Analysis of the failure of the large standard parts of Shenyang Thermal Power Station
High-temperature fastener failure analysis focuses on the hardness of cylinder and main valve bolts. The test results show that the hardness of the broken bolt ranges from HB=265 to 290, while the normal value is between HB=240 and 270, indicating an abnormally high hardness. This increased hardness significantly contributes to the early failure of the bolts. The materials commonly used for high-temperature fasteners are 20Cr1Mo1VTiB and 25Cr2Mo1V. From a material perspective, 20Cr1Mo1VTiB exhibits better permanent plasticity and lower creep resistance compared to 25Cr2Mo1V. As a result, 25Cr2Mo1V is more prone to fracture, which has been confirmed through practical applications. All fasteners in Shenyang use these materials, but the observed failures mainly involve 25Cr2Mo1V bolts. Metallographic analysis was conducted near the fracture area using a 25% nitric acid and 2% picric acid alcohol solution for etching. The microstructure revealed a tempered shellfish pattern, with visible network boundaries. Oxidation cracks were found to have extended into the substrate. Impact toughness tests were carried out at different positions along the bolt: samples were taken 10–65 mm from the fracture before tempering, and 70–130 mm after heat treatment (after tempering). Each group had 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 showed a dramatic drop in impact toughness after operating for 79,400 hours. The impact toughness of the broken bolts fell far below the acceptable minimum standard. Long-term operation of high-temperature fasteners in the 400–550°C range leads to a significant reduction in impact toughness, resulting in brittle fractures. In the Shenyang Thermal Power Plant, the fasteners made of 25Cr2Mo1V operate at 535°C for extended periods. Each shutdown cycle is equivalent to multiple tempering treatments, and the slow cooling process after furnace shutdown further exacerbates embrittlement. Experimental data shows that 25Cr2Mo1V steel becomes severely embrittled after 50,000 hours of operation. The bolts in units 3 and 4 have operated for nearly 90,000 hours without replacement or restoration, leading to high residual stress and extremely low toughness. Any external impact could cause failure at points of stress concentration or existing damage. Based on the above analysis, the primary causes of high-temperature fastener failure in the Shenyang Thermal Power Plant are material embrittlement and accumulated stress. To prevent or reduce such failures, several measures should be implemented: first, during each overhaul, high-temperature fastening bolts should be inspected and maintained as necessary. They should be disassembled, cleaned, and lubricated regularly. Unqualified bolts must be replaced immediately to ensure safe operation. Second, the degree of embrittlement can be assessed through hardness testing. For 25Cr2Mo1V bolts, the ideal hardness range is HB 240–270. If the hardness drops below HB 200, the strength is greatly reduced, and the bolt should not be used. If the hardness exceeds HB 285, restorative heat treatment can be applied, and the bolt can be reused after passing inspection. Electromagnetic Flowmeter,Ultra Mag Flow Meter,Electromagnetic Meter,Electromagnetic Flow Sensor Jiangsu Pinpai Technology Co., Ltd. , https://www.jspingpa.com