Abstract: The IN alloy bolts used in the reheat valve of the steam turbine of the ultra-supercritical unit are frequently fractured, and the fracture forms are diverse. In order to study the factors affecting its fracture behavior, the durable fracture test of the bolt material under different stresses and temperatures was carried out, and the metallographic examination and fracture analysis of the fracture sample were carried out. The results show that the aging brittleness of the alloy bolt is large, the durability plasticity is very low, and the fracture of the specimen is mainly along the grain brittleness, and with the extension of the fracture time, the oxidation characteristics and fracture corrosion characteristics at the crack source become more and more obvious, and the fracture form gradually changes to the axial fracture along the crystal. Keywords:in Alloy bolt;Fracture;Aging brittleness;Crack source;CLC Number: TG Document Symbol Code: A Article Number: At the end of the century, the American company Specialmetals developed a new type of anti-oxidation, low expansion and high temperature alloy for aero engines. sagbo). In recent years, with the rise of ultra-supercritical steam turbine technology, higher requirements have been put forward for the performance of steam turbine high-temperature bolts, and the alloy is also used as a bolt material for steam turbine units [ ] GH and R and other traditional superalloy bolts have a chromium content above the mass fraction), good oxidation and corrosion resistance, but the high chromium content will make the Curie temperature of the ferromagnetic superalloy drop below room temperature [ ] to make its linear expansion coefficient higher than that of the connector (valve or cylinder), and the thermal conductivity is poor. In alloy not only has the high creep fracture strength and relaxation resistance of traditional high-temperature alloys, but also has a linear expansion coefficient similar to that of martensitic heat-resistant steelsWhen the alloy is used to reheat the bonnet bolts of the turbine valve, the operating temperature is on the left and right, the design steam pressure is MPa, and there is no alternating load. In Since the alloy bolt was put into operation, it has been fractured with extreme frequency throughout the country. Peng et al. [] believe that the bolt fracture is not related to excessive preload, but is caused by the fatigue crack propagation along the oxidation damage zone caused by stress-promoted grain boundary oxidation. Jiang Tao et al. [ ] found that the local high temperature of the heating rod when a broken bolt was heated and tightened, caused by the burning of the bolt hole material and the source of cracks, which in turn caused the bolt to break. Based on the failure analysis results of fractured bolts in a power plant, the author pointed out that grain boundary oxidation cracks caused by unqualified aging treatment are the main reasons for the fracture of some in alloy bolts. In alloy bolt fracture typeAccording to the statistical results of alloy bolt fracture of ultra-supercritical steam turbine in Jiangsu Province, the operation time at fracture is h;There are two main types of fractures: rough and uneven mud-like fractures (as shown in the figure) without crack propagation characteristics and fine fractures (as shown in the figure) with crack propagation characteristics. Fractures caused by macroscopic defects generated by manufacturing and fatigue fractures caused by the fracture of existing bolts in the same group of bolts that cause other bolts to be subjected to alternating loads are not discussed in this article. 
For the sake of description, the fracture type shown in the figure is named quasi-fracture, which is generally located at the transition of the smooth rod arc (where the working stress is maximum), and the fracture metallographic indicates that there are dendritic bifurcation cracks similar to stress corrosion (as shown in the figure) and oxidation characteristics of the inner wall (as shown in the figure) at the crack source of the inner hole wall;The fracture type shown in the figure is named quasi-fracture, and this kind of fracture is generally located at the bottom of the heating hole (where the stress concentration degree is very high due to the existence of the outer thread and the inner hole processing groove), and the oxidation corrosion characteristics of the fracture are not obvious, and the fracture is brittle along the crystal (as shown in the figure) at high times
According to the basic characteristics of the fracture, it is preliminarily judged that the quasi-fracture is a long-term (low-stress) fracture along the crystalline oxidation, and the quasi-fracture is a short-term (high-stress) fracture along the crystalline brittleness. In order to verify this point of view, samples were taken on non-defective alloy bolts to carry out persistent fracture tests under different temperatures and stresses, and the fracture characteristics of the specimens were analyzed, the main influencing factors of fracture were found, and preventive measures were proposed. Test Materials and Test Methods The in alloy bolt specimen used in the test is from a complete bolt under the maintenance and replacement of a power plant, and the hardness is HBW after standard thermal treatmentVacuum heat treatment furnace is adopted, and the solution treatment process is air cooling after heat preservation h, and the aging treatment process is (heat preservation h after furnace cooling (h) and aging treatment process is (heat preservation h after furnace cooling (h) to (heat preservation h after air cooling. Brinell hardness test is carried out according to the GBT Brinell hardness test for metal materials Part I: test method, and the tester is an HBS digital Brinell hardness tester. The high-temperature endurance strength test is prepared and tested in accordance with the requirements of the GBT metal uniaxial tensile creep test method, and the test instrument is RD and RCL type high-temperature creep and endurance strength testing machines. The test temperature is to adjust the initial stress to obtain a persistent fracture sample with different fracture times, and the longest fracture time is controlled within h. Test Results and Analysis Endurance Performance Diagram Two sets of endurance strength test data at different test temperatures, it can be seen that temperature has a great influence on endurance strength, and the specimen still has a fairly high endurance strength, but the endurance strength decreases sharply. In addition, from the slope of the long-lasting strength curve, it can be seen that the aging enhancement effect is significant. The figure shows the long-lasting plasticity of the sample at different temperatures and similar fracture times, and the persistent plasticity can be seen to be very low (the plasticity increases after h), and the cross-sectional shrinkage is also very low, and the cross-sectional shrinkage reflects the ability of the material to be locally necked and deformed [ ] is reflected in the macroscopic brittle fracture, so the material is very brittle when it is operated at and below, and the temperature is its brittleness sensitive temperature. 
Metallographic examination and fracture morphology analysis The fracture metallographic examination showed that the fracture form of the long-lasting specimen was crystal penetration, and the internal pores gradually increased with the extension of fracture timeand the fracture form of the next durable specimen is along the grain (as shown in the figure), there are no internal holes, and the microstructure is the austenite grain boundary distribution of the primary phase and the fine secondary phase of the grain boundary distributionOxide scale and oxidation cracks (as shown in Fig.) appear on the surface of the lower durable specimen, and with the extension of time, the oxide scale becomes more continuous and thicker, and the oxidation cracking is more [as shown in Fig. b];;Due to the short fracture time, there is no oxide scale or oxidation crack on the surface. 
Select a long-term test sample that reflects the temperature and time variation: b (h), b (h), c (h), c (h), a (h).Observing the microscopic morphology of the fracture area (as shown in the figure), it can be seen that there are oxidative corrosion products in the fracture zone of samples B, C and C, and the energy spectrum analysis shows that these corrosion products contain a large amount of oxygen (see table), and an internal defect is found in the crack source area of sample A. Observing the microscopic morphology of the extended area (as shown in the figure), it can be seen that sample b is a dimple fracture with a dimple and holesa Specimen is a typical rock sugar-like grain fracture, which is relatively obvious once because it is not covered by corrosive productsSamples b, c, and c are fractures of the corrosion pattern along the intercrystalline cracks, and the longer the fracture time, the more obvious the corrosion characteristics become, and the morphology along the crystalline is gradually invisible. 
The results of the durability test show that the material has a strong tendency to harden with age, while the aging hardening disappears under the test, and the durability decreases sharply, and the material has good ductility. This coincides with the actual fracture mode of the in alloy bolt, which operates at a temperature sensitive to aging embrittlement, and the fractures that occur are brittle fractures without obvious deformation. The observation of the crack source of the long-lasting specimen fracture is found that the crack source of the specimen is located on the free surface of the specimen, and the crack source of the high-stress short-term long-lasting specimen tends to be the defect of the surface itself, and with the extension of time, the crack source is often the oxidation cracking of the surfaceThe fracture mode also changed from age-like brittle fracture mode to anthyloxy-like brittle fracture mode. Temperature also has a significant effect on the oxidation resistance of in alloys, and some literature shows that due to the different oxides generated at different temperatures, the oxidation resistance of the alloy is higher than that of the alloy, so the applicability of in the use of in alloy bolts remains to be studied. Fracture characteristics, causes and prevention The fracture morphology of the fracture is similar to that of the C and C persistent specimens, and the free surface of the specimen adjacent to the fracture is oxygenated and the section is covered by oxidation corrosion products. This type of fracture has enough time to oxidize the grain boundaries, the initial stress is relatively low, and the oxygen gradually diffuses in the grain boundaries under the promotion of stress, and the resulting oxides embrittle the grain boundaries and gradually form intergranular cracks. Due to the uncertainty of the propagation of the crystalline oxygenation crack, a rough uneven fracture is formed that is basically perpendicular to the direction of the principal stress and has no crack propagation characteristics, and the fracture mode is long-term crystalline oxidation fracture. The main cause of similar fracture is along the crystalline oxidation, which can be classified as a kind of stress corrosion, often originates from the small oxidation defect of the inner wall and the position of the working stress, such as the smooth rod reducing, the small oxidation defect of the inner wall may be caused by the local high temperature heating and burning of the heating rod, and may also be caused by the surface oxidation cracking after long-term operationTo prevent this kind of cracking, attention should be paid to prevent local overheating of the inner wall when the heat is tightened, and the quality of bolt heat treatment should be improved, especially the quality of aging treatment. A large number of studies have shown that whether or not aging treatment is carried out has an important impact on the microstructure and properties of the IN alloy used in the steam turbine [when the secondary phase content reaches the area fraction], it is more obvious to improve the crack propagation resistance of the alloy and the stress resistance to accelerate the oxidation of grain boundaries [ ] Fracture characteristics, causes and prevention The fracture morphology is similar to that of the b (h) sample in the figure, which is mainly along the crystal morphology, and there are traces of slight oxidation corrosion on the grain surface, indicating that the high-temperature oxygenation time of this type of sample is short, and the stress plays a dominant role. b The initial stress of the specimen is mpa, and if it breaks after test h, the stress will be higher, which is much higher than the working stress of the bolt during steady operation, so such fractures may occur at the beginning of start-up, before the preload is too large and the stress is relaxed (and there is also a temperature difference stress), Or it occurs when the load is too large or forcefully dismantled. Similar fractures often occur at the bottom of the heating hole, where there are screws on the outside and a retract groove on the inside, and the stress concentration is large when subjected to external loads, and local overtemperature is easy to occur when heated tightly, and these factors tend to cause short-term grain brittle fracture of the bolt. Quasi-fracture is dominated by stress, and often occurs in the position where the degree of stress concentration is very high, such as heating hole bottom retract groove, nut tightening first tooth etc.;It can be prevented by reducing the degree of stress concentration, such as improving the processing accuracy, the bolt is designed as a through hole, etc., and the bolt should be avoided at the same time because the bolt is subjected to a large impact load by being strongly disassembled and assembled. Conclusion ( ) Temperature has a great influence on the long-lasting fracture behavior, and the alloy has a strong aging hardening tendency and considerable strength in the early stage of operation, but the aging brittleness is greater and the long-lasting plasticity is low. (Time also has a great influence on the fracture mode: for a long time, the oxidative cracking on the surface becomes the source of cracking, and the fracture form tends to form a rough and uneven oxidative corrosion fracture along the crystalline oxidation pattern, and the stress plays a role in promoting the diffusion of oxygen in the grain boundaries;The time is short, the oxidation of grain boundaries is small, the stress is dominant, and the aging brittleness of the material leads to the occurrence of brittle fractures along the grain at the defects at the high stress position, and such fractures are easy to occur at the stress concentration on the inner and outer surfaces and the processing defect location. ( In alloy bolt failure modes are mainly divided into brittle fracture mode along crystalline oxidation and aging brittle fracture mode, through-hole design avoids the processing problem at the bottom of the heating hole, can effectively reduce the occurrence of aging brittle fracture, but has no preventive effect on crystalline oxidation brittle fracture. Improving the grain boundary oxidation resistance of materials through standardized heat treatment (especially aging treatment) is the main way to solve the frequent cracking of in alloy bolts, and it is also necessary to prevent local overheating of the inner wall of the bolt when the bolt is strongly disassembled and the hot tightening. 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