Superaustenitic stainless steels

Austenitic SSTs with alloying element contents (particularly nickel and/or molybdenum) higher than the conventional 300-series SSTs are commonly categorized as superaustenitic SSTs (see Table 14.7). In some cases, they 

Chemical Composition of Cast Superaustenitic Stainless Steel 

CU5MCuC is the cast version of Incoloy 825 (trademark of Inco Alloys Intemational) although columbium is substituted for titanium. Titanium will oxidize rapidly during air melting columbium will not. CU5MCuC has corrosion resistance and weldability similar to CN7M. It has equal corrosion resistance in sulfuric, nitric, and phosphoric acids, seawater, and other environments. It is also highly resistant to chloride SCC. 

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Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

Pitting potential increased with increase in chromium contents >20 wt%, and molybdenum of 2-6 wt%. Recent results, applying microelectrochemical techniques, confirmed that even in the superaustenitic stainless steels molybdenum strongly improves the repassivation behavior but has no influence on pit initiation.27 The corrosion resistance of aluminum alloys is totally dependent on metallurgical factors.52, (Frankel)5... 

Titanium offers a much broader range of corrosion resistance and its applications are similar to those of the superaustenitic stainless steels and some nickel... 

Use material which is not susceptible to SCC thermally stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

With chromium contents of >20 wt% and 2-6 wt% of molybdenum the pitting potential of steels is increased. Molybdenum in superaustenitic stainless steels... 

For good pitting resistance, PRE must be more than 32. The higher the PRE, the better the pitting resistance of the steel. This formula is based upon the fact that primary protection of steel comes from chromium content. Mo addition strengthens the passive chromium layer, and nitrogen further enhances the pitting resistance. Because of this, the superaustenitic stainless steel, which contains 4%-5% Mo (SMO 254), has a PRE of above 40. 

Lower critical pitting temperature as compared to the high-molybdenum superaustenitic stainless steels such as Alloy 926 [13]. 

Heino, S, Knutson-Wedel, M., Karlsson, B (1998). Precipitation in a high nitrogen superaustenitic stainless steel. High Nitrogen Steels 1998 Conference proceedings, TransTechPublications Ltd, Switzerland, 1998, pp. 143-148... 

The superaustenitic stainless steels were developed to provide alloys with better resistance to localized corrosion. Included in this family of stainless steels are those that have improved pitting resistance, those that have improved crevice corrosion resistance, and those that have good general corrosion resistance to strong acids. 

This is a superaustenitic stainless steel in the 6-moly alloy family that is designed for maximum resistance to pitting and crevice corrosion. Its chemical makeup will be found in Table 11.1. 

Superaustenitic stainless steels 254SMO, AL6XN, 3127hMo... 

Besides the reports of MIC described above that are mainly related to water with relatively low salt content such as fresh water, an increase in the corrosion potential has also been reported for stainless steels exposed to natural seawater. The ennoblement of the corrosion potential has been observed for various stainless steel compositions (i.e., austenitic, ferritic, duplex, and superaustenitic stainless steels) exposed in natural seawater with different salinities and at different temperatures Copyright 2002 Marcel Dekker, Inc. 

The first successful major use of stainless steel for seawater systems was in the Gullfaks oilfield in the Norwegian offshore sector where Avesta 254SMO (21% Cr, 18% Ni, 6% Mo, 0.2% N) was adopted. The reason for this selection was the need for a material resistant to alternate exposure to seawater and sulfide-containing oil in the storage/ballast spaces in concrete platforms. Several thousand tonnes of superaustenitic stainless steel are now in service, mainly on offshore platforms. ... 

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