Heat-resisting steels

Fig. 11.6 Oxidation resistances of carbon steel and stainless steel heat-resistant alloys with different compositions after 1000 h exposure in air from 1100 to 1700 °F [4]. Courtesy of United States Steel.

Stainless steel contains at least 10.5 % chromium and a maximum of 1.2 % carbon and is therefore per definition high alloyed. Based on their microstructure, stainless steels can be subdivided into ferritic, martensitic, ferritic-austenitic, and austenitic steels. According to their usage properties, stainless steels can be categorized into corrosion-resistant steels, heat-resistant steels, and high-temperature steels (Klocke 2010). 

In this section, the prototype of membrane-type partial oxidation (MPOX) reformer combining the oxygen permeable membrane of (Ceo.85Smo.i5)02—15vol%MnFe204 with dimensions of 3.6 cm x 3.6 cm x 135 pm and a ferric stainless-steel heat-resistant separator is demonstrated, and its reforming properties will be discussed (Takamura, Ogawa, Suehiro, Takahashi, Okada, 2008). 

Sizing Low-carbon steel, hi -silioongray or ductile iron, AISI H13 tool steel, stainless steels, heat-resistant alloys Graphite suspension, boron nitride... 

There are large-scale operations using direct-heat resistance furnaces. These are mainly in melting bulk materials where the Hquid material serves as a uniform resistor. The material is contained in a cmcible of fixed dimensions which, coupled with a given resistivity of the material, fixes the total resistance within reasonable limits. The most common appHcation for this type of direct-heat electric resistance furnace is the melting of glass (qv) and arc furnaces for the melting of steel (qv). 

The materials of constmction of the radiant coil are highly heat-resistant steel alloys, such as Sicromal containing 25% Cr, 20% Ni, and 2% Si. Triethyi phosphate [78-40-0] catalyst is injected into the acetic acid vapor. Ammonia [7664-41-7] is added to the gas mixture leaving the furnace to neutralize the catalyst and thus prevent ketene and water from recombining. The cmde ketene obtained from this process contains water, acetic acid, acetic anhydride, and 7 vol % other gases (mainly carbon monoxide [630-08-0][124-38-9] ethylene /74-< 3 -/7, and methane /74-< 2-<7/). The gas mixture is chilled to less than 100°C to remove water, unconverted acetic acid, and the acetic anhydride formed as a Hquid phase (52,53). 

Niobium is important as an alloy addition in steels (see Steel). This use consumes over 90% of the niobium produced. Niobium is also vital as an alloying element in superalloys for aircraft turbine engines. Other uses, mainly in aerospace appHcations, take advantage of its heat resistance when alloyed singly or with groups of elements such as titanium, tirconium, hafnium, or tungsten. Niobium alloyed with titanium or with tin is also important in the superconductor industry (see High temperature alloys Refractories). 

II) foil, tape heat-resistant ahoys, steels stmctural turbiae parts... 

Redistillation. For certain appHcations, especially those involving reduction of other metal compounds, better than 99% purity is required. This can be achieved by redistillation. In one method, cmde calcium is placed in the bottom of a large vertical retort made of heat-resistant steel equipped with a water-cooled condenser at the top. The retort is sealed and evacuated to a pressure of less than 6.6 Pa (0.05 mm Hg) while the bottom is heated to 900—925°C. Under these conditions calcium quickly distills to the condensing section leaving behind the bulk of the less volatile impurities. Variations of this method have been used for commercial production. Subsequent processing must take place under exclusion of moisture to avoid oxidation. 

Table 17. Cast Stainless Steels, Corrosion and Heat-Resistant Grades...

Table 19. Composition of Heat-Resisting Chromium Steels...

Some units have been built and successbilly operated with simple slot-type distributors made of heat-resistant steel. This requires a heat-resistant plenum chamber but eliminates the frequently encountered problem of corrosion caused by condensation of acids and water vapor on the cold metal of the distributor. 

Stainless steel, ferritic 17% Cr type 0 2 0 2 <400 Wronglit, cast, clad Good Good 7S 6.0 AlSl type 430 ASTM corrosion- and heat-resisting steels... 

TABLE 28-13 Standard Cast Heat-Resistant Stainless Steels ... 

Actually, in many cases strength and mechanical properties become of secondaiy importance in process applications, compared with resistance to the corrosive surroundings. All common heat-resistant alloys form oxides when exposed to hot oxidizing environments. Whether the alloy is resistant depends upon whether the oxide is stable and forms a protective film. Thus, mild steel is seldom used above 480°C (900°F) because of excessive scaling rates. Higher temperatures require chromium (see Fig. 28-25). Thus, type 502 steel, with 4 to 6 percent Cr, is acceptable to 620°C (I,I50°F). A 9 to 12 percent Cr steel will handle 730°C (I,350°F) 14 to 18 percent Cr extends the limit to 800°C (I,500°F) and 27 percent Cr to I,I00°C (2,000°F). 

Fig. 20-11 Potential-time curves of an enamelled container with built-in stainless steel heat exchanger as a function of equalizing resistance, R. Curve 1 container potential in the region of the heat exchanger. Curve 2 heat exchanger potential in the voltage cone of defects in the enamelling. Curve 3 heat exchanger potential outside the voltage cone of the defects.

Thermal Expansion. Alloys differ in their thermal expansion, but the differences are modest. Coefficients for the ferritic grades of steel are perhaps 30 percent below those of the austenitic steels at best, while expansion of the nickel-base austenitic types may be no more than 12 to 15 percent less than tho.se of the less expensive, iron-base, austenitic, heat-resistant alloys. Unfortu-... 

Protective footwear includes shoes, boots with steel toecaps, full boots, e.g. Wellingtons. The choice of material determines durability, acid resistance, oil resistance, heat resistance, non-slip characteristics, impact resistance etc. 

Other metals used are chromium, lead, and aluminum. Electro-chromium coated sheet is used for the tops of steel drinks cans. Aluminium coated sheet provides a combination of corrosion and heat resistance ideal for car exhaust pipes. 

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