Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-06-29
2001-11-06
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S327000
Reexamination Certificate
active
06312532
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a ferritic-austenitic stainless steel with high contents of Cr, N, Cu and W in combination with relatively low contents of Ni and Mo. The material is suitable for applications where high resistance to corrosion is desire, especially in acidic or basic environments, where you have high chloride contents at the same time.
BACKGROUND OF THE INVENTION
The following discussion of the state of the art should not be construed as an admission of prior art.
Duplex steel is characterized by a ferritic-austenitic structure, where both phases have different compositions. Modern duplex steel will mainly be alloyed with Cr, Mo, Ni and N. The duplex steel grade SAF 2507 (UNS S32750) has high contents of Cr, Mo and N for a high resistance to pitting corrosion. This resistance is often described as a PRE-number (PRE =Pitting Resistance Equivalent=%Cr+3.3% Mo+16N). The alloy is consequently optimized with respect to this property and has a good resistance in many acids and bases, but the alloy is above all developed for resistance in chloride environments. The elements Cu and W have been used as alloying additions. Consequently has for example the steel grade DP3W (UNS S39274) which has an analogous composition as SAF 2507, but it is alloyed with 2.0% W as substitute for a share of the Mo-content in the alloy. The steel grade Uranus 52N+(NS S32529) has similarly an analogous composition as SAF 2507, but it is alloyed with 1.5% Cu with the purpose to improve the resistance in acid environments. The steel grade Zeron 100 is a further steel grade which is analogous to SAF 2507, but this is alloyed with both about 0.7% Cu and 0.7% W. The steel grade DTS 25.7NWCu (UNS S39277) is in this composition very similar to SAF 2507, besides that it is alloyed with about 1.7% Cu and 1.0% W. In relation with that it is alloyed with W, a PRE formula was produced, which also includes the element W with a weight corresponding the halve of this for Mo., i.e.—PRENW=%Cr+3.3(%Mo+0.5% W)+16N. All described steel grades have a PRE number, irrespective to the calculation method, that is over 40.
Another type of ferritic-austenitic alloy with high resistance to chloride is the steel grade described in the Swedish Patent 9302139-2 or U.S. Pat. No. 5,582,656. This type of alloy is characterized by Mn 0.3-4%, Cr 28-35%, Ni 3-10%, Mo 1-3%, Cu maximum 1.0% and W maximum 2.0%, and has a high PRE number, generally over 40. The biggest difference compared with the established superduplex steel SAF 2507 and others is that the contents of Cr and N are higher in this steel grade. This steel grade has been used in environments where the resistance to intergranular corrosion and corrosion in ammonium carbamates is of importance, but the alloy also has a very high resistance to chloride environments.
SUMMARY OF THE INVENTION
One purpose of the present invention is to provide a material with high resistance to chloride environments, at the same time the material has extraordinary properties in acidic and basic environments combined with good mechanical properties and high structural stability. This combination can be very useful in applications within for example the chemical industry, where you have problems with corrosion caused by acids and at the same time have a contamination of the acid with chlorides, which further amplifies the corrosive effect. These properties of the alloy in combination with a high strength lead to advantageous design solutions from an economic point of view. There are certainly existing materials with very good properties in acid environments, but these are often steels with high contents of Ni, which makes the costs of such materials excessively high. Another disadvantage with austenitic steel compared with duplex alloys is that the strength in the austenitic steel is usually considerably lower.
In the present day situation there are no duplex stainless steels described that are optimized for this combination of properties, and which then attain those good properties which are described here.
According to one preferred aspect, the present invention provides a ferrite-austenite steel alloy having good warm workability, high resistance to crevice corrosion and good structural stability, comprising in weight-%:
C maximum 0.05%;
Si maximum 0.8%;
Mn 0.30-4.0%;
Cr 27.0-35.0%;
Ni 3.0-10.0%;
Mo 0-3.0%;
N 0.30-0.55%;
Cu 0.5-3.0%;
W 2.0-5.0%;
S maximum 0.010%; and
balance Fe and normally occurring steelmaking additions for deoxidization and hot ductility;
wherein the alloy comprises 30-70 volume % ferrite and the balance austenite.
REFERENCES:
patent: 5284530 (1994-02-01), Azuma et al.
patent: 5298093 (1994-03-01), Okamoto
patent: 5582656 (1996-12-01), Kangas et al.
patent: 5733387 (1998-03-01), Lee et al.
patent: 501321 (1995-01-01), None
patent: 96/39543 (1996-12-01), None
Burns Doane Swecker & Mathis L.L.P.
Sandvik AB
Yee Deborah
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