Bevel gears play a crucial role in the transmission system of automobiles. They are responsible for reducing the speed, increasing the torque, and changing the direction of torque rotation. In this study, we conducted a detailed analysis of the abnormal failure fracture of the active bevel gear, which is an important transmission component in the steering system of automobiles.
1. Introduction
The main reducer in the automobile transmission system functions to decrease the rotational speed, increase the torque, and alter the direction of torque rotation. It consists of one or several pairs of reduction gear pairs, with the gear with fewer teeth driving the gear with more teeth to achieve speed reduction. The conical gear transmission is used to change the direction of torque rotation. The active bevel gear is a vital transmission component in this system, which inputs power and transmits it to the driven gear to control the steering system of the automobile and adjust the linear speed of the inner and outer wheels, thereby enabling the automobile wheels to turn smoothly.
The processing technology of the bevel gear includes: blanking -> hot forging -> normalizing (pre-heat treatment) -> rough machining -> finish machining -> carburizing and quenching -> tempering -> fine grinding to form [1]. The pre-heat treatment can eliminate the uneven distribution of the austenite structure, relieve the stress generated during the forging process, reduce the chance of cracks, improve the cutting performance, and prepare for the final quenching and tempering heat treatment [2 – 3].
The automobile main reducer gear studied in this paper experienced abnormal fracture before reaching the normal service life. The design standard of this component requires a surface effective hardening layer depth of 0.8 – 1.1 mm and a surface hardness of 32 – 48 HRC for the matrix hardness. Therefore, a comprehensive analysis and detection were carried out to provide certain theoretical guidance for later production.
2. Analysis and Detection
2.1 Macroscopic Observation
Most of the teeth of the active bevel gear were fractured, and macroscopic cracks had also appeared at the root of the unbroken teeth. By observing the fracture surface, obvious crack propagation traces of Beach marks could be found on two of the broken teeth, indicating low-cycle fatigue [4] fracture. Figure 1 shows the physical image of the active bevel gear.
2.2 Chemical Composition
The chemical composition of the tooth surface layer was determined using a direct reading spectrometer. The material of the gear was 20CrMnTiH steel, and the analysis results of the chemical elements are shown in Table 1, which meet the requirements of the content of each element in the “Alloy Structural Steel Standard” GB / T 3077 – 2015.
2.3 Hardness Test
Samples were taken from the residual tooth surface and the interior of the matrix, and the Rockwell hardness of each part was measured respectively. The test results are shown in Table 2. The detected value of the matrix is close to the lower limit of the design requirement. The average hardness value of the gear surface is lower than the design requirement, and the hardness is uneven, with a large gradient in the hardness difference. This indicates the presence of abnormal tissue on the surface, and the inhomogeneity of the tissue causes the hardness difference.
2.4 Determination of the Depth of the Surface Effective Carburizing Hardening Layer
Samples were taken at three positions on the surface layer, and the average depth of the surface effective hardening layer of the gear was measured using a micro Vickers hardness tester, which was 0.86 mm (required 0.8 – 1.1 mm). According to the standard (GB / T 9450 – 2005 “Determination and Verification of the Depth of Carburizing and Quenching Hardening Layer of Steel Parts”), the measurement result exceeds the required minimum value and meets the design requirements.
2.5 Metallographic Analysis
Metallographic samples were prepared at the broken teeth, and after rough grinding, fine grinding, polishing, and observation under a metallographic microscope, no obvious non-metallic inclusions were found in the vicinity of the fracture and the matrix, only a small amount of spherical oxides were present, all below grade 1, as shown in Figure 2. Therefore, it is not the main reason for the gear fracture.
After the polished samples were etched with 4% nitric acid alcohol, washed, dried, and microscopically observed, the microstructure of the carburized hardening layer on the surface of the unbroken gear was fine needle martensite + a small amount of residual austenite [5], as shown in Figure 3. Its microstructure belongs to the normal product after quenching; the microstructure of the core is lath-shaped bainite + sorbite, as shown in Figure 4. This microstructure will lead to a reduction in the bending strength of the gear [6 – 7]. However, considering its location at the core and not the main stress-bearing part, it has little impact on the gear fracture.
There is a network-like non-martensitic structure on the surface of the broken gear root, and the depth of the test is about 0.04 mm, as shown in Figure 5. After carburizing and carbonitriding, the ideal structure on the surface of the quenched part should be fine needle-like high-carbon martensite. However, due to many uncontrollable factors in the heat treatment process and processing technology, some non-martensitic mixed structures such as bainite and troostite (pearlite type) are formed on the gear surface, resulting in serious quality defects. If the depth of the non-martensitic structure exceeds the standard seriously, the surface hardness of the part will be low in mechanical properties, affecting the hardness gradient and resulting in uneven test hardness.
The gear tooth root has a non-martensitic structure, and the national automotive industry standard QC / T 262 – 1999 “Metallographic Inspection of Automobile Carburized Gears” stipulates that the deepest non-martensitic structure on the gear surface shall not exceed 0.02 mm. The surface non-martensitic depth of this gear is 0.04 mm and penetrates along the original austenite grain boundary in a network-like manner [8]. The deeper non-martensitic structure seriously reduces the surface hardness and wear resistance of the gear, as well as the fatigue limit [9], and fine cracks are initiated from the grain boundary or the stress concentration area of the oxide, forming a crack source, causing the gear to fracture due to insufficient bending strength during later service meshing.
3. Discussion of Test Results
The gear surface has a deeper non-martensitic structure that penetrates along the original austenite grain boundary in a network-like manner. This structure seriously weakens the strength of the gear surface and grain boundary, reduces the wear resistance and fatigue life of the gear (under the same force, the early initiation of the crack source or the accumulated degree of crack damage will mostly reduce the fatigue coefficient of the gear, thereby reducing the fatigue limit life). The presence of the non-martensitic structure first causes uneven hardness on the gear surface, making the gear prone to stress concentration and the appearance of fatigue crack sources during service. The continued expansion of multiple crack sources eventually leads to gear fracture, greatly shortening the fatigue life of the gear, which is the main reason for the gear failure and fracture [10].
4. Improvement Measures and Effects
There are two main ways to address the source of the non-martensitic structure: one is to minimize the elements that are preferentially selectively oxidized during material selection (the order of preferential oxidation of different elements is C > Ce > Ba > Mg > Al > Ti > Si > B > V > Nb > Mn > Cr > Cd > Fe > P > Mo > Sn > Ni > As > Cu); the second is to reduce the oxidizing components of the carburizing atmosphere (such as reducing the oxygen partial pressure, etc.). To solve the current domestic gear problems, the second option is more acceptable to manufacturers.
Specific measures include: (1) The presence of the non-martensitic structure indicates that there is an oxidizing atmosphere in the heat treatment furnace. The cleanliness of the carburizing atmosphere in the furnace should be improved, and the sealing of the heat treatment furnace should be strictly controlled. The exhaust time of the heat treatment furnace can be appropriately extended to make the carburizing atmosphere in the furnace more pure. (2) The overall hardness of the surface layer of this failed gear is relatively low. Appropriately increasing the surface hardness can improve the contact fatigue strength of the tooth surface. Ensure the cleanliness of the workpiece surface before carburizing to improve the surface hardness and uniformity. (3) During the carburizing process, due to the higher carbon potential at the tooth root and the slower cooling rate than other parts, non-martensitic structures are prone to form at the tooth root. It is necessary to appropriately increase the quenching cooling rate of the gear to achieve the purpose of reducing or eliminating this defect.
5. Conclusion
Based on the improvement suggestions, the heat treatment process was adjusted (including the oxidation atmosphere and quenching cooling process), and sampling and observation were conducted on the new batch of gears. No obvious network-like surface non-martensitic structure was found. Completely eliminating this structure requires high requirements for the material and heat treatment process. The adjustment of the process has greatly improved the organizational defects on the surface layer of the gear. From the final analysis results and the improved effect, it can be known that the analysis method in this paper is highly effective and can provide certain guidance for the processing technology of the gear.
