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High-temperature fastener failure test study hardness We test the hardness of the cylinder and the main valve bolt. The test result is that the hardness of the broken bolt is between HB=265~290, and the normal value is HB=240～270, indicating high hardness. It is an important reason for accelerating bolt breakage.
The materials used in high-temperature fasteners are mostly 20Cr1Mo1VTiB and 25Cr2Mo1V. From the material point of view, 20Cr1Mo1VTiB has better permanent plasticity than 25Cr2Mo1V, and creep creep is small, so 25Cr2Mo1V is easier to break than 20Cr1Mo1VTiB. This is also proved by practical application results. The materials of Shenyang hot and high temperature fasteners are all of the above, but the fracture failure is 25Cr2Mo1V.
Tissue observation was taken near the fracture, etched with 25% nitric acid, 2% picric acid alcohol solution, and organized into tempered shellfish. Metallographic structure test was performed, and the obvious network boundary was observed. Oxidation cracks have spread to the substrate. Impact toughness 400× metallurgical structure tempering impact on steel impact toughness Sampling (before tempering) at 10 to 65 mm from the bolt fracture, and sampling at 70 to 130 mm from the bolt fracture after heat treatment (after tempering) Do the impact test (4 in each group, average) and the results.
5Cr2Mo1V steel bolts before and after tempering steel toughness bolt name and state tempering treatment situation impact toughness (J / cm2) hardness 3 machine main valve bolts, after running 79400h fracture tempering before tempering is not difficult to see, the impact toughness of broken bolts has been Far below the standard lower limit allowable value.
Failure of high-temperature fasteners The steel has a significantly reduced impact toughness after long-term operation in the 400-550 °C region, and the fracture phenomenon is brittle fracture. The fastening bolts of the Shenyang Thermal Power Plant (material 25Cr2Mo1V) are operated at 535 °C for a long time, and the hundreds of shutdown groups are equivalent to tempering the fastening bolts several times, and the temperature of the components after the furnace shutdown is lowered. Very slow, causing the high temperature fastening bolts to be in a slow cooling state, and the impact toughness of the tempered alloy steel will be significantly reduced under the slow cooling state. The experiment shows that the 25Cr2Mo1V steel has a serious degree of embrittlement after running for 50,000 hours, while the high temperature bolts of the 3 and 4 furnaces of Shenyang Thermal Power Plant have been running nearly 90,000 hours, and these bolts have never been dismantled and replaced since they were put into operation. , and no restorative treatment has been carried out, so the bolt itself has a large stress and extremely low brittleness. If there is an external impact, the bolt can be broken at the stress concentration or damage.
Through the above analysis, we can conclude that the cause of high temperature fastener fracture in Shenyang Thermal Power Plant is caused by material embrittlement and stress. Measures to prevent or reduce the failure of high-temperature fasteners 1) Each time the unit is overhauled, the existing high-temperature fastening bolts should be checked and repaired as necessary inspection and maintenance items, and disassembled, cleaned and lubricated on time. Replace the unqualified bolts completely and replace them when conditions are not allowed to ensure safe operation. 2) The method of checking the degree of embrittlement can be tested by the hardness test method: for the bolts of 25Cr2Mo1V steel, the hardness value should be between HB240 and HB270. For the bolts with HB<200, the strength is greatly reduced, and it is not suitable to continue using it. . For the bolts with HB>285, the method of restorative heat treatment can be adopted, and it can be used after passing the inspection.
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