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The wax oil centrifugal pump in the second set of atmospheric and vacuum oil refining workshop is a cantilever centrifugal pump. The design requirements of the pump shaft are made of 35CrMo, which has been quenched and tempered and has a hardness of HB=269~302. The impeller material is G25 steel, and the static balance tolerance is 8g. There is an M24×1.5 thread on the end of the pump shaft for fixing the impeller. The pump head is 150m, the volume flow rate is 200m3/h, the rotation speed is 2950r/min, and the wax oil density is 0.919kg/m3. Since December 1995, many shaft-breaking accidents have occurred in this pump. The broken parts are all at the thread undercut groove at the shaft end. The time interval between shaft breaking is getting smaller and smaller, with the shortest being only about one month. At the same time, honeycomb-like perforations appeared in many places on the surface of the impeller, which affected the normal operation of the device. 1 Analysis and calculation 1.1 Macroscopic analysis of the fracture surface of the pump shaft (1) Macroscopic observation of the fracture site of the failed pump shaft revealed that the macroscopic fracture surface can be clearly divided into 3 areas: Fatigue crack source area, fatigue crack expansion area and final fracture area. If you carefully observe the edge of the shaft, you can see several primary fatigue crack steps, indicating that there are multiple sources of fatigue cracks at this fracture. These fatigue crack sources reflect the serious stress concentration at the undercut groove at the end of the shaft. The fatigue crack expansion area accounts for most of the total fracture area, and the final fracture area is very small, indicating that the tensile stress on this shoulder is small. Since the transition fillet radius R is very small here, a large stress concentration will occur. The cyclic load acts on the thread undercut portion where the stress concentration is the largest, causing fatigue fracture of the pump shaft. The cyclic load is the axial impact force caused by the dynamic unbalance caused by the weight loss of the impeller. If the impeller has not been replaced for a long time, the dynamic imbalance caused by it will become more serious, and the time for the pump shaft to break will become shorter and shorter. (2) Metallographic analysis: Samples were taken near the fracture and the metallographic structure of the shaft was found to be mainly tempered troostite. Many lath-like martensite forms can still be seen in the structure, as shown in Figure 1. This shows that when the shaft was undergoing quenching and tempering treatment, the high temperature tempering temperature or time was not in place, and the tempered sorbite structure was not obtained, resulting in an increase in the notch sensitivity of the shaft and accelerating the initiation and expansion of fatigue cracks. (3) Hardness test: Samples were taken for the hardness test. The Rockwell hardness HRC values were 31, 32, 31, and 34 respectively. These values are higher than the HB value specified in the drawing, which further indicates that the structure is non-tempered sorbite. (4) Scanning Electron Microscope (SEM) Analysis After sampling, the micromorphology of the pump shaft fracture was analyzed with a scanning electron microscope. It can be seen that there are fatigue stripes in the fatigue crack source area and fatigue crack expansion area, as shown in Figures 2 and 3, indicating that the failure is caused by fatigue fracture, and the micromorphology of fatigue crack expansion is a cleavage pattern. 1.2 Impeller (1) Macroscopic observation and analysis There are many honeycomb-shaped perforations on the surface of the impeller at the suction port of the pump, and there are obvious friction marks on the surface of the hub. The front and rear surfaces of the impeller are densely covered with corrosion pits, and the casting structure is relatively loose. From the perspective of the overall assembly, the weightless perforation part of the impeller corresponds to the fatigue crack source point at the shaft end. (2) Chemical composition analysis. The contents of various components are:: w(C)=0.162%, w(Si)=0.218%, w(Mn)=0.119%, w(P)=0.015%, w(S)=0.030%. From the chemical composition, the content of carbon and manganese is low. (3) Metallographic analysis: The structural characteristics are massive and acicular ferrite + pearlite, which is a typical Widmansite structure, as shown in Figure 4. The grains are coarse and the average grain size is level 3. The pearlite content is low, which is related to the low carbon content, and there are inclusions between the crystals. This shows that the casting quality of the impeller is poor, with a large amount of shrinkage porosity, uneven metallographic structure, and severe segregation, which leads to uneven metal surface conditions. The presence of non-metallic inclusions causes gaps in the metal, causing physical unevenness and incompleteness. Because the wax oil has a high temperature, a high acid value, and contains elements such as sulfur, the area around the inclusions becomes the origin of pitting corrosion. It can be inferred from the pitting corrosion on the impeller surface that the local perforated parts are where inclusions are enriched, and the low manganese content may be caused by the accumulation of this manganese-rich sulfide. At the same time, segregation in the metallographic structure causes the weakening of grain boundaries, so that the combination of pitting corrosion and intergranular corrosion accelerates local corrosion. (4) Calculation of the net positive suction pressure head of the pump. Since the allowable cavitation margin Δh of the pump is unknown, it is solved here based on the suction specific rotation number.: In the formula, n is the rotation speed, r/min ； qV is the volume flow rate, m3/min ； S is the suction specific revolution of the pump. For centrifugal pumps of ordinary design, no matter how large the specific speed is, 1200 can be taken. Therefore, it can be calculated that Δh=7.4m. 2 Improvement measures ① Increase the transition radius R of the undercut groove. It is recommended that R=1.5~2mm to improve the degree of stress concentration in this part. ②The heat treatment process is carried out strictly in accordance with the drawing requirements to ensure that the shaft obtains good comprehensive mechanical properties. ③When installing the impeller, the pretightening force of the nut should be appropriate and should not be too large. ④The impeller is surface treated to improve corrosion resistance. 3 Conclusion After the pump shaft and impeller of the wax oil centrifugal pump were modified according to the above measures, it was installed and put into use after the device was overhauled in May 1997. It has been running for more than a year and is running well without any shaft breakage accidents, which proves that the modification is successful.