Effect Of Shot Peening And Rare Earth Oxide Coating On The Oxidation Resistance Of Fe-Cr Alloys

The efficiency of conventional fossil power plants is a strong function of the steam temperature and pressure. Researches have been pursuing to increase both the temperature and pressure worldwide since the energy crisis in the 1970s. The need to reduce CO2 emission recently also becomes an incentive to improve the efficiency. Steam temperatures of the fossil power units with higher efficiency are now around 600℃. These lead to the consideration of materials capable of operating under higher stresses at ever increasing temperatures. One of the key factors to limite the materials using is their susceptibility to steam oxidation.It is inevitable that the metal is oxidized when it contacts with high temperature water vapor or steam. The higher the temperatue, the more quickly the scale grows. It results in three potential problems. Firstly, the thickness of the tube wall is decreased quickly and then the stress is increased and creep ruptures will happen. Secondly, the increased insulation of the tube material from the cooling fluid by the low thermal conductivity of the oxide scale leads to an increase in metal temperature, thus, oxidation and creep may be accelerated. The third concern is that the thicker oxide scale may spall more easily when the boiler is cooled down. On restart, the spalled oxide scale may lodge somewhere in the system with the potential for causing tube blockages. It is notable that long-term overheating failures due to flow restriction from exfoliated oxide scale is the second most important cause of boiler tube failures, thus increased attention must be paid to the oxide scale growth and exfoliation.Electrophoresis deposition of CeO2 coating and shot peening treatment were used for Fe-Cr alloy in this thesis. The purpose was to reduce the oxidation rate and to avoid or soothe the overheating failures due to oxide exfoliation.Electrophoresis deposited CeO2 coating was used for pure iron and Fe-Cr alloys such as T91, TP304H and HR3C. Shot peening treatment was used for TP304H and HR3C. The oxidation kinetics was studied under isothermal condition in water vapor. The oxide scales were then studied with XRD, SEM and EDS.The results show that the oxidation resistance of pure iron, T91, TP304H and HR3C was improved by CeO2 coating. The effect of CeO2 coating on the oxidation resistance of HR3C is the most remarkable. The oxidation resistance of TP304H with composite coating (CeO2+Cr2O3) is better than that with CeO2 coating only.Shot peening can significantly improve the oxidation resistance of TP304H and HR3C. The oxidation resistance of TP304H with shot peening is better than HR3C, and it is even better than HR3C with shot peening. The mass gain of shot peened TP304H is only 3.7% that of un-peened TP304H. The combined treatment of shot peening and CeO2 coating is superior to shot peening only.The inner surface of TP304H reheater tubes were shot peened with self-designed shot peening apparatus for small diameter tubes. The oxide scale thickness at the inner surface of shot peened reheater tube was only 3% of the untreated one after service for 7474h at 620-630℃, and no exfoliation occurred on tubes with shot peening. The effect of shot peening on the oxidation resistance was remarkable.CeO2 coating obviousely affects the oxidation behavior of pure iron, T91 and TP304H at the initial transient oxidation stage. The coating can introduce lower oxygen partial pressure, promote the formation of Cr rich oxides, and inhibit iron oxides; The Ce rich oxide band is located inside the scale after oxidation, it has no obvious effect on the diffusion process of anion and cations, so its effect on oxidation rate is unsignificant at the steady oxidation stage. Gold marker was used to define the mass transport direction in the study of T91. Analysis of the scale on T91 steel shows that Ce rich oxide band is located at the interface of the inner equiaxed layer and the outer columnar layer after oxidation, and this Ce rich band is not consistent with the original surface. A new oxide nucleating and growing site (reaction front) was induced at the inner surface of the Ce rich band. Analysis of the scale on TP304H steel shows that the adherence of the scale is improved by CeO2 coating. Ce rich oxide band is located between the inner and outer layers. CeFeO3 and CeO2 are characterized in the Ce rich band.The electrodeposited CeO2 coating can improve the oxidation resistance of HR3C greatly in water vapor. The diffusion process was changed from inward oxygen diffusion and outward iron diffusion to predominantly inward oxygen diffusion. Shot peening introduces cold worked, high energy region at the surface, Cr diffusion is accelerated along the slip bands, dislocations and copious amounts of grain boundaries at elevated temperatures, and Cr rich oxides form at the early stage of oxidation, which acts as a strong barrier against further oxidation. The formation of Fe3O4 layer is inhibited. TP304H is a kind of metastable austenitic stainless steel. Shot peening can induce martensite at the surface. The shot peening induced martensite still exists at the testing temperature. The existence of shot peening induced martensite at the surface may play an important role in Cr diffusion and affect the oxidation behavior of TP304H steel significantly.