Fracture Cause of Turbine High-Temperature Bolt

With the continuous improvement of domestic thermal generator assembly capacity and thermal efficiency, the steam pressure of steam turbine is also getting higher and higher, which also puts forward higher requirements for the material performance of high-temperature components of steam turbine.

For example, high temperature bolts on steam turbines play an important role in ensuring the air tightness of the cylinder split.

High temperature bolts are generally made of heat strength materials with excellent high temperature performance.

20Cr1Mo1VNbTiB steel is a pearlite heat strength steel, which has high endurance strength and good anti relaxation performance, and is commonly used to manufacture high-temperature bolts for steam turbines.

During the operation of the generator unit in a power plant, the high-temperature bolt of the gate was broken and failed.

The unit has been operating for 47341h, the bolt material is 20Cr1Mo1VNbTiB steel, and the bolt specification is M56mm × 4mm × 310mm.

Researchers conducted a series of inspections and analyses on the broken bolt, and selected another unbroken bolt of the same batch and material for comparison to find out the reason for the fracture of the high-temperature bolt, hoping that similar accidents would not occur again.

1. Physical and chemical inspection

1.1 Macro observation

Fracture Cause of Turbine High-Temperature Bolt 1

Fig. 1 Macro appearance of the broken bolt

Fracture Cause of Turbine High-Temperature Bolt 2

Fig. 2 Macro morphology of fracture surface of the fractured bolt

The overall morphology and section morphology of the broken bolt are shown in Figure 1 and Fig. 2.

It can be seen that the fracture is at the bolt rod, and the distance from the fracture to the end face is about 130mm.

The section is flat, granular, without plastic deformation, and it is a typical brittle fracture.

By observing the external surface of the bolt, it can be seen that the thread surface is smooth without dents, cracks, rust, burrs or other defects that cause stress concentration.

1.2 Chemical composition analysis

Wire cutting sampling was carried out in the middle of the screw of the broken bolt and the comparison bolt.

After the cutting surface was smoothed on sand paper and cleaned with alcohol, the full spectrum vertical direct reading spectrometer was used to analyze its chemical composition.

From the experimental results, it can be seen that its chemical composition meets the composition requirements of 20Cr1Mo1VNbTiB steel in DL/T 439-2018 Technical Guidelines for High Temperature Fasteners in Fossil Fuel Power Plants.

1.3 Mechanical property test

Take hardness sample, tensile sample and U-notch impact sample on the broken bolt and unbroken bolt respectively, conduct tensile test at room temperature with universal material testing machine, measure their tensile strength, yield strength and elongation after fracture, measure their impact absorption energy at room temperature with impact testing machine, and measure their Brinell hardness with Brinell hardness tester. See Table 1 for test results.

Table 1 Mechanical Property Test Results of Bolts

Sample

Yield strength Rp0.2/MPa

Tensile strength Rm/MPa

Elongation after fracture A/%

Impact absorbed energy/J

Hardness/HB

End face   Transverse load surface 20mm from the end face

Measured value of broken bolt

953

1071

15

25

290

278

Measured value of unbroken bolt

952

1066

17

86

266

269

standard value

≥735

≥834

≥12

≥39

252~302

The results show that the hardness, tensile strength, yield strength and elongation after fracture of the bolt meet the standard requirements, but the impact absorption energy of the broken bolt is only 25J, which is far lower than the standard requirements, indicating that the material of the broken bolt is brittle and has low resistance to impact load.

The impact absorption energy of unbroken bolt is 86J, meeting the standard requirements.

1.4 Macro organization inspection

The end faces of fractured bolts and unbroken bolts and the cross sections at the fracture surfaces of fractured bolts shall be examined for macrostructure according to the recommended method in DL/T 439-2018, as shown in Fig. 3.

Fracture Cause of Turbine High-Temperature Bolt 3

Fig. 3 Macro morphology of bolt end face

Under the light of different angles, the fractured bolt end face and the cross section at the fracture surface all present polygonal particle patches of different colors and light brightness, and coarse grains can be seen by naked eyes.

The average diameter of grains can be seen to be more than 2mm by using a magnifying glass, that is, the microstructure of the fractured bolt is composed of macro coarse grains.

The formation of coarse grains in 20Cr1Mo1VNbTiB steel is related to production processes such as manufacturing and heat treatment, and belongs to overheating structure.

Coarse grains can be formed when the temperature during hot working reaches above Ac1 (the initial temperature for pearlite to austenite transformation), or when the steel is heated several times to above the austenitic recrystallization temperature and then cooled rapidly.

Under the long-term high temperature and high pressure environment, the carbide in the overheated structure gradually precipitates, which will reduce the impact toughness of the material.

1.5 Metallographic inspection

Take samples from the bolt and screw for metallographic inspection, as shown in Fig. 4.

Fracture Cause of Turbine High-Temperature Bolt 4

Fig. 4 Microstructure of bolt

It can be seen that the microstructure of the fractured bolt is bainite, the grain size is grade 1, and the row bainite in the crystal is cross distributed, showing a frame structure.

The microstructure of the unbroken bolt is fine crystalline bainite with a grain size of grade 5.

The metallographic inspection results show that the grain size of the broken bolt does not meet the requirements of the DL/T 439-2018 Technical Guidelines for High Temperature Fasteners of Dali Power Plant that the grain size is Grade 5.

1.6 Fracture analysis

The fracture morphology of the broken bolt was observed by scanning electron microscope.

Fracture Cause of Turbine High-Temperature Bolt 5

Fig. 5 Fracture Micromorphology and Energy Spectrum Analysis Results of Broken Bolts

The fracture morphology is shown in Fig. 5a), and the fracture surface has been completely covered by dense oxide.

The oxide is Fe2O3 through energy spectrum analysis, and the energy spectrum (EDS spectrum) is shown in Fig. 5b).

Due to the fracture of the broken bolt during operation, the fracture surface is exposed to high temperature for a long time, resulting in the formation of dense oxide film on the fracture surface.

Therefore, it is difficult to directly observe the fracture information of the bolt from the fracture surface.

The researchers observed the fracture morphology of materials through the fracture surface of tensile and impact samples, and judged the fracture mode of bolts.

Fracture Cause of Turbine High-Temperature Bolt 6

Fig. 6 Fracture Micromorphology of Tensile and Impact Specimens

The fracture surfaces of tensile and impact samples were observed under scanning electron microscope.

The fracture morphology of tensile and impact samples is shown in Fig. 6.

Figures 6a) and 6b) show the tensile and impact fracture morphology at low magnification.

It can be seen that the shear lip area of the tensile fracture section is very small, accounting for about 15% of the fracture area.

The whole section is dominated by radiation area, and there is basically no fiber area.

The toughness of the material is very poor according to the macro morphology of the fracture.

Figure 6c) shows the tensile fracture morphology under high magnification observation.

The fracture surface is mainly cleavage, with a small amount of dimple inclusions.

The whole fracture surface morphology conforms to the quasi cleavage fracture characteristics.

Fig. 6d) shows the impact fracture morphology under high power observation.

It can be seen that the section is basically a radiation area, and the entire section is mostly a cleavage surface.

Fig. 6 shows that the fracture of the sample belongs to cleavage fracture, indicating that the material is very brittle.

2. Analysis and discussion

It can be seen from the above physical and chemical inspection results that the chemical composition, room temperature tensile property, hardness and other indicators of the bolts meet the standard requirements.

The fracture bolt structure has coarse grains, the grain size is grade 1, and the row bainite cross distributed in the grains is a frame structure, which leads to the material brittleness;

The fracture characteristics of tensile and impact fracture surfaces show that the material is cleavage fracture, and the impact absorbed energy is far lower than the standard requirements, which again proves that the material is very brittle.

High temperature bolts of steam turbine work under complex working conditions such as high temperature, high stress and steam corrosion, which requires bolt materials to have high high temperature creep endurance strength, low linear expansion coefficient, good anti relaxation performance, good stress corrosion resistance, low notch sensitivity and good oxidation resistance.

However, the fractured bolt material has coarse grains, so its abnormal structure leads to excessive brittleness of the material and can not withstand the impact stress caused by startup, shutdown and unit load fluctuation.

3. Conclusions and Suggestions

The high-temperature bolt of the turbine has coarse grains, and has been in service for a long time under the high-temperature and high-pressure environment, resulting in the gradual precipitation of carbides and the decline of material impact toughness.

Under the impact stress caused by unit startup, shutdown and unit load fluctuation, the high-temperature bolt has brittle fracture.

It is recommended that thermal power plants conduct 100% ultrasonic inspection and 100% hardness inspection on turbine high-temperature bolts during unit maintenance, and replace the bolts that fail to pass the ultrasonic inspection and hardness inspection.

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