Influence Of Surface Modification On Steam Oxidation Of Super304H

Super304H austenitic heat resistant steel, recently developed by Sumitomo Metal Industries of Japan and Mitsubishi Heavy Industries of Japan, is based on 18/8 Cr-Ni stainless steel alloyed containing about 3%Cu and a small amount of Nb, N. Super304H austenitic heat resistant steel is widely used in ultra-super critical power plants. Now service data show that Super304H has the typical structure of double oxide film when it works at less than 600 ℃ on water vapor. The outer layer film is the Fe-rich oxide, and the inner layer film is Fe-Cr spinel phase. With the extension of working time, there are a large number of holes appearing between the inner and outer oxide film, and the outer oxide film will spall on those holes. The spalling oxide blocks the superheater and reheater pipes and the water vapor flow, which may cause pipe explosion accident. With the development of 600 ℃/620 ℃/620 ℃/30-35 MPa double reheat technology, Super304H is still one of the preferred materials of superheater and reheater pipes. According to estimates, the requirements of 620 ℃ ultra-supercritical unit-will not be met unless the outer tube temperature reaches 640 ℃-680 ℃. However, the using of Super304H is inhibited by its poor antioxidant capacity.In order to study the influence of surface modification on steam oxidation of Super304H, as well as the influence of temperature and the oxygen partial pressure on steam oxidation of Super304H after surface modified. As-received Super304H (softening treatment at high temperature+cold large strain processing+ solution treatment) was cut into flakes, and in order to obtain the smooth-surfaced without coarse scratches and uniform roughness samples, the flakes were polished on sand papers. The samples were shot peening, aluminized, and then oxidation experiments were carried out at different conditions. The experimental temperatures are 620 ℃ and 650 ℃. The dissolved oxygen contents are lOppb and 5ppm. Non-continuous weighing method is used to characterize the oxidation kinetics curves. Scanning electron microscope, Electronic Differential System and X-ray diffraction are used to characterize the phase and morphology of oxide film. The results are showed as bellows.(1) The antioxidant capacity of Super304H can be improved by shot peening and aluminized. The oxidation kinetics curves of Super304H which is reacted at 620 ℃ 10ppb is a straight line. The oxide film of Super304H sample is formed by Cr2O3, and the oxidation kinetics curves of Super304H after shot peening is a parabola line. The oxide film of Super304H after shot peening is also formed by Cr2O3, and the oxidation kinetics curve of Super304H after aluminized is a parabola line. The oxide film of Super304H after aluminized is also formed by Al2O3.(2) With the steam temperature rising, Super304H oxide film is gradually transformed into a double layer structurer. The oxidation behavior of shot peening and aluminized Super304H is substantially independent of temperature.(3) The oxidation weight the morphology of oxide film and the phase of oxide film of Super304H is influenced by the oxygen partial pressure. When the dissolved oxygen contents rises to 5ppm, the oxide film of Super304H is spalling after 500h. The oxidation behavior of shot peening and aluminized Super304H is substantially independent of the dissolved oxygen contents. With the increase of the dissolved oxygen contents, k of shot peening and aluminized Super304H increases and Q of shot peening and aluminized Super304H increases.