Duplex alloys

Some duplex alloys have even better pitting resistance than type 316 and should be considered in severely pitting media. Titanium is virtually immune to chloride pitting and cupro-nickel alloys are used for condensers where sea-water is the coolant high pitting resistance in this duty is claimed for Cu-25Ni-20Cr-4-5Mo. 

KCNS concentration and increased the peak potential for their study of sensitization of duplex alloy 2205 (40). 

With the desired microstructure, these alloys are resistant to hydrogen stress cracking and much more resistant to chloride stress corrosion cracking than are the austenitic stainless steels. (The threshold temperature for chloride stress corrosion cracking of duplex alloys in neutral pH aqueous chlorides is about 300°F [150°C].) The chloride stress corrosion cracking resistance of the duplex alloys is similar to that of superaustenitic alloys such as Alloy AL-6XN. Because they contain about 50% ferrite, the duplex stainless steels are more susceptible to hydrogen embrittlement. 

Duplex alloys with a composition around Fe-26Cr-6.5 Ni-3Mo with low carbon and nitrogen have been found to be resistant to chloride SCC, pitting corrosion, and IGC in the as-welded condition (48). 

All processes still require use of oxygen for pasava-tion in the synthesis loop. Metallurgical advances have reduced the amount required. Snamprogetti now utilizes a bimetallic zirconium/25-22-2 (Ni, Cr, Mo) tube in its stripper. The corrosion rate for zirconium in urea service is nil. Toyo utilizes a duplex alloy (ferrite-austenite), which requires less ojqjgen. Stamicarbon working with the Swedish steel producer Sandvik, has patented a proprietary material called Saferex, which requires very little oxygen future plants wnll use this new material,... 

At low plastic strain amplitudes (ACp/ 2<2xl0 ), the deformation is mainly accommodated by the softer austenitic phase of the duplex alloy. Movement of the screw dislocations in the a-phase is very difficult because of the low temperature behavior of this phase at 300 K. Cyclic deformation of the austenitic phase then controls the fatigue properties of the duplex alloy. Because of the large reversibility of the cyclic strain, delayed transgranular crack initiation occurs in this phase (Fig. 5-34a). This explains the good fatigue resistance of the two-phase alloy at Agp/2< 10" as shown in the Coffin-Manson curve (Fig. 5-33). 

Corrosion fatigue tests under a controlled potential have been conducted in the passive region at the rest potential of the duplex alloy (Eq=- 2Q raVsce) d at a more anodic potential ( anod.=+150 mVsc ) for a given strain rate e=10 s . This strain rate has been chosen because it corresponds to the more marked corrosion fatigue life reduction at both applied potentials. The Coffin-Manson curves under such conditions are given in Fig. 5-36. The following features can be observed ... 

Figure S-37. Coffin-Manson curves at fi = 10 s" for duplex alloy at a free corrosion potential and at =-500 mVscE-...

In this example, the mechanical and electrochemical coupling effects between the a-and y-phase have been shown to be the key for understanding the cyclic deformation behavior of the duplex alloy under various electrochemical conditions. 

Due to the high chromium contents, duplex alloys are sensitive to 885 (475°C) embrittlement. This generally limits their usage to 600°F (SIS C) maximum for pressure vessels. Due to the presence of nickel, chromium, and molybdenum they are also susceptible to the formation of affect mechanical properties and corrosion resistance due to alloy depletion. The temperature range of 1100°F (593 C)-1600°F (882°C) and most rapidly at about 1450°F (788°C). The deleterious effects of phase formation are not obvious at the elevated temperature but can become a factor at room temperature. The formation of a phase in these alloys is sufficiently rapid to have an effect on properties due to slow cooling (air) after anneal. A measurable effect as a result of exposure in this temperature range due to welding has been demonstrated. 

MoPlus stainless steel is a trademark of Carpenter Technology. It is a two-phase (duplex) alloy with approximately 45% austenite distributed within a ferrite matrix. Alloy S32950 displays good resistance to chloride SCC, pitting corrosion, and general corrosion in many severe environments. The chemical composition is shown in Table 12.2. 

The chemical composition of Ferralium 255 is shown in Table 12.4. This is a duplex alloy with austenite distributed within a ferrite matrix. This alloy has a maximum service temperature of 500°F (260°C). 

Welding of duplex alloys can also be somewhat difficult due to the potential for forming the a phase. Welding filler material containing about 1-2% more nickel than the casting is normally used when the castings will be re-solution heat-treated. Filler material with 3% additional nickel is used when castings are not re-solution heat-treated. ... 

Chromium-rich duplex alloys, such as 326 and Uranus, are highly resistant to intergranular attack because ... 

Casting bronzes containing zinc are called gun phase is formed, and the bronzes are termed as metals. beta bronzes or duplex alloys. The tensile strength... 

The most critical corrosion conditions are present within the high-pressure strippers. Therefore, even higher grade materials like 25Cr22Ni2Mo (Stamicarbon) or duplex alloys (Mitsui Toatsu Chemicals) are required [13]. 

Chromium carbides and nitrides Negative Precipitation of carhides/nitrides causes Cr-depleted zones that are selectively attacked in certain corrosive media. In older generations of duplex alloys, nitrides were frecpiently present in welded joints and in base metal with coarse microstmcture. This has rarely been the reason for a corrosion failure. 

These duplex alloys have been used in Europe for many years therefore, guidelines relating to austenite-ferrite phase distribution are available. It has been shown that to ensure resistance to chloride see, welds should contain at least 25% ferrite. To maintain a good phase balance for corrosion resistance and mechanical properties (especially ductility and notch toughness) comparable to the base metal, the average ferrite content of the weld should not exceed 60%. 

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