Austenitic stainless steels properties

Nickel—Iron. A large amount of nickel is used in alloy and stainless steels and in cast irons. Nickel is added to ferritic alloy steels to increase the hardenabihty and to modify ferrite and cementite properties and morphologies, and thus to improve the strength, toughness, and ductihty of the steel. In austenitic stainless steels, the nickel content is 7—35 wt %. Its primary roles are to stabilize the ductile austenite stmcture and to provide, in conjunction with chromium, good corrosion resistance. Nickel is added to cast irons to improve strength and toughness. 

Structural Properties at Low Temperatures It is most convenient to classify metals by their lattice symmetiy for low temperature mechanical properties considerations. The face-centered-cubic (fee) metals and their alloys are most often used in the construc tion of cryogenic equipment. Al, Cu Ni, their alloys, and the austenitic stainless steels of the 18-8 type are fee and do not exhibit an impact duc tile-to-brittle transition at low temperatures. As a general nile, the mechanical properties of these metals with the exception of 2024-T4 aluminum, improve as the temperature is reduced. Since annealing of these metals and alloys can affect both the ultimate and yield strengths, care must be exercised under these conditions. 

Above temperatures of 900°F, the austenitic stainless steel and other high alloy materials demonstrate inereas-ingly superior creep and stress-rupture properties over the chromium-molybdenum steels. For furnace hangers, tube supports, and other hardware exposed to firebox temperatures, cast alloys of 25 Cr-20 Ni and 25 Cr-12 Ni are frequently used. These materials are also generally needed because of their resistanee to oxidation and other high temperature corrodents. 

Furnace tubes, piping, and exchanger tubing with metal temperatures above 800°F now tend to be an austenitic stainless steel, e.g., Type 304, 321, and 347, although the chromium-molybdenum steels are still used extensively. The stainless steels are favored beeause not only are their creep and stress-rupture properties superior at temperatures over 900°F, but more importantly because of their vastly superior resistance to high-temperature sulfide corrosion and oxidation. Where corrosion is not a significant factor, e.g., steam generation, the low alloys, and in some applications, carbon steel may be used. 

Nonmagnetic drill collars are manufactured from various alloys, although the most common are Monel K500 (approximately 68% nickel, 28% copper with some iron and manganese, and 316L austenitic stainless steel). A stainless steel with the composition of 0.06% carbon, 0.50% silicon, 17-19% manganese, less than 3.50% nickel, 12% chromium, and 1.15% molybdenum, with mechanical properties of 110 to 115 Ksi tensile strength is also used. 

Fig. 8.27 Some compositional modifications of 18/8 austenitic stainless steel to produce special properties. Dashed lines show compositional links to other alloy systems (after...

It is important to use the stable austenite stainless steels grades for H storage containers, because metastable austenite can be transformed from y to a or y - e - Ot after cooling or deformation, which hydrogen brittleness can be occurred. Table 1 gives the effect of thermal Hi charging on mechanical properties. The condition of thermal H2-charged is 300 "C. 10 days, 10 MPa, H2. In the Table 1,20 is... 

Auxiliary piping welded to alloy steel casings shall be of a material with the same nominal properties as the casing material or shall be of low carbon austenitic stainless steel. Other materials compatible with the casing material and intended service may be nsed with the purchaser s approval. 

Stainless steel 316L material used for piping and equipment shows considerable corrosion resistance because of the beneficial effect of molybdenum on the surface properties. It is also observed that the surface treatment (pre-reduced, polished, passivated and chemically treated surfaces) of stainless steel equipment and piping reduces the corrosion process in seawater applications. The corrosion resistance of stainless steel in seawater applications can also be enhanced by bulk alloying the stainless steel with nitrogen, chromium, molybdenum and nickel by converting the stainless steel into super austenitic stainless steel. From leaching studies it is also observed that the release of iron, chromium and nickel from the super austenitic stainless steel to seawater is considerably... 

Table 10 Properties of Femtic and Austenitic Stainless Steels for Converter Housing...

This family of stainless accounts for the widest usage of all the stainless steels. These materials are nonmagnetic, have face-centered cubic structures, and possess mechanical properties similar to the mild steels, but with better formability. The AISI designation system identified the most common of these alloys with numbers beginning with 300 and resulted in the term 300 series stainless. Table 3 lists the chemical analyses of some standard austenitic stainless steels and compares them to a few materials from other families of materials. 

Materials such as austenitic stainless steels, nickel-based alloys, and titanium alloys can be used as materials for pressure vessel components in cryogenic applications at temperatures as low as 200°C. Alloy steels have brittle transition points making their impact properties at low temperatures unsuitable for pressure applications. Closures and bolts must also be made of materials that remain ductile at low temperatures. 

Heat treatment is more widely used than cold work to strengthen metals because final properties can be tailored to design requirements and additionally achieve much higher strength levels. Not every metal may be heat treated. For example, the only way of strengthening austenitic stainless steels (300 series) is by cold work. 

Heat-affected zone (HAZ) is the volume of parent metal in which the mechanical properties and/or the microstructure have been changed by the heat of welding or thermal cutting. For most welds in carbon and low-alloy steels, the HAZ is a band, usually about 1/8 in. (3 mm) wide, adjacent to the fusion line of the weld. In austenitic stainless steels, a narrow, secondary HAZ may be generated some distance from the fusion line as illustrated in Figure 21.4. 

Nickel (face-centered cubic) is a major addition to iron-chromium alloys and with 8 to 22 wt% Ni forms the basis of the austenitic stainless steels. The major influence of nickel is to permit the formation of face-centered-cubic solid solution alloys, which generally have more favorable metallurgical properties than the body-centered-cubic,... 

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