Fracture failure analysis of the main shaft of a large sewage treatment equipment

In view of the main shaft fracture failure accident occurred in the service process of a sewage treatment equipment, this paper carried out the fracture macro analysis, chemical composition analysis, material mechanical properties testing, metallographic structure analysis, fracture micro morphology analysis of the failure of the main shaft. The analysis results show that there are cold cracks in the weld and toe of the main shaft of the equipment, and the cracks expand under the action of rotating and bending alternating loads. The main shaft speed of the equipment is 3r/min, the load change frequency is very small, and the corrosive medium has sufficient time to play a role in the crack tip, fatigue cracks further propagated, leading to corrosion fatigue fracture of the main shaft. So in order to prevent this kind of failure, it is suggested to improve the welding process regulations for the equipment of the main shaft and rationally select welding materials.


Introduction
The sewage treatment equipment works in humid and high stress environment, and there are many attachments on outside surface of metal parts, thus forming a corrosive environment [1][2][3].In addition, most of the structural parts of sewage treatment equipment are connected by welding.If the welding process is improper, it is easy to produce cold cracks.The main shaft structure of the equipment is under high static and dynamic load, crack propagation under the action of corrosive medium and alternating stress, which is easy to produce corrosion fatigue failure and reduce the service life of the equipment [4][5][6].Therefore, it is very important to analyze the failure mechanism of the main shaft of sewage treatment equipment in corrosive environment to evaluate its structural safety and reliability [7,8].This paper takes the main shaft of a sewage treatment equipment as the research object, comprehensively analyzes its failure reasons, and puts forward relevant improvement measures and suggestions.

Macro analysis
The main shaft specification is Φ273mm×16mm steel tube, the material is Q345A, and the test standard is GB/T 1591-2008.It was observed that one side of the sample stiffener was fractured, and the fracture surface expanded along the radial direction in a step shape at the position of the stiffener as depicted in Figure 1(a).There was corrosion on the fracture surface and the inner wall of the main shaft tube, and the thickness of the main shaft wall was uneven.The minimum wall thickness was 15.19mm after measurement.The observation shows that the fracture surface is in a step shape near the stiffener, and the fracture in the crack propagation areas has fatigue characteristics [9][10][11][12].In Figure 1(b), there is an obvious annular crack at the position of the stiffener at the other end of the main shaft.According to GB/T 4336-2016 standard, ARL4460 direct reading spectrometer was used to detect the chemical composition of the main shaft.The chemical composition is shown in Table 1.According to the results in the table, the chemical composition of the main shaft meets the requirements of GB/T 1591-2008 standard for Q345A, and the carbon equivalent CEV(%) meets the requirements of GB/T 1591-2008 standard for Q345A, but the carbon equivalent is at the upper limit.

Mechanical property test
The longitudinal plate tensile sample was taken from the main shaft tube section far away from the failure part.According to GB/T 228.1-2010 standard, the SHT4106 material testing machine was used to test the tensile property of the main shaft at room temperature.The test results are shown in Table 2.As can be seen from Table 2, the tensile properties of the main shaft all meet the requirements of GB/T 1591-2008 for Q345A.
Table 2. Experimental results of the main shaft tension at room temperature.

Charpy impact test
According to GB/T 229-2007 standard, the longitudinal Charpy V-notched impact sample (10×10×55mm) was taken from the main shaft tube section far from the failure site.The PIT752D-2 impact testing machine was used to carry out Charpy impact test, and the test results were shown in Table 3.The test results show that the impact performance of the main shaft meets the requirements of GB/T 1591-2008 for Q345A.

Vickers hardness test
The weld position at the fracture of the main shaft was sampled, and the Vickers hardness at the weld was tested with the KB30BVZ-FA Vickers hardness tester according to GB/T 4340.1-2009standard.The test results are shown in Table 4.The test results show that the hardness value of heat affected zone is obviously higher than that of base metal and weld.Metallographic samples were taken at the fracture of the main shaft sample, and the optical microscope(OM) was applied to observe that the tube body was banded, mainly composed of ferrite and pearlite, as shown in Figure 2.There is a crack in the weld of the stiffener, which extends to the tube body and has the characteristics of transcrystallization, as shown in Figure 3.In the microstructure of the heat affected zone of the weld, bainite and martensite exist in the fine crystal zone, as shown in Figure 4.
In order to further test the crack initiation position, metallographic analysis was carried out on the other side of the main shaft fracture.Heat affected zone (HZ) was found near the external surface of the fracture near the tube, as shown in Figure 5.When the HZ was enlarged, the result was martensite + bainite, as shown in Figure 6.No abnormality was found in the other fracture structures, which were ferrite + pearlite, so the welding cold crack was initiated at the welding toe.

Electron microscopic analysis
VEGAⅡ scanning electron microscope(SEM) was used to observe the micro-morphology of the fracture samples.The low-magnification morphologies and high-magnification morphologies are shown in Figure 7 and Figure 8.The crack source area is in the weld or weld toe, and the width of the source area is less than 1mm.Most of the fractures are crack propagation areas, which are relatively flat, and some areas have intergranular characteristics when observed at high magnification.

Discussion
Although the main shaft material Q345A meets the standard provisions, the carbon equivalent is at the upper limit, so it needs to be preheated before welding, and the welding wire also needs to be preheated [13,14].The mechanical property test shows that the tensile properties of the main shaft meet the requirements of GB/T 1591-2008 for Q345A.Charpy impact test results show that the impact performance of the main shaft meets the requirements of GB/T 1591-2008 for Q345A.Vickers hardness test (HV10) showed that the hardness value of heat affected zone was significantly higher SEM MAG:1.50kx20μm

Intergranular characteristics
than that of base metal and weld.The microstructure analysis of the main shaft shows that the structure is mainly composed of ferrite and pearlite (F+P), and the distribution is banded.The crack analysis shows that there are cracks in the weld between the main shaft and the stiffener, which extend to the tube body.
The fracture microstructure analysis shows that the heat affected zone at the fracture is martensite and bainite, and the other fracture microstructures are ferrite and pearlite without any abnormality.Therefore, the cold welding crack starts at the weld and toe [15][16][17].
The macroscopic morphology of fracture has fatigue characteristics, but the microscopic morphology has intergranular characteristics [18].The energy spectrum analysis of fracture surface shows that the main elements of fracture surface are O, P and Fe.
Although the main shaft material Q345A meets the standard GB/T 1591-2008, its carbon equivalent is in the upper limit.Due to improper welding process, cold welding cracks occur in the weld and weld toe, which becomes the fatigue crack source.Under the action of the rotating and bending alternating load of the main shaft, the crack propagates.Sewage is corrosive and contains corrosive media, such as dissolved oxygen.The speed of the main shaft is only 3r/min, the frequency of load change is very small, and the corrosion medium has enough time to play a role in the crack tip, leading to further crack propagation.so the failure mode of the main shaft is corrosion fatigue fracture [19,20].

Conclusions
The main shaft material of the equipment meets the requirements of GB/T 1591-2008 for Q345A.The fracture property of the main shaft of the equipment is corrosion fatigue, and the welding cold crack produced at the weld and weld toe is the crack source.It is suggested to formulate welding process regulations through welding process evaluation test, including pre-welding preheating and post-welding heat preservation process regulations for tubes and wires.Optimized tube and welding wire to further improve the weldability of materials.Ultrasonic or magnetic particle inspection is carried out on the formed weld.

Figure 1 .
Figure 1.(a) Macroscopic morphology of fracture of the main shaft; (b) Annular crack morphology at the position of the main shaft stiffener.

Figure 2 .
Figure 2. Metallographic structure of the main shaft.

Figure 4 .
Figure 4. Microstructure of heat affected zone of weld.

Figure 6 .
Figure 6.Organization of heat affected zone.

Table 1 .
The chemical composition of the main shaft.

Table 3 .
Charpy impact test results of main shaft.

Table 4 .
Vickers Hardness test results of the main shaft weld (HV10).