As fundamental components for transmitting motion and power, spur gears find extensive application across various engineering fields. The conventional manufacturing route for high-performance spur gears typically involves forging a blank followed by machining the tooth profile. Traditional free forging methods, however, are characterized by low production efficiency and significant post-forging machining allowances, leading to material waste and compromised mechanical properties due to the interruption of continuous metal flow lines. In contrast, flashless forging, also known as closed-die forging, offers a compelling alternative. This precision forging approach not only increases material utilization from approximately 40% to over 70% but also enhances gear strength by more than 20% and improves production efficiency by around 40%. For widely used spur gears, the manufacturing process directly impacts critical service performance metrics such as noise generation and operational lifespan. Given the hereditary nature of the microstructure and properties in steel materials, stringent control from the forging stage is paramount. This study investigates the precision forging technology for spur gears, encompassing material behavior analysis, process design, numerical simulation, and practical manufacturing trials.
Research Materials and Methodology
The material selected for this investigation is a low-alloy steel, with its chemical composition detailed in Table 1. The specific parameters of the target spur gears are listed in Table 2.
| C | Si | Mn | P | S | Cr | Ni | Mo | Nb | Balance |
|---|---|---|---|---|---|---|---|---|---|
| 0.20 | 0.18 | 0.58 | 0.004 | 0.002 | 1.70 | 1.60 | 0.28 | 0.02-0.06 | – |
| Module m (mm) | Number of Teeth z | Pressure Angle α (°) | Helix Angle β (°) | Face Width b (mm) | Addendum Coefficient ha* | Profile Shift Coefficient x | Accuracy Grade |
|---|---|---|---|---|---|---|---|
| 4 | 21 | 25 | 0 | 32 | 1 | 0 | 6 |
To establish a foundational understanding of the material’s forgeability and response to thermal-mechanical processing, a series of experiments were conducted. Specimens were prepared under varying forging ratios (degree of deformation) and subsequent normalizing temperatures. The forging ratio (Y) is defined as the ratio of the initial cross-sectional area to the final cross-sectional area:
$$Y = \frac{A_0}{A_1}$$
The specimen preparation matrix is summarized in Table 3.
| Forging Ratio | Specimen Dimension & Normalizing Temperature | ||
|---|---|---|---|
| 880°C | 910°C | 950°C | |
| 2 | φ36 × 160 | φ36 × 160 | φ36 × 160 |
| 3 | φ30 × 250 | φ30 × 250 | φ30 × 250 |
| 4 | φ26 × 320 | φ26 × 320 | φ26 × 320 |
These specimens were subjected to tensile testing, impact testing (at room and low temperatures), and metallographic examination to evaluate non-metallic inclusions, grain size, and the severity of banded structure. This data provides critical insights for determining optimal forging parameters for the spur gears.
Results and Discussion: Material Behavior and Process Design
1. Influence of Forging Process on Material Properties
The mechanical properties and microstructural ratings obtained from the prepared specimens are consolidated in Table 4.
| Specimen ID | Rp0.2 (MPa) | Rm (MPa) | A (%) | Z (%) | Impact Energy (J) | Inclusion Level | Grain Size Grade | Banded Structure Grade | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Test1 | Test2 | Test1 | Test2 | Test1 | Test2 | Test1 | Test2 | RT | LT | ||||
| Forging Ratio 2, 880°C | 903 | 909 | 1023 | 1046 | 16 | 14 | 55 | 42 | 100 | 56 | A0,B0,C0,D0 | 10 | 2 |
| Forging Ratio 2, 910°C | 907 | 913 | 1046 | 1048 | 17 | 17 | 53 | 52 | 106 | 52 | A0,B0,C0,D0 | 10 | 0 |
| Forging Ratio 2, 950°C | 917 | 935 | 1058 | 1076 | 15.5 | 16 | 52 | 48 | 94 | 50 | A0,B0,C0,D0 | 10 | 0 |
| Forging Ratio 3, 880°C | 947 | 922 | 1070 | 1053 | 15.5 | 16 | 53 | 56 | 80 | 50 | A0,B0,C0,D0 | 10 | 2 |
| Forging Ratio 3, 910°C | 883 | 922 | 1018 | 1061 | 11.5 | 14.5 | 29 | 39 | 98 | 30 | A0,B0,C0,D0 | 10 | 2 |
| Forging Ratio 3, 950°C | 949 | 960 | 1084 | 1086 | 15 | 11 | 47 | 28 | 78 | 48 | A0,B0,C0,D0 | 10 | 0 |
| Forging Ratio 4, 880°C | 932 | 913 | 1066 | 1049 | 13.5 | 16 | 37 | 51 | 90 | 58 | A0,B0,C0,D0 | 10 | 0 |
| Forging Ratio 4, 910°C | 920 | 914 | 1063 | 1080 | 12.5 | 10 | 35 | 28 | 80 | 46 | A0,B0,C0,D0 | 10 | 0 |
| Forging Ratio 4, 950°C | 963 | 960 | 1107 | 1096 | 14 | 8 | 45 | 28 | 90 | 58 | A0,B0,C0,D0 | 10 | 0 |
The analysis of the test data reveals significant trends. The microstructure of the forged steel consistently exhibited a fine grain size of grade 10, and the banded structure was effectively controlled to a level of 2 or better. A clear correlation was observed: a larger forging ratio (greater deformation) combined with a higher normalizing temperature resulted in superior microstructural characteristics, notably a finer and more homogeneous structure with minimal banding. Furthermore, the yield ratio (Rp0.2/Rm) showed a discernible decreasing trend with increasing forging ratio. This relationship is crucial for selecting the initial billet size for the precision forging of spur gears, as it influences the final strength and ductility balance. The constitutive behavior of the material at high temperatures, essential for process simulation, can be described by a simplified power-law relationship:
$$\sigma = K \epsilon^n$$
Where $\sigma$ is the flow stress, $\epsilon$ is the strain, $K$ is the strength coefficient, and $n$ is the strain-hardening exponent. The values of $K$ and $n$ are strongly temperature-dependent, as indicated by the stress-strain curves obtained at various temperatures.
2. Precision Forging Process Design for Spur Gears
Based on the material study, the manufacturing route for the spur gears was re-engineered. The conventional process (involving free forging, rough turning, and gear hobbing/shaping) was replaced with a precision forging-centric process: Billet preparation → Precision flashless forging → Finish machining of end faces and bore → Carburizing heat treatment → Finish grinding.
Forging Process Analysis: Considering the technical requirements and structural characteristics of spur gears, a closed-die forging method was selected for hot forming. The initial forging temperature was set between 1000°C and 1100°C to reduce deformation resistance and improve the formability and precision of the tooth profile, thereby achieving a more complete metal flow line pattern.
Forging Design: The forging was designed based on the final gear dimensions. A grinding allowance of 0.6 mm was added to the tooth flanks, while the root diameter was left without allowance. To ensure uniform stock for grinding, a positive profile shift of +0.15 was incorporated. The outer diameter and end faces included machining allowances. A flash gutter or overflow cavity was designed to minimize forming load and accommodate any excess material. The bore, being a region of extrusion during forging, was also designed with a machining allowance.
Billet Design: Cylindrical bar stock was chosen as the starting material. Applying the principle of volume constancy in plastic deformation ($V_{billet} \approx V_{forging}$), the billet dimensions were calculated. To facilitate easy placement into the die cavity, the heated billet diameter should be slightly smaller than the root diameter of the gear teeth in the die. The initial billet size was determined to be φ50 mm × (84 ± 1) mm.
Die Design: To ensure complete and precise filling of the complex tooth geometry, a two-stage forging sequence (pre-forging followed by finish forging) was employed. Ejector pins were integrated into the die design for part removal. The finish forging ejector was designed to match the tooth profile for uniform ejection force. The upper and lower dies were mounted to the press ram and bed using T-slot bolts. To withstand the extremely high pressures during the closed-die forging of spur gears, especially at the tooth corners, a prestressed die assembly was utilized. The core insert containing the tooth profile is shrink-fitted into one or more concentric stress rings. This creates beneficial compressive pre-stresses at the interface, which counteract the tensile stresses induced by the forging load, significantly enhancing the die’s load-bearing capacity and service life. The design of the closed-die forging assembly and the prestressed tooth die insert were finalized for manufacturing.
3. Finite Element Analysis of Closed-Die Forging
Prior to physical trials, a finite element analysis (FEA) was conducted to simulate the closed-die forging process of the spur gears. This simulation aimed to analyze the forming characteristics, verify the designed parameters, and predict forming loads and potential defects.
Simulation Setup: The billet was modeled as a plastic body with temperature-dependent flow stress data. The dies were modeled as rigid bodies. Shear friction with a factor of 0.3 was applied at the billet-die interfaces. The initial forging temperature was set to 1050°C, and the press speed was 300 mm/s.
Simulation Results: The simulation successfully captured the progressive filling of the die cavity. At a ram displacement of 41.2 mm, the lower portion of the gear teeth was nearly fully formed, while the middle and upper sections began to take shape. Complete filling of the tooth profile was achieved at a displacement of 42.9 mm, resulting in a uniformly formed gear. The analysis of the temperature field revealed that areas in direct and prolonged contact with the die, such as the tooth root and tip regions, experienced more significant temperature changes due to heat transfer. The evolution of forming load was also monitored. The load increased gradually as the material deformed and filled the cavity, followed by a sharp, near-vertical rise as the final corners of the tooth profile were filled, reaching the maximum load. This peak load $F_{max}$ is critical for press selection and die stress analysis. The deformation process can be related to the effective strain $\bar{\epsilon}$, which is a function of the displacement and the geometry of the spur gears:
$$\bar{\epsilon} = f(\Delta h, m, z, b)$$
where $\Delta h$ is the ram displacement, $m$ is the module, $z$ is the number of teeth, and $b$ is the face width.
Forging Trials and Production Implementation
Guided by the results of the material study, process design, and FEA, the closed-die forging tools were manufactured, and physical trials were conducted. The actual closed-die forging assembly and a successfully produced precision forged spur gear are shown in the documentation from the trials. A macro-etch test was performed on a sectioned forged gear to examine the flow lines. The results confirmed that the metal flow lines followed the contour of the gear teeth continuously and were not severed, which is a hallmark of a sound forging process and contributes to enhanced fatigue performance of the final spur gears.
Based on the successful trial, the final process parameters for small-batch production were established:
- Process Flow: Induction Heating → Upsetting → Pre-forging → Finish Forging → Ejection.
- Billet Size: φ50 mm × 84 mm.
- Forging Temperature: Die preheat: 200-300°C; Billet start temperature: 1050°C.
- Press Speed: 0.3 m/s.
- Lubricant: Water-based graphite.
This closed-die forging process was successfully implemented for the small-batch manufacturing of the target spur gears (module 4 mm, 21 teeth). The process demonstrates significant advantages for producing spur gears with similar dimensions and high production volumes, such as planetary gears. It offers a substantial increase in material utilization, improved production efficiency, and most importantly, superior mechanical properties and longer service life due to the optimized microstructure and continuous grain flow. This contributes to more sustainable manufacturing practices.