High-temperature bolts are one of the important components in steam turbine equipment. They are connected with high-temperature components such as steam turbine cylinders, main steam valves, regulating valves and steam pipes. It is necessary to ensure that the unit does not leak during operation. A company has recently broken high-profile door and bolt bolts, which seriously threatened the safe operation of the unit. Fortunately, the unit was found to be timely during the maintenance period, and did not cause brittle fracture of the bolt to injure people. The failure of high temperature fasteners is worth learning.
1 event passing and processing
In the maintenance process of a #4 unit of a power plant, the M20×80 high-profile door bonnet bolts (see Figure 1) were found to be broken, of which 13 were 4-1 door-closing, 11 were 4-2, and 13 were 4-3. 4-4 There are 8 broken doors, a total of 45, and the broken bolts reach 70%. The power plant randomly sent the broken bolts for inspection and replaced all 64 bolts. In response to this unsafe situation, the power plant conducted a similar problem check on the spare #2 unit and found that there were also bolt breaks (10), of which 2-3 high-profile door bolts broke the most, reaching 7.
figure 1
figure 2
2 reasons analysis
The #4 unit of the power plant has a total operation of 24,906 h, starting and stopping 9/9 times. The design material of the high-profile door bonnet bolt is 20CrlMolVTiB, and the macro-fracture has a brittle fracture morphology (see Figure 2), which has an overload fracture tendency. The crack starts at the tip of the first threaded thread. The fracture is rough and crystallized. Some of them belong to the old crack. The surface has severe rust and no metallic luster. The other part is a new fracture with metallic luster. It is a crystalline fracture. This phenomenon indicates that the bolt has cracks before the fracture, which is the fracture that occurs after the crack develops to a certain extent.
Physical and chemical analysis results: 1 bolt chemical composition analysis found that the content of alloying elements are within the standard requirements, consistent with the design material; 2 bolt hardness test results are between 180-200HB, far below the standard requirements (255-293HB) The mechanical properties are obviously reduced; 3 the fracture bolts are sampled and sent for inspection, and the metallographic structure is coarse and the grain size is 2 grade.
In summary, the coarse crystals appear in the 20CrlMolVTiB bolt production process, resulting in a decrease in creep fracture toughness. Under high temperature and long-term operation, the mechanical properties are further reduced, and the performance does not meet the operational requirements, resulting in large-area fracture of the bolt.
3 high temperature bolt 20CrlMolVTiB fracture factor
3.1 has creep brittleness
Due to its own characteristics, 20CrlMolVTiB bolts have serious tissue heritability, and they often appear coarse crystal phenomenon in the production process, that is, polygon particle spots with different color and brightness under different angles of light appearing in the low-power test. Block (mixed crystal). The presence of coarse crystal grains inevitably leads to a decrease in creep fracture toughness, and the use performance cannot meet the operational requirements.
3.2 Stress concentration effects
The thread of the bolt is equivalent to the notch. If the thread processing quality is not good, the radius of the root of the thread is too small, so there is a high stress peak at this sharp notch. At the same time, the bolt structure design is unreasonable, and there is no rounded corner in the transition zone between the bolt and the working neck, causing severe stress concentration on the first thread of the thread. According to the data, the stress concentration on the first thread is equivalent to about 50% of the total stress.
3.3 Operational impact
If the unit starts and stops repeatedly and the running load is unstable, the turbine operating temperature will fluctuate, so that the temperature gradient will change along the length of the bolt, resulting in certain thermal stress, resulting in low thermal fatigue and cyclic fatigue. If frequent start and stop and load changes The bolt will be subjected to creep and fatigue interactions.
3.4 Influence of bolt deflection during installation
When the bolt is installed, the bolt is not perpendicular to the flange, or the joint surface of the nut and the washer is skewed, and the unevenness of the flange is uneven, which causes the bolt to be skewed after being tightened. Under the influence of the deflection, the bolt will have a large bending stress, resulting in uneven axial stress along the circumference of the bolt.
4 preventive measures
The procurement of bolts should be carried out at a fixed point. The metal supervisors should understand the production process and quality assurance system, and communicate with the professionals of the manufacturer to ensure that the bolt production and inspection are carried out according to the requirements, thus improving the qualification rate of new bolts entering the factory. New bolts must be inspected before storage, and can not be inspected without inspection. Strict implementation of DLT438 standard requirements and company arrival acceptance system. Since 20CrlMolVTiB steel bolts have serious genetic heritability, it is recommended to increase the proportion of grain level inspection and thoroughly control the grain. Coarseness causes brittle fracture of the bolt.
4.1 Strengthen the supervision of the unit maintenance bolts
After long-term operation of the bolt, physical and chemical inspection is required to detect defects in time and master the change of the structural properties of the material. Focus on the following:
(1) 25Cr2MoV and 25Cr2Mo1V steel bolts with a steam temperature of 510 ° C or higher and a tendency of hot brittleness;
(2) bolts that have not been broken in the broken bolt group;
(3) For bolts with frequent disassembly and assembly, the inspection cycle should be shortened;
(4) high-pressure cylinder high-temperature section bolts, speed control valves and main valve bolts;
(5) Pre-operation inspection found bolts with abnormal hardness and black mesh austenite grain boundaries;
(6) For high temperature bolts greater than or equal to M32, 100% non-destructive testing (ultrasonic flaw detection, magnetic particle inspection, penetrant inspection, endoscope observation center hole) shall be dismantled;
(7) Strictly implement the requirements of DLT438 standard, creep measurement, material compounding, hardness testing, ultrasonic testing and metallographic analysis;
(8) In any case, the broken bolts shall be dissected and failed.
(9) For the high temperature bolts used in the peaking unit, the inspection ratio should be increased.
4.2 Strict implementation of bolt replacement and scrapping system
1) The bolts that meet the following conditions after the operation test shall be replaced. 1 hardness exceeds the standard; 2 metallographic structure has obvious black mesh austenite grain boundary; impact energy of 3U-shaped notch: speed control valve bolt and bolts loaded and unloaded by torque method, Ak≤47J; Bolts for loading and unloading hydraulic tensioners, Ak≤24J;
2) Bolts that meet one of the following conditions shall be scrapped: 1 the creep deformation of the bolt reaches 1%; 2 the bolts have been found to be cracked; 3 the hot brittleness occurs, and the bolt replacement is achieved; 4 the shape is seriously damaged and cannot be repaired and restored; 5 bolts in the center of the local burn melt.
5 summary
In the process of large-scale repair or daily maintenance, the bolts below the M32 specification are generally under-resourced, and the inspection items are single. The inspection standards before and after the operation are too low. It has always been a blind spot for metal supervision and control in the industry. management.
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