Alloying microalloying

The reformer tubes typically operate at maximum temperatures of 1,600°F to 1,700°F and are designed for a minimum stress-to-rupture life of 100,000 operating hours. A 35/25 Ni/Cr alloy is used that is modified with niobium and microalloyed with trace elements such as titanium and zirconium. Smaller tube diameters provide better heat transfer and cooler walls. This reduces tube and fuel costs and increases tube life. But more tubes increases the pressure drop. The optimum inside tube diameter is 4 to 5 in. The wall thickness may be as low as 0.25 inch with a length of 40 to 45 ft. The lane spacing between tube rows must be enough to avoid flame impingement from the burners. Typical spacing is 6 to 8 feet. 

Microalloying of Ni-base alloys with rare earths improves ductility, stress rupture life and creep resistance as shown in Table 12.9. The improved properties are due to desulphurisation caused by rare earths. Addition of cerium to inconel 600 and Hstalloy improves their hot workability. 

This chapter summarizes important contributions on the AB5/AB and elemental hydrides. Examples of microalloying in LaNis- and FeTi-based intermetallic hydrides have been emphasized, and there has been some discussion on elemental hydrides. Classification of testing methods for durability of alloys/ intermetallics were presented, examples of intrinsic and extrinsic tests that include cycling and aging have been reported for AB5/AB and other alloys. More recent results on extrinsic tests on lightweight complex hydrides (imide-amide) that were cycled between Li2NH/LiNH2 phases were also reported. 

The Cu-Al-Ni alloys are advantageous because of their higher stability at higher temperatures compared with the Cu-Zn-Al alloys. However, second-phase precipitation cannot be suppressed and embrittles the Cu-Al-Ni alloys and precludes cold working, i.e. such alloys can only be hot finished (Van Humbeek and Delaey, 1989 Hodgson, 1990). Thermomechanical treatments and microalloying additions - in particular Mn, Ti, and Zr - are used for... 

Wol] studied the influence of microalloying with Ti on the plastic deformation of Fe-40Cr alloys. A study of surface mechanical properties of Ti implanted Cr-Fe alloys has been performed by [1988Sas]. 

Reformer tubes typically operate at maximum temperatures of 1600 to 1700°F and are designed for a minimum stress-to-rupture life of 100,000 operating hours. Today s preferred metallurgy is a 35/25 Ni/Cr alloy modified with niobium and microalloyed with trace elements including titanium and zirconium. 

Whereas both Fe3Al and FeAl (<40 at.% Al) are intrinsically ductile, they are susceptible to embrittlement at room temperature in the presence of moisture. This phenomenon is because of the strong affinity of freshly exposed surfaces of the alloys for oxygen. When water is the oxidant, this results in the release of hydrogen which can then enter the alloy. This problem has been mitigated by microalloying with, for example, Cr, Zr, and C and by control of microstructure (McKamey etal., 1991 Lynch and Heldt, 1994 Stoloff et al., 1994 McKamey, 1996 Liu et al., 1998). 

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