2019 - cone gear shaft failure analysis and improvement
06 -
25 Qiao Yanfang li-ye ma tiger rock (
Heaven and earth in ningxia bull-running industrial group co. , LTD. )
As shown in figure 1 cone gear shaft is the key parts of a product, material for crnimo7-18
6, carburizing and quenching 58 to 62 HRC pattern request, heart of 40 ~ 43 HRC hardness.
Technological process: forging & rarr;
Coarse car & rarr;
Conditioning & rarr;
Semi-finishing & rarr;
Carburizing and quenching & rarr;
Finish machining.
Fracture occur in use process, breaking location steps had occurred in the tail end of the shaft.
Figure 11.
Macroscopic observation and fracture analysis of the fracture site focused on the two, one is the steps of the rounded, fracture is qi, also is the key way at the end of the shaft, due to the use of harsh, shaft fracture most polluted, coarse step judgment of the end of the shaft stress is too large, belongs to brittle fracture.
2.
Chemical composition inspection from the chemical composition, composition of volatile, heat treatment process is not easy to accurately control, therefore, from raw materials, control the range of material ingredient C, Cr, purpose is according to the actual composition adjust each furnace heat treatment process.
3.
Hardness, and microstructure inspection according to GB/T132981991 sampling metal microstructure inspection method, using the cutting in split biopsy hardness, the crackle sample do metallographic analysis, the results are shown in figure 2.
Biopsy hardness, hardness of from the diameter of axle laterally to the core of 44.
1HRC、43.
1 hrc, 42岁。
1 hrc, 41岁。
7HRC、41.
3HRC。
Hardness show that fracture trunnion from outside to inside, hardness change is very small, the hardness gradient is very small.
At the same time, the microstructure from outside to inside the difference is small, all is ferrite and martensite + according to JB/T6141.
3 -
The 1992 'classification of the microstructure of carburizing retained austenite grade 2, martensite, carbide grade 2, the core ferrite.
4.
Surface stress test by test, found that the shaft surface tensile stress, demonstrates the process of machining shaft after carburizing and quenching stress concentration phenomenon.
Failure cause analysis: look from the test results, fracture trunnion from outside to inside, hardness is conform to the requirements of the drawings,
40～43HRC）
Changes, but is very small, small hardness gradient, microstructure and a huge difference, at the same time shows that in the process of carburizing, quenching, the diameter of axle are hardening.
Second, due to the effect of surface tension stress of the shaft, in use process, cannot reduce the stress concentration by the right amount of plastic deformation and prevent the crack extension.
At the same time, because the sharp corners and slotting machining bad mark more accelerated the formation and propagation of crack.
5.
The improved method (
1)
Increasing axial split step transition fillet, lower roughness and reduce stress.
（
2)
Through the experiments, delay the carburizing and quenching cooling time and cooling intensity, can effectively improve organization, reduce fracture phenomenon.
Improved after the heat treatment process of microstructure results as shown in figure 3.
（
3)
Different internal stress of the diameter of the shaft after heat treatment, carburizing and quenching process should also be different.
（
4)
Caused by chemical composition fluctuation is bigger, carburizing and quenching heat treatment process is very difficult to control, therefore, to control the material range of key elements such as carbon, to narrow the hardenability bandwidth, reduce material can effectively improve axis organization condition, slow down the shaft fracture phenomenon.
（
5)
Delay the carburizing and quenching cooling time and the cooling intensity and control the chemical composition of materials can't use at the same time, otherwise it will reduce the strength of the shaft, should according to different levels of materials using a different technique.
（
6)
Through the advanced technology of mg can reduce the stress of the shaft surface, reduce the stress concentration phenomenon.
In short, the failure of shaft fracture is caused by various reasons, only from the components of the structure design, machining, heat treatment process and chemical composition of materials' multi-pronged, would slow down the life of the shaft, to improve the manufacturing quality.
Source: the metal processing (
Hot working)
》
