Impact of Sn Element on Casting Defects in CN7M Alloy and Mitigation Strategies

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In industrial applications such as petroleum, chemical, and nuclear power systems, valve components cast from CN7M austenitic stainless steel are critical due to their superior corrosion resistance. However, uncontrolled residual elements like tin (Sn) can severely degrade casting quality. This study investigates Sn-induced crack formation mechanisms in CN7M castings and proposes effective countermeasures.

1. Characterization of Sn-Related Casting Defects

Massive intergranular cracks were observed in CN7M valve castings produced using substandard master alloys. The crack propagation exhibited three distinct features:

  1. Macroscopic network-like distribution across variable cross-sections
  2. Oxidized fracture surfaces following columnar grain boundaries
  3. Microscopic Sn-rich precipitates at grain boundaries (0.05–0.06 wt%)
Table 1. Chemical Composition of Cracked CN7M Castings (wt%)
Batch C Si Mn Cr Ni Cu Sn
1 0.024 0.86 0.88 19.47 28.03 3.33 0.058
2 0.031 0.79 0.91 19.68 28.16 3.29 0.051
3 0.028 0.83 0.90 19.71 28.22 3.27 0.055

2. Thermodynamic Analysis of Sn Segregation

The equilibrium partition coefficient for Sn in austenite can be expressed as:

$$
k_{Sn} = \frac{C_s}{C_l} \approx 0.02-0.05
$$

where \( C_s \) and \( C_l \) represent Sn concentrations in solid and liquid phases, respectively. This low partitioning coefficient promotes Sn enrichment at final solidification zones, particularly at grain boundaries.

3. Crack Formation Mechanism

Sn-induced casting defects develop through three synergistic effects:

  1. Intermetallic Formation: Sn combines with Cu to create low-melting compounds (e.g., Cu6Sn5, melting point ≈415°C)
  2. Grain Boundary Embrittlement: Reduced interfacial energy at boundaries:
    $$
    \gamma_{gb} = \gamma_0 – \Gamma_{Sn}RT\ln(1+X_{Sn})
    $$
    where \( \gamma_0 \) = intrinsic boundary energy, \( \Gamma_{Sn} \) = Sn adsorption coefficient
  3. Thermal Stress Concentration: Thermal gradients during cooling generate stresses exceeding boundary strength:
    $$
    \sigma_{thermal} = E\alpha\Delta T \left(1 – \frac{2\nu}{1-\nu}\right)
    $$

4. Process Optimization for Defect Mitigation

Revised melting practices achieved Sn reduction from 0.06% to 0.005% through:

Table 2. Optimized Charge Composition for 1-ton Heat
Material Mass (kg) Sn Content (ppm)
316 Stainless 495 <50
Electrolytic Mn 5 <10
Nickel Plate 232 <20
Electrolytic Cu 32 <30

The improved process eliminated casting defects through:

  • Strict Sn limitation: \( X_{Sn} \leq 0.008\% \)
  • Controlled cooling rates: \( \frac{dT}{dt} \leq 25°C/min \) between 1200–900°C
  • Enhanced melt purification: Oxygen activity \( a_O \leq 3ppm \)

5. Quality Verification

Post-optimization analysis confirmed:

  • Zero crack occurrence in 500+ castings
  • Grain boundary Sn levels <0.005% (EDS verification)
  • Improved mechanical properties:
    $$
    \sigma_b \geq 485MPa,\quad \delta \geq 40\%
    $$

6. Conclusion

This study demonstrates that controlling Sn content below 0.01% effectively eliminates casting defects in CN7M components. The developed methodology provides a systematic approach for managing residual elements in high-performance austenitic stainless steel castings.

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