Any grade of steel that contains 11.5 % or more chromium. There are more than 70 standard types of stainless steel and many special alloys. These steels are produced in the wrought form (AISI types) and as cast alloys (ACI types). Generally, all are iron based, with 12 to 30 percent chromium, 0 to 22 percent nickel, and minor amounts of carbon, columbium, copper, molybdenum, selenium, tantalum, and titanium.Wrought Stainless Steels:1.Martensitic Alloys: characteristically magnetic and hardenable by heat treatment are oxidation resistant. They are exemplified by Type 410 (UNS S41000). Contain 12 to 20 percent chromium with controlled amount of carbon and other additives. Their corrosion resistance is inferior to austenitic stainless steels, and is generally used in mildly corrosive environments and for cutlery, turbine blades, and high-temperature parts.2.Ferritic Stainless: characteristically magnetic but not hardenable by heat treatment. Contain 15 to as much as 30 percent Cr with low carbon content (0.1 percent). The higher chromium content improves its corrosive resistance. Type 430 (UNS S43000) widely used in nitric acid plants is a typical example. Corrosion resistance is rated good, although ferritic alloys are not good against reducing acids such as HCl.3.Austenitic Stainless: widely used in bioprocessing, are characteristically non-magnetic, not hardenable by heat treatment, and are the most corrosion resistant of the three groups. These steels contain 16 to 26 percent chromium, 6 to 22 percent nickel. Carbon is kept low (0.08 percent) to minimize carbide precipitation. To avoid precipitation, special stainless steels stabilized with titanium, columbium, or tantalum, have been developed (types 321, 347, 348). Another approach to the problem is the use of low-carbon steels such as 304L and 316L, with 0.03 percent maximum carbon.Type 302 is the basic alloy of this group. Types 304 (UNS S30400) and 304L are low-carbon versions of 302. Types 316 (UNS S31600), 316L, and 317 (UNS S31700), with 2.5 to 3.5 percent molybdenum, are the most corrosion resistant.Cast Stainless Alloys: are widely used in pumps, valves, and fittings. All corrosion resistant alloys have the letter C plus a second letter (A to N) denoting increasing nickel content. Numerals indicate maximum carbon. Typical members of this group are CF-8, similar to 304 stainless, CF-8M, similar to 316, and CD-4M Cu, which has improved resistance to nitric, sulfuric, and phosphoric acids.High Performance Alloys: because the weaknesses sometimes encountered in the ferritic and 18-8 (18 Cr-8 Ni) austenitic grades 304, 316 and variations thereof, new and better “super” stainless steels have been developed. These are Superferritic Grades, Duplex Grades, and Superaustenitic Grades. Of these three, the high-performance austenitic grades have all the weldability and fabricability of conventional 18-8 varieties, coupled with nitrogen induced strength comparable to the duplex grades and a very high resistance to chloride pitting and stress corrosion cracking.The most notable low carbon, high purity superaustenitic stainless steel (nickel-based alloy technology) is the 6 Mo (6% Molybdenum) known by its trade name AL-6XN (UNS N08367) or “6 Moly” stainless steel. Its basic chemical composition being 20.0-22.0 Cr, 23.5-25.5 Ni, 6.0-7.0 Mo, 0.18-0.25 N, 0.03 C max, and Fe balance, it is the material of choice for many modern high performance-piping systems, and is available in standard pipe sizes and all commercial sizes of tubing.Nickel-Based Alloys: there are a few series of these alloys in use by the Chemical Processing Industry (CPI); among these, the most widely recognized are:The Inco (International Nickel Co.) 200 series, for example, Nickel 200 and 201, which are widely used in the chemical process industries, are commercially pure nickels.The 300 series are precipitation and dispersion strengthened low-alloyed grades.The 400 series are nickel-copper alloys (non-ferrous alloys), well known as Monel alloys.The 500 series are the precipitation-hardened 400 alloys, such as Monel K500.The 600 series also known as Inconel alloys are nickel-chromium alloys, such as Alloy 625The 700 series also known as Inconel alloys are precipitation-hardened nickel-chromium alloys.The 800 series are nickel-iron-chromium alloys also known as Incoloy alloys.The 900 series are precipitation-hardened nickel-iron-chromium alloys.The 1000 series are also known as Hastelloy B (USN N10001) - 61% Ni, 28% Mo, 5.5% Fe, 1% Cr, available as wrought and cast, resistant to all concentrations of hydrochloric acid at all temperatures, and Hastelloy C (USN N10002)– 54% Ni, 16% Mo, 5.5 Fe, 15.5 Cr, resists all concentrations of hydrochloric acid at room temperature, wet and dry chlorine, hypochlorite, and chloride solutions.As noted in the previous descriptions, the corrosion resistance factor is the paramount concern when it applies to the proper selection of materials applying to the CPI at large. The current selection process is an almost continuous progression of small steps, each step containing one or more alloys of increasing corrosion resistance that can be summarized as:1.304 and 316 stainless steels and their L grades2.Duplex stainless steels3.Superaustenitics stainless steels, in particular “6Mo”4.Ni, Cr, Mo family commonly called the “Alloy C family”5.Cobalt based alloys with high corrosion and wear-resistance6.Titanium alloys, referred as chemically pure (CP)
