Austenitic alloy

The addition of molybdenum to the austenitic alloy (types 316, 316L, 317, and 317L) provides generally better corrosion resistance and improved resistance to pitting. 

Sulfur Corrosion Chromium is the most important material in imparting resistance to sulfidation (formation of smfidic scales similar to oxide scales). The austenitic alloys are generally used because of their superior mechanical properties and fabrication qualities, despite the fact that nickel in the alloy tends to lessen resistance to sulfidation somewhat. 

Aging (extended exposure to an elevated temperature) of an austenitic alloy can strengthen the material and increase its resistance to damage. Thus such alloys can be... 

Figure 53.3 illustrates a pit in a stainless steel such as type 534 or 316 austenitic alloy. Pitting starts at heterogeneity in the steel surface, such as an outcropping sulfide inclusion, the shielded region beneath a deposit or even a discontinuity in the naturally present oxide film caused by a scratch or embedded particle of abrasive grit. This initiation phase of pitting corrosion may take seconds... 

Coriou, M., Grail, L, Mahieu, C. and Pelas, M., Sensitivity to Stress Corrosion and Intergranular Attack of High-nickel Austenitic Alloys , Corrosion, 22, 280 (1966)... 

Maximum corrosion rates used by Du Pont for various alloys are given in Table 19.4, and most users of the test consider average corrosion rates of 0-46-0-61 mm/y and 0-76 mm/y to represent the upper limits for satisfactory resistance for wrought austenitic alloys and cast austenitic alloys, respectively. Streicher considers that if the corrosion rate for each period increases over that for the previous period the alloy is susceptible. 

A 213 Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater, and Heat-Exchanger... 

Steel It has a higher C content (usually 0.5-1%) and is harder than soft bon. An important property of steel is that it may be hardened. If heated to bright redness (to obtain an austenitic alloy) and suddenly cooled quenched), by putting it in water, oil, etc, it becomes hard and brittle due to the formation of the very hard martensite. Brittleness can be removed by tempering (that is by a carefully heating for a short time at, say, 250-300°C) to release or dimmish the internal strains resulted from quenching. 

It is important to use the stable austenite alloys for hydrogen container materials, such as, FeNiCr stainless steel, FeNiCr strengthened with N and Mn stainless steel, /-precipitated strengthening superalloy. 

The class of austenitic stainless steels is by far the most widely used. Adding nickel considerably improves the resistance to corrosion, which can be enhanced further by adding molybdenum. Both ferric and austenitic alloys cannot be hardened by heat treatment. Among different types of stainless steels the most used in process industries are of austentic types. The main are briefly described. 

At temperatures above 300°C Holinski found that molybdenum disulphide produced embrittlement of stainless steel. He suggested that free sulphur released at these temperatures reacted with nickel in austenitic alloys to deposit nickel sulphide preferentially at grain boundaries, thus leading to a form of stress corrosion. Knappwost similarly reported that molybdenum disulphide reacted with iron at 700°C to produce ferric sulphide and free molybdenum, and Tsuya et al showed that it reacted more rapidly with iron and nickel than with silver or copper in a vacuum of 10 Torr above 500°C. The reaction with copper was in fact slow above 500°C but very rapid about 700 C. 

Whether decarburization will be an issue for internal combustion engines burning H2 is difficult to predict from existing information. Low-alloy carbon steels begin to decarburize at temperatures around the operating temperature of exhaust valves, but exhaust valves and valve seats are made from high-alloy steels, austenitic alloys, and superalloys where the carbon is much more stable than low-alloy carbon steels. The hardenable martensitic valve stems of exhaust valves may experience decarburization over extended periods, and this would lead to accelerated wear because of the softened surface that results from decarburization. 

Austenitic alloys also make use of the concept of stabilization. Stainless types 321 and 347 are versions of type 304 stabilized with titanium and niobium, respectively. These elements will preferentially combine with carbon that comes out of solid solution during weld solidification. Rather than a loss of corrosion resistance associated with formation of harmful chromium carbides, the carbides of titanium and niobium are not detrimental to corrosion resistance. The austenitic family of stainless also prompted another approach to avoiding the effects of chromium carbide precipitation. Because the amount of chromium that precipitated was proportional to the carbon content, lowering the carbon could prevent sensitization. Maintaining the carbon content to below about 0.035% vs. 

Although more formable than the ferritic alloys, they are not as ductile as the austenitic family of alloys. Welding requires more care than with the austenitic alloys because of a greater tendency toward compositional segregation and sensitivity to weld heat input. Improper fabrication techniques can result in equipment that falls short of expectations for corrosion resistance and mechanical properties. 

Low-melting-point metals are of particular concern. Molten copper, zinc, or aluminum will attack the grain boundaries of austenitic alloys preferentially. Copper alloy clamps or fixtures used to hold work, whereas welding has been known to leave smears of metal that have subsequently caused cracking. Zinc from galvanized steel or paint primers has also been known to contaminate weld joints. 

NACE International. Current edition. Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Stress Corrosion Cracking During Shutdown of Refinery Equipment. NACE RP0170. Houston, TX NACE International. 

Two generalizations are frequently made with respect to the cracking response of the stainless steels in chloride-bearing environments. One is that the ferritic stainless steels are immune to cracking relative to the austenitic alloys. Although the cracking tendency is much lower, cracking of ferritic stainless steels has been encountered when chlorides are present. This tendency has been reduced with the development of ferrit-... 

There is also alarge group of austenitic alloys with compositions ranging to 100% Ni, 50% Cr, 16% Mo, and controlled amounts of Nb, Cu, Ti, andW. ... 

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