11921 Grades of Stainless Steel

Austenitic Stainless Steels Austenitic grades are highly ductile and have good toughness properties. The alloys are nonmagnetic and have excellent weldability. Table 11.11 provides a summary of the important austenitic grades and their uses.

Ferritic Stainless Steels Ferritic grades, although having a lower corrosion resistance than the austenitic grades, have excellent structural strength at high temperatures. In addition, if suitable precautions are taken to avoid sensitization, these alloys have good resistance to SCC. However, the ductile/brittle transition temperature of the ferritic grades is above room temperature, and this limits the section size of these grades. Table 11.12 summarizes the types and uses of the ferritic grades.

TABLE 11.11 Examples of Austenitic Stainless Steel Grades and Their Uses

AISI Series Comment and Uses

200 Series Basic 18Cr/8Nia type used in the food industry where minimal corrosion resistance required

300 Series 304 and 304L—food industry, transportation, chemical industry, heat exchangers, piping and tubing. L grade used where welding is required. Typical corrosion rates;b (mm/y) 0.025 (general), 0.25 (crevice).

Superaustenitic 254 SMO,c 6% Mo for improved pitting and crevice corrosion resistance. Marine and offshore applications aCompositions given in weight percent, e.g., 18Cr/8Ni represents 18%Cr and 8%Ni.

bUnless stated corrosion rates are based upon contact with seawater.

cManufactured by AVESTA.

TABLE 11.12 Examples of Ferritic Stainless Steel Grades and Their Uses

AISI Type Comments and Uses

400 Series 430. Basic ferritic grade (17Cr)—Furnace parts below 850°C, heat exchangers, tubing and piping. Typical corrosion rates; (mm/y), 0.025 (general), 5.0 (crevice).

Superferritic 29Cr/4Mo. Severe corrosive environments, e.g., 10% boiling sulfuric acid, petrochemical industries.

Martensitic Stainless Steel Martensitic grades, unlike the austenitic and ferritic types, are amenable to hardening by heat treatment, like that of carbon steels. The corrosion resistance of these grades is generally lower than the austenitic and ferritic stainless steels. These grades are used where corrosion resistance is not of paramount importance, but other properties such as hardness or wear resistance are required. Table 11.13 summarizes the types and uses of these grades.

Duplex Stainless Steels Duplex grades have microstructures composed of both austenite and ferrite, although austenite/martensite or ferrite/martensite mixed microstructures, in principle, are also classed as duplex stainless steel. The duplex grades have an excellent combination of toughness, strength, weldability, and corrosion resistance: see Fig. 11.19. Typically the grades contain between 18 and 25%Cr, 4 and 8%Ni, and 2 and 4%Mo. Table 11.14 summarizes the types and uses of theses grades.

Precipitation-Hardening (PH) Stainless Steels PH stainless steels provide a range of grades with high strength and good hardness. Unfortunately, the PH grades have limited corrosion resistance and at best are parallel with that of the 304 grade. They are used in the aircraft industry where high strength is required.

TABLE 11.13 Examples of Martensitic Stainless Steel Grades and Their Uses

AISI Type Comments and Uses

400 Series 410. Basic type (12Cr)—steam and water valves, pump and steam turbine parts.

420. Higher carbon content for higher hardenability—cutlery and surgical instruments.

431. Higher Cr and Ni content—food industry, valves and separators.

400 Series 410. Basic type (12Cr)—steam and water valves, pump and steam turbine parts.

420. Higher carbon content for higher hardenability—cutlery and surgical instruments.

431. Higher Cr and Ni content—food industry, valves and separators.

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