| At present, the martensitic precipitation hardening stainless steel has been used as the main structure material of aviation, compared with domestic aluminum alloy and structural steel, with unmatched performance.15-5PH stainless steel, due to its good heat resistance, high strength and long service life at high temperature, and has been widely used in aerospace, the manufacture of aircraft parts and other components. But,15-5PH steel causes oxide layer peeling off and instability of oxidation phenomenon at high temperature oxidation environment, which will cause corresponding parts losing efficacy. So this article mainly discusses the oxidation behavior of high temperature oxidation environment of the15-5PH steel, and hope the research results can provide strong theoretical support for the steel using in our country in aerospace and other fields of application data, lay the foundation for the popularization and application in China.First, using the experiment device of high temperature oxidation resistance furnace, and using the discontinuous weighing method to researching oxidation kinetics of15-5PH steel at high temperature, at the same time, studing the oxide of the steel structure and its formation mechanism. Then using XL-30ESEM environmental scanning electron microscopy to observe microstructure of oxide film, with a Kevex spectrometer analysis of various elements on the oxide layer, use the D8Advance (Super speed) diffraction analysis phase of oxide layer.The oxidation experiment results show that, the15-5PH steel at450℃.600℃and700℃temperature oxide oxidation kinetics equation follows the parabolic law,△m=ktz. In the process of oxidation, oxidation peeling phenomenon has not appeared at700℃and below temperature, showing the good oxidation resistance. But at800℃,after50h, the oxide layer appears local peeling phenomenon and instability oxidation.The oxide film (layer) growth pattern of15-5PH steel can be summarized as, in15-5PH steel, compared with oxide reaction element, such as Fe and Ni, the affinity between Cr and O is higher, so the Cr oxidizing at the start of the oxidation. In oxidation of initial period, the oxide film thickness increases gradually. Under700℃and below tempreture oxidation environment, along with the Cr oxide content increasing gradually in the oxide film, when complete oxidation film coating on metal substrate surface, because of the Cr oxide with spinel structure has good high temperature oxidation resistance, preventing oxygen further contact with the metal substrate, so the reaction gradually slow down.In15-5PH steel oxide layer, compared with chromium ions, iron ions are easier to spread, especially at the environment of800℃. With iron ions quickly spread to outer oxide film, due to oxygen directly into the internal, forms porous and containing chromium oxidation film; Iron spreads outward along the oxide grain boundary diffusion, forms oxide film, on the surface of the oxide film, where is easy to forming large particles oxide pellets with thorn on its shape, when these oxide pellets of large particles further development, gradually gathered together into a large piece of flake oxide layer, and the stress from the oxide layer will result micro cracks, and damages the integrity of the Cr2O3easily. In addition, the metal matrix is beginning to happen austenitizing at800℃, Cr ions in the martensite matrix diffusion velocity is greater than which in the austenite, so the chromium content of oxidation layer near15-5PH steel substrate reduces sharply by its diffusion, without enough amount of chromium diffusion to intimal oxide layer, can not form a sufficient quantity of chromium oxide, so it’s unable to form a complete good oxidation resistance oxidation film. With destruction and regeneration of the oxide film happening alternately, makes instability oxidation at800℃. This shows that the instability of oxidation is the cycle of destruction and reconstruct of oxide film with good high temperature oxidation resistance. |
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