Duplex stainless steel

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. 

Duplex stainless steels. Duplex stainless steels have provided a fruitful area for CALPHAD calculations and have been an example of where high levels of success have been achieved for practical materials. An early study (Hayes 1985) was able to demonstrate that reasonable predictions for amounts of austenite could be obtained for a variety of different duplex stainless steels, demonstrating the... 

The acid pulping process uses sodium bisulfite or magnesium bisulfite in a pulp digester at a pH of 3. Type 316 stainless steel is normally used as a minimum alloy because the sulfur dioxide can be converted into sulfuric acid. Sulfuric acid can corrode the stainless steel depending on the pH, temperatiire, and pressure of the system. SCC has been observed in the HAZ of weldments in the pulp digesters in the presence of sodium hydroxide. Because of SCC susceptibility of the austenitic stainless steels, duplex stainless steel 2205 is often used in pressure vessels and tanks. 

Austenitic stainless steel, duplex stainless steel, nickel superalloys, titanium, graphite... 

Carbon steel IS 2002 Grade A, ASTM A 560, SA-516, A-179 Carbon-manganese steel Carbon-molybdennm steel Chromium-molybdenum steel Chromium-molybdenum-vanadium steel Chromium-nickel stainless steel Duplex steel, super duplex steel Non-ferrous alloys of Ni, Cu, Al, Ti... 

In the 1970s—when strength considerations became an issue for stainless steels—duplex versions were developed, which also increased resistance to chloride stress corrosion cracking. In the 1980s stainless steels with higher molybdenum were formulated to solve problems with localized corrosion encountered in aggressive environments. 

The problems in the food and beverage industry are typically unique to specific services within those industries. Many of the chloride-resistant stainless steels, duplex stainless steels [5], and high-alloy materials have been used in specific applications with good success [3]. 

Stress corrosion cracking (SCC) is a special corrosion form, which is seen as fine-branched transgranular cracks in the material. SCC is caused by a combination of mechanical stress, the presence of chlorides and elevated temperature. At high temperature and high mechanical stress. SCC of 316-type stainless steel can occur at chloride levels even below 1000 ppm. The answer to this is to use a more corrosion-resistant stainless steel or a duplex stainless steel. Duplex stainless steels normally do not show SCC at temperatures below 100°C. 

Stainless steels (DIN EN 10088-1/3) 12% Cr steel, austenitic stainless steel, duplex stainless steel X3CrNiMol3-4 (A 182 Gr. F 6 NM), X20Cr 13 (A 276 Type 420), X2CrNil9-l 1 (A 182 Gr. F 304L), X2CrNiMoN22-5 (A 182 Gr. F 51) Alloyed high-grade steels... 

The problem of welding INOR-8-stainless steel duplex tubing is being studied. Experiments have indicated that proper selection of alloy ratios and weld design will assure welds that will be satisfactory in liigh-tempera-ture service. 

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. 

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). 

Materials of Construction and Operational Stress. Before a centrifugal separation device is chosen, the corrosive characteristics of the Hquid and soHds as weU as the cleaning and saniti2ing solutions must be deterrnined. A wide variety of materials may be used. Most centrifuges are austenitic stainless steels however, many are made of ordinary steel, mbber or plastic coated steel. Monel, HasteUoy, titanium, duplex stainless steel, and others. The solvents present and of course the temperature environment must be considered in elastomers and plastics, including composites. 

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,...

Table 15. Duplex (Ferrite + Austenite) Grades of Stainless Steel...

Plain tubes (either as solid wall or duplex) are available in carbon steel, carbon alloy steels, stainless steels, copper, brass and alloys, cupro-nickel, nickel, monel, tantalum, carbon, glass, and other special materials. Usually there is no great problem in selecting an available tube material. However, when its assembly into the tubesheet along with the resulting fabrication problems are considered, the selection of the tube alone is only part of a coordinated design. Plain-tube mechanical data and dimensions are given in Tables 10-3 and 10-4. 

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. 

Duplex stainless steels are mostly composed of alternate austenite and ferrite grains. Their structure improves resistance to chloride-induced stress corrosion cracking. In certain reducing acids, such as acetic and formic, preferential attack of the ferrite is a serious problem. 

Chlorides have probably received the most study in relation to their effect on corrosion. Like other ions, they increase the electrical conductivity of the water so that the flow of corrosion currents will be facilitated. They also reduce the effectiveness of natural protective films, which may be permeable to small ions the effect of chloride on stainless steel is an extreme example but a similar effect is noted to a lesser degree with other metals. Turner" has observed that the meringue dezincification of duplex brasses is affected by the chloride/bicarbonate hardness ratio. 

Proc. Conf Duplex Stainless Steels. (Ed. Lula, R. A.), ASM (1983)... 

Table 8.2 Typical chemical compositions for duplex stainless steels (after Hochmann etal.)...

Times to failure for various stainless steels tested in MgClj have been shown to increase with increasing proportions of martensite present Perhaps the role of martensite under anodic dissolution conditions is comparable to that of ferrite in duplex stainless steels where the enhanced dissolution of one phase prevents crack initiation in the other. There is, of course, another aspect of martensitic transformation that should be mentioned, i.e. the transformation of austenite to martensite either in the bulk material or at a growing crack tip that can give increased susceptibility to... 

Similarly it seems that retained austenite may be beneficial in certain circumstances , probably because the austenite acts as a barrier to the diffusion of hydrogen, although in high concentrations (such as those obtained in duplex stainless steels) the austenite can also act as a crack stopper (i.e. a ductile region in the microstructure which blunts and stops the brittle crack). 

The H2SO4-CUSO4 test, unlike the Huey test, is specific for susceptibility due to chromium depletion and is unaffected by the presence of submicro-scopic a-phase in stainless steels containing molybdenum or carbide stabilisers. It can be used, therefore, with confidence to test susceptibility in austenic (300 series) and ferritic (400 series) stainless steels and in duplex austeno-ferritic stainless steels such as Types 329 and 326. 

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

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. 

The duplex range of stainless steels can be readily cast, wrought and machined. Problems can occur in welding, due to the need to keep the correct balance of ferrite and austenite in the weld area, but this can be overcome using the correct welding materials and procedures. 

---