The other half of the fracture can be seen on the flange of the fixed valve cover base, which is the actual arrangement of the 16 bolts. Taking the east-west direction as the ordinate and the north-south as the abscissa, it can be seen that the northeast side, that is, the fourth quadrant position is the position of the bolts of No.3 and No.8, the mechanical test results of the bolt-based fracture bolts of No.15 and the bolt numbering test of the corresponding fracture characteristics. Temperature b/MPa0.2/MPa5/%ak/(Jcm-2)akv/(Jcm-2) Hardness HB fracture characteristics 1540°C575～58017.0～17.572.0, 73.5 first break, flush, lighter stripes 2 room temperature 59272622540°C143 is more broken, flush, and clear stripes 5 room temperature 93483017.058.0 later broken, two sections 6 room temperature 82567918.069.0332296540 °C56517.076.0146 first break, flush, clear stripes 9 room temperature 81067216.055.030152349540 °C59017.079 .5 first break, flush, radiation stripe clearing 10 room temperature 562626210540 ° C 50, 55 first break, flush, radiation stripe clear 11540 ° C 130, 125, 130262 later broken, two sections 12 room temperature 255 after the break, three Section 13 room temperature 83069918.066.054 ~ 882123513540 °C70016.568.061 first break, flush, clear stripes clear 14 room temperature 87075920.065.0551026214540 °C74516.566.0 is earlier than the first, flat, stripe shallow 16 room temperature 79469817.061.0 later broken, two A section of the note: (1) Blackbody data indicates failure; (2) akV data has no standard and is for reference only.
The 16 bolts in the fracture are actually in the east, so the west side of the corresponding second and third quadrants is the first fault zone. This coincides with the direction in which the door cover flies out. Most of the fractures, that is, the macroscopic features of the fractures are obvious radial stripes. Some sources of fracture are easy to discern, and are located at the emission point of the radial stripe at the root of the thread, while others are not obvious. These macroscopic features are typical brittle fractures.
The mechanical properties test carried out the mechanical properties test after the intact part of the bolt fracture was cut. See the sampling conditions of the bolts at room temperature and high temperature mechanical properties, the corresponding fracture characteristics and test results. Among them, the impact test has two types of gaps, U-shaped and V-shaped. It is the standard value of mechanical properties of 20Cr1Mo1V1 material.
20Cr1Mo1V1 standard value standard b/MPa0.2/MPa5/%ak/(Jcm-2) hardness HBDL439-91 room temperature 735637156069248～277 (manual) 540/°C652～65710.0～10.573.5～76.0127～1422.1 room temperature mechanical properties test spot check The tensile strength of all the 6 samples was acceptable; the elongation of the plasticity index was acceptable, but the reduction of the area was 2 unqualified.
Among the 7 samples of the bolt hardness, 3 were qualified and 4 were unqualified and low. The reason for the analysis is that the hardness is on the impact sample, and the impact sample is taken from the core of the bolt, so the hardness value is low. Only one of the five samples with room temperature impact toughness was qualified, and one of the titanium-containing elements was used. The other four 20Cr1Mo1V1 were unqualified (the results of the two notch forms), and both were lower than the standard by more than 10%, and the lowest was 9 The impact toughness of the bolt is only 43.5% of the standard, which is lower than the standard value of 56.5%.
Anti-failure measures and recommendations (1) Check whether the raw material inspection certificate of the batch of bolts of the manufacturer is qualified and complete; check the heat treatment during the manufacturing process, the heat treatment curve of the bolts of the batch, etc.; check the factory inspection report and the certificate of conformity.
(2) Strengthen the supervision work of all links in the manufacture of bolts. According to the regulations, the raw material inspection report, re-inspection report, processing quality, process and temperature curve are supervised and inspected. Strictly check the quality certification documents at the time of delivery and keep them on file for future reference.
(3) Before installation and use, high-temperature bolts shall be strictly inspected according to the inspection items required by relevant regulations. Bolts suspected of having problems, such as 20Cr1Mo1V1 bolts with abnormal metallographic structure, may be subjected to destructive mechanical performance tests, with emphasis on impact toughness.
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