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Friday, 9 July 2010

Materials Views_2010-05-28 reports superior hyper-duplex corrosion resistant stainless steel

Materials Views_2010-05-28 reports superior hyper-duplex corrosion resistance. "The Influence of microstructure on the corrosion resistance of a newly developed hyperduplex stainless steel has been studied by a team of Brazilian  Metallurgists; S. S. M. Tavares, J. M. Pardal, A. Loureiro , E. Ponzio,  J. A. de Souza from the Universidade Federal Fluminense -  (Brazil)"

"Hyperduplex UNS S32707 is a newly developed austenitic-ferritic (dual phase or duplex) stainless steel. The steel contains about 27%Cr, 7%Ni, 4.5%Mo, and 0.4%N, which results in a pitting resistance equivalent factor (PRE) equal to 49. In this study, the pitting corrosion resistance of this new grade of stainless steel was investigated by varying the microstructure using different thermal processes. The critical pitting temperature measurement and cyclic polarization tests confirm the high corrosion resistance of the hyperduplex steel in the solution treated condition. However, deleterious phases form easily during thermal processing and cause a drastic decrease in the corrosion resistance.

Jounal, Authors and ABSTRACT

Materials Views Summary

Materials Views' Martin Grolms gives the reader some some of the essentials:eg.
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.

Experimental:
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.

Results-Microstructure:
Afterwards some specimens had unequal austenite/ferrite proportions, and other were δ-phase precipitated.

The experimental procedure and results are further summarised in Materials views

Findings:
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.

Applications:
“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”.

“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”.

References:
MaterialsViews with enlarged size micro-image.
Wiley Interscience

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Other References
Duplex Stainless Steels (Conference proceedings / American Society for Metals)Duplex Stainless Steels (Conference proceedings / American Society for Metals)

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