Casting defects in large screw compressor cylinders made of ZG15Cr13 martensitic steel pose significant challenges due to the material’s inherent brittleness and susceptibility to cracks during solidification. This article details a systematic approach to address cracks, porosity, and shrinkage defects through optimized welding repair protocols, nondestructive testing, and dimensional verification.
1. Classification and Detection of Casting Defects
Typical casting defects observed in ZG15Cr13 cylinders include:
| Defect Type | Characteristics | Detection Method |
|---|---|---|
| Surface cracks | Linear discontinuities (≤150 mm) | PT (Penetrant Testing) |
| Subsurface porosity | Clustered voids (≤270 mm) | UT (Ultrasonic Testing) |
| Shrinkage cavities | Irregular cavities at hot spots | Visual + PT |
The defect severity assessment follows:
$$ S_d = \frac{A_d}{A_t} \times 100\% $$
Where \( S_d \) = defect severity index, \( A_d \) = defective area, \( A_t \) = total inspected area.
2. Defect Removal and Crack Control
For crack termination, crack stop holes are implemented using:
$$ D_{hole} = 2 \times t_{wall} + 5\ \text{mm} $$
Where \( D_{hole} \) = hole diameter, \( t_{wall} \) = wall thickness. This prevents crack propagation while minimizing welding distortion.
3. Welding Repair Methodology
A hybrid welding strategy combines martensitic and austenitic materials:
| Layer | Process | Material | Hardness (HV) |
|---|---|---|---|
| Base | SMAW | E410-15 | 280-320 |
| Surface | GTAW | ER309L | 180-220 |
Preheating temperature calculation:
$$ T_p = 150 + 70 \times \log(t_{mat}) $$
Where \( T_p \) = preheat temperature (°C), \( t_{mat} \) = material thickness (mm). Post-weld heat treatment maintains:
$$ T_{PWHT} = 620 \pm 15^{\circ}\text{C}\ \text{for}\ 2\ \text{hours} $$
4. Quality Verification Protocol
The three-stage validation process includes:
| Stage | Test | Acceptance Criteria |
|---|---|---|
| 1 | PT Inspection | GB/T 9443 Class I |
| 2 | Hydrostatic Test | 0.81 MPaG × 1 hour |
| 3 | Helium Leak Test | ≤1×10-6 mbar·L/s |
Dimensional tolerances are verified using:
$$ \Delta G = \frac{D_{act} – D_{nom}}{D_{nom}} \times 100\% \leq 0.05\% $$
Where \( \Delta G \) = geometric deviation.
5. Process Optimization Outcomes
Field data from repaired compressors shows:
| Parameter | Pre-Repair | Post-Repair |
|---|---|---|
| Vibration (mm/s) | 8.2 | 2.1 |
| Leak Rate (%) | 12.7 | 0.9 |
| MTBF (hr) | 4,200 | 18,500 |
The success formula for casting defect repair:
$$ Q_{repair} = \frac{(H_{base} – H_{weld}) \times \sigma_{UTS}}{E_{residual}} \geq 1.5 $$
Where \( Q_{repair} \) = quality index, \( H \) = hardness, \( \sigma_{UTS} \) = ultimate tensile strength, \( E_{residual} \) = residual stress.
6. Preventive Measures for Casting Defects
Modified foundry parameters reduce defect occurrence:
| Parameter | Original | Optimized |
|---|---|---|
| Pour Temp (°C) | 1,680 | 1,620 |
| Mold Preheat (°C) | 200 | 350 |
| Feeding Ratio | 1.8 | 2.3 |
The thermal gradient control equation:
$$ \nabla T = \frac{T_{pour} – T_{mold}}{t_{solid}} \leq 25^{\circ}\text{C/s} $$
This prevents shrinkage-related casting defects through controlled solidification.