Austenitic containing nickel

Super austenitic, high nickel, stainless steels, containing between 29 to 30 per cent nickel and 20 per cent chromium, have a good resistance to acids and acid chlorides. They are more expensive than the lower alloy content, 300 series, of austenitic stainless steels. 

If the total amount of other metals present in iron exceeds about 5%, the alloy is sometimes called a high-alloy steel. Most stainless steels are in this category because the chromium content is between 10% and 25%, and some types also contain 4% to 20% nickel. Stainless steels, so-called because of their resistance to corrosion, are of several types. The form of iron having the fee structure is known as y-Fe or austenite. Therefore, one type of stainless steel (that also contains nickel) is known as austenitic stainless steel because it has the austenite (fee) structure. Martensitic stainless steels have a structure that contains a body-centered... 

Discaloy [Westinghouse], TM for an austenitic iron-base alloy containing nickel, chromium, and relatively small proportions of molybdenum, titanium, silicon, and manganese. This alloy is precipitation-hardened and was developed primarily to meet the need for improved gas-turbine disks, one of the most critical components of jet engines. 

The carbon potentials in liquid alkali metals are important in material compatibility problems. Carbon as a minor component of several materials of technical importance strongly influences the strength and ductility of the materials. The alkali metals have the ability to wet the surfaces of metals or alloys. In this state they tend to exchange carbon until they reach the chemical equilibrium. The carbon exchange between sodium and austenitic chromium nickel steels is extensively studied. As is shown in Fig. 8, in which the chemical activities of carbon in sodium and in the Crl8-Ni9 steel are compared as functions of temperature sodium containing 0.1 wppm carbon decarburizes an austenitic steel with a carbon content of 0.05 w-% carbon at a temperature of 650 °C and carburizes the same steel at 550 °C. 

Precipitation processes of this kind are always caused by heat treatments, snch as sensitizing annealing, that are inappropriate for the alloy in question. For the austenitic chronuum-nickel-molybdenum steels used for the fabrication of chemical plant equipment, the critical tanperature range is 400-800°C. Chromium depletion through formation of chromium-rich carbides, mostly of the type (M23Cg), is the main cause of intergranular corrosion in these steels. The precipitation of chromium nitrides of importance only that the chromium-rich nitride (CrjN) can initiate intergranular corrosion, especially in ferritic steels. Since the intermetalUc phases in stainless steels contain appreciably less chromium than carbides and nitrides and their deposition is far slower, the chromium depletion related to these phases is minimal. 

In the high silicon austenitic chromium-nickel special steel XI CrNiSi 18 15, which has high resistance to concentrated nitric acid, susceptibility to intergranular corrosion depends on the separation of a carbide and on a 3t-phase of type M (C,N)2, containing chromium, nickel, silicon,... 

Steel made from scrap will have at least traces of nickel because scrap invariably contains a small quantity of austenitic stainless steel, which contains nickel. Removal of nickel from steel is very difiicult, so it is left in the steel. 

Mo containing austenitic chromium-nickel steels are less susceptible in the simultaneous presence of chloride ions and oxygen [133]. 

On the corrosion behaviour of austenitic chromium-nickel-(molybdenum) steels with and without addition of nitrogen with special consideration of their strain in chloride containing solutions)... 

Experimental work related to chromium-carbide-carbon equilibrium on a nickel base alloy (15% chromium, 10% iron, balance Ni), showed that the carbon solubility in nickel base alloys is lesser than in austenitic steels. Nickel-based alloys containing chromium can be sensitized in the temperature range 500-700°C. 

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. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

Austenitic stainless steels are the most corrosion-resistant of the three groups. These steels contain 16 to 26 percent chromium and 6 to 22 percent nickel. Carbon is kept low (0.08 percent maximum) to minimize carbide precipitation. These alloys can be work-hardened, but heat treatment will not cause hardening. Tensile strength in the annealed condition is about 585 MPa (85,000 Ibf/in"), but workhardening can increase this to 2,000 MPa (300,000 Ibf/in"). Austenitic stainless steels are tough and ducdile. 

Austenitic cast irons (either flake graphite irons or nodular graphite irons) are produced by mixing in nickel from 13-30%, chromium from 1-5% and copper from 0.5-7.5 (to lower nickel-containing grades to augment the corrosion resistance at lower cost). 

Stainless and heat-resisting steels containing at least 18% by weight chromium and 8% nickel are in widespread use in industry. The structure of these steels is changed from magnetic body centered cubic or ferritic crystal structure to a nonmagnetic, face-centered cubic or austenitic crystal structure. 

The austenitic steels can be used at very low temperatures (low-alloy ferritic steels containing 9% nickel down to -196°C) without the risk of brittle fracture [23]. 

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