Ferritic stainless chromium steel

For the pressure of 825 bar forgings of high strength ferritic-martensitic material have to be used. These steels are normally not suitable for welding. Because of special corrosion requirements and in order to avoid the use of liners, a martensitic precipitation-hardening stainless chromium steel was used for the cylinder and the covers. The clamps were made of a low alloy high strength steel. 

Besides heat-treatment, the nickel content of the alloy also affects SCC behavior. Fig. 1-18 (eopson, 1959). erNi steels containing 10% nickel have the greatest susceptibility to see in boiling Mgei2 solution. With increasing nickel content See susceptibility is reduced and nickel-based alloys are resistant. Ferritic nickel-free stainless chromium steels are also highly resistant. 

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

Ferritic Stainless Steels. These steels are iron—chromium alloys not hardenable by heat treatment. In alloys having 17% chromium or more, an insidious embrittlement occurs in extended service around 475°C. This can be mitigated to some degree but not eliminated. They commonly include Types 405, 409, 430, 430F, and 446 (see Table 4) newer grades are 434, 436, 439, and 442. 

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. 

Chromium diffusion applied to a low-carbon steel produces a surface that has the characteristics of ferritic stainless steel, such as AISI446 to a depth about 0.1 mm. When diffusion is applied to a high-carbon steel, a surface rich in chromium carbides is formed. This has a hardness greater than 1000 VHN, which provides good resistance to abrasion. 

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. 

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

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. 

The precipitation-hardening stainless steels are proprietary grades hardened by both the martensitic transformation and precipitation hardening. These contain higher amounts of chromium (16-17% ) and nickel (4- 7%) than (he 12% chromium ferritic alloys. These steels are normally used at lower temperatures than the 12% chromium ferritic superalloys. 

The distinction between martensitic steels and other steels is not sharp. Some ferritic stainless steels such as AISI 430 steel (UNS S4300) or the 3Cr 12 alloy (UNS S41003), can be partially martensitic. Conversely, low-carbon martensitic steels such as AISI 410S (UNS 41008) and 416 (UNS S41603) might substantially ferritic. The lower chromium alloy content steels such as AISI 500 series heat-resistant steels also have many characteristics of martensitic steels. 

Since ferritic stainless steels contain more carbon than other classes, they are relatively harder to weld and shape than other varieties, which have historically limited their applications. However, since the 1960s, processes such as argon oxygen decarburization have resulted in steels with less carbon, allowing for smaller concentrations of chromium to be used. As a result, the price for ferritic stainless steel has dramatically dropped, and a number of applications now employ these materials - more than 2/3 of which include automotive exhaust systems. 

Interconnects are used to electrically connect adjacent cells and to fnnction as gas separators in cell stacks. High-temperatnre corrosion of interconnects is a significant issne in the development of SOFCs. Ferritic stainless steels have many of the desired properties for interconnects bnt experience stability issnes in both the anode and cathode environments. The dnal environments canse an anomalons oxidation for which a mechanistic understanding has yet to be determined. Protective coatings from non-chromium-containing condnctive oxides snch as (Mn,Co)304 spinels look promising bnt need further development. 

Ferritic stainless steels contain more chromium than martensitic stainless steels to improve corrosion resistance. They cannot be hardened by quenching and tempering and have limited ductility in thicker sections. The base alloy is 430. They are used for heat exchanger tubes. See Figure 4-5. 

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