PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Vacuum Heat Treatment Analysis of Gear Fracture Failure

Bevel gears are very important mechanical parts in the industry. The main features are that they can withstand large loads, work smoothly, and have low noise and vibration. They are widely used in the main reducer of automobiles and play a vital role in the transmission of power and motion. role. The failure of gears is mainly related to working conditions, including stress conditions, loads, temperatures and environments. The main factors of gear failure include design, material, processing, installation, environment and use.

Gear fracture analysis is an important technical means to judge the fracture form and mechanism of gears. The fracture morphology can truly record the origin, expansion and fracture process of cracks. Therefore, it is not only the basis for studying the microscopic mechanism of the fracture process, but also the analysis of fracture causes. reliable basis. Since the residual stress on the gear surface has an important influence on the fatigue life of the gear, the detection and analysis of the residual stress has become a new failure analysis and detection method.

1.Background investigation of broken and failed gears

The failed gear is two broken teeth of the driven wheel of the gear pair of the middle axle of a common traction vehicle. The driving wheel is intact, and its related parts, the axle housing, are intact, and there is no interference with the gear pair. The threads of the driving wheel, driven wheel and assembly nuts are intact, and there is no potential looseness. influences. The material used for the gear is 22CrMoH steel. This material has high economy and suitable mechanical properties. It is mostly used for the driving gear and driven gear of the rear axle of medium-sized automobiles. The general technical requirements for heat treatment of heavy-duty carburized gears are: the effective hardened layer depth at 1/2 tooth height is 1.7-2.1mm, the surface hardness is 58-63HRC, the core hardness is 33-45HRC, the martensite level is 1-4, and the residual austenitic Grades 1 to 4 of tensite, grades 1 to 2 of carbides, grades 1 to 4 of core structure, surface non-martensite thickness ≤ 0.02mm, and grain size higher than grade 6.

The failed gear structure is shown in the figure below.

2.Fracture macroscopic morphology analysis

When a metal part is subjected to a stress greater than the critical strength of the material, the part usually breaks and a fracture is formed at the fracture. Fracture analysis is the most important analysis method among various failure analysis methods, and the fracture location is usually at the weakest part structure or material organization.

The appearance of fracture is not only related to the composition, organization and structure of the part, but also related to the force condition of the part when it breaks and the external use environment. Since the section includes a large amount of information about the entire process of the fracture failure of the sample, by observing the morphology of the fracture and analyzing the relevant data recorded, it is possible to study the nature, cause, form, and stress state of the fracture and many other information. . A fracture is an open crack, which includes the entire process from the initiation of the crack to the final fracture, while other cracks may be accompanied by the fracture, which usually does not include the information at the beginning of the fracture. The purpose of macroscopic fracture analysis is to determine the crack source area, extension area and final fracture area, and to find out the mechanism of fracture production, which is the basis for completing failure analysis.

Through the macro-fracture observation (see figure), the driving wheel is intact, and the driven wheel has broken teeth on the 1# and 2# teeth, and the 1# tooth has obvious bright streaks, which is a typical form of contact fatigue fracture. Adjacent broken teeth have no obvious fatigue fracture characteristics, showing dark gray impact ductile fracture. Two fracture features with different properties appear in the same component system. In this case, the first fracture site should be determined first. The basic principle is that there are both fatigue fractures and impact ductile fractures. Generally, fatigue fractures occur first. From this analysis, it can be concluded that the 1# tooth breaks first, and the 2# tooth is interrupted by the fragments of the 1# tooth. Further macroscopic observation of the 1# tooth shows that the fracture of the 1# tooth has both fatigue fracture characteristics and ductile fracture characteristics. When both exist at the same time, the crack source region exists on the side of the fatigue crack. The conclusion of the macroscopic macroscopic analysis is that the subsurface layer of the tooth surface near the tooth root on the concave surface of 1# tooth first appears microcracks, and as the fatigue stress increases, the surface carburized layer first appears crushed (crushed), and the cracks further centripetal The part expands, and it becomes unstable and fractures at the junctional shear lip, and the broken fragments interrupt the 2# tooth. The microscopic morphology analysis of the crack source area, shear lip and core ductile fracture of 1# tooth will be carried out below.

The source of the fracture crack is located in the middle and lower part of the concave surface of the gear, and other secondary microcracks are generated during the crack propagation process. Under the action of variable load stress, microcracks are first generated at the surface layer of the occlusal surface near the tooth root in the contact area of the middle part of the concave surface of the gear, and the tooth surface Crushed and peeled off. Then the crack spreads toward the center in a herringbone pattern, and finally breaks at the shear lip, and the quasi-cleavage morphology of dimples can be seen in the instantaneous fracture zone of the center. On both sides of the crack, there are no obvious inclusions and second phase defects. The fracture nature was determined to be a contact fatigue crack.

1) The source of the fracture crack is located in the middle and lower part of the concave surface of the gear, and other secondary microcracks are also generated during the crack propagation process.

2) The microscopic morphology of the crack source is characterized by contact fatigue, and the microscopic morphology of the core is characterized by dimples and cleavage.

3.Residual stress detection and analysis

Gear strengthening shot peening is one of the effective ways to improve the contact fatigue strength of tooth surface. Since shot peening significantly improves its fatigue performance by increasing the surface compressive stress, it is more effective for workpieces subjected to high-frequency fatigue loads. The residual compressive stress formed by shot peening can offset part of the applied load. During shot peening, small-sized spherical steel shots hit the surface of the workpiece to form compressive stress. The impact of each shot will cause a certain plastic deformation of the metal, and finally a permanent compressive stress state is formed because the surface cannot be fully restored. As a surface strengthening process, shot peening can form residual compressive stress on the surface, which is equivalent to 55% to 60% of the ultimate tensile strength of the material, and the surface of the workpiece is just the place where cracks are easy to initiate. For gears that are vacuum carburized and quenched in a vacuum furnace, the resulting compressive stress can reach 1177-1725MPa, which can greatly improve the fatigue performance. The depth of the compressive stress layer is a function of the peening intensity (or peening energy) and increases with shot size or shot velocity. From the comparative verification data of the residual stress test, after the surface strengthening process, the residual compressive stress on the surface of the tooth root is increased from 824MPa to 1252MPa, and the contact fatigue performance has been greatly improved.

4.Conclusion

Through gear fracture analysis, physical and chemical analysis, metallographic structure and residual stress analysis, the reasons for the failure of heavy-duty carburized gears are as follows:

1) The crack source is located on the middle and lower surface of the concave surface of the gear, and the failure fracture form is contact fatigue fracture.

2) The internal oxidation on the tooth surface and tooth root weakens the grain boundary and reduces the contact fatigue strength of the gear.

3) The purity and uniformity of the driven gear material slightly exceed the standard, and the deformation of the gear after vacuum heat treatment and press quenching is inconsistent. When meshing with the driving gear, stress concentration occurs due to partial load, which accelerates the fracture of the gear.

5.Suggestions and measures

1) After the gear is vacuum carburized, vacuum quenched and vacuum tempered in a vacuum furnace, because of the presence of an inner oxide soft layer, it is easy to generate stress concentration during shot peening treatment, which becomes the starting point of fatigue crack source. It is recommended to select low-Si materials on the premise of ensuring the hardenability of the material, try to control the content of alloy elements such as Cr, Mn, Ti, V, etc., and use purification treatment for carburizing raw material gas (methanol or natural gas) to reduce oxidizing components, such as water, The content of impurities such as sulfur can reduce the degree of internal oxidation of the product after heat treatment.

2) Strengthening shot peening can improve the bending fatigue strength and contact fatigue strength of gears, which is an important way to improve the anti-seizing ability of gears and increase the service life of gears.

3) Control the purity and segregation of inclusions from the source of the material, cooperate with the implementation of the press quenching process to reduce heat treatment deformation, improve the consistency of the contact area, and improve the working conditions.