니켈기 초내열합금의 고온산화 및 기계적 특성

Abstract

Hydrogen is considered as a clean and green energy source to replace fossil fuels. This promotes the development of high-temperature gas-cooled reactors (HTGRs) to be one of the most promising Gen-IV nuclear systems. The HTGRs are known as high-efficiency systems designed for the economical production of hydrogen and electricity. Approximately 90% of the HTGRs heat will be used to generate electricity and 10% to produce hydrogen. The intermediate heat exchanger (IHX) of HTGRs is generally operated at a pressure of approximately 8 MPa and temperatures over 900oC. Thickness of material for IHX has been known that it could vary from 0.2 mm to 2 mm according to conventional or compact IHX design. Alloy 617 is recently known as primary structural materials for the IHX. This alloy is a solid-solution strengthening austenitic Ni-based superalloy showing excellent strength, creep-rupture strength, corrosion resistance, and oxidation resistance at high temperatures. Main corrosion features of Alloy 617 during high temperature aging are presented as (i) formation of the a Cr2O3 external oxide layer on the surface, (ii) formation of Al2O3 internal oxide along the grain boundaries, and (iii) development of carbide free zone below external oxide layer. It is known that the corrosion features of Alloy 617 cause a deterioration of mechanical properties such as creep rupture life. Thus, the corrosion features at surface will have a dramatic effect on the mechanical properties with decreasing thickness of material for IHX. In this study, the corrosion features of Alloy 617 were investigated by various temperatures and environments. Also, Alloy 617 was studied for improvement of oxidation resistance and mechanical properties by surface coating and microstructure modification. The surface oxide layer of Alloy 617 was composed of external oxide layer and internal oxide by aging at 900oC, 950oC, 1000oC, and 1050oC in air and He atmosphere. The internal oxide was formed along the grain boundary and confirmed as Al2O3. The depth of internal oxide was increased with increasing exposure time and/or temperatures. The depth of internal oxide in air atmosphere was deeper than that in He atmosphere because of the low oxygen partial pressure in He atmosphere. Under external tensile stress, stress concentration was increased with increasing depth of internal oxide, whereupon crack propagation will be accelerated along the grain boundary with increasing depth of internal oxide. By high temperature aging, the external oxide layer mainly consisted of Cr2O3 with small amounts of TiO2 and MnCr2O4 on surface. The formation of the Cr2O3 induced a Cr-depleted zone below the external oxide layer. The Cr-depleted zone causes a deterioration of mechanical properties because of depletion of alloying elements. The external oxidation is different for quantitative evaluation through thickness of scale because of delamination of scale. Thus, it was considered that the Cr-depleted zone could be used as the quantitative indicator for evaluation of the external oxidation because this region resulted from the formation of Cr2O3. Also, a carbide free zone was tried to evaluate the external oxidation. The carbide free zone was formed by dissolution of Cr-rich M23C6 carbide in Cr-depleted zone below the external oxide layer. The Cr-depleted zone will be able to induce the additory solution of Cr and/or carbon because of depletion of Cr due to the external oxidation. It is believed that the carbide free zone was induced by formation of Cr-depleted zone because Cr and/or carbon in the Cr-rich M23C6 carbide could be dissolved in Cr-depleted zone. The depths of carbide free zone were continuously increased with increasing exposure time, aging temperatures and depth of Cr-depleted zone. Eventually, it is believed that the carbide free zone will be used as quantitative indicator for the evaluation of external oxidation instead of Cr-depleted zone. Also, the carbide free zone will be induced for deterioration of mechanical properties such as creep resistance, because of dissolution of carbides limiting the grain boundary sliding. To improve the oxidation and corrosion resistance and to increase the service lifetime at high temperatures, a surface coating was carried out using Al-pack cementation techniques. The aluminide coating on Alloy 617 consisted of Ni-aluminide layers, such as δ-Ni2Al3 and β-NiAl, and inter-diffusion zone, after Al-pack cementation of high temperature high Al activity. After high temperature aging at 950oC, the Ni-aluminide layer was transformed from δ-Ni2Al3 to β-NiAl due to the outward Ni diffusion. The aluminide coating on Alloy 617 was not observed the delamination after aging at 950oC. During the high temperature aging, the aluminide coating on Alloy 617 was presented for an excellent oxidation and/or corrosion resistance by formation of protective oxide layer, which was identified to Al1.54Cr0.46O3. For the improvement of internal oxidation resistance, the microstructure modification, such as grain refinement, of Alloy 617 was carried out by recrystallization after cold rolling. The carbides in as-received Alloy 617 mainly consisted of Cr-rich M23C6 carbide. The carbides in cold-rolled Alloy 617 were distributed along the elongated grain boundary by the cold rolling, and were fractured during severe plastic deformation. The Cr-rich M23C6 carbide was also dissociated by dislocations introduced by severe plastic deformation, whereupon area fraction of this carbide was greatly decreased in cold-rolled Alloy 617. The 50% cold-rolled specimen recrystallized by annealing at 1050oC for 1 h, showed the grain size of approximately 5.2 μm. The dissociated Cr-rich M23C6 carbides were re-precipitated to the fine Cr-rich M23C6 carbides (0.5 μm) along the grain boundary during the recrystallization. The as-received Alloy 617 (AR, 71 μm) and grain-refined Alloy 617 (GR, 5.2 μm) were aged at 950oC for 2000 h in He atmosphere. The internal oxide spread along the grain boundaries via branch-like growth because the internal oxidation mainly occurred along the grain boundaries. In AR, which has larger grains, the internal oxide was formed deep in the matrix. On the other hand, the depth of internal oxides in GR, which has smaller grains, was relatively shallow in near surface. The branches of the internal oxide were densely stretched because of grain refinement, wherein branches are very close to each other. The average depth of the internal oxide increased with prolonged exposure time. The extension of the internal oxide depth in GR was approximately half of that in AR. These differences in internal oxide depth affected crack propagation under tensile stress. The longer internal oxides in AR induced the rapid crack propagation by a relatively higher stress concentration resulting in larger cracks along the grain boundary, while a number of short cracks located near the surface were observed in GR. Thus, crack propagation was restricted to GR by the even distribution of stress and reduction of stress concentration because of the densely stretched internal oxide.