KEYWORDS: Chemical analysis, Particles, Scanning electron microscopy, Analytical research, Statistical analysis, Crystals, Testing and analysis, Spherical lenses, Chemical elements, Absorption
The high-temperature fastening bolt of the high-pressure main steam valve of the steam turbine of 330MW unit in a thermal power plant breaks during operation. In this paper, the causes of bolt fracture are analyzed by means of macro morphology observation, fracture SEM detection, microstructure detection, mechanical property detection and chemical composition detection. The results show that during the long-term high-temperature service, the matrix structure of the fastening bolt of the high-pressure main throttle valve is aging and carbide particles are gradually precipitated. With the increase of operation time, the carbide particles continue to aggregate and grow up and distribute along the grain boundary, and a large number of creep holes and creep cracks are formed on the grain boundary. A large number of coarse carbides and creep cracks lead to the decline of mechanical properties of bolts, especially the deterioration of impact toughness and tensile strength; The accumulation of creep cracks becomes the crack source. Under the impact load formed during the action of high-pressure main throttle valve, the stress concentration part of the bottom of the first thread of the bolt external thread cracks, and the cracks continue to expand, resulting in the brittle fracture of the bolt.
Bearing bush spring is very important to the economic and stable operation of steam turbine. The supporting spring of turbine rotor bearing bush of 330MW unit in a thermal power plant broke during operation, resulting in unit shutdown. In order to find out the cause of spring fracture, this paper adopts macro morphology observation, fracture SEM detection, microstructure detection and chemical composition detection. The results show that the spring has original crack defects. In addition, the spring squeezes and wears with the inner wall of the mounting hole during operation, resulting in the damage of the outer surface of the steel wire. Moreover, this position is where the maximum shear stress is borne, so the crack source is initiated on the surface of the steel wire. Under the cyclic variable load caused by the vibration of steam turbine bearing bush, the crack continues to expand in the form of fatigue, and the effective bearing area of spring continues to decrease until fracture occurs.
The connecting nut between phase B drain valve and gas tank of 51DK circuit breaker of high voltage shunt reactor of No. 1 bus in a substation was broken and failed. In order to find out the cause of fracture, the fractured brass connecting nut was comprehensively detected and analyzed by means of appearance morphology analysis, chemical composition analysis, microstructure detection and fracture micro area analysis. The results show that the chemical composition of brass nut is unqualified, the content of Cu element is low, and the content of Pb, Fe and impurity elements is high, which is easy to produce stress corrosion tendency, and the overheating structure is formed due to improper forging process, resulting in poor strength and toughness of the nut; Under the combined action of sulfide and stress, the stress corrosion cracking of externally threaded pipe occurs at the stress concentration part of the bottom of the first circle of thread, and gradually expands, so that the effective bearing area decreases, and finally breaks under the action of internal working pressure.
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