Chromium content

Chromium is the most effective addition to improve the resistance of steels to corrosion and oxidation at elevated temperatures, and the chromium—molybdenum steels are an important class of alloys for use in steam (qv) power plants, petroleum (qv) refineries, and chemical-process equipment. The chromium content in these steels varies from 0.5 to 10%. As a group, the low carbon chromium—molybdenum steels have similar creep—mpture strengths, regardless of the chromium content, but corrosion and oxidation resistance increase progressively with chromium content. 

Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

Standard Wrought Steels. Steels containing 11% and more of chromium are classed as stainless steels. The prime characteristics are corrosion and oxidation resistance, which increase as the chromium content is increased. Three groups of wrought stainless steels, series 200, 300, and 400, have composition limits that have been standardized by the American Iron and Steel Institute (AlSl) (see Steel). Figure 8 compares the creep—mpture strengths of the standard austenitic stainless steels that are most commonly used at elevated temperatures (35). Compositions of these steels are Hsted in Table 3. 

Steels iu the AISI 400 series contain a minimum of 11.5% chromium and usually not more than 2.5% of any other aHoyiag element these steels are either hardenable (martensitic) or nonhardenable, depending principally on chromium content. Whereas these steels resist oxidation up to temperatures as high as 1150°C, they are not particularly strong above 700°C. Steels iu the AISI 300 series contain a minimum of 16% chromium and 6% nickel the relative amounts of these elements are balanced to give an austenitic stmcture. These steels caimot be strengthened by heat treatment, but can be strain-hardened by cold work. 

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 %. 

Another set of nickel aHoys, which have a high chromium content, a moderate molybdenum content, and some copper, are the ILLIUM aHoys. These cast aHoys are wear and erosion resistant and highly resistant to corrosion by acids and alkaHes under both oxidizing and reducing conditions. 

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). 

The higher chromium—iron alloys were developed in the United States from the early twentieth century on, when the effect of chromium on oxidation resistance at 1090°C was first noticed. Oxidation resistance increased markedly as the chromium content was raised above 20%. For steels containing appreciable quantities of nickel, 20% chromium seems to be the minimum amount necessary for oxidation resistance at 1090°C. 

Although a minimum content of 12wt% Cr is required to impart the stainless characteristic to steels, much effort has been appHed to develop new grades of stainless steel having significantly reduced chromium contents without unacceptable degradation of corrosion resistance and other properties. There has been some modest success in this endeavor (34,53—56). 

No data are available on casting shrinkage, but in alloys having the same approximate nickel and chromium contents, pattern-makers usually make allowances for roughly 3% linear casting shrinkage. 

Ferritic stainless contains 15 to 30 percent Cr, with low carbon content (0.1 percent). The higher chromium content improves its corrosive resistance. Type 430 is a typical example. The strength of ferritic stainless can be increased by cold working but not by heat treatment. Fairly ductile ferritic grades can be fabricated by all standard methods. They are fairly easy to machine. Welding is not a problem, although it requires skilled operators. 

Another useful element in imparting oxidation resistance to steel is silicon (complementing the effects of chromium). In the lower-chromium ranges, silicon in the amounts of 0.75 to 2 percent is more effective than chromium on a weight-percentage basis. The influence of 1 percent silicon in improving the oxidation rate of steels with varying chromium contents is shown in Fig. 28-26. 

Hydrogen Atmospheres Austenitic stainless steels, by virtue of their high chromium contents, are usually resistant to hydrogen atmospheres. 

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

Alloys having relatively high chromium contents. Type 446 stainless steel, and. 60 Cr/50 Ni display improved fuel... 

Type 309-This is a 23/14 steel with greater oxidation resistance than 18/10 steels because of its higher chromium content. 

The high-chromium casting alloys (50% nickel, 50% chromium and 40% nickel, 60% chromium) are designated for use at temperatures up to 900 C in furnaces and boilers Ared by fuels containing vanadium, sulfur and sodium compounds (e.g., residual petroleum products). Alloys with lower chromium contents cannot be used with residual fuel oils at temperature above 6S0 C because the nickel reacts with the vanadium, sulfur and sodium -impurities to form compounds that are molten above 650 C [27]. 

Chrom-gehaltf m. chromium content, -gelatine, /. chromatized gelatin, bichromated gelatin, -gelb, n. chrome yellow (lead chromate) Cologne yellow (lead chromate and... 

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. 

Fig. 1.8(a) Intergranular precipitation of chromium carbide particles in a sensitised austenitic stainless steel and the consequent chromium-depleted zones adjacent to the grain boundaries, (b) variation of the chromium content across a grain boundary in a sensitised austenitic stainless steel (l8Cr) and (c) intergranular corrosion of a sensitised austenitic stainless steel... 

The ease with which stainless steels can passivate then increases with the level of chromium within the alloy and so materials with higher chromium content are more passive (i.e. conduct a lower passive current density) and passivate more readily (i.e. the critical current density is lower and the active/passive transition is lower in potential). They are also passive in more aggressive solutions the pitting potential is higher. 

Steigerwald, R. F., Effect of Chromium Content on Pitting Behaviour of Fe-Cr Alloys , Corrosion, 22, 107 (1966)... 

Fig. 3.10 Effect of chromium content on the corrosion of buried steel (after Romanoff )...

The grades with the 410 or 420 numerals are the basic 13% chromium type with varied carbon content. The additions of sulphur or selenium (possibly with phosphorus) to some grades (416 group) is to improve machinability. 431S29 has increased chromium content to improve corrosion resistance, but reference to Fig. 3.11 shows that such addition alone would lead to a mixed martensite-5-ferrite structure with certain disadvantages to mechanical properties. The nickel addition is to limit ferrite content. 

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