Austenitic—ferritic duplex steels

Schiapfer, H. W., Weber, J. (1986), Austenitic-Ferritic Duplex Steels - Alloy-Design, Properties, Applications, Sulzer Pump Division. 

TABLE 25-12 Standard Wrought Austenitic/Ferritic Duplex Stainless Steels... 

The four stainless steel types are austenitic ferritic duplex and martensitic. The discovery of stainless steel (12.8% Cr, Fe alloy) is due to the work22 of H. Brearly in 1912. The approximate composition of austenitic and ferritic stainless steels is listed in Table 4.5. 

Stainless steels can be divided into four categories, based on their microstructure ferritic, austenitic, martensitic and austenitic-ferritic (duplex). Only specific grades of austenic and duplex stainless steel are currently used in concrete, although also a ferritic type with 12% chromium has been proposed [5-9]. In some countries also clad bars, i. e. bars with a carbon-steel core and an external layer of stainless steel are used. 

Duplex austenitic-ferritic stainless steel SAF 2540 resistant to dil. HCl < 3 wt.% up to lOOX... 

Susceptibility of duplex stainless steels to formation of undesirable intermetallic phases is typically done using ASTM A 923 (Test Methods for Detecting Detrimental Intermetallic Phase in Wrought Duplex Austenitic/Ferritic Stainless Steels). The application of ASTM A 923 to testing of weldments is recommended in TAPPI TIP 0402-29 [196]. 

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. 

Austenitic-ferritic duplex (1.4462) 450 Cr-Mo H2-resistant steel (1.7779) 500 Weldable... 

Duplex stainless steels (ca 4% nickel, 23% chrome) have been identified as having potential appHcation to nitric acid service (75). Because they have a lower nickel and higher chromium content than typical austenitic steels, they provide the ductabdity of austenitic SS and the stress—corrosion cracking resistance of ferritic SS. The higher strength and corrosion resistance of duplex steel offer potential cost advantages as a material of constmction for absorption columns (see CORROSION AND CORROSION CONTROL). 

Fig. 5. Metastable Fe—Ni—Cr "temary"-pliase diagram where C content is 0.1 wt % and for alloys cooled rapidly from 1000°C showing the locations of austenitic, duplex, ferritic, and martensitic stainless steels with respect to the metastable-phase boundaries. For carbon contents higher than 0.1 wt %, martensite lines occur at lower ahoy contents (43). A is duplex stainless steel, eg. Type 329, 327 B, ferritic stainless steels, eg. Type 446 C, 5 ferrite + martensite D, martensitic stainless steels, eg. Type 410 E, ferrite + martensite F, ferrite + pearlite G, high nickel ahoys, eg, ahoy 800 H,...

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

High-alloy pipeline steels (e.g. austenitic-ferritic or duplex) have been used where the product stream demands materials with better corrosion resistance than carbon steel. In practice the external corrosion resistance of these materials cannot be guaranteed, so cathodic protection is employed to protect areas which may be subject to corrosion. 

Duplex, and super-duplex stainless steels, contain high percentages of chromium. They are called duplex because their structure is a mixture of the austenitic and ferritic phases. They have a better corrosion resistance than the austenitic stainless steels and are less susceptible to stress corrosion cracking. The chromium content of duplex stainless steels is around 20 per cent, and around 25 per cent in the super-duplex grades. The super-duplex steels where developed for use in aggressive off-shore environments. 

Austenitic-ferritic Chromium-Nickel Steel X5CrNiMoCu255 (W.No. 1.4596) The duplex steel obtains the corrosion resistance from Cr, Ni and Mo. The strength... 

There are four main classes of stainless steel (austenitic, ferritic, ferritic-austenitic (duplex) and martensitic), and within these, a variety of different grades. The names ferritic and austenitic follow from their structures ferrite (P-Fe) and austenite (y-Fe) lattices hosting the alloying elements. The presence of Cr promotes the formation of the ferrite structure, while the austenite lattice forms when Ni is introduced. While ferritic and martensitic stainless steels are magnetic, austenitic stainless steel is non-magnetic. Further additives to some stainless steels are molybdenum (which improves corrosion resistance) and nitrogen (which adds strength and improves corrosion resistance). 

Other properties. The coefficient of thermal expansion of concrete is about 10 °C . That of ferritic steels is not very different (about 1.2 X 10 as in usual carbon-steel reinforcement) that of austenitic steels is higher (about 1.8 X 10 ) austenitic-ferritic steels are in an intermediate position. The higher thermal expansion of austenitic and duplex stainless steels is not believed to cause any problems in concrete, and no cases of damage due to differential expansion have been reported [6]. Furthermore, the thermal conductivity of austenitic stainless steel is much lower than that of carbon steel and thus the increase in temperature throughout the steel is delayed. 

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