Austenitic stainless steels application

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. 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

The selection of materials for high-temperature applications is discussed by Day (1979). At low temperatures, less than 10°C, metals that are normally ductile can fail in a brittle manner. Serious disasters have occurred through the failure of welded carbon steel vessels at low temperatures. The phenomenon of brittle failure is associated with the crystalline structure of metals. Metals with a body-centred-cubic (bcc) lattice are more liable to brittle failure than those with a face-centred-cubic (fee) or hexagonal lattice. For low-temperature equipment, such as cryogenic plant and liquefied-gas storages, austenitic stainless steel (fee) or aluminium alloys (hex) should be specified see Wigley (1978). 

IP-2.2.7(d). The strength reduction factor represents the reduction in yield strength with long-term exposure of the material to elevated temperatures and, in the absence of more applicable data, shall be taken as 1.0 for austenitic stainless steel and 0.8 for other materials. For castings, the basic allowable stress shall be multiplied by the casting quality factor, Ec. Where the allowable stress value exceeds two-thirds of yield strength at temperature, the allowable stress value must be reduced as specified in para. IP-2.2.7(c). Wind and earthquake forces need not be considered as acting concurrently. At temperatures warmer than 427°C (800°F), use 1.33... 

The stress intensification factors in Appendix D of ASME B31.3 have been developed from fatigue tests of representative piping components and assemblies manufactured from ductile ferrous materials. The allowable displacement stress range is based on tests of carbon and austenitic stainless steels. Caution should be exercised when using eqs. (la) and (lb) (para. IP-2.2.10) for allowable displacement stress range for some nonferrous materials (e.g., certain copper and aluminum alloys) for other than low-cycle applications. 

The stress values for austenitic stainless steels in this Table may not be applicable if the material has been given a final heat treatment other than that required by the material specification or by reference to Note (25) or (26). 

Non-metallic wear part materials, which are proven to be compatible with the specified process fluid, may be proposed within the above limits. See 5.7.4.C. Such materials may be selected as wear components to be mated against a suitably selected metallic component such as hardened 12 % Cr steel or hardfaced austenitic stainless steel. Materials may be used beyond these limits if proven application experience can be provided, and if approved by the purchaser. ... 

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... 

Chloride stress cracking corrosion was also a problem when austenitic stainless steels were covered with thermal insulation. The NACE Paper also discusses the protective coatings for stainless steels and provides sandblasting and coating application guides for stainless and carbon steels. 

Hydrogen has a tendency to adsorb and dissociate at material surfaces, the atomic hydrogen then diffuses into the material and causes embrittlement and diffusion. Materials suitable for hydrogen applications are mainly austenitic stainless steel and aluminum alloys [12, 29]. 

The most widely used austenitic stainless steel is Type 304, known as 18—8. It has excellent corrosion resistance and, because of its austenitic stmcture, excellent ductihty. It may be deep-drawn or stretch formed. It can be readily welded, but carbide precipitation must be avoided in and near the weld by cooling rapidly enough after welding. Where carbide precipitation presents problems. Types 321, 347, or 304L may be used. The applications of Types 304 are wide and varied, including kitchen equipment and utensils, dairy installations, transportation equipment, and oil-, chemical-, paper- (qv), and food-processing (qv) machinery. 

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. 

The austenitic stainless steel A-286 has also been used in pressure vessels for hydrogen gas containment, although reported experience with this steel is limited to specialized laboratory components [27, 47] and not extensive commercial application. Nonetheless, the material has filled a unique niche, where relatively large diameter, seamless pressure vessels have been fabricated... 

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