Stainless steels ferritic-austenitic

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. 

In many stainless steels, ferrite is precipitated from the residual melt in the interdendritic areas together with austenite, (for example in steel 407, figure 10), and a partition ratio P Q has been calculated as ... 

Damaging environments that cause cracking may differ for austenitic, compared to martensitic or ferritic, stainless steels. For austenitic steels, the two major damaging ions are hydroxyl and chloride (OH and CF). A boiling, relatively concentrated chloride that hydrolyzes to a shghtly acid pH, such as FeCF or MgCh, can cause cracking of thick sections of stressed 18-8 within hours. A... 

Practice for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels. (Ferritic Stainless Steels are now in ASTM A 763.)... 

Amongst the stainless steels, ferritic Cr steels and the austenitic CrNi steels with and without added molybdenum must be considered as passivatable materials for domestic installations. These materials are of particular interest for hot water pipes and hot water tanks, as well as for cold water pipes. The stainless steels suitable for domestic installations are summarised in Table 16. 

Within the class of stainless steels, ferritic chromium steels as well as austenitic chromium-nickel steels, with and without additional molybdenum, can be used as passivatible materials in waste water installations. Table 4 lists the most frequently used chromium-nickel steels, which are also listed in DIN EN 1124-1 [26]. 

Which of the two stainless steels, ferritic or austenitic, would sensitize more under the given conditions ... 

Ferritic stainless steels like austenitic stainless steels are... 

Austenitic Stainless Steels Ferritic Stainless Steels Iron- and Nickel-Base Alloys... 

Austenitic stainless steel. Although austenitic stainless steel is readily welded, weld metal and HAZ cracking can occur. Weld metal solidification cracking is more likely in fully austenitic structures, which are more crack sensitive than those containing a small amount of ferrite. The beneficial effect of ferrite has been attributed largely to its capacity to dissolve harmful impurities that would otherwise form low melting-point segregates and interdendritic cracks. 

When good resistance to aqueous sulfide corrosion is required, ferritic stainless steels or duplex stainless steels can be substituted for austenitic stainless steel. Ferritic stainless steels, such as Type 405 (S40500) or Type 430 (S43000), are not susceptible to chloride SCC. The duplex stainless steels have a mixed ferritic-austenitic structure and are resistant to chloride SCC, but not to highly aggressive chloride environments. 

The stream from the reactor consisting of a mixture of urea, unconverted ammonium carbamate, excess water, and NH, is fed into the top of the stripper. The ACES stripper utilizes a ferrite—austenite stainless steel, as do the carbamate condensers. The reactor and scmbber are constmcted with 316 L urea-grade stainless steel. 

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. 

Nickel—Iron. A large amount of nickel is used in alloy and stainless steels and in cast irons. Nickel is added to ferritic alloy steels to increase the hardenabihty and to modify ferrite and cementite properties and morphologies, and thus to improve the strength, toughness, and ductihty of the steel. In austenitic stainless steels, the nickel content is 7—35 wt %. Its primary roles are to stabilize the ductile austenite stmcture and to provide, in conjunction with chromium, good corrosion resistance. Nickel is added to cast irons to improve strength and toughness. 

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,...

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

Martensitic Stainless Steels. The martensitic stainless steels have somewhat higher carbon contents than the ferritic grades for the equivalent chromium level and are therefore subject to the austenite—martensite transformation on heating and quenching. These steels can be hardened significantly. The higher carbon martensitic types, eg, 420 and 440, are typical cutiery compositions, whereas the lower carbon grades are used for special tools, dies, and machine parts and equipment subject to combined abrasion and mild corrosion. 

Other Metals. Metals such as the austenitic series. Types 301—347, and the ferritic series. Types 409—446, of stainless steels may be enameled, as well as a number of other alloys (17). The metal preparation usually consists of degreasiag and grit blasting. Copper, gold, and silver are also enameled. These metals are usually prepared for appHcation by degreasiag. Copper is pickled usiag either a nitric acid [7697-37-2] or a sulfuric acid [7664-93-9] solution, followed by a dilute nitric acid dip. Silver may be pickled in hot dilute sulfuric acid followed by a dip in a nitric acid solution (18). 

Stainless steels include ferritic stainless (Fe-Cr-Ni alloys with < 6% Ni) and austenitic stainless (Fe-Cr-Ni alloys with > 6.5% Ni). 

Many stainless steels, however, are austenitic (f.c.c.) at room temperature. The most common austenitic stainless, "18/8", has a composition Fe-0.1% C, 1% Mn, 18% Cr, 8% Ni. The chromium is added, as before, to give corrosion resistance. But nickel is added as well because it stabilises austenite. The Fe-Ni phase diagram (Fig. 12.8) shows why. Adding nickel lowers the temperature of the f.c.c.-b.c.c. transformation from 914°C for pure iron to 720°C for Fe-8% Ni. In addition, the Mn, Cr and Ni slow the diffusive f.c.c.-b.c.c. transformation down by orders of magnitude. 18/8 stainless steel can therefore be cooled in air from 800°C to room temperature without transforming to b.c.c. The austenite is, of course, unstable at room temperature. Flowever, diffusion is far too slow for the metastable austenite to transform to ferrite by a diffusive mechanism. It is, of course, possible for the austenite to transform displacively to give... 

Straight chromium ferritic stainless steels are less sensitive to stress corrosion cracking than austenitic steels (18 Cr-8 Ni) but are noted for poor resistance to acidic condensates. 

Table 3.13. Compositions of Ferrite/Austenite Stainless Steels ...

Residual stresses occur from welding and other fabrication techniques even at very low stress values. Unfortunately, stress relief of equipment is not usually a reliable or practical solution. Careful design of equipment can eliminate crevices or splash zones in which chlorides can concentrate. The use of high-nickel stainless steel alloy 825 (40% nickel, 21% chromium, 3% molybdenum and 2% copper) or the ferritic/austenitic steels would solve this problem. 

The corrosive and mechanical effects of flow are observed in pipes, especially at bends and downstream of flow disturbances, tube and shell heat exchangers, valves and pumps. More corrosion and/or harder materials are used in such areas. Austenitic stainless steels work harden and hence are superior in flowing conditions to ferritic stainless steels of otherwise similar corrosion resistance. Hard... 

Ferritic stainless steels have inferior corrosion resistance compared with austenitic grades of equivalent chromium content, because of the absence of nickel. Stress corrosion cracking can occur in strong alkali. 

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. 

There is no evidence that any particular crystal structure is more readily corroded than any other. For example, the difference in the corrosion behaviour of austenitic and ferritic stainless steels is, of course, due to compositional rather than structural differences. 

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