Failure Analysis of High Strength Hexagon Head Bolt Fracture of Engine Base

A batch of hexagonal head bolts of M12X30mm and performance grade of 12.9 produced by a fastener factory, the material is 42CrMo steel, the manufacturing standard is GB/T5782-2000, the surface of the bolt is blackened, and the preparation process is wire rod. Spheroidizing annealing - pickling - phosphating - saponification - cold drawing - cold forming molding thread processing - cleaning - heat treatment - washing - oxidation (blackening) - washing - immersion antirust oil. The bolt is an engine base bolt. Eight of them are installed on an engine. After 24 hours of installation, two bolts were found to be broken. Due to the timely detection, although the engine components were not seriously damaged, it also affected the normal operation of the engine. In order to prevent the bolt from breaking again, the author analyzed the cause of the bolt failure.

1 Physical and chemical test and results can be seen, the bolt break is flush, no plastic deformation, the section is perpendicular to the axis, which is brittle fracture, and there is no obvious corrosion trace near the fracture. Cut a piece of sample from the fracture of the sample with wire cutting, first clean it with alcohol, then clean it in acetone solution for 20 minutes with ultrasonic cleaning machine, then wash it with deionized water, then dry it, observe the shape of the fracture, and analyze its chemistry. ingredient. The side surface perpendicular to the cross-section of the crack source was used as the end face of the metallographic sample, and then mechanically ground and polished, and then etched with a nitric acid solution having a volume fraction of 4%, and the microstructure was observed and the hardness was measured.

More fine hairline spreads through the grain boundary and has more tearing edges. This is a typical microscopic feature of the hydrogen-brittle fracture. The extension zone has a river-like pattern similar to the quasi-cleavage fracture, and its fracture mechanism is also Belonging to the cleavage fracture, see. At high magnification, the microscopic morphology of the instantaneous break zone is the shear dimple, see (d).

1.3 Microstructure The sample was taken longitudinally along the centerline of the bolt and the metallographic specimen was prepared by hot damascene method. The microstructure was observed using a 4XC type optical microscope. It can be seen that the structure of each part of the bolt is completely the same, and it is a tempered sorbite structure, which is a normal balanced structure. The surface of the bolt has a carburized layer of 0.1 mm deep, indicating that the surface hardness of the bolt is high. There is a straight crack at the root of the thread, and the crack is cracked along the crystal without branching.

1.4 Hardness The TH300 hardness tester is used to test the hardness of the root, surface and core of the failed bolt, according to the requirements of GB/T3098.1-2000. The surface hardness of the bolt is 518HV0.3, which is about 80HV0.3cm higher than the core hardness (the range of hardness required for the core is 385435HV0.3), which does not meet the standard requirements; and the core hardness of the bolt is 434HV10, although it meets the requirements, but the overall The level is close to the upper limit of hardness and individual points exceed the upper limit of hardness. Table 1 shows the hardness of the root of the failed bolt thread.

1.5 Chemical composition The chemical composition of the bolt is analyzed by direct reading spectrometer. It can be seen from Table 2 that the chemical composition of the failed bolt meets the hardness of the root of the failed bolt of GB/T3077. Table 2 The chemical composition (mass fraction) of the failed bolt The value of a 1999 regulation.

1.6 Hydrogen content The sample was sampled in the area of ​​the bolt thread segment at a distance of 03 mm from the surface, and the hydrogen content was measured using a H-3000 type hydrogen analyzer. The results show that the hydrogen content of the failed bolt sample is 10mgkg1, and the hydrogen content of the normal bolt should be less than 3mg kg1, which indicates that the hydrogen content of the subsurface region of the broken bolt is higher.

2 Analysis of fracture causes The structure of the failure bolt is tempered sorbite. The bolt fracture has a macroscopic bright area. The microscopic existence of “chicken-like” morphology, the crack along the crystal root is not bifurcated, these are typical characteristics of hydrogen embrittlement. . When the bolt is installed and subjected to tensile stress during storage, the stress is concentrated here due to stress concentration at the root of the thread.

According to the results of the above test, according to the criterion of failure of hydrogen embrittlement fracture, the failure of the class 12.9 high-strength bolt should belong to hydrogen embrittlement fracture. First, in various microstructures, the general order of sensitivity to hydrogen embrittlement is martensite, upper bainite (coarse bainite), lower bainite (fine bainite). Sorbite, pearlite, austenite, and thus the Sorbite structure of the bolt is a hydrogen-hardened sensitive structure. Secondly, when the strength is greater than 1 200 MPa, the hydrogen content in the material can cause hydrogen embrittlement at 5 kg1; according to the test results of hardness and hydrogen content, the tensile strength is estimated to be 1200 MPa, and the subsurface hydrogen content is 10 mg kg1; The tensile strength of the bolt is 1 000 MPa, and the surface hardness and the hardness of the core are equivalent. Since the failure bolt may be too high in strength due to improper heat treatment, the hardness difference between the inner and outer portions is large, so that the sensitivity to hydrogen embrittlement is increased.

Again, the working stress on the bolt is mainly static tensile stress, and there is stress concentration at the root of the thread. Finally, the surface of the hydrogen brittle fracture macro-fracture is clean, no corrosion products, the fracture is flush, and there are radioactive patterns; the hydrogen brittle micro-fracture is intergranular fracture, the grain outline is clear, and the grain boundary is accompanied by deformation lines (hairline or chicken claw marks) ), fewer secondary cracks, more tearing ribs or dimples, these features can be found on the failed bolt.

Dai Leyang, et al.: The cause of the explosion of the superheater of a tanker auxiliary boiler has resulted in significant pearlite spheroidization and more creep creeping cracks (as shown), indicating that the boiler has a long-term Overpressure or overheating is possible. Upon inquiry, the engineer was informed that the boiler sometimes had overpressure operation, mainly because some port oil-filling tasks were relatively tight and often urged the crew to work quickly. In addition, the outer wall of the squib has obvious oxidation, and the inner wall has more serious scaling, which will exacerbate the excessive temperature of the tube wall, causing the failure tube to overheat for a long period of time, eventually causing tissue aging or even partial decarburization, resulting in a decrease in its permanent strength. In addition, since the oil content of the oil of the tanker boiler is large, the scale layer inside the superheater tube is easily piled up locally, and the scale layer of the peeled off is also easy to adhere to each other, resulting in a small flow area of ​​the steel pipe or even blockage, which may It is the direct cause of the local overheating of the blasting point of the failed tube.

On the other hand, since the thermal expansion coefficient of the brittle-scale layer on the inner wall of the squib is different from that of the base metal of the pipe wall, it is easily cracked under the action of alternating thermal stress, and then peeled off by steam. When the scale layer is cracked, the hot water in the tube suddenly contacts the high temperature pipe wall through the crack of the scale layer and suddenly vaporizes, resulting in local expansion pressure. This alternating expansion pressure causes the wall material of the pipe wall to be broken, and the longitudinal crack is induced on the outer wall of the blasting site where the strength of the base body has decreased, and gradually spreads toward the inner wall. The inner wall of the steel pipe at the peeling layer of the fouling layer is easily subjected to stress concentration induced by steam corrosion, and then many longitudinal micro-cracks are gradually formed in the inner pipe wall at the crack of the scale layer. Under the long-term interaction of the longitudinal cracks on the outer wall and the inner wall of the steel pipe, the effective thickness of the pipe wall is reduced. When the pipe is reduced to a certain extent, the superheater pipe will have a long-time overheating pipe burst accident.

3 Conclusions and recommendations Due to the long-term over-temperature operation of the superheater tube of the tanker auxiliary boiler, the pearlite of the steel tube matrix is ​​spheroidized and creep damage occurs. The permanent strength of the steel tube decreases, and the fouling inside the tube exacerbates the overheating of the steel tube, eventually leading to Burst tube.

In the operation and management of the tanker auxiliary boiler, the engineer should be strictly required to start the boost according to the boost curve, and it is strictly forbidden to catch up with the abnormal conditions such as the boosting of the fire; secondly, it is necessary to adhere to the regular test of the water quality and the release of the furnace water treatment agent. To reduce fouling, it is necessary to strengthen the observation of the condensate observation cabinet to prevent the leakage of cargo oil through the return pipe to the hot water well, pollute the boiler water and increase the fouling, thereby reducing the heat transfer efficiency of the water pipe and causing long-term over-temperature.

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