Research on Compound Forming Characteristics of Spiral Bevel Gears by Casting and Forging

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1. Introduction

Spiral bevel gears are critical components in gear transmission systems, directly influencing transmission efficiency and service life. Traditional forming processes for spiral bevel gears, such as forging from steel ingots, are energy-intensive, material-wasteful, and result in non-uniform microstructures. To address these challenges, casting-forging composite forming technology has emerged as a promising solution, combining the advantages of casting and forging to enhance mechanical properties and reduce environmental impact. This study investigates the microstructural evolution and optimal process parameters for spiral bevel gears produced via casting-forging composite forming.

2. Spiral Bevel Gear Casting and Forging Composite Forming Process

2.1 Traditional Forming Process

The traditional forming process for spiral bevel gears involves multiple steps:

  1. Steel ingot casting
  2. Billet opening
  3. Blanking
  4. Upsetting
  5. Punching
  6. Ring rolling
  7. Machining

However, this process has significant drawbacks, including high material and energy consumption, poor product quality, and non-uniform internal structures.

2.2 Casting-Forging Composite Forming Process

The casting-forging composite forming process streamlines production by:

  1. Casting molten metal into a pre-shaped blank.
  2. Forging the cast blank into the final gear shape using precision dies.

This approach reduces process steps, minimizes reheating, and improves microstructure homogeneity. The specific process for spiral bevel gears is:

  • Molten iron pouring → Casting blank → Rough forging → Finish forging.

2.3 Process Parameters

Key parameters for the casting-forging process are listed in Table 1.

ParameterValue/Range
Material20CrMoH low alloy steel
Casting temperature1520°C
Pouring time518 s
Forging die materialH13 tool steel
Forging deformation10%, 20%, 30%, 40%
Forging speed10–40 mm/s
Forging temperature850–1150°C
Environment temperature25°C
Die preheating temperature200°C
Friction coefficient0.65

3. Microstructural Analysis

3.1 As-Cast Microstructure

The microstructure of as-cast spiral bevel gears (Figure 1) consists of pearlite (gray-black) and ferrite (white) phases, with no recrystallization. This structure is characterized by coarse grains and segregation.

![Figure 1: Microstructure of as-cast spiral bevel gear (50 μm magnification)](Figure 1)

3.2 Effect of Forging Parameters on Microstructure

3.2.1 Forging Deformation

Increasing forging deformation from 10% to 40% results in significant grain refinement (Figure 2). Dynamic recrystallization initiates at grain boundaries and high dislocation density regions, leading to uniform, fine grains at 40% deformation.

Deformation (%)Grain Size (μm)Recrystallization
10CoarseNone
20ModeratePartial
30FineExtensive
40Very fineComplete

![Figure 2: Microstructure under different forging deformations (50 μm magnification)](Figure 2)

3.2.2 Forging Temperature

Optimal forging temperature (1050°C) produces the finest grains (Figure 3). Lower temperatures (850°C) result in incomplete recrystallization, while higher temperatures (1150°C) cause grain coarsening due to excessive thermal energy.

Temperature (°C)Grain Size (μm)Microstructure Features
850CoarsePartial recrystallization
950FineUniform recrystallization
1050Very fineOptimal refinement
1150CoarseOverheated structure

![Figure 3: Microstructure under different forging temperatures (50 μm magnification)](Figure 3)

3.2.3 Forging Speed

Reducing forging speed from 40 mm/s to 20 mm/s promotes dynamic recrystallization and grain refinement (Figure 4). Higher speeds limit recrystallization, leading to uneven microstructures.

Speed (mm/s)Grain Size (μm)Recrystallization Uniformity
10FineHigh
20Very fineExcellent
30ModerateModerate
40CoarsePoor

![Figure 4: Microstructure under different forging speeds (50 μm magnification)](Figure 4)

4. Optimization of Process Parameters

Based on microstructural analysis, the optimal casting-forging parameters for spiral bevel gears are:

  • Casting temperature: 1520°C
  • Pouring time: 518 s
  • Forging deformation: 40%
  • Forging temperature: 1050°C
  • Forging speed: 20 mm/s

These parameters yield a microstructure with fine, uniform grains and improved mechanical properties (Table 2).

PropertyAs-CastOptimized Composite
Tensile strength (MPa)450680
Yield strength (MPa)280520
Elongation (%)1218

5. Conclusion

  1. Microstructural Evolution: Casting produces a non-recrystallized structure of pearlite and ferrite, while forging induces dynamic recrystallization, resulting in fine, uniform grains.
  2. Parameter Influence:
    • Deformation: Higher deformation enhances grain refinement.
    • Temperature: 1050°C balances recrystallization and grain growth.
    • Speed: Lower speeds promote uniform recrystallization.
  3. Optimal Process: The recommended parameters achieve superior mechanical properties and microstructure homogeneity.
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