CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Treatment on Microstructure and Properties of Laser Cladded Ni/316L Overlay Layer
Literature Overview
This study investigates the influence of post-weld heat treatment (PWHT) on the microstructural evolution and mechanical performance of a nickel-based (Ni) layer deposited by laser cladding onto a 316L stainless steel substrate. The work is particularly relevant to engineers dealing with bimetallic pressure vessels and heat exchanger tubes where nickel-based overlay layers are applied to resist severe corrosion environments such as high-temperature sulfuric acid, hydrochloric acid, and molten salt media. The authors systematically examined the as-deposited condition and three different PWHT regimes, providing a comprehensive comparison that is directly applicable to engineering practice.
Core Technical Content
Laser Cladding Process Parameters
The laser cladding process employed a fiber laser with a maximum output power of 40 kW, operating at a power level of 2.5–3.0 kW. The scanning speed ranged from 1000 to 2000 mm/min, with a powder feed rate of 8–12 g/min. The powder used was a commercial Ni-based alloy (similar to Stellite 6 composition, containing Cr, Mo, and Co) deposited onto an AISI 316L substrate plate. The key process window for achieving a defect-free single-pass cladding with a dilution ratio below 15% was identified as follows:
| Parameter | Range | Optimal Value |
|---|---|---|
| Laser power | 2.5–3.5 kW | 3.0 kW |
| Scanning speed | 800–2200 mm/min | 1500 mm/min |
| Powder feed rate | 6–14 g/min | 10 g/min |
| Powder particle size | 30–75 μm | 45–60 μm |
| Dilution ratio | 10–25% | <15% |
| Cladding thickness | 0.3–0.8 mm per pass | 0.5 mm |
Microstructural Analysis
In the as-deposited condition, the Ni overlay layer exhibited a columnar dendritic microstructure with primary dendrites growing perpendicular to the substrate interface. The interdendritic regions contained intermetallic phases, predominantly Ni₃(Fe,Cr) and Cr₂₃C₆, which are hard and brittle. The 316L substrate near the fusion boundary showed a narrow heat-affected zone (HAZ) approximately 80–120 μm wide, with a martensitic transformation in the 316L due to the rapid cooling rate exceeding 1000 °C/s.
After PWHT at 1050 °C for 2 hours followed by air cooling, the columnar dendritic structure was partially recrystallized, and the interdendritic carbides underwent significant spheroidization. The volume fraction of Cr₂₃C₆ decreased from approximately 12 vol% to 5 vol%, while the matrix hardness dropped from 480 HV to 320 HV. A more refined grain structure was observed at the fusion boundary, with grain size reduced from about 15 μm to 8 μm.
Mechanical Properties and Corrosion Resistance
| Condition | Hardness (HV) | Tensile Strength (MPa) | Elongation (%) | Pitting Corrosion Potential (mV vs SCE) |
|---|---|---|---|---|
| As-deposited | 480 | 720 | 8 | -180 |
| PWHT 1050°C/2h | 320 | 580 | 15 | -120 |
| PWHT 900°C/2h | 400 | 650 | 11 | -150 |
| PWHT 800°C/2h | 450 | 700 | 9 | -170 |
The PWHT at 1050 °C significantly improved ductility and corrosion resistance but at the cost of hardness and tensile strength. The 900 °C treatment offered a balanced compromise, maintaining adequate hardness while improving ductility and corrosion resistance compared to the as-deposited condition. The pitting corrosion potential shift of +60 mV after 1050 °C PWHT indicates substantially improved resistance to chloride-induced pitting.
Key Technical Insights and Engineering Implications
Dilution Control
One critical observation is that the dilution ratio between the Ni overlay and 316L substrate directly affects the effectiveness of PWHT. When dilution exceeds 20%, the overlay layer begins to inherit the sensitization tendency of the 316L matrix, leading to chromium depletion at grain boundaries. This is particularly problematic in chloride-containing environments. The study confirms that maintaining dilution below 15% is essential for achieving the full corrosion-resistant benefit of the Ni-based overlay, which aligns with recommendations in ASME Section IX and API 934 for dissimilar metal overlay applications.
Heat Treatment Selection Criteria
The choice of PWHT temperature must be carefully balanced against the service requirements. For applications demanding high wear resistance (e.g., slurry pump impellers, valve trim), the as-deposited condition or a moderate PWHT at 800 °C may be preferred. For corrosion-critical applications such as hydrogenation reactor internals or acid storage vessels, a full solution treatment at 1050 °C is recommended, accepting the reduction in hardness. The 900 °C treatment represents a practical compromise for dual-service components requiring moderate wear and corrosion resistance.
Bond Strength Considerations
The bond strength between the Ni overlay and 316L substrate was measured at 385 MPa in the as-deposited condition and increased to 420 MPa after 1050 °C PWHT. This improvement is attributed to the dissolution of brittle intermetallic phases at the interface and the formation of a more homogeneous diffusion zone. According to GB/T 150 and ASME VIII Div.1, the minimum acceptable bond strength for clad plate pressure vessels is typically 50 ksi (345 MPa), and all tested conditions met this requirement.
Practical Recommendations
For engineers designing bimetallic components with laser-cladded Ni overlays on 316L substrates, the following recommendations are derived from this study:
- Limit the dilution ratio to below 15% by optimizing laser power, scanning speed, and powder feed rate during the cladding process.
- Apply PWHT at 1050 °C for 2 hours for maximum corrosion resistance, or at 900 °C for a balanced property set.
- Conduct bond strength testing in accordance with ASTM G129 or equivalent to verify interface integrity after PWHT.
- Perform intergranular corrosion testing per ASTM A263 to confirm that PWHT has eliminated sensitization in the overlay layer.
- Consider multi-pass cladding with interpass temperature control below 150 °C to minimize HAZ softening in the substrate.
Study Insights and Reflections
This literature provides valuable guidance on the trade-offs inherent in post-weld heat treatment of laser-cladded Ni/316L systems. The most striking finding is that the as-deposited condition, despite its superior hardness, exhibits the poorest corrosion resistance due to the presence of continuous interdendritic carbides and residual stresses. This underscores a fundamental principle in overlay welding engineering: mechanical properties and corrosion resistance are not always positively correlated, and the selection of post-weld treatment must be driven by the primary failure mode expected in service. For pressure vessel applications governed by GB/T 150 and NB/T 47002, the additional consideration of weld residual stress and its effect on stress corrosion cracking susceptibility further reinforces the importance of appropriate PWHT selection. The study also highlights the critical role of dilution control, which remains one of the most challenging aspects of laser cladding in industrial production environments.