. For hyperduplex steel CPT values above 90 have been reported. However, precipitation of tertiary phases such as sigma (δ), chi (χ), and Cr2N often decreases the CPT.

At the Brazilian Fluminense Federal University (UFF) investigations were conducted to get a deeper insight into the microstructure and corrosion properties of the new hyperduplex stainless steel. A tube of steel, with a diameter of 12.5 mm and thickness of 2 mm, was purchased under the solution treated condition. Small pieces of this tube were cut for the study. One of the specimens represented the original hyperduplex tube, while the other ones were produced by six different thermal procedures. Afterwards some specimens had unequal austenite/ferrite proportions, and other were δ-phase precipitated.

After processing, the specimens were prepared for microscopic analysis and electrochemical corrosion (cyclic polarization) test. Cyclic polarization tests were conducted in a conventional three-electrode cell using a Pt foil as auxiliary electrode, and a saturated calomel electrode as reference. The working electrode was constructed by using the hyperduplex specimen embedded in epoxy resin. The working solution was artificial sea water. The CPTs were also measured according to the ASTM E150 standard. The current and temperature were measured during the experiment. The microstructure was analyzed using a light optical microscope and scanning electron microscope. Different metallographic etches were used to reveal specific microstructural features. Austenite phase quantification was performed using Image Tools software for image analysis.

The CPT measurement and cyclic polarization test confirmed the high corrosion resistance of the hyperduplex steel in the solution treated condition. However, deleterious phases formed easily during thermal processing and caused a drastic decrease in the corrosion resistance. Besides, it must be considered that the specimen studied was very small and for industrial applications large pieces have to be heated to a higher temperature to assure a good homogenization and to avoid sigma precipitation under cooling.

The corrosion resistance of hyperduplex stainless steel is higher than the other austenitic–ferritic steels, since a CPT higher than 92°C was obtained. The Critical PRE values are approx. 45–55ºC for solution treated duplex steels and 80–90ºC for superduplex steels.

“The first application of hyperduplex stainless steels seems to be as heat exchange tubes used in the petroleum platforms in Brazil”, says Sérgio S.M. Tavares from UFF. “The material has mechanical and corrosion resistance superior to superduplex steels, which makes it very attractive in the off-shore equipments. High Cr and Mo content makes it more susceptible to embrittlement phenomena associated to intermetallic precipitation. The challenge lies in the development of reliable welding procedures for the hyperduplex steel”.

S. S. M. Tavares, J. M. Pardal, E. Ponzio, A. Loureiro and J. A. de Souza ; DOI: 10.1002/maco.200905386