Analysis and Improvement of Plasticity in Copper-Containing Steel Castings

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Introduction

Copper-containing steel castings have gained significant attention in marine engineering due to their exceptional combination of high strength, ductility, low-temperature impact toughness, and weldability. However, recent quality inspections revealed abnormal fractures in large castings (maximum wall thickness: 300 mm) after heat treatment, where plasticity indices fell below specifications despite satisfactory strength metrics. This study investigates the root cause of plasticity degradation through comprehensive fracture analysis and proposes effective mitigation strategies.

Material and Methods

The steel casting was produced via electric arc furnace melting with LF+VD refining and argon-protected pouring. Key chemical compositions are shown in Table 1.

Table 1: Chemical Composition of Copper-Containing Steel Casting (wt.%)
Element C Si Mn P S Ni Cr Cu
Content ≤0.13 ≤0.50 ≤0.80 ≤0.020 ≤0.030 ≤1.60 ≤0.30 ≤0.80

The heat treatment protocol consisted of three stages:

Table 2: Heat Treatment Parameters
Process Temperature (°C) Duration (h) Cooling
Pre-treatment 640 ± 10 48 Furnace
Normalizing 920 ± 10 7 Air
Tempering 620 ± 10 10 Air

Mechanical Performance Anomalies

Post-treatment mechanical testing revealed insufficient elongation (18% vs required 20%) despite acceptable strength and toughness (Table 3).

Table 3: Mechanical Property Comparison
Parameter ReL (MPa) Rm (MPa) A (%) Z (%) KV-40°C (J)
Required ≥370 ≥490 ≥20 ≥40 ≥27
Measured 406 526 18.0 57 47

Fractographic Analysis

SEM examination revealed:

  1. Minimal necking with circular dimples (24× magnification)
  2. Quasi-cleavage facets within dimples (200-1000×)
  3. Smooth surfaces at defect centers indicating hydrogen-induced microvoids

Hydrogen Diffusion Theory

The hydrogen concentration evolution during dehydrogenation heat treatment follows:

$$ U = \frac{H}{H_0} = \Phi\left(\frac{D\tau}{R^2}, \frac{P}{Q}, \frac{r}{R}\right) $$

Where:

  • $U$: Hydrogen concentration parameter
  • $D$: Diffusion coefficient (cm²/s)
  • $\tau$: Treatment duration (h)
  • $R$: Casting radius (cm)

The dimensionless F0 parameter determines dehydrogenation efficiency:

$$ F_0 = \frac{D\tau}{R^2} $$

Table 4: Dehydrogenation Efficiency at Various Durations
Duration (h) F0 Residual H (ppm)
72 0.2475 3.0
144 0.4950 1.0
216 0.7425 0.4

Process Optimization

Modified heat treatment parameters:

  1. Extended dehydrogenation at 640°C for 144 hours
  2. Enhanced argon shielding during pouring
  3. Strict mold drying protocols (>24 h at 250°C)

Validation Results

Post-optimization mechanical tests showed significant improvement:

Table 5: Post-Treatment Mechanical Properties
Parameter A (%) Z (%) KV-40°C (J) Residual H (ppm)
Initial 18.0 57 47 5.5
Optimized 25.0 70 72 0.5

Conclusion

This study demonstrates that hydrogen embrittlement constitutes the primary mechanism for plasticity degradation in thick-section copper-containing steel castings. Through systematic dehydrogenation treatment design and process control, the elongation increased by 38.9% while maintaining other mechanical properties. The proposed methodology provides critical insights for manufacturing high-performance marine steel castings.

Critical Factors in Steel Casting Production

Key considerations for optimizing copper-containing steel castings:

$$ \text{Hydrogen Control Index} = \frac{\text{Initial H} – \text{Final H}}{\text{Treatment Time}} \times \sqrt{\text{Section Thickness}} $$

  1. Maintain molten steel hydrogen ≤ 1.5 ppm through VD refining
  2. Implement real-time argon shielding monitoring during pouring
  3. Apply multi-stage dehydrogenation for castings >200 mm thickness

The developed approach significantly enhances the reliability of steel castings in critical marine applications, providing a scientific basis for large-scale component manufacturing.

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