|
18-8 series improved coarse grain austenitic stainless steel for its good high temperature strength, corrosion anti-flue gas oxidation and relatively low cost has been the favored of the world, is widely used in high-temperature heating surface tube row (including the final superheater and reheater). However, this type of austenitic stainless steel in high temperature steam environment sometimes serious steam oxidation and subsequent oxidation of the skin area of ??spalling problem, resulting in the superheater and reheater tube overheating tube rupture caused by the accumulation of oxide clogged abnormal wear occurs or nozzles, blades, impellers divisions caused due to the erosion of the oxide particles carried in the steam of the steam chamber member. This article mainly oxidation behavior in the environment of high temperature steam medium of TP347H, and TP304H Steel research, I hope to help give TP347H, and TP304H steel power plant safe operation. First Thermal Power Research Institute indigenously developed laboratory steam oxidation unit, high-temperature steam oxidation kinetics of TP347H and TP304H steel continuous weighing, studies show that as the temperature rises, TP347H and TP304H steel oxidation corrosion gradually worsened. At 560 ° C, the temperature of 590 ° C and 620 ℃ TP347H, and TP304H steel oxidation kinetics equation follow the Δm = ktz parabolic law. The above two kinds of steel and at these temperatures during the oxidation process no spalling of the oxide layer. And 590 ℃ to 620 ℃ the TP347H, and TP304H steel unit area weight the fastest growing. Then by means of scanning electron microscopy and energy dispersive spectroscopy in-depth understanding of the oxidation of TP347H and TP304H steel corrosion products (oxide layer), the organizational structure and the formation mechanism. The study found growth mode: of TP347H and TP304H steel surface oxide first formed on the surface of particulate oxides, followed by the oxide surface to generate crystals of the object, these crystal lateral growth to form a continuous oxide film, crystalline oxides further grown, started the longitudinal growth of small holes formed between the crystalline oxide, crystalline oxides continue to grow with time, between the holes disappear, then the crystalline oxide generate a large number of whiskers. And with increasing oxidation temperature, the faster this growth process. This shows that as the temperature rises, the antioxidant capacity of the metal weakened. The TP347H and TP304H inner wall oxide is constituted by two layers of oxide, the outer oxide relatively loose, the outer oxide of Fe2O3, the inner oxide densification, the inner hygiene is the composition of the spinel oxide (Fe, Cr) and NiO ; the interface between the inner layer and the outer layer of oxides corresponding to the original metal surface between the inner and outer layers under normal circumstances by the sizes of the holes are connected with the oxidation time, the increase in the thickness of the oxide layer. The growth process of TP347H, and TP304H steel can be summarized as follows: oxide metal interface uneven, uneven thickness of the inner layer of oxidation to the metal substrate to the crater-like growth. Growing oxide pit in front of non-suppression film pits on both sides of the suppression film, oxidation pits forward oxidation rate significantly greater than the oxide, pit morphology was open to both sides of the oxidation rate. When the oxide pits in front of the growth of grain boundary, the grain boundary Cr atoms fast oxidation pits in front of the formation to inhibit membrane, oxidation pits into stagnation period, its form was closed. Inhibition of film containing high Cr suppress the poor Cr with the presence of film in front of metal and Ni-rich belt. In general, the dimples on both sides of suppressing the thickness of the film are greater than the oxidation of Cr and the amount of Ni and the front metal depleted CR and the thickness of the Ni-rich zone and extent of the pit in front of inhibition of the thickness of the film-containing, containing Cr and the amount of Ni and the front the metal poor Cr and Ni-rich with the thickness and extent. Two oxidation pit side of confluence, unoxidized metallic particles are often left behind. The uneven distribution of chemical elements of the dimple-like oxidized inner oxide large, the inner oxide the residual broken suppression film exists, which indicates that the oxidation process is an oxidation inhibiting film is formed on the leading edge, and is beyond the process.
|