Fe-Cr-Al alloys

There is an obvious overlap among various applications categories. An example of the overlap is alumina which is both a structural refractory ceramic as well as a catalyst support. The additives modify the interconversion of various AI2O3 phases and the high surface area of y-Al203 is maintained by the added 3 wt% ceria or lanthana. Additives like yttria stabilize zirconia with respect to inertness and mechanical stability. Addition of yttrium or lanthanide to Fe-Cr-Al alloys reduces the spallation of oxide film. 

Van Deventer, E.H. and V.A. Maroni, Hydrogen permeation characteristics of some Fe-Cr-Al alloys. Journal of Nuclear Materials, 113, 65 (1983). 

Materials used in metallic mat technology are Fe-Cr-Al alloys. One of the main worries about using metallic materials is the possibility of oxidation or even corrosive phenomena that may affect the functionality and sometimes even the continuity of the component. When components have to work at high temperatures, as in the case of combustion systems, the kinetics of oxidation and/or corrosion undergo considerable accelerations unless surface phenomena stimulate the formation of... 

The Fe-Cr-Al alloys have been used for some hme as heating elements (that is as material for resistors) for kilns, in competition with more expensive alloys with a high Ni content (Ni-Cr, Ni-Cr-Si, Ni-Cr-Fe-Si). The Fe-Cr-Al alloys normally have a chromium content of about 20-22% and an aluminum content of 5-5.5% while the content of yttrium, the most common alloying "auxiliary" element, is normally lower than 0.1%, if present. In comparison with the Ni-Cr and Ni-Cr-Fe alloys, the Fe-Cr-Al alloys have a lower linear thermal expansion coefficient. Mechanical resistance is sufficiently high for the alloys containing 22% of chromium and with an aluminum content of 4.5-5.5% and, therefore, 72.5-73.5 of iron. 

The Fe-Cr-Al alloys are normally used in oxidative afmospheres at temperatures between 850°C and I350°C reducing atmospheres cause negative effects unless the components have been previously air-oxidized. Their use is not recommended in reducing atmospheres and even less in carburizing atmospheres. 

It is also vital that a constant supply of aluminum is guaranteed through the diffusion from the substrate in order to ensure the surface is totally covered with alumina. Otherwise Cr203 and Fe-Cr spinel would be obtained mainly. The addition of the already mentioned RE in contents of less than 0.1% increase the resistance to oxidation of the Fe-Cr-Al alloys. Moreover the alumina grains are reduced in size. 

Aspect of the various types of mats after the exposure in saline mist for (a) 100 hours, (b) 500 hours, (c) 800 hours the samples marked with 1 and 2 are made in a RieUo mat in a Fe-Cr-Al alloy without an intentional addition of RE. Sample 3 is made in a Riello mat in a Fe-Cr-Al alloy with an intentional addition of RE samples 4 and 5, respectively, the "longitudinal" and "transversal" orientation of the different producer mats. 

Figure 5.36 Scanning electron micrographs of (a) the surface and (b) the cross-section of an Fe-Cr-Al alloy which was oxidized at 1000 °C and cooled to room temperature. Buckling of the alumina scale is evident.

The importance of the latter mechanism has been illustrated in experiments where hydrogen annealing of nickel-base single crystals - and Fe-Cr-Al alloys lowered the sulphur contents to very low levels and resulted in dramatic improvements in the adherence of alumina films to the alloys. This is illustrated in Figure 5.39 where the cycUc oxidation behaviour of the Fe-Cr-Al (low S) alloy, which is the same alloy as Fe-Cr-Al (normal S) that has been desulphurized by hydrogen annealing, is comparable to that of the Y-doped alloys. 

Figure 5.39 Cyclic oxidation kinetics for several Fe-Cr-Al alloys exposed at 1100°C.

Figure 5.40 Cross-section scanning electron micrograph showing the alumina scale formed on a Y-doped Fe-Cr-Al alloy after cychc oxidation at 1100°C for 525 h.

Corrosion studies of the CrFeSi compound in an industrial galvanizing bath (containing aluminium) were conducted by [2005Liu]. During these experiments it was shown that a layer of Fe-Cr-Al alloys was formed at the surface conferring good resistance against corrosion. 

However, such a buckle is stable and will not propagate to cause decohesion failure by delamination unless the strain-energy release rate also satisfies Equation (8.13). Examples of buckling of alumina films from an Fe-Cr-Al alloy are presented in Chapter 5 of Reference [2]. 

Influence of rare earths as alloy additions. The addition of rare earths to AI2O3-forming alloys not only reduces the rate of oxidation as shown in fig. 8 (Parshin et al. 1990), but also enhances scale adhesion, particularly under cyclic conditions (see fig. 9, Tien and Pettit 1972). Golightly et al. (1976) and Wukusick and Collins (1964) have also studied the effect of Y additions on the oxidation of Fe-Cr-Al alloys in air, while Amano et al. (1979) have examined the influence of Ce additions. Jedlinski, Borchardt... 

Jedlinski, J., G. Borchaidt and S. Mrowec, 1991, The elfect of reactive elements on the oxidation behaviour of Fe-Cr-Al alloys, in Microscopy of Oxidation, Proc. Int. Conf., Cambridge, 1990, eds M.J. Bennett and G.W. Lorimer (Institute of Metals, London) p. 278. 

FIGURE 15.1. Dependence on the aluminum content of the sulfidation rate of ternary Fe-Cr-Al alloys containing 20% chromium. (Drawn from data of Mrowec, S. and Wedrychowska, M., OxidMet, 1979,13, 481. With permission.)... 

Multiple additions of minor elements to Fe-Cr-Al alloys affect the failure mechanisms of the oxide scales. There are two broad categories, cohesive failure after rapid initial growth and adhesive failure following slow scale... 

Because of their excellent oxidation resistance, Fe-Cr-Al alloys are commonly used construction materials for components that have to operate at high temperature. The oxidation resistance relies on the formation of slowly growing, well adherent alumina-based surface oxide scales, which form during high temperature exposure. Examples of the application of Fe-Cr-Al alloys are heating element strips or wires, fibre-based domestic and industrial burners, car catalyst carriers, furnace tubes, etc. 

The commercial Fe-Cr-Al alloys studied were the wrought alloys Aluchrom... 

Here tg denotes the time to breakaway (in h), Cq the initial A1 content in (wt.%), Cb the critical A1 content at which breakaway occurs (in wt.%), p the alloy density (in g cm ), h, e, the initial foil or specimen thickness (in cm), k and n the kinetics constants defined in Eq. 22.1. In the calculations it was assumed that the critical A1 content in Eq. 22.2 is independent of temperature and/or specimen thickness. The value of Cb was set to 0.3 wt.% as found for thin-foil Fe-Cr-Al alloys after oxidation at 1200°C e.g. in [5, 6]. It should be mentioned that, in former studies, indications were found that the Cb value depends on temperature. However, to the best of our knowledge, no extensive experimental data are available yet. In the diagram all the lifetimes are given in a normalised way, setting the obtained result at 1050°C for the short cycle (15/5 s) to 100%. 

The test results clearly show that the time to failure is related not only to the time at temperature but strongly depends on the number of cycles for a given oxidation time. For instance, in the case of an oxidation temperature of 1050°C the lifetime for the 120/15 s cycle is approximately six times longer than for the 15/5 s cycle. Because the Fe-Cr-Al alloys studied exhibit only very poor creep strength at high temperature, it is likely that for the thin metal strips used, plastic deformation of the metal takes place during every cycle. The amount of this plastic deformation caused by creep of the metal depends on the oxide-to-metal thickness ratio and accumulates over the testing time. 

Kor2] Kornilov, I.I., New Heat-Resistant Fe-Cr-Al Alloys wifli High Electrical Resistance (in Russian), Izv. Akad. NaukSSSR, Ser. Khim., (5), 751-757 (1940) (Experimental, 15) [1943Mon] Mondolfo, L.F., Al-Cr-Fe , in Metallography of Aluminium Alloys , John Wiley Sons Inc., New York, 70-71 (1943) (Phase Diagram, Review,, , 1)... 

Chu] Chubb, W., Alfant, S., Bauer, A.A., Jablonowski, E.J., Schober, F.R., Dickenson, R.F., Constitution, Metallurgy and Oxidation Resistance of Fe-Cr-Al Alloys , Batelle Memorial Institute, Columbus (1958) (Phase Diagram, Experimental,, , 66)... 

Tys] Tyszko, Z., Oles, A., Niziol, Z., Stracture and Physical Properties of Fe-Cr-Al Alloys (in French), Cercle d Etudes deMetaux, 13,473-492 (1977) (Experimental, Crys. Structure, Phys. Prop., 20)... 

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