Analysis of the failure of the large standard parts of Shenyang Thermal Power Station
High-temperature fastener failure testing was conducted to evaluate the hardness of the cylinder and main valve bolts. The test results showed that the hardness of the broken bolt ranged from HB=265 to 290, which is higher than the normal range of HB=240 to 270. This indicates excessive hardness, which significantly contributes to the early failure of the bolts. The materials commonly used for high-temperature fasteners include 20Cr1Mo1VTiB and 25Cr2Mo1V. From a material perspective, 20Cr1Mo1VTiB exhibits better permanent plasticity and lower creep properties compared to 25Cr2Mo1V. As a result, 25Cr2Mo1V is more prone to fracture under similar conditions. This has been confirmed through practical application data. All high-temperature fasteners in Shenyang are made from these two materials, but the failures observed were mainly related to 25Cr2Mo1V components. A metallographic analysis was performed near the fracture area using an etching solution of 25% nitric acid and 2% picric acid in alcohol. The microstructure revealed a tempered martensitic structure with clear intergranular networks. Oxidation cracks had propagated into the base material. Impact toughness tests were carried out at 400× magnification, focusing on the tempered steel structure. Samples were taken from the bolt fracture area: one group from 10–65 mm before tempering and another from 70–130 mm after heat treatment. Each group contained four samples, and the average impact toughness values were recorded. For 5Cr2Mo1V steel bolts, the impact toughness significantly decreased after tempering. For example, the main valve bolts of machine No. 3, after operating for 79,400 hours, showed a dramatic reduction in impact toughness, far below the acceptable minimum standard. This highlights the degradation of mechanical properties over time. Long-term operation in the 400–550°C temperature range leads to a significant decrease in impact toughness of high-temperature fasteners. This results in brittle fracture behavior. At the Shenyang Thermal Power Plant, the fasteners (made of 25Cr2Mo1V) operate at 535°C for extended periods. Frequent shutdowns effectively simulate multiple tempering cycles. After shutdown, the cooling process is very slow, causing the fasteners to remain in a slow-cooling condition. This further reduces the impact toughness of the tempered alloy steel. Experimental results show that 25Cr2Mo1V steel becomes severely embrittled after 50,000 hours of operation. In the case of Shenyang’s 3rd and 4th units, the bolts have operated for nearly 90,000 hours without replacement or any restoration treatment. As a result, they are under high residual stress and have extremely low ductility. Any external impact could lead to immediate fracture at stress concentration points or existing damage sites. Based on the above analysis, it can be concluded that the primary causes of high-temperature fastener fractures in the Shenyang Thermal Power Plant are material embrittlement and accumulated stress. To prevent or reduce such failures, the following measures are recommended: 1) During each unit overhaul, all high-temperature fastening bolts should be inspected as part of the maintenance plan. Bolts should be disassembled, cleaned, and lubricated regularly. Any unqualified bolts must be replaced immediately to ensure safe operation. 2) The degree of embrittlement can be assessed using hardness testing. For 25Cr2Mo1V bolts, the acceptable hardness range is between HB=240 and HB=270. If the hardness is below HB=200, the strength is significantly reduced, and the bolt should not be used anymore. For bolts with hardness above HB=285, a restorative heat treatment can be applied, and the bolt can be reused only after passing inspection. 80G Fm Radar Level Meter,80G Liquid Level Sensor,Level Monitoring Gauge,Anti-Corrosion High Precision Radar Level Gauge Jiangsu Pinpai Technology Co., Ltd. , https://www.jspingpa.com