The material of the sun gear (see Fig. 1) is 8620RH, and the main chemical components are (mass fraction,%) WC = 0.18 ~ 0.23, WMn = 0.7 ~ 0.9, WNi = 0.4 ~ 0.7, WCr = 0.4 ~ 0.6, WMo = 0.15 ~ 0.25, and WSi = 0.15 ~ 0.3.
Fig. 1 Sketch of sun gear
Product technical requirements:
The minimum hardness at the 2 / 3 tooth height along the central axis from the tooth top is 25HRC, the surface bainite / pearlite at the 2 / 3 tooth height is ≤ grade 1, the sub surface bainite is ≤ grade 2, the surface Bainite / pearlite at the tooth bottom is ≤ grade 7, and the sub surface bainite is ≤ grade 4 (the surface Bainite / pearlite level is divided into grade 1-7 according to the amount of black massive structure and the tendency to form a continuous network;
The bainite grade of the sub surface layer is divided into grade 1-7 according to the trend of continuous network formation of bainite.
The surface hardness is 59HRC, and the hardened layer depth is required to be 1-1.55mm.
The thickness of the sun gear teeth is 168.42mm.
During the heat treatment process, the hardened layer depth at 1 / 2 of the tooth thickness shall be in the middle of the layer depth range, and the structure level of black massive bainite / pearlite on the surface layer at 2 / 3 of the tooth height shall meet the technical requirements, making it difficult to adjust the heat treatment process parameters.
After many tests, the production process was finally determined, so as to discuss with everyone.
1. Test method
The test is carried out in the box furnace, and the heat treatment process curve is shown in Fig. 2.
The test was conducted in three times, numbered 1, 2 and 3.
The No. 1 process is strongly infiltrated for 506min and quenched with general quenching oil G oil;
The No. 2 process is strongly infiltrated for 443min and quenched with fast quenching oil k;
The No. 3 process is strongly infiltrated for 303min and quenched with K oil.
See Table 1 for heat treatment process.
Fig. 2 heat treatment process curve
Table 1 three heat treatment processes of sun gear
| Process No.: | Heat treatment process | Quenching medium |
| 1 | Strong carburizing potential 1.2%, 925 ℃, 506min; Cool down to 865 ℃ for quenching. | G oil |
| 2 | Strong carburizing potential 1.2%, 925 ℃, 443min; Cool down to 865 ℃ for quenching. | K oil |
| 3 | Strong carburizing potential 1.2%, 925 ℃, 303min; Cool down to 865 ℃ for quenching. | K oil |
2. Test results and analysis
See Table 2 for the test results of three heat treatment processes of the sun gear.
Table 2 test results of three heat treatment processes of sun gear
| Number | Carburizing hardened layer depth / mm | Surface hardness / HRC | Heart hardness / HRC | Retained austenite content | Intergranular oxide/ μ m | Decarbonization | Direct quenching carbide |
| 1 | 1.33 | 65 | 29 | ≤24% | 24~32 | nothing | nothing |
| 2 | 1.55 | 64 | 33 | 24% yuan 30% | 12~24 | nothing | nothing |
| 3 | 1.29 | 64 | 30 | 24% | 12~24 | nothing | nothing |
| Microcrack | 2 / 3 tooth height | Tooth bottom | Conclusion | ||||
| Surface Bainite / pearlite | Subsurface bainite | Surface Bainite / pearlite | Subsurface bainite | ||||
| 1 | nothing | >5 | 1 | 7 | <5 | unqualified | |
| 2 | nothing | 1 | <1 | 7 | 3 | qualified | |
| 3 | nothing | 1 | 1 | 6 | 3 | qualified | |
(1) The depth of the black reticulated intergranular oxide at the top of 2 / 3 teeth is more than 24μm, as shown in Fig. 3a.
The black massive surface Bainite / pearlite structure is greater than grade 5, forming a discontinuous reticulation trend, as shown in Fig. 3b, the subsurface bainite at the bottom of the teeth is less than grade 5, forming a discontinuous reticulation trend, which is relatively dense, as shown in Fig. 3c, and does not meet the requirements of the heat treatment process.
(2) The hardened layer depth is 1.55mm, which is the upper limit of the required range of partial layer depth.
The bainite / pearlite on the surface layer at the tooth bottom reaches grade 7 to form a lamellar black belt.
As shown in Fig. 3d, it can not meet the process requirements well.
The test results not only meet the metallographic requirements of the heat treatment process, as shown in Fig. 3E, but also make the hardened layer of the sun gear deep in the middle of the technical requirements.
（a）
(b)
（c）
（d）
（e）
Fig. 3 metallographic structure of sun gear after three heat treatment processes
Intergranular oxide does not corrode.
It is observed under the microscope as a black network structure, which is generally formed by internal oxidation of alloying elements.
The depth of formation of the black network is mainly seen;
Under the light microscope, the surface Bainite / pearlite is slightly corroded by 4% nitric acid alcohol to form a black massive structure (see Fig. 3b, 3d and 3e), which is a non martensitic structure formed due to internal oxidation and poor alloy elements resulting in decreased hardenability.
It is mainly seen that the black structure forms a network.
The results from the test show that the depth of intergranular oxide is greater than 24 μ m. The surface Bainite / pearlite structure is greater than grade 5, and the subsurface bainite at the tooth bottom is less than grade 5.
Speculated causes:
① It may be due to the large moisture and air content in the feed gas of the box type furnace or the poor sealing of the equipment, which indirectly brings in a large number of oxygen atoms, which causes the internal oxidation of the alloy elements, resulting in the deep depth of the intergranular oxide, and also reduces the hardenability of the parts, generating a large number of black massive Bainite / pearlite.
② Quenching cooling rate is insufficient.
The more intense the quenching cooling, the greater the cooling rate, the less Bainite / pearlite structure formed.
③ Quenching medium.
During the quenching process, the steam film rupture time increases due to the oxidation of oil and insufficient stirring, which intensifies the tendency of increasing surface Bainite / pearlite. Improving the quenching ability of oil can effectively inhibit the formation of surface Bainite / pearlite.
According to the above forming factors, ① and ② factors shall remain unchanged: the equipment and feed gas shall remain unchanged, the cooling and stirring rate shall remain unchanged, and ③ factors shall be changed: ordinary quenching oil G oil shall be replaced with rapid quenching oil k oil, and No. 2 process shall be developed.
It is found that the metallographic structure of the sun gear has been greatly improved and meets the requirements of heat treatment process.
It indicates that the tightness of the test equipment and the purity of the feed gas meet the requirements of the heat treatment process.
The No. 3 process is developed without changing the equipment, feed gas, cooling and stirring rate and rapid quenching oil medium to shorten the strong penetration time.
The test results show that the No. 3 process can well meet the requirements of heat treatment process.
3. Conclusion
(1) High purity feed gas shall be used as the heat treatment gas source as far as possible.
The equipment shall be regularly maintained to ensure the tightness of the equipment and reduce the content of air and water mixed into the furnace.
(2) It is recommended to use rapid quenching oil for the heat treatment process of the sun gear to reduce the content of intergranular oxide, surface bainite / pearlite and sub surface bainite.
