Abstract: This article focuses on the precision forging technology of spur gear, especially those made of 17Cr2Ni2MoVNb steel. It discusses the effects of different forging ratios and normalizing temperatures on the material’s properties, the design and simulation of the forging process, and the manufacturing of spur gear forgings. The results show that the closed forging process can achieve small-batch production of spur gear with complete forging streamlines and improved mechanical properties.
1. Introduction
Spur gear is fundamental components for transmitting motion and power and are widely used in engineering fields. The traditional manufacturing process of spur gears often involves free forging or ordinary die forging to produce blanks, followed by machining operations such as cutting teeth and holes. However, this process has several drawbacks, including low material utilization, poor gear strength due to interrupted flow lines, and low production efficiency. In contrast, precision forging techniques offer significant advantages, such as improved material utilization, enhanced mechanical properties, and reduced production costs.
The 17Cr2Ni2MoVNb steel is selected for this study due to its excellent mechanical properties and potential for use in high-performance spur gear. The objective of this research is to develop a precision forging process for 17Cr2Ni2MoVNb steel spur gear, including the determination of optimal forging parameters, the design of forgings and dies, and the simulation and verification of the forging process.
2. Experimental Materials and Process
2.1 Materials
The material used for the spur gear in this study is 17Cr2Ni2MoVNb steel. The chemical composition of the steel is shown in Table 1.
| Element | Content (mass fraction) |
|---|---|
| Cr | a228 |
| Ni | 760 |
| Mo | a02-6 |
| V | Q002 |
| Nb | 00 a |
| C | Q58 |
| S | L0 |
| Mn | Q18 |
| Si | 10 20 |
The spur gear parameters are listed in Table 2.
| Parameter | Value |
|---|---|
| Modulus (mm) | |
| Number of teeth | 21 |
| Pressure angle (°) | 25 |
| Helix angle (°) | 32 |
| Tooth width (mm) | |
| Addendum coefficient | |
| Modification coefficient | |
| Accuracy grade |
2.2 Equipment and Process
The forging equipment used in this experiment is an NPS2500/4000 type 2500t screw press, equipped with a 500kW×2 dual-station intermediate frequency induction heating furnace. The process involves heating the billet to a suitable temperature and then forging it into the desired shape using a closed forging die.
3. Experimental Results and Analysis
3.1 Mechanical Properties and Microstructure of Forged Samples
3.1.1 Sample Preparation and Testing
Samples were prepared by varying the forging ratio and normalizing temperature. The samples were then subjected to tensile, impact, and metallographic tests to determine their mechanical properties and microstructure. The sample preparation details are shown in Table 3.
| Forging Ratio | Sample Specification (mm) |
|---|---|
| At normalizing temperature 880°C | 36×160(21) |
| At normalizing temperature 910°C | 36×160(22) |
| At normalizing temperature 950°C | 36×160(23) |
| At forging ratio 3 | 30×250(31) |
| At forging ratio 3 and normalizing temperature 910°C | 30X250(32) |
| At forging ratio 3 and normalizing temperature 950°C | 30X250(33) |
| At forging ratio 4 | 26X320(41) |
| At forging ratio 4 and normalizing temperature 910°C | 26X320(42) |
| At forging ratio 4 and normalizing temperature 950°C | 26X320(43) |
The test results are presented in Table 4.
| Sample Number | Rpa2 (MPa) | Rm (MPa) | A (%) | Z (%) | Impact Absorption Energy (J) | Non-metallic Inclusion Level | Grain Size Grade | Banded Structure Grade | |
|---|---|---|---|---|---|---|---|---|---|
| 21 | 903 | 1023 | 16 | 55 | 100 | Ao.B0.Co.DO | 2 | ||
| 909 | 1046 | 14 | 42 | 80 | 54 | 10 | |||
| 22 | 907 | 1046 | 17 | 53 | 106 | 52 | 10 | 0 | |
| 913 | 1048 | 17 | 52 | 90 | 58 | Ao.B0.Co.DO | |||
| 23 | 917 | 1058 | 15.5 | 52 | 94 | 50 | Ao.B0.Co.DO | 10 | 0 |
| 935 | 1076 | 16 | 48 | 98 | 56 | ||||
| 31 | 947 | 1070 | 15.5 | 53 | 80 | 50 | Ao.Bo.Co.DO | 10 | 2 |
| 922 | 1053 | 16 | 56 | 92 | 50 | ||||
| 32 | 883 | 1018 | 11.5 | 29 | 98 | 30 | Ao.B0.Co.DO | 10 | 2 |
| 922 | 1061 | 14.5 | 39 | 66 | 34 | ||||
| 33 | 949 | 1084 | 15 | 47 | 78 | 48 | Ao.B0.Co.DO | 10 | 0 |
| 960 | 1086 | 11 | 28 | 80 | 44 | ||||
| 41 | 932 | 1066 | 13.5 | 37 | 90 | 58 | Ao.B0.Co.DO | 10 | 0 |
| 913 | 1049 | 16 | 51 | 120 | 62 | ||||
| 42 | 920 | 1063 | 125 | 35 | 80 | 46 | Ao.B0.Co.DO | 10 | 0 |
| 914 | 1080 | 10 | 28 | 86 | 50 | ||||
| 43 | 963 | 1107 | 14 | 45 | 90 | 58 | Ao.B0.Co.DO | 10 | 0 |
| 960 | 1096 | 8 | 28 | 94 | 56 |
3.1.2 Results Analysis
The results show that the banded structure grade of 17Cr2Ni2MoVNb steel after forging is ≤2, and the grain size grade is 10. Moreover, as the forging ratio increases (greater deformation) and the normalizing temperature increases, the banded structure and grain size improve. At the same time, as the forging ratio increases, the yield ratio shows a certain downward trend. These findings provide important data for the selection of raw material sizes for spur gear precision forging.
3.2 Precision Forging Technology of Spur Gear
3.2.1 Forging Process Analysis
The closed forging method is adopted for the forging process. The initial forging temperature of the blank is set between 1000 – 1100 °C to reduce deformation resistance and improve the forming accuracy of the teeth and obtain a more complete metal flow line morphology.
3.2.2 Forging Design
Based on the typical sample size specifications of the gear, considering a grinding allowance of 0.6mm for the tooth profile, no grinding allowance at the tooth root, and adding a displacement of +0.15 to ensure uniform grinding allowance, an additional machining allowance is added for the outer diameter to account for possible insufficient filling at the tooth tip. A machining allowance is also left for the end face, and a shunt cavity is set to reduce forming resistance and accommodate excess metal. The inner hole is an extrusion deformation area and is left with a machining allowance. The forging drawing designed according to these parameters.
3.2.3 Blank Design
The blank is selected as a bar. According to the forging deformation characteristics, the volume of the forging drawing and the blank should be approximately equal, and after heating, it should be slightly smaller than the diameter of the die tooth root circle to facilitate placement in the die cavity for closed forging. The initially determined blank size is 50×(84±1) mm. The blank size design.
3.2.4 Die Design
3.2.4.1 Design of Pre-forging and Final Forging Dies for Closed Forging
To ensure complete spur gear forming, a pre-forging + final forging process is adopted, and the part is ejected by a push rod to ensure the final forging forming accuracy. The push rod is designed to have a clearance fit with the tooth cavity shape, so that the forging is evenly stressed during ejection, thereby improving the precision of the forged spur gear. The upper and lower dies are connected to the equipment bed and slider using T-type screws. The general layout of the closed forging die.
3.2.4.2 Tooth Profile Die Design
To enhance the deformation ability and service life of the die, a prestressing method is usually adopted. The die is clamped in one or more stress rings. At the contact surface between adjacent stress rings, an inward prestressing force is generated. After several layers are superimposed and applied to the innermost die, the forging pressure transmitted through the forging can be fully offset, increasing the maximum pressure that the die can withstand, improving the stress condition of the die, and enabling the die to withstand higher pressures. The tooth profile die with a single stress ring.
3.2.5 Closed Forging Finite Element Analysis
3.2.5.1 Blank Material Parameters
According to the high-temperature rheological characteristics of the material, the stress-strain curves of the material at different temperatures are measured. The results show that when the strain rate is the same, the higher the material temperature, the smaller the rheological stress.
3.2.5.2 CAE Analysis of the Forming Process
The forging material is set as an elastoplastic body, and the die material is set as a rigid body. The shear friction condition is adopted between the blank and the die, and the friction factor is taken as 0.3. The forging temperature is set as 1050 °C, and the extrusion speed of the upper and lower punches is 300mm/s. The simulation results of spur gear deformation at different reduction amounts.
Analysis shows that when the reduction amount is 41.2 mm, the bottom part of spur gear teeth is basically filled, and the middle and upper parts of spur gear teeth start to gradually form; when the reduction amount is 42.9mm, the forming is completed, and a complete tooth profile is obtained. At this time, the forming is relatively uniform along spur gear teeth.
The temperature and forming force of the forging after simulation forming are analyzed. The temperature change at different reduction amounts. During the precision forging process of spur gear, because the bottom part of spur gear first contacts the die, with the change of the pressure applied by the punch and the reduction amount, the blank starts to deform from the tooth root part until the tooth profile is completely filled. During the forming process, due to the direct interaction between the tooth root and the bottom part of the tooth tip and the die, the temperature change in this part is relatively high. The force change on the top of the die with the extension of time, and as spur gear is formed, the change of the load gradually increases, and when the corner of the tooth part is formed, the load speed increases linearly and reaches the maximum.
3.2.6 Results of Closed Forging of Spur Gear
Based on the above design of spur gear forgings, blanks, and closed forging dies, and the analysis of simulation results, it is considered that the above design scheme is feasible. The closed precision forging die and the spur gear forging physical samples are verified. The closed forging die, and the spur gear precision forging physical sample.
3.2.7 Distribution of Forging Flow Lines
After dissecting the precision forged spur gear, the distribution of forging flow lines is measured. The flow line distribution is basically consistent with the shape of the forging, indicating that the forging flow lines are basically complete and not damaged.
3.3 Small-batch Manufacturing of Spur Gear by Closed Forging
According to the above experimental results, the closed forging process flow for spur gear is determined as: intermediate frequency induction heating → flattening → pre-forging → final forging → ejection. The blank size is φ50mm×84mm. The forging temperature is: the die preheating temperature is 200 – 300 °C, and the initial forging temperature is 1050 °C. The forging hammer speed is 0.3m/s, and the lubricant is water-based graphite. The small-batch manufacturing of spur gear by closed forging is completed.
4. Conclusions
- Through different forging tests and analysis, the changes in the microstructure and mechanical properties of 17Cr2Ni2MoVNb steel after changing forging parameters such as forging ratio and normalizing temperature are obtained. The banded structure grade after forging is ≤2, the grain size grade is 10, and as the forging ratio increases and the normalizing temperature increases, the banded structure and grain size improve. At the same time, as the forging ratio increases, the yield ratio shows a certain downward trend.
- The closed forging process simulation analysis of spur gear is completed, as well as the design and manufacturing of spur gear forging drawings and closed forging dies. The precision forging manufacturing of spur gear with a modulus of 4mm and 21 teeth is realized, and the forging flow lines of the experimental spur gear is complete and in accordance with the forging process state.
- Using the “intermediate frequency induction heating → flattening → pre-forging → final forging → ejection” closed forging process flow for spur gear, with a blank size of φ50mm×84mm, forging temperatures of die preheating temperature 200 – 300 °C and initial forging temperature 1050 °C, and a forging hammer speed of 0.3m/s, and using water-based graphite as a lubricant, the small-batch manufacturing of spur gear with a modulus of 4mm and 21 teeth is completed.
- The closed forging process is used to achieve small-batch manufacturing of spur gear in this article, which can be extended and applied to the manufacturing of planetary gears with similar sizes and a large base number. It not only improves material utilization and processing efficiency but also provides a manufacturing method for meeting the requirements of long product life, obtaining better mechanical properties, improving gear quality, and increasing service time.