Metallurgy creep

PTFE is produced by the free radical polymerization process. While it has outstanding thermal and corrosive resistance, it is a marginal engineering material because it is not easily machinable. PTFE has low tensile strength, resistance to wear, and low creep resistance. Molding powders are processed by press and sinter methods used in powder metallurgy. It can also be extruded using ram extruder techniques. 

Creep and fracture in crystals are important mechanical processes which often determine the limits of materials application. Consequently, they have been widely studied and analyzed in physical metallurgy [J. Weertmann, J.R. Weertmann (1983) R.M. Thomson (1983)]. In solid state chemistry and outside the field of metallurgy, much less is known about these mechanical processes [F. Ernst (1995)]. This is true although the atomic mechanisms of creep and fracture are basically independent of the crystal type. Dislocation formation, annihilation, and motion play decisive roles in this context. We cannot give an exhaustive account of creep and fracture in this chapter. Rather, we intend to point out those aspects which strongly influence chemical reactivity and reaction kinetics. Illustrations are mainly from the field of metals and metal alloys. 

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. 

Stress Distribution and High Temperature Creep Rate of Discontinuous Fiber Reinforced Metals, Acta Metallurgies et Materialia, 38, 1941-1953 (1990). 26. A. G. Evans, J. W. Hutchinson, and R. M. McMeeking, Stress-Strain Behavior of Metal Matrix Composites with Discontinuous Reinforcements, Scripta Metallurgica et Materialia, 25, 3-8 (1991). 

Flat tensile creep specimens were machined from the blocks so that the longitudinal specimen axes were either parallel to the plane containing the majority of the long axes of the fibres for the squeeze-cast composites or parallel to the extrusion direction for powder metallurgy materials. Constant stress tensile creep tests were carried out at temperatures from 423 to 523 K. The applied stresses ranged from 10 to 200 MPa. Creep tests were performed in purified argon in tensile creep testing machines with the nominal stress maintained constant to within 0.1% up to a true strain of about 0.35. Almost all of the specimens were run to final fracture. 

G. Sauthoff. Creep Behaviour and Creep Mechanisms in Ordered Intermetallics. In C.T. Liu, R.W. Cahn, and G. Sautholf (eds.) Ordered Intermetallics - Physical Metallurgy and Mechanical Behaviour. Kluwer Acad. Publ. Dordrecht (1992) 525-539. 

The industrial uses of tellurium are limited. In metallurgy, tellurium is used as an additive to improve alloys. The addition of tellurium improves the creep strength of tin and the mechanical properties of lead. Powdered tellurium is used as a secondary vulcanizing agent in various types of rubbers (natural rubber and styrene-butadiene rubbers) as it reduces the time of curing and endows the rubbers with increased resistance to heat and abrasion. In addition, tellurium and its compounds have been used as oxidation catalysts in organic syntheses. Due to its photoelectric properties, tellurium and its compounds are also employed in the semiconductor and electronics industry. In much smaller quantities, tellurium is used in pottery glazes. For further details, see Fishbein (1991). 

There is no general empirical or mathematical equation to relate creep strain to metallurgy or temperature since it is a combination of all these factors. One common equation for creep strain is the Norton-Badey power law ... 

K. S. Raja, S. A. Namjoshi, D. A. Jones, Corrosion-creep interaction of stainless alloys in acid chloride solutions. Metallurgical and Materials Transactions A Physical Metallurgy and Materials Science, 36A(5) (2005) 1107-1120. 

D. Argence, A. Pineau, Predictive metallurgy applied to creep-fatigue damage of austenitic stainless steels, in Proc. of the Donald McLean Symposium, Structural Materials, The Institute of Materials, 1995, pp. 229—257. 

F. V. Lend and G. S. Ansell, Creep Mechanisms and their Role in the Sintering of Metal Powders, in Modern Developments in Powder Metallurgy Vol. 1 (H. H. Hausner, ed.), Plenum Publishing, New York, 1966, p. 281. 

Fig. 20. EfTect of yttrium addition on creep rupture lives of powder metallurgy (PM) and ingot metallurgy (IM)...

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