The oxidation behavior of three different zones of welded Incoloy 800H alloy
Introduction
Superalloys are considered for using in many power-generating facilities (Davis, 1997, Rai et al., 2004), which can be used in heat-exchange tubes of High Temperature Gas Reactor (HTGR) of nuclear-power plant (Fiorrenttin, 1987). The chemical composition of Incoloy-800H (800H) was shown in Table 1, and Ni content is more higher than stainless steel which the mechanical properties and oxidation resistance of 800H alloy become better. In general, the oxidation resistance of an alloy strongly depends on the dense nature of the scale and its good adhesion to the substrate at elevated temperatures. Typically, a chromia-former alloy with adequate amounts of Cr content (≥20 wt.%) may expect to form a full-dense, continuous Cr2O3 layer in the scales, which effectively blocks outward diffusion of cations and inward transport of oxygen, thereby providing an excellent benefit effect against further oxidation (Wood and Stoot, 1987, Cabet and Rouillard, 2009). In addition, depending on alloy composition, several Cr-containing spinels, such as (M,Cr)3O4 where M = Fe, Ni, Mn, or Ti, were always observed on top of the chromia layer (Asteman et al., 2001, Tempest and Wild, 1982, Laverde et al., 2004). The diffusivities of those cations (Fe, Mn, Ni and Ti) were much faster than those of Cr ions in the Cr2O3 sub-lattices, having the order of DMn > DFe > DNi > DCr (Kim et al., 2009, Lobnig et al., 1992). Thus, the formation of various spinels may also provide a protective layer to reduce the oxidation rates.
In addition, a proper welding process is applied to assemble the parts of heat-exchangers, and it is expected that the properties of those welded alloys may significantly change. Thus, it is of essence to fully understand the welding effect on the possibility of materials degradation in the 800H alloy when applied in practical industries during thermal activated processes, especially when water vapor presents in the environments. According to various positions of heat-exchangers from the reactor, the operation temperature could be from 950 °C at the entrance side to 390 °C at the exit side (Fujikawa et al., 2004). Thus, the main goal of this work is to systemically study the oxidation behavior of the welded 800H alloy in dry/wet air at 950 °C. The effects of welding and water vapor on oxidation kinetics and reaction mechanism of the 800H superalloy are explored.
Section snippets
Experimental
A commercial 800H sheet (100 cm × 60 cm × 0.6 cm) was directly purchased from the local supplier. The welding process was done by TIG under an atmosphere of Ar. Two alloy plates with V-slot of 45° were assembled, and Inconel-82 (the composition also shown in Table 1) was chosen as the filler, as suggested by AWS-A5.14 specification (AWS, 2011). The welded 800H alloy produced three different zones with after welding process were obtained three different zones, as shown in the optical micrograph and
Oxidation kinetics
Parabolic plots of oxidation kinetics for the three different zones of welded 800H alloy in dry and wet air at 950 °C are shown in Fig. 3. In general, the dry-air oxidation kinetics of the three zones of welded 800H obeyed a parabolic-rate law, having a simple mass-gain trend throughout the exposure period of time. The parabolic-rate constants (kp values) were determined by the equation of (mass gain per unit area) = kp(t)1/2 at the steady-state stage, and the calculated kp values (in the
Conclusions
Based on the results and research, following several conclusions can be drawn.
- 1.
The oxidation kinetics of three different zones of welded 800H alloy followed the parabolic-rate law in dry-air oxidation, indicating that diffusion is the rate-controlling step during oxidation. The dry-air oxidation rates of the alloys followed by the fast-to-slow rank of 800H-HAZ > 800H-SUB > 800H-MZ.
- 2.
The scales formed on the alloys were composed of Cr2O3 and FeCr2O4 for both 800H-SUB and 800H-HAZ, and MnCr2O4 for
Acknowledgments
Partly financial support by the National Science Council of Republic of China under the Grant Nos. of 100-NU-E-019-001-NU and NSC 99-3113-Y-042-001 is greatly acknowledged. TGA equipment support by the National Taiwan Ocean University under the Grant No. of NTOU-RD972-04-03-01-01 is also appreciated.
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