Corn harvester bridge main shaft broken shaft for failure analysis

Corn harvester bridge main shaft broken shaft 2020 - reason for failure analysis 02 - 20 Cao Chunyu Lv Feng zhao zhejiang macros ( National crop harvest machinery products quality supervision and inspection center) Abstract: cloth such as hardness, metallographic test methods to analyze the causes of bridge main transmission shaft fracture. Results show that the main reason for the broken shaft is: tempering process, as a result of the shaft cross section is not completely hardening, axis of metallographic organization for abnormal conditioning, lead to shaft overall mechanical performance can't satisfy the work requirements, in the process of harvesting, bridge bearing large torque, easy to fracture. Key words: failure analysis; 40 cr steel; Metallographic examination 0 quote an enterprise newly production of corn harvester harvest after sales operations across the bridge main shaft frequently appear rupture phenomenon, therefore the enterprises entrust my unit of the fracture reason for failure analysis. Corn harvester in the process of harvesting operations, the bridge main for the main shaft torque for the work of the work transfer power. If the parts failure, harvesting machine cannot work, directly affect the harvest work. Therefore, the author carries on the failure analysis, in order to put forward the feasible solution. 1 shaft the technical requirements of the shaft to entrust outsourcing machining enterprises, technical documents for the design drawings, 40 cr main requirements for material, heat treatment process and tempering, the tempering hardness 221 ~ 269 HBW, design 40 mm in diameter, microstructure design drawings without being asked. 2 test method to observe the appearance, of shaft fracture morphology, analysis of microstructure, composition, hardness test, on the basis of analyze the invalidation cause of shaft. See from table 3 and table 4, the hardness of sample A and sample B the core to meet the design requirements, but the data is close to the design requirements under the threshold. 2. Preliminary analysis 1 fracture sample as shown, two axis for the same batch, the same specification products, machining process and heat treatment process is exactly the same. For the convenience of expression and distinguish, make up for the broken shaft sample A ( As shown in figure 1, figure 2) Not the use of shaft made up for the sample B ( See figure 3) 。 Can be seen from the figure 3, the sample B machining keyway, tested, and precision meet the design requirements, and no mark damage on the stress concentration. As can be seen from the figure 1, the sample fracture morphology is A shiny crystal, and normal stress direction is vertical, and no obvious plastic deformation, should belong to brittle fracture. These collection crystal crack in figure in the circle, as a result, the crack source should be circle in the figure. 2. 2 sample composition analysis two axis is the same material and detect via direct reading spectrometer analysis, the main chemical components are shown in table 1 of the sample. Can be seen from table 1 shaft in line with the GB/T3077 - Requirements for the chemical composition of 40 cr steel in 1999, as shown in table 2. 2. 3 samples of brinell hardness test of sample A and sample B respectively for brinell hardness test, the sample A brinell hardness sample cut piece of fracture 2 cm distance, when you cut out by cooling, to prevent overheating, organization change and hardness. Test results are shown in table 3 and table 4. 2. 4 microstructure analysis for brinell hardness test block of the rest of the broken shaft parts, clean and cut the sample along the diameter into four parts, take part, cleaning oily be soiled, dirt on the surface of the fracture; With the grinding wheel grinding fracture slightly flat ( Pay attention to the cooling water in order to avoid sample when grinding organization change because of the heat) ； The smooth, clean, dry the sample, after manual ordinal number in from coarse to fine sandpaper grinding, polishing (again Machine polishing) ； Used with dryer for 4% nitric acid alcohol solution after etching sample surface, in 500 times metallographic microscope to view of cross section metallography, strip and block ferrite, cause the hardness values close to the design value only offline, because the tempering process, the shaft was not completely hardening. The shaft adopts the critical hardening of 40 cr steel is 30 ~ 40 mm in diameter, and the shaft design is only 40 mm in diameter, in theory, the shaft should be overall hardening, the core should be tempered sorbite, troostite should not appear, strip and block ferrite, probably because actual quenching temperature is too high, make steel grain growth, quenching middle of the back get thick martensite, and the subsequent tempering temperature is too low, so that martensite not fully decomposition, part into austenite, remain large brittleness, in use easy to cause the brittle fracture. Therefore, think outsourcing processing enterprises in the problem during quenching. The bridge axis enterprise outsourcing processing, wood is installed on the grain harvest machinery parts, did not appear rupture phenomenon, in the shaft is installed on the corn harvest machinery, began to appear broken shaft phenomenon, the reason is that the bridge axis for the moment when the corn harvest machine, take more load, there was a problem because the shaft heat treatment, the axial mechanical performance is decreasing, under the high torque, to failure. Because the bridge axial cross section is not completely hardening, and quenching and tempering process problems, axis of metallographic organization for abnormal conditioning, lead to shaft overall mechanical performance can't satisfy the work requirements, in the process of harvesting, bridge bearings by bigger torque, easy to fracture. Suggestions to strengthen quality management and control, modify the design drawing, increase the metallographic organization, organization of concrete surface and the core are tempered sorbite, level 1 ~ 4 level, improve the hardness range, specifically for 240 ~ 280 HBW, timely communication with outsourcing processing enterprises at the same time, solve the problems existing in the heat treatment process. （ Source: agricultural machinery use and maintenance)