Failure analysis of frequent fracture accident of torsional shear type high strength bolts for steel structure of a thermoelectric 2×350MW unit project in Northwest China

In this paper, the frequent breakage of high-strength torsional shear bolts used in steel structure of a 2×350MW thermoelectric unit in northwest China is taken as an example. By means of macro and micro fracture analysis, physical and chemical inspection, the causes of frequent breakage of high-strength bolts are analyzed and the improvement measures are put forward. The results show that the failure of the 20MnTiB steel high-strength bolt after assembly is hydrogen embrittlement fracture, and the main cause of the failure is the presence of zinc layer on the bolt surface and a large number of zinc microholes on the shallow surface.


Introduction
High strength bolts are the basic parts of industrial connections and are widely used in all walks of life.Its main function is to tighten and connect the connected parts.Hydrogen embrittlement fracture is a common failure mode of high-strength bolts during service [1][2][3].Due to the delay and concealment of hydrogen embrittlement fracture, it is difficult to detect whether hydrogen embrittlement fracture will occur in bolts through conventional inspection procedures in advance, so its hazard is greater than other fracture forms [4][5].Literature shows that hydrogen embrittlement of high-strength bolts has caused serious losses to steel structure industry, electric power road and bridge industry, automobile industry and aerospace industry [6].
Since August 2016, during the boiler steel frame installation process of the 2×350MW off-site expansion project of a thermal power company in Northwest China, the frequent breakage accident of high-strength torsional shear bolts used for 10.9 grade steel structure has occurred, and 169 bolts have been broken as of March 17, 2017.In this paper, the causes of frequent breakage of high strength bolts are systematically analyzed, which provides scientific basis for the investigation and treatment of similar problems.

2.2.Measurement of structural dimensions
The transition fillet between bolt head and screw was measured for 6 bolts in good condition。

2.3.Material Analysis
Material chemical composition analysis was performed on the bolt head and screw core of the unbroken bolts No. L1 and L2, and material fluorescence spectrum analysis was performed on the bolt head, screw core and surface of the unbroken bolts No. L1 and L2, and the bolt core and surface of the broken bolts M1, M2, M3 and M4.

2.4.Zinc layer thickness measurement
The thickness of zinc layer was measured for sample number L3, L4, M5 and M6.

Mechanical property test 2.5.1.Normal temperature tensile test
The standard samples of broken bolt bolts numbered G1 and G2 were prepared for tensile test at room temperature.

2.5.2.Impact test
Three samples were randomly selected from the broken bolt bolt of No. G3, and the impact test was carried out at +10℃.Three samples were randomly selected from the broken bolt of No. G4 to carry out impact test at +0℃.Three samples were randomly selected from the broken bolt of No. G5 to carry out impact test at -10℃.Three samples were randomly selected from the broken bolt of No. G6 to carry out impact test at -20℃.

2.5.3.Microhardness test
The 300mN load force microhardness test was carried out from the main crack source side at 0.5mm distance from the bolt head of L3 unbroken sample 1mm away from the main crack.

2.6.Microstructure analysis
Metallographic test was carried out in the area near the main crack of L3 sample.

2.7.Determination of hydrogen content
The hydrogen content of the screw cross section near the fracture surface of L5 and L6 samples was determined.

3.1.Structural dimension measurement
The transition fillet between bolt head and screw was measured for 6 bolts in good condition.The maximum value of r was 1.5059mm, and the minimum value of R was 0.8486mm, both of which were smaller than the minimum value of 1.6mm required by standard GB/T 3632-2008.The analysis results show that the bolt surface contains zinc element.The measurement results show that there is a zinc layer on the bolt surface and the thickness of the zinc layer is not uniform.The thickness of the zinc layer at the bolt head is greater than that of the screw.

3.4.Microhardness test
Figure 1.Microhardness of bolt head cross section(HV) The test results show that the microhardness of the cross section of the bolt head is 376.1 ~474.1, which exceeds the GB/T 3632-2008 standard.There is a sudden hardness drop in the transition corner area on both sides, and the Vickers hardness at the main crack source is lower than the crack tip.

3.5.Determination of hydrogen content
The hydrogen content of the screw cross section near the fracture surface of L5 and L6 samples was determined, and the measurement results are shown in Table 7： Table 3. Determination of hydrogen

Sample number Sampling site Measured value H-L5
Cross section of screw near fracture surface 1ppm H-L6 Cross section of screw near fracture surface 2ppm 3.6.Fracture analysis

3.6.1.Macro analysis of fracture
The fracture at the bolt head of broken bolts numbered M1, M2, M3 and M4 was analyzed, and the macro morphology of the fracture was shown in Figure 2.
M1 fracture M2 fracture M3 fracture M4 fracture Figure 2. Macroscopic morphology of fracture It can be seen from the macro fracture morphology that the bolt breaks at the connection between the bolt head and the screw, and there are primary cracking zone and secondary fracture zone at the fracture.The cracking source in the primary cracking zone is located in the transition rounded zone between the bolt head and the screw, and there are reddish-brown corrosion products on the surface.The length of the cracking source is about two-thirds to the whole circle, and the depth of the primary cracking zone of the M1 fracture is 3.0405 ~3.4327mm.The secondary fracture zone is clean and even without obvious plastic deformation.The surface of the fracture is crystalline grain, and the surface of the fracture has the characteristics of flash facet under sunlight.

3.6.2.Electron microscope and energy spectrum analysis 3.6.2.1.M1 fracture
Granular impurities can be seen at the crack source in the primary crack zone, and the granular impurities are accompanied by point-like particles.By energy spectrum analysis, the particulate impurities are metal oxides, and the main component of the point particles is zinc.

Figure 3. SEM morphology of crack source in primary crack zone
The primary cracking zone is typical intergranular, with hair lines or "chicken feet" deformation lines along the grain boundaries, secondary cracks and micro-pores.

Figure 4. SEM morphology of primary cracking zone
The secondary fracture expansion zone is a quasi-cleavage type with mixed morphology dominated by cleavage with a small amount of dimples, and the crack growth is dominated by intergranular cracking with a small amount of ductile cracking.The content of zinc in M1 fracture decreases gradually from the outside of the bolt head to the inside of the fracture, and there is an uneven distribution of zinc in the primary fracture zone.In summary, the primary fracture zone shows the characteristics of intergranular cleavage fracture, and there are hair lines or "chicken feet" deformation lines on the grain boundaries, and there are intergranular secondary cracks and micropores.The secondary fracture extension zone is of quasi-cleavage type, and the crack propagation is mainly intergranular cracking with a small amount of ductile cracking.The overall performance is hydrogen embrittlement fracture.High strength bolts of grade 10.9 and above are prohibited from hot-dip galvanizing in engineering applications, but the surface of the failed bolt contains a large amount of zinc, and the thickness of the zinc layer is 0.514 ~11.30μm.A large number of metallic zinc inclusions penetrated into the shallow surface.Zinc with a melting point of 419.5℃ once existed in the matrix as a liquid, and the gas had a great solubility in the liquid zinc.After solidification, micropores were formed in the matrix, that is, a large number of micropores were formed on the shallow surface of bolts.The transition fillet between bolt head and screw is lower than the standard requirements, and the stress concentration coefficient of this part is greater than the design requirements.Due to structural and heat treatment reasons, the hardness of the transition corner of the screw and bolt head is significantly lower than that of the other parts, resulting in stress concentration and deformation of this part under the action of tensile stress.As well as a large number of microholes in the shallow surface of the bolt, the local hydrogen concentration in the shallow surface of the transition rounded corner area is high.Due to high local stress concentration, reduced load capacity and hydrogen embrittlement, a primary crack starts in the transitional rounded corner area and expands inward.After a certain depth of crack expansion, the short-term force equilibrium is reached due to the change of applied load, so the macro performance is a primary crack.The secondary fracture is a delayed fracture, the hydrogen embrittlement inside the bolt is caused by the presence of excessive hydrogen, and the stress concentration coefficient is further increased by the presence of primary crack.Low stress amplitude, reduced effective bearing area, high stress concentration factor and hydrogen embrittlement result in secondary delayed fracture.

Conclusion
To sum up, the main reasons for the frequent fracture accidents of torsional shear type high-strength bolts for steel structures are as follows： 1.The failure of 20MnTiB steel high-strength bolt after assembly is hydrogen embrittlement fracture.2.The presence of zinc layer on the surface of the bolt and the presence of a large number of zinc microholes on the shallow surface is the main cause of bolt failure.

Figure 5 .
Figure 5. SEM morphology of secondary fracture extension area

Figure 6 .Figure 7 .Figure 8 .
Figure 6.SEM morphology of transient fault zone A large number of granular metal inclusions can be seen in the transient fault area, and there are a large number of micropores with a maximum diameter of 40μm around the metal inclusions.The metal inclusions are zinc by energy spectrum analysis.3.6.2.2.M2、M3、M4 fracture This analysis covers 16 broken bolt heads, 27 broken screws and 5 cracked bolts.

Table 2 .
Measurement of zinc layer thickness /μm