Research On Mechanism Of Erosion Induced By Wet Steam

Liquid impact erosion is a kind of material breakage caused by the loose small particles impact on material surface with a certain speed and angle, which is the key damage form of the last stage blades of steam turbine. Since the ferric oxides will be peeled from the inside of the superheated tube and the welding spatters and residues of steam pipe will be mixed into the steam, the micro-particles inevitably exist in the wet steam, then the damage style and damage degree will be changed. Therefore, the effect of micro-particles should be considered during the wet steam erosion.Firstly, a series of experiments were performed on the self-developed experimental apparatus, and the characteristics of the surface damage induced by erosion of wet steam containing micro-particles are studied. Indentations and scratches caused by micro-particles impact and cracks caused by water-drops impact are not the only damage styles after the steel surface was eroded by wet steam containing micro-particles. Craters also appear on the surface. Further analyses indicate that micro-particle is one of the most important factors to cause the crater generation. Besides the abrasion, the additional effects enhance liquid erosion damage, and decrease the damage threshold velocity of liquid impact due to the effect of micro-particles containing in wet steam.Secondly, the formation mechanism of crater is analyzed. The lips appeared on the edge of craters and the cracks appeared around and inside of craters indicate the mechanical behaviors are the main causes to generate crater. The results of numerical calculations, elemental composition detection and experiments indicate that the forces are not from the water-drops or micro-particles impact to solid wall. The shock waves and micro-jets caused by micro-bubbles collapse near the solid wall are the resource of mechanical behaviors. The cavitation erosion occurs during the erosion of wet steam containing micro-particles. The anti-cavitation erosion performance of material surface can be enhanced significantly by increasing the surface electron affinity.Finally, by means of crater recognition and information statistics as an evaluation methodology of damage degree, the effects of micro-particle concentrations, sizes and shapes are investigated. With the increment of concentration, the number of crater increases. When the concentration is higher than 1.0 g/m~3, the number of crater not changes any more. The sizes have significant effects on erosion damage degree. When the sizes are from 0.5μm to 6.5μm, the surface damage is mainly caused by crater generation. While the sizes are bigger than 6.5μm, micro-particle's cutting is the main damage style for material surface. The shapes have little effects on the crater generation. But the damage degree caused by micro-particles impact has related to shape, and the irregular particles cause more serious damage than that of spherical particles.