CLADDING TECHNOLOGY SHANXI CO., LTDPenetration Cracking Analysis in CuNi Alloy Cladding on 16Mn Steel Plate
Literature Overview
This study focuses on the formation mechanism and prevention of penetration cracks in copper-nickel (CuNi) alloy overlay welds deposited on 16Mn low-alloy steel substrates. CuNi alloys, particularly 90-10 CuNi and 70-30 CuNi grades, are widely used in seawater-cooled heat exchangers and marine applications where resistance to biofouling and galvanic corrosion is essential. The 16Mn steel substrate provides structural strength at a fraction of the cost of full CuNi construction. However, the significant mismatch in thermal expansion coefficients and solidification behavior between the substrate and overlay creates a high susceptibility to hot cracking and cold cracking, with penetration cracks being the most damaging defect type.
Crack Formation Mechanism
The study identifies three primary mechanisms contributing to penetration crack formation:
- Solidification cracking in the dilution zone: The CuNi alloy solidifies with a wide mushy zone, and the presence of sulfur and phosphorus impurities from the 16Mn substrate promotes liquid film formation at grain boundaries during solidification, leading to hot tearing.
- Thermal stress cracking: The coefficient of thermal expansion for 90-10 CuNi is approximately 17.2 × 10⁻⁶/K, while 16Mn steel is approximately 12.0 × 10⁻⁶/K. This 43% mismatch generates tensile stresses at the fusion boundary during cooling, which can exceed the yield strength of the dilution zone.
- Hydrogen-induced cracking: Hydrogen generated from moisture in the flux or electrode coating diffuses into the dilution zone during welding. The high carbon equivalent of the 16Mn HAZ (CE ≈ 0.45–0.55) combined with hydrogen accumulation creates conditions favorable for delayed cracking, typically occurring 2–48 hours after welding.
Process Optimization and Countermeasures
The study proposes several countermeasures that have been validated through both experimental and production trials:
| Countermeasure | Mechanism | Effectiveness |
|---|---|---|
| Preheating to 200–250 °C | Reduces cooling rate, promotes hydrogen diffusion | Reduces cold cracking by 60–70% |
| Post-weld heat treatment (650 °C × 2 h) | Relieves residual stress, promotes hydrogen outgassing | Essential for thick sections (>15 mm) |
| Low-hydrogen flux selection | Reduces hydrogen pickup at source | Reduces HIC susceptibility |
| Multi-pass welding with intermediate stress relief | Limits cumulative thermal stress | Recommended for sections >20 mm |
| Using a transition layer (e.g., Ni-based alloy) | Buffers thermal expansion mismatch | Reduces dilution zone stress by 30–40% |
Metallographic Analysis and Defect Characterization
Metallographic examination reveals that penetration cracks typically initiate at the fusion boundary and propagate through the dilution zone into the base metal. The crack morphology is intergranular in the dilution zone, consistent with solidification cracking, and becomes transgranular as it propagates into the base metal, indicating a transition to stress-driven fracture. SEM analysis of crack surfaces shows dendritic Cu-rich phases and eutectic Cu-Ni compounds, confirming that the cracking occurs during solidification when the liquid fraction is between 0.1 and 0.3.
The study also identifies that the weld composition in the dilution zone, when analyzed by optical emission spectroscopy (OES), typically contains 15–35% Cu and 5–15% Ni, which falls within the brittle intermetallic region of the Cu-Fe binary phase diagram. This compositional range is particularly susceptible to cracking during solidification.
Engineering Practice and Standard Compliance
In accordance with GB/T 151 and NB/T 47002, CuNi cladding on steel substrates requires qualification testing that includes:
- Bond strength testing per ASTM A263 (minimum 300 MPa for CuNi overlays)
- Hydrostatic testing at 1.25 times design pressure
- Ultrasonic testing of the cladding layer and fusion boundary
- Macrographic examination of cross-sections to verify cladding thickness and uniformity
A practical case from seawater heat exchanger fabrication showed that implementing a two-layer strategy—a first layer of Ni-Fe transition alloy followed by the 90-10 CuNi overlay—reduced penetration crack incidence from 12% to below 2% in production runs.
Study Insights and Reflections
This literature provides valuable insight into the metallurgical challenges of dissimilar metal cladding. The identification of the dilution zone composition as the critical factor in crack susceptibility has direct implications for welding procedure specification. Engineers should carefully control the dilution rate by adjusting heat input, travel speed, and electrode composition to ensure that the dilution zone composition falls within a ductile region of the phase diagram. The study also highlights the importance of post-weld heat treatment as a mandatory step for CuNi cladding on steel, a requirement that is sometimes overlooked in production environments where schedule pressure is high. The combination of preheat, controlled heat input, transition layer deposition, and post-weld stress relief represents a comprehensive approach that addresses all three crack formation mechanisms identified in this research.