| As a leading edge technology in the iron and steel industry, CSP(Compact Strip Production) line can produce steels continuously and automatically from molten steel to finished products. Wuhan Iron and Steel Co. imported and set up CSP line in 2009. Hearth roller, one of the key equipment in CSP line, is used for moving hot steel billets, its working temperature can reach 1050-1250°C, which is a serious challenge to materials. On this background, a new superalloy 22Co-21Ni-29Cr-2.2Nb-Fe(Co22 for short) was developed to make roller rings. This alloy has good high-temperature properties with low cost, however, the high temperature oxidation resistance of the alloy still needs further research. In this paper, some work on the isothermal oxidation and cyclic oxidation behavior of the Co22 alloy was done.Isothermal oxidation behavior of Co22 alloy without Si and Mn was investigated at 1050-1250°C in air, analyzed with scanning electronic microscopy and X-ray diffraction, and the oxidation kinetics were investigated by weighing method. The results show that oxidation mass gain of the samples at 1050, 1150 and 1250°C followed the secondary parabolic law, and the oxidation rate constants at each temperature were 1.02×10-10, 5.13×10-10, and 2.95×10-9 g2cm-4s-1, respectively. The oxide layers formed on all specimens consisted of Cr2O3,(Fe,Ni,Co,Cr)3O4 spinels and CrNbO4. A continuous Cr2O3 layer formed exactly on top of substrate as the inner layer, while the(Fe,Ni,Co,Cr)3O4 spinels grown on the surface of Cr2O3 layer, even formed a continuous but non-protective outer layer. The mechanism of oxide layer formation is suggested to be a three-stages process: firstly, Cr2O3 is formed as a continuous and protective layer, then Fe, Co, Ni, Nb ions diffuses to scale-gas interface to form spinels and nodules, finally, a continuous, but porous(Fe,Ni,Co,Cr)3O4 spinels layer is formed as an outer layer.Cyclic oxidation behavior of three Co22 alloys with different Si or Mn contents was investigated at 1050°C for 60 cycles(1h oxidation and 30 min cooling) in air, analyzed by weighing method, scanning electronic microscopy and X-ray diffraction. The results show that the mass of three alloys all increased, in the form of parabolic, in the early period of oxidation, then decrease occurred, but in the later period, the situation changed, the mass of Co22 alloy with 1.5wt.% Si and 1.0wt.% Mn slowly increased with the increase of cycles, and the mass of alloy with 1.5wt.% Si and 2.0wt.% Mn slowly decreased, while two-stage of quick drop was observed on the oxidation of the alloy with 0.5wt.% Si and 2.0wt.% Mn. After oxidation, a continuous layer of Cr2O3 was formed as the main body of protective oxide layers, SiO2 was distributed discontinuously at the oxide layer-substrate interface and even penetrated into the matrix, while spinels, containing Cr, Mn, Fe, Co, Ni and so on, formed on the top of Cr2O3 layer. Si can promote the formation of SiO2, thereby enhancing the adhesion of oxide layer and substrate which could improve the resistance of cyclic oxidation. Mn has some significant beneficial effect on the oxidation resistence in the early period of oxidation, but it also contribute to the spallation of the oxide layer. In this study, the Co22 alloy with 1.0wt.% Mn and 1.5wt.% Si has the best cyclic oxidation resistance in the oxidation at 1050°C. |
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