With increasing demands on railway vehicle speed and load capacity, the quality requirements for steel castings in bogie systems have intensified. This paper presents a systematic improvement strategy addressing defects observed during the production of axle box bodies – critical load-bearing components in metro vehicle bogies. Through structural analysis and process optimization, we achieved significant quality enhancements in these thin-walled box-type steel castings.
1. Structural Characteristics and Process Challenges
The axle box body, made from ZG25MnCrNiMo (Grade C steel), features complex geometry with wall thickness ranging 20-30 mm. Key process parameters include:
| Parameter | Value |
|---|---|
| Weight | 95 kg |
| Shrinkage allowance | 2% |
| Pouring temperature | 1,560-1,580°C |
| Molding material | CO₂-hardened sodium silicate sand |
The original casting process exhibited multiple defects including shrinkage porosity (12% occurrence), sand inclusion (8%), and dimensional inaccuracies (5% rejection rate). Through defect analysis, we identified three critical improvement areas:
$$ f(x) = k \cdot \frac{V}{A} \cdot \sqrt{\frac{T_m – T_p}{\rho \cdot c}} $$
Where k represents the mold material constant, V/A the volume-surface area ratio, and Tm, Tp the melting and pouring temperatures respectively.
2. Defect Analysis and Solutions
2.1 Sand Erosion in Thin Sections
The 50mm-deep rib cavity between large/small axle sleeves showed consistent sand erosion. Our solution combined:
- Reinforced core design with embedded steel pins
- Modified core binder composition (Table 1)
| Parameter | Original | Improved |
|---|---|---|
| Compressive strength (MPa) | 1.2 | 2.5 |
| Erosion resistance index | 65 | 92 |
| Shakeout performance | Grade C | Grade B |
2.2 Shrinkage Porosity in Thick Sections
The original riser design failed to maintain directional solidification in the 245mm-high axle sleeve. Modified riser configuration achieved better feeding efficiency:
$$ L_f = \frac{4.5 \cdot t}{\sqrt{M}} $$
Where Lf is feeding distance, t section thickness, and M the casting modulus. Key improvements included:
- Increased riser quantity from 2 to 3
- Changed riser shape from elliptical to circular
- Reduced inter-riser distance by 25%
3. Process Optimization Details
3.1 Gating System Redesign
Relocating the ingate from central ribs to machined surfaces eliminated crack formation. The new gating ratio (1:1.5:2) ensures laminar flow while maintaining adequate feeding pressure:
$$ v = \frac{Q}{A} = \frac{0.85 \cdot g \cdot H^{0.5}}{\mu} $$
Where v is metal velocity, Q flow rate, and H metallostatic head.
3.2 Core Positioning System
The redesigned spring seat core achieved ±0.5mm positioning accuracy through:
- Dual lateral locators instead of single bottom support
- Reduced core seat clearance from 3mm to 1mm
- Triangular reinforcement ribs in core structure
4. Production Verification
Implementing these improvements in 10 trial castings yielded:
| Quality Indicator | Original | Improved |
|---|---|---|
| Surface defects per m² | 3.2 | 0.4 |
| UT rejection rate | 15% | 0% |
| Machining allowance consistency | ±2.5mm | ±0.8mm |
The optimized steel casting process demonstrates how systematic analysis and targeted improvements can significantly enhance the quality of complex railway components. This methodology provides a template for addressing similar challenges in heavy-section steel castings across industries.