Stainless Steels

[meles meles]

STAINLESS STEEL (WROUGHT)

Except for the precipitation-hardening (PH) stainless steels, wrought stainless steels are commonly designated by a three-digit numbering system of the American Iron and Steel Institute. Wrought austenitic stainless steels constitute the 2XX and 3XX series and the wrought ferritic stainless steels are part of the 4XX series. Wrought martensitic stainless steels belong either to the 4XX or 5XX series. Suffix letters, such as L for low carbon content or Se for selenium, are used to denote special compositional modifications. Cast stainless steels are commonly known by the designations of the Alloy Casting Institute of the Steel Founders Society of America, which begin with letters CA through CN and are followed by numbers or numbers and letters. Powder compositions are usually identified by the designations of the Metal Powder Industries Federation.

Of the austenitic, ferritic, and martensitic families of wrought stainless steels, each has a general-purpose alloy. All of the others in the family are derivatives of the basic alloy, with compositions tailored for special properties. The stainless steel 3XX series has the largest number of alloys and stainless steel 302, a stainless “18-8” alloy, is the general-purpose one. Besides its 17 to 19% chromium and 8 to 10% nickel, it contains a maximum of 0.15% carbon, 2% manganese, 1% silicon, 0.4% phosphorus, and 0.03% sulfur. 302B is similar except for greater silicon (2 to 3%) to increase resistance to scaling. Stainless steels 303 and 303Se are also similar except for greater sulfur (0.15% minimum) and, optionally, 0.6% molybdenum in 303, and 0.06 maximum sulfur and 0.15 minimum selenium in 303Se. Both are more readily machinable than 302. 304 and 304L stainless steels are low-carbon (0.08% and 0.03 maximum, respectively) alternatives, intended to restrict carbide precipitation during welding and, thus, are preferred to 302 for applications requiring welding. They may also contain slightly more chromium and nickel. 304N is similar to 304 except for 0.10 to 0.16% nitrogen. The nitrogen provides greater strength than 302 at just a small sacrifice in ductility and a minimal effect on corrosion resistance. 305 has 0.12% maximum carbon but greater nickel (10.5 to 13%) to reduce the rate of work hardening for applications requiring severe forming operations. S30430, as designated by the Uni- fied Numbering System, contains 0.08 maximum carbon, 17 to 19% chromium, 8 to 10% nickel, and 3 to 4% copper. It features a still lower rate of work hardening and is used for severe cold-heading operations. 308 contains more chromium (19 to 21%) and nickel (10 to 12%) and, thus, is somewhat more corrosion and heat resistant. Although used for furnace parts and oil-refinery equipment, its principal use is for welding rods because its higher alloy content compensates for alloy content that may be reduced during welding. See Table S.10

(Wrought Stainless Steels).

Stainless steels 309, 3095, 310, 310S, and 314 have still greater chromium and nickel contents. 309S and 310S are low-carbon (0.08% maximum) versions of 309 and 310 for applications requiring welding. They are also noted for high creep strength. Stainless steel 314, which like 309 and 310 contains 0.25% maximum carbon, also has greater silicon (1.5 to 3%), thus providing greater oxidation resistance. Because of the high silicon content, however, it is prone to embrittlement during prolonged exposure at temperatures of 649 to 816°C. This embrittlement, however, is only evident at room temperature and is not considered harmful unless the alloy is subject to shock loads. These alloys are widely used for heaters and heat exchangers, radiant tubes, and chemical and oil-refinery equipment.

Stainless steels 316, 316L, 316F, 316N, 317, 317L, 321, and 329 are characterized by the addition of molybdenum, molybdenum and nitrogen (316N), or titanium (321). Stainless steel 316, with 16 to 18% chromium, 10 to 14% nickel, and 2 to 3% molybdenum, is more corrosion and creep resistant than 302- or 304- type alloys. Type 316L is the low-carbon version for welding applications; 316F, because of its greater phosphorus and sulfur, is the “free-machining” version; and 316N contains a small amount of nitrogen for greater strength. Stainless steels 317 and 317L are slightly richer in chromium, nickel, and molybdenum and, thus, somewhat more corrosion and heat resistant. Like 316, they are used for processing equipment in the oil, chemical, food, paper, and pharmaceutical industries. Type 321 is titanium- stabilized to inhibit carbide precipitation and provide greater resistance to intergranular corrosion in welds. Type 329, a high-chromium (25 to 30%) low-nickel (3 to 6%) alloy with 1 to 2% molybdenum, is similar to 316 in general corrosion resistance but more resistant to stress corrosion. Stainless steel 330, a high-nickel (34 to 37%), normal chromium (17 to 20%), 0.75 to 1.5% silicon, molybdenumfree alloy, combines good resistance to carburization, heat, and thermal shock.

Stainless steels 347 and 348 are similar to 321 except for the use of columbium and tantalum instead of titanium for stabilization. Type 348 also contains a small amount (0.2%) of copper. Both have greater creep strength than 321 and they are used for welded components, radiant tubes, aircraft-engine exhaust manifolds, pressure vessels, and oil-refinery equipment. Stainless steel 384, with nominally 16% chromium and 18% nickel, is another lowwork- hardening alloy used for severe coldheading applications.

The stainless steel 2XX series of austenitics comprises 201, 202, and 205. They are normal in chromium content (16 to 19%), but low in nickel (1 to 6%), high in manganese (5.5 to 15.5%), and with 0.12 to 0.25% carbon and some nitrogen. Types 201 and 202 have been called the low-nickel equivalents of 301 and 302, respectively. Type 202, with 17 to 19% chroinium, 7.5 to 10% manganese, 4 to 6% nickel, and a maximum of 1% silicon, 0.25% nitrogen, 0.15% carbon, 0.06% phosphorus, and 0.03% sulfur, is the general-purpose alloy. Type 201, which contains less nickel (3.5 to 5.5%) and manganese (5.5 to 7.5%), was prominent during the Korean war due to a nickel shortage. Type 205 has the least nickel (1 to 1.75%), and the most manganese (14 to 15.5%), carbon (0.12 to 0.25%), and nitrogen (0.32 to 0.40%) contents. It is said to be the low-nickel equivalent of 305 and has a low rate of work hardening that is useful for parts requiring severe forming operations.

Like stainless steels in general, austenitic stainless steels have a density of 7750 to 8027 kg/m3. Unlike some other stainless steels, they are essentially nonmagnetic, although most alloys will become slightly magnetic with cold work. Their melting range is 1371 to 1454°C, specific heat at 0 to 100°C is about 502 J/kg · K, and electrical resistivity at room temperature ranges from 69 × 10–8 to 78 × 10–8 Ω · m. Types 309 and 310 have the highest resistivity, and 201 and 202 the lowest.

Most are available in many mill forms and are quite ductile in the annealed condition, tensile elongations ranging from 35 to 70%, depending on the alloy. Although most cannot be strengthened by heat treatment, they can be strengthened appreciably by cold work. In the annealed condition, the tensile yield strength of all the austenitics falls in the range of 207 to 552 MPa, with ultimate strengths in the range of 517 to 827 MPa. But cold-working 201 or 301 sheet just to the half-hard temper increases yield strength to 758 MPa and ultimate strength to at least 1034 MPa. Tensile modulus is typically 193 × 103 to 199 × 103 MPa and decreases slightly with severe cold work. As to high-temperature strength, even in the annealed condition most alloys have tensile yield strengths of at least 83 MPa at 815°C, and some (308, 310) about 138 MPa. Types 310 and 347 have the highest creep strength, or stress-rupture strength, at 538 to 649°C. Annealing temperatures range from 954 to 1149°C, initial forging temperatures range from 1093 to 1260°C, and their machinability index is typically 50 to 55, 65 for 303 and 303Se, relative to 100 for 1112 steel.

Among the many specialty wrought austenitic stainless steels are a number of nitrogenstrengthened stainless steels: Nitronic 20, 32, 33, 40, 50, and 60; 18-18 Plus and Marinaloy HN and 22; and SAF 2205 and 253MA. Nitrogen, unlike carbon, has the advantage of increasing strength without markedly reducing ductility. Some of these alloys are twice as strong as the standard austenitics and also provide better resistance to certain environments. All are normal or higher than normal in chromium content. Some are also normal or higher than normal in nickel content, whereas others are low in nickel and, in the case of 18-18 Plus, nickel-free. Nitronic 20, a 23% chromium, 8% nickel, 2.5% manganese alloy, combines high resistance to oxidation and sulfidation and was developed for engine exhaust valves. Unlike austenitics in general, it is hardenable by heat treatment. Solution treating at 1177°C, water quenching, and aging at 760°C provide tensile strengths of 579 MPa yield and 979 MPa ultimate. SAF 2205, an extra-low-carbon (0.03%), 22% chromium, 5.5% nickel, 3% molybdenum alloy, is a ferritic-austenitic alloy with high resistance to chloride- and hydrogen-sulfideinduced stress corrosion, pitting in chloride environments, and intergranular corrosion in welded applications.

The wrought ferritic stainless steels are magnetic and less ductile than the austenitics. Although some can be hardened slightly by heat treatment, they are generally not hardenable by heat treatment. All contain at least 10.5% chromium and, although the standard alloys are nickel-free, small amounts of nickel are common in the nonstandard ones. Among the standard alloys, stainless steel 430 is the generalpurpose alloy. It contains 16 to 18% chromium and a maximum of 0.12% carbon, 1% manganese, 1% silicon, 0.04% phosphorus, and 0.03% sulfur. Stainless steel 430F and 430FSe, the “free-machining” versions, contain more phosphorus (0.06% maximum) and sulfur (0.15% minimum in 430F, 0.06 maximum in 430FSe). 430FSe also contains 0.15% minimum selenium, and 0.6% molybdenum is an option for 430F. The other standard ferritics are stainless steels 405, 409, 429, 434, 436, 442, and 446. 405 and 409 are the lowest in carbon (0.08% maximum) and chromium (11.5 to 14.5% and 10.5 to 11.75%, respectively), the former containing 0.10 to 0.30% aluminum to prevent hardening on cooling from elevated temperatures, and the latter containing 0.75% maximum titanium. Type 429 is identical to 430 except for less chromium (14 to 16%) for better weldability. Types 434 and 436 are identical to 430 except for 0.75 to 1.25% molybdenum in the former and this amount of molybdenum plus 0.70% maximum columbium and tantalum in the latter; these additives improve corrosion resistance in specific environments. Types 442 to 446 are the highest in chromium (18 to 23% and 23 to 27%, respectively) for superior corrosion and oxidation resistance, and in carbon (0.20% maximum). Type 446 also contains more silicon (1.50% maximum).

These standard alloys melt in the range of 1427 to 1532°C, thermal conductivities of 21 to 27 W/m · K at 100°C, and electrical resistivities of 59 to 67 µΩ · cm at 21°C. In the annealed condition, tensile yield strengths range from 241 to 276 MPa for 405 to as high as 414 MPa for 434, with ultimate strengths of 448 to 586 MPa and elongations of 20 to 33%. For 1% creep in 10,000 h at 538°C, 430 has a stress-rupture strength of 59 MPa. Typical applications include automotive trim and exhaust components, chemical-processing equipment, furnace hardware and heat-treating fixtures, turbine blades, and molds for glass. Wrought martensitic stainless steels are also magnetic and, as they are hardenable by heat treatment, provide high strength. Of those in the stainless steel 4XX series, 410, which contains 11.5 to 13.0% chromium, is the general- purpose alloy. The others, 403, 414, 416, 416Se, 420, 420F, 422, 431, 440A, and 440C, have similar (403, 414) or more chromium (16 to 18% in the 440s). Most are nickel-free or, as in the case of 414, 422, and 431, low in nickel. Most of the alloys also contain molybdenum, usually less than 1%, plus the usual 1% or so maximum of manganese and silicon. Carbon content ranges from 0.15% maximum in 403 through 416 and 416Se, to 0.60 to 0.75% in 440A, and as much as 1.20% in 440C. Type 403 is the low-silicon (0.50% maximum) version of 410; 414 is a nickel (1.25 to 2.50%)- modified version for better corrosion resistance. Types 416 and 416Se, which contain 12 to 14% chromium, also contain more than the usual sulfur or sulfur, phosphorus, and selenium to enhance machinability. Type 420 is richer in carbon for greater strength, and 420F has more sulfur and phosphorus for better machinability. Type 422, which contains the greatest variety of alloying elements, has 0.20 to 0.25% carbon, 11 to 13% chromium, low silicon (0.75% maximum), low phosphorus, and sulfur (0.025% maximum), 0.5 to 1.0% nickel, 0.75 to 1.25% of both molybdenum and tungsten, and 0.15 to 0.3% vanadium. This composition is intended to maximize toughness and strength at temperatures to 649°C. Type 431 is a higher-chromium (15 to 17%) nickel (1.25 to 2.50%) alloy for better corrosion resistance. The high-carbon, high-chromium 440 alloys combine considerable corrosion resistance with maximum hardness. The stainless steel 5XX series of wrought martensitic alloys — 501, 501A, 501B, 502, 503, and 504 — contain less chromium, ranging from 4 to 6% in 501 and 502, to 8 to 10% in 501 B and 504. All contain some molybdenum, usually less than 1%, and are nickel-free. Most of the 4XX alloys can provide yield strengths greater than 1034 MPa and some, such as the 440s, more than 1724 MPa. The martensitic stainless steels, however, are less machinable than the austenitic and ferritic alloys and they are also less weldable. Forging temperatures range from 1038 to 1232°C. Most of the alloys are available in a wide range of mill forms and typical applications include turbine blades, springs, knife blades and cutlery, instruments, ball bearings, valves and pump parts, and heat exchangers.

The wrought PH stainless steels are also called age-hardenable stainless steels. Three basic types are now available: austenitic, semiaustenitic, and martensitic. Regardless of the type, the final hardening mechanism is precipitation hardening, brought about by small amounts of one or more alloying elements, such as aluminum, titanium, copper, and, sometimes, molybdenum. Their principal advantages are high strength, toughness, corrosion resistance, and relatively simple heat treatment. Of the austenitic PH stainless steels, A-286 is the principal alloy. Also referred to as an ironbase superalloy, it contains about 15% chromium, 25% nickel, 2% titanium, 1.5% manganese, 1.3% molybdenum, 0.3% vanadium, 0.15% aluminum, 0.05% carbon, and 0.005% boron. It is widely used for aircraft turbine parts and high-strength fasteners. Heat treatment (solution treating at 981°C, water or oil quenching, aging at 718 to 732°C for 16 to 18 h and air cooling) provides an ultimate tensile strength of about 1035 MPa and a tensile yield strength of about 690 MPa, with 25% elongation and a Charpy impact strength of 87 J. The alloy retains considerable strength at high temperatures. At 649°C, for example, tensile yield strength is 607 MPa. The alloy also has good weldability and its corrosion resistance in most environments is similar to that of 3XX stainless steels. The semiaustenitic PH stainless steels are austenitic in the annealed or solution-treated condition and can be transformed to a martensitic structure by relatively simple thermal or thermomechanical treatments. They are available in all mill forms, although sheet and strip are the most common. True semiaustenitic PH stainless steels include PH 14–8 Mo, PH 15–7 Mo, and 17–7PH. AM-350 and AM-355 are also so classified, although they are said not truly to have a precipitation-hardening reaction.

The above PH steels are lowest in carbon content (0.04% nominally in PH 14-8Mo, 0.07% in the others). PH 14-8Mo also nominally contains 15.1% chromium, 8.2% nickel, 2.2% molybdenum, 1.2% aluminum, 0.02% manganese, 0.02% silicon, and 0.005% nitrogen. PH 15-7Mo contains 15.2% chromium, 7.1% nickel, 2.2% molybdenum, 1.2% aluminum, 0.50% manganese, 0.30% silicon, and 0.04% nitrogen. 17-7PH is similar to PH 15-7Mo except for 17% chromium and being molybdenum- free. AM-350 contains 16.5% chromium, 4.25% nickel, 2.75% molybdenum, 0.75% manganese, 0.35% silicon, 0.10% nitrogen, and 0.10% carbon. AM-355 has 15.5% chromium, 4.25% nickel, 2.75% molybdenum, 0.85% manganese, 0.35% silicon, 0.12% nitrogen, and 0.13% carbon. In the solution-heattreated condition in which these steels are supplied, they area readily formable. They then can be strengthened to various strength levels by conditioning the austenite, transformation to martensite, and precipitation hardening. One such procedure, for 17-7PH, involves heating at 760°C, air cooling to 16°C, then heating to 565°C and air-cooling to room temperature. In their heat-treated conditions, these steels encompass tensile yield strengths ranging from about 1241 MPa for AM-355 to 1793 MPa for PH 15-7Mo.

After solution treatment, the martensitic PH stainless steels always have a martensitic structure at room temperature. These steels include the progenitor of the PH stainless steels, Stainless W, PH 13-8Mo, 15-5PH, 17-4PH, and Custom 455. Of these, PH 13-8Mo and Custom 455, which contain 11 to 13% chromium and about 8% nickel plus small amounts of other alloying elements, are the higher-strength alloys, providing tensile yield strengths of 1448 MPa and 1620 MPa, respectively, in bar form after heat treatment. The other alloys range from 15 to 17% in chromium and 4 to 6% in nickel, and typically have tensile yield strengths of 1207 to 1276 MPa in heat-treated bar form. They are used mainly in bar form and forgings, and only to a small extent in sheet. Age hardening, following high-temperature solution treating, is performed at 427 to 677°C.