Duplex stainless steel alloy

Duplex stainless steel alloys are a mixture of ferritic (400 series) and austenitic (300 series) metals. They provide 1) resistance to stress corrosion and fatigue, 2) pitting resistance, 3) are suitable for a wide temperature range (-50°C to 280°C) and 4) are cost effective. In urea plants, duplex stainless steel is used to construct strippers, decomposers, condensers and pipe lines88. 

An interesting class of materials that should be considered for evaporator service are the duplex stainless steel alloys. These high-performance stainless steel alloys have a duplex stmcture. The alloy contains both the austenitic and ferritic phases... 

Flow Lines Pipe and Internal Corrosion Duplex Stainless Steel Alloy... 

Cathodic protection and external coating Duplex stainless steel alloy (22% Cr)... 

Magar [32] evaluated inhibitors in white water by means of the linear polarization resistance technique in a search for effective inhibitors to prevent corrosion of bronze suction rolls. A galvanodynamic testing method was used by DeBerry and Ellis [33] in order to study inhibition of corrosion of a duplex stainless steel alloy. 

High-temperature and pressure tests (weight loss, electrochemical measurements, and U-bends) were conducted by Honda et al. [152] with austenitic, ferritic, and duplex stainless steel alloys in alkaline sulfide solutions at 150°C. Dupo-iron [160] reported on SCC tests conducted in white liquors using the constant load test. 

CO2 corrosion often occurs at points where there is turbulent flow, such as In production tubing, piping and separators. The problem can be reduced it there is little or no water present. The initial rates of corrosion are generally independent of the type of carbon steel, and chrome alloy steels or duplex stainless steels (chrome and nickel alloy) are required to reduce the rate of corrosion. 

The enhanced strength and corrosion properties of duplex stainless steels depend on maintaining equal amounts of the austenite and ferrite phases. The welding thermal cycle can dismpt this balance therefore, proper weld-parameter and filler metal selection is essential. Precipitation-hardened stainless steels derive their additional strength from alloy precipitates in an austenitic or martensitic stainless steel matrix. To obtain weld properties neat those of the base metal, these steels are heat treated after welding. 

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,...

For firewater, steel pipes are used but corrosion products can block sprinklers. Cement asbestos pipes are utilized but pressure limitations restrict their use. For critical applications, including offshore oil installations, cupronickel alloys and even duplex stainless steels are used. Fire-retardant grades of fiber-reinforced plastics are now available. 

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. 

The metallurgy selected for construction of a firewater pump is dependent on the properties of the water source to be used. For fresh water sources (i.e., public water mains), cast iron is normally adequate although bronze internals may be optional. Brackish or sea water utilization will require the use of highly corrosion resistance materials and possibly coatings. Typically specified metals include alloy bronze, monnel, ni-resistant, or duplex stainless steels sometime combined with a corrosion resistant paint or specialized coating. 

Validation of the database. This is the final part in producing an assessed database and must be undertaken systematically. There are certain critical features such as melting points which are well documented for complex industrial alloys. In steels, volume fractions of austenite and ferrite in duplex stainless steels are also well documented, as are 7 solvus temperatures (7 ) in Ni-based superalloys. These must be well matched and preferably some form of statistics for the accuracy of calculated results should be given. 

Materials known to exhibit inconsistent electrical properties are high chromium alloys such as 17-4 PH, duplex stainless steel and ASTM A 479 grade XM -1 9. 

Appendix A contains a materials selection guide for aerated freshwater systems. As indicated in Note 27 of Appendix A, in freshwater systems, admiralty brass should be limited to a maximum pH value of 7.2 from ammonia and copper-nickel alloys and should not be used in waters containing more sulfides than 0.007 mg/L The materials selection guide is also satisfactory for seawater, although pump cases and impellers should be a suitable duplex stainless steel or nickel-aluminum-bronze (properly heat treated). Neoprene-lined water boxes should be considered. For piping, fiber-reinforced plastic (up to 150 psi [1,035 kPa] operating pressure) and neoprene-lined steel should also be considered. Titanium and high-molybdenum SS tubes should be considered where low maintenance is required or the cost can be justified by life expectancy. 

Duplex stainless steels contain both ferrite and austenite in approximately equal amounts Alloy 2205 is an example. Figure 21.9 illustrates the microstructure of a duplex stainless steel microstructure in plate material. Typically, the duplex stainless steels contain 17 wt% or more chromium and <7% nickel. The more corrosion-resistant types contain at least 2% molybdenum. They are much stronger than the austenitic stainless steels, permitting a thinner section thickness. Thus, while they may cost more per pound, they may cost less per piece. 

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. 

Hydrogen embrittlement can occur in carbon and low-alloy steels, in ferritic and martensitic stainless steels, and in duplex stainless steels. It is normally not a problem in either the austenitic stainless steels or nickel-based high alloys. Hydrogen can dissolve in a steel as a result of a number of phenomena (1) Corrosion creates nascent hydrogen, usually in the presence of a cathodic poison. 

Leali Tranquilli, R, Merolli, A., Gabbi, C., Cacchioli, A., and Gonizzi, G. (1994) Evaluation of different preparations of plasma-spray hydroxyapatite coatings on titanium alloy and duplex stainless steel... 

Duplex stainless steels offer high strength, coupled with resistance to abrasion and erosion and to stress corrosion cracking. It is claimed that Ferralium alloy... 

Leali Tranquilli P, Merolli A, Palmacci O, Gabbi C, Cacchiolo A, Gonizzi G (1994) Evaluation of different preparation of plasma-spray hydroxylapatite coating on titanium alloy and duplex stainless steel in the rabbit. J Mater Sci Mater in Med 5 345-349... 

---