Creep rupture strength

It was concluded that 1.5Ti-3.0Al should be considered the most suitable alloy for ECC flue gas expander rotors. A 1,400-mm diameter expander rotor disk was manufactured using this alloy. Test specimens removed from the disk rim showed that the disk had equivalent tensile properties at both room and elevated temperatures, and the same creep rupture strength as that of AISI 685. 

For resistance against fatigue, Nimonic 75 has been used with Nimonic 80 and Nimonic 90. Nimonic 75 is an 80-20 nickel-chromium alloy stiffened with a small amount of titanium carbide. Nimonic 75 has excellent oxidation and corrosion resistance at elevated temperatures, a reasonable creep strength, and good fatigue resistance. In addition, it is easy to press, draw, and mold. As firing temperatures have increased in the newer gas turbine models, HA-188, a Cr, Ni-based alloy, has recently been employed in the latter section of some combustion liners for improved creep rupture strength. 

If some other criterion such as creep-rupture strength is of primary importance, the alloy choice may be restricted. Here it would be necessary to have thennal fatigue comparisons only for the alloys that pass the primary screening. When alloy selection reaches this stage some further cautions are in order. 

Good impact strength at low temperatures and excellent creep rupture strength. 

Oc is the creep rupture strength at a time equivalent to N cycles... 

Example 2.21 A rod of plastic is subjected to a steady axial pull of 50 N and superimposed on this is an alternating axial load of 100 N. If the fatigue limit for the material is 13 MN/m and the creep rupture strength at the equivalent time is 40 MN/m, estimate a suitable diameter for the rod. Thermal effects may be ignored and a fatigue strength reduction factor of 1.5 with a safety factor of 2.5 should be used. 

Engineers and metallurgists have developed alloys to comply economically with individual codes. In Germany, where design stress is determined from yield strength and creep-rupture strength and no... 

Of the common alloying elements in steel, molybdenum is the niiisl effective in increasing creep—rupture strength, and the carbon— molybdenum steels generally have more than twice the creep—rupture strength of plain carbon steel at the same temperature. The most commonly used steels for high temperature service contain from 0.5 to 1.5% molybdenum. 

Chromium is the most effective addition to improve the resistance of steels to corrosion and oxidation ar elevated temperatures, and Ihe chromium—molybdenum steels are an important class of alloys for use in steam power plants, petroleum refineries, and chemical-process equipment. The chromium content in these steels varies from 0.5 to Ill s. As a group, the low carbon chromium—molybdenum steels huve similar creep—rupture strengths, regardless of the chromium content, hut corrosion and oxidation resistance increase progressively vvith chromium content. Most of the chromium — molybdenum steels are used in the annealed or in the normalised and tempered condition some ol the modified grades have better properties in the quench and tempered condition. 

The highly alloyed austenitic stainless steels arc proprietary inodilica-(ions of the standard AISI 316 stainless steel. These have higher creep-rupture strengths than ihe standard steels, yet retain the good corrosion resistance and forming characteristics of the standard austenitic stainless steels. 

For reformer outlet manifolds the normal metallurgy choice is a wrought type of Alloy 800 H. It has sufficient ductility and thermal-shock resistance during startup and shutdown. The cast version of Alloy 800 H provides a cost-effective, alternate material with a higher creep-rupture strength, low tendency for embrittlement and good ductility. Hot reformed-gas transfer lines are usually refractory-lined with an interior of Alloy 800 sheathing88. 

Flow within molds in processes such as IM and resin transfer molding (RTM), or through dies in the extrusion process, can orient the molecules of the resin as well as short or long fibers. This orientation can result in the designed properties desired or, if not properly processed, can result in inferior properties which may become evident in the form of reduced resistance to crazing, low impact strength, lowered creep rupture strength, etc. 

Recently a new-generation tube material has emerged, called Micro Alloy [1490]-[1493]. This contains not only niobium but also titanium and zirconium (or lanthanum), and has improved creep rupture strength still further. Table 24 shows the chemical composition of the tube materials. 

For desulfurizers, coking, and catalytic reforming units, many of the pressure vessels operating with metal temperatures of 700 to 900°F are constructed, from carbon-V2 Mo or IV4 Cr-V2 Mo alloy steels. These steels have marked creep-rupture strength properties over carbon steel. For example, compare the design stress value of 15,000 psi for IV4 Cr-V2 Mo steel at 900°F with the 6,500 psi value for carbon steel at the corresponding temperature. 

Rhenium (Re) differs from the other refractory metals (Nb, Ta, Mo and W) in that it has an hep structure, and does not form carbides. Because it does not have a ductile-to-brittle transition temperature. Re retains its ductility from subzero to high temperatures. In addition, it can be mechanically formed and shaped to some degree at room temperature. It also has a very high modulus of elasticity that, among metals, is second only to those of Ir and Os. Compared with other refractory metals. Re has superior tensile strength and creep-rupture strength over a wide temperature range. 

The results are shown in Table 24.6 where the maximum stress on the tube between the regenerative and the recuperative burner is almost the same for both tube inside pressures of 0-0.6 MPa. These values are thought to be low enough than the creep rupture strength at 900°C with a long working time—about... 

Creep rupture strength experiments have been conducted in the ZEMAK I - IV laboratory facilities in Julich. Fig. 2-15 shows some measurement results. Also fatigue tests with periodic stress - strain impact have been made to simulate load changes. 

Fig. 2-15 Creep rupture strength experiments for different alloys under consideration for HTGRs [32]... 

Since experimental creep rupture times rarely exceed 10" h, it is necessary to extrapolate the data, using a straight line extension of the ductile rupture line on the log-log graph. The British Gas Specification for polyethylene pipe required the 50 year creep rupture stress cr o > 10 MPa. The International Standard ISO 9080 classifies polyethylene as PE80 if the lower confidence limit of the 50 year creep rupture strength lies between 8.0 and 9.9 MPa, and as PEIOO if it lies between 10.0 and 11.9 MPa. 

Fundamental questions about factors that control the creep rates of ceramic materials have not been answered. The effects of carbon and solid solution dopants on the creep rate of SiC materials need to be better understood. The role of intragranular stacking faults on P-SiC creep rates should also be determined. Furthermore, a determination must be made as to whether the microstructure of a-SiC is intrinsically more creep resistant than the microstrueture of p-SiC. For oxide ceramics, the role of microstructure in controlling creep rate and creep rupture strength must he determined, partieularly for multiphase microstructures. 

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