Temperature metal

Appllca.tlons. The principal appHcations of nickel-base superalloys are in gas turbines, where they are utilized as blades, disks, and sheet metal parts. Abcraft gas turbines utilized in both commercial and military service depend upon superalloys for parts exposed to peak metal temperatures in excess of 1000°C. Typical gas turbine engines produced in the United States in 1990 utilized nickel and cobalt-base superalloys for 46% of total engine weight (41). However, programs for future aerospace propulsion systems emphasize the need for lightweight materials having greater heat resistance. For such apphcations, intermetallics matrix composites and ceramic composites are expected to be needed. 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

The zinc is normally melted in a gas, oU, or coal-fired reverberatory furnace with a capacity up to 100 tons or in a low frequency induction furnace with a capacity of a few tons. The more highly aUoyed compositions are more effectively melted and mixed in low frequency induction furnaces. The furnace must be refractory-lined to eliminate iron pickup by the molten metal. The metal temperature is maintained below 500°C to minimize loss by oxidation. A ladle is used to transfer the metal for casting into molds the pouring temperature is usuaUy ca 440°C. Zinc scrap is not generaUy suitable for remelting because it may contain undesirable impurities. 

Unlike the aircraft turbiae, the ground-based gas turbiae operates continuously at the same power setting, usually 60—80% of maximum power output except when starting. Air and fuel flow patterns are constant and the combustor can be tuned to minimise smoke formation and high metal temperatures. 

The combustion chamber of a modem steam generator is a large water-cooled chamber in which fuel is burned. Firing densities are important to ensure that the chamber wall metal temperatures do not exceed the limits of failure of the tubes. Firing densities are expressed in two ways volumetric combustion intensities and area firing intensities. The volumetric combustion intensity is defined by equation 23,... 

Si minimum metal temperature expected Basic allowable stress at MPa Idp/im (ksi)... 

Design Temperature The design temperature is the material temperature representing the most severe condition of coincident pressure and temperature. For uninsulated metallic pipe with fluid below 38°C (100°F), the metal temperature is taken as the fluid temperature. 

With external insulation, the metal temperature is taken as the fluid temperature unless service data, tests, or calculations justify lower values. For internally insulated pipe, the design metal temperature shall be calculated or obtained from tests. 

S/, = S from Table 10-49 at maximum (hot) metal temperature normally expected during operation or shutdown... 

Specification P no. Grade Temper Size range, in Minimum tensile strength, kip/in" Minimum yield strength, kip/in" Notes Minimum temperature, (IS) Metal temperature, F (22) ... 

This is a product specification. No design stresses are necessary. Limitations on metal temperature for materials covered by tliis specification are ... 

Base-metal P numberf Weld-metal analysis A numberf Material group Nominal waU thickness Minimum specified tensile strength, base metal Metal temperature range h/in, nominal wall Minimum time, b Brinell hardness, maximum... 

When pressure tests are conducted at metal temperatures near the ductile-to-brittle transition temperature of the material, the possibility of brittle fracture shall be considered. 

Subsection C This subsection contains requirements pertaining to classes of materials. Carbon and low-alloy steels are governed by Part UCS, nonferrous materials by Part UNF, high-alloy steels by Part UHA, and steels with tensile properties enhanced by heat treatment by Part UHT. Each of these parts includes tables of maximum allowable stress values for all code materials for a range of metal temperatures. These stress values include appropriate safety fac tors. Rules governing the apphcation, fabrication, and heat treatment of the vessels are included in each part. 

Equipment which is electrically driven and operated with metal temperatures exceeding 425 K should be provided with auxihaiy drives or power sources. Loss of rotation of a heated calciner or high-temperature dryer carrying a hea y bed of hot solids will quickly result in sagging of the cyhnder due to nonuniform cooling. 

FIG. 27-40 Elffect of departure from nucleate boiling (DNB) on tube-metal temperature. 

Turbine-Blade Cooling The turbine inlet temperatures of gas turbines have increased considerably over the past years and will continue to do so. This trend has been made possible by advancement in materials and technology, and the use of advanced turbine bladecooling techniques. The olade metal temperature must be kept below 1400° F (760° C) to avoid hot corrosion problems. To achieve this cooling air is bled from the compressor and is directed to the stator, the rotor, and other parts of the turbine rotor and casing to provide adequate cooling. The effect of the coolant on the aerodynamic, and thermodynamics depends on the type of cooling involved, the temperature of the coolant compared to the mainstream temperature, the location and direction of coolant injection, and the amount of coolant. 

Water Cooling Water is passed through a number of tubes embedded in the blade. The water is emitted from the blade tips as steam to provide excellent cooling. This method keeps blade metal temperatures below 1000° F (538° C) however, a full application of this method is not expected until the year 2000. 

Blade material and cooling flow (blade metal temperature)... 

Since SCC is often dependent on environmental factors other than stress and exposure to a specific corrodent, appropriate alteration of these other factors may be effective. For example, a reduction in metal temperature, a change in pH, or a reduction in the levels of oxygen or oxidizing ions may reduce or eliminate SCC. 

Eiimination. Careful use of appropriate welding procedures and techniques, as these relate to metal temperature, is necessary, especially when welding thin-walled tubes. 

Critical factors. Slag entrapment can occur if weld-metal temperature is too low or if solidification is too rapid. 

Figure 20.7 shows that up to 1960 turbine inlet temperatures were virtually the same as the metal temperatures. After 1960 there was a sharp divergence, with inlet temperatures substantially above the temperatures of the blade metal itself - indeed, the gas temperature is greater than the melting point of the blades. Impossible Not at... 

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