CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Treatment on Microstructure and Properties of Ni/316L Laser Cladding Layer
Literature Overview
This 2022 study published in Hot Working Technology (热加工工艺) by Wang Hao, Li Xin, Qu Yulong, and Dong Hui investigates the effect of post-weld heat treatment on the microstructure and mechanical properties of a layered Ni/316L laser cladding deposit. The research is particularly relevant to the oil and gas drilling industry, as the authors are affiliated with PetroChina Changqing Drilling Company and the Drilling and Production Engineering Technology Research Institute. The Ni/316L layered cladding is designed to provide a combination of the exceptional corrosion resistance of nickel and the good mechanical properties and moderate cost of 316L stainless steel, making it suitable for downhole tools and drilling equipment exposed to aggressive formation fluids.
Core Technical Content
The study employs laser cladding (also known as laser melting or laser surfacing) to deposit alternating layers of nickel and 316L stainless steel powder on a substrate, followed by various heat treatment conditions to evaluate their effect on the overlay's microstructure and properties.
| Parameter | Value / Range | Description |
|---|---|---|
| Laser power | 2-6 kW | Depends on powder feed rate and travel speed |
| Powder feed rate | 5-20 g/min | Controls deposition rate and layer thickness |
| Travel speed | 100-400 mm/min | Affects heat input and cooling rate |
| Powder composition (Ni layer) | Pure Ni or Ni-based alloy | Provides corrosion resistance |
| Powder composition (316L layer) | 316L stainless steel powder | Provides mechanical properties |
| Layer thickness per pass | 0.3-0.8 mm | Typical for single-pass laser cladding |
| Total overlay thickness | 2-6 mm | Multiple passes to achieve desired thickness |
| Shielding gas | Argon | Prevents oxidation during cladding |
The heat treatment conditions investigated typically include:
- Solution treatment: 1050-1150 °C, 1-2 h, water quench
- Aging treatment: 700-800 °C, 2-4 h, air cool
- Combined solution + aging: Solution treatment followed by aging
- Stress relief: 500-600 °C, 1-2 h, furnace cool
Microstructural Evolution with Heat Treatment
The as-deposited microstructure of the Ni/316L layered cladding exhibits several distinctive features:
- Columnar grains growing perpendicular to the substrate surface, driven by the steep thermal gradient during laser cladding.
- Fine dendritic structure within the grains, with very small dendrite arm spacing due to the extremely high cooling rates (10^3 to 10^5 K/s) inherent to laser cladding.
- Interface characteristics: The Ni/316L interfaces show evidence of interdiffusion, with a thin diffusion zone (typically 5-20 μm) where the composition transitions from Ni-rich to Fe-rich.
- Residual stress: The as-deposited overlay contains significant tensile residual stresses due to the rapid solidification and thermal contraction during each pass.
The effect of heat treatment on this microstructure is profound:
- Solution treatment (1050-1150 °C): Dissolves the δ-ferrite that forms in the 316L layers during solidification, homogenizes the composition at the Ni/316L interfaces, and reduces residual stresses. The columnar grain structure may partially transform to equiaxed grains through recrystallization.
- Aging treatment (700-800 °C): Promotes the precipitation of carbides and intermetallic phases, particularly at the Ni/316L interfaces. This can improve hardness but may reduce ductility and corrosion resistance if excessive.
- Combined solution + aging: This two-step treatment achieves the best balance of properties. The solution treatment homogenizes the microstructure and dissolves harmful phases, while the subsequent aging precipitates fine strengthening particles that improve mechanical properties without significantly compromising corrosion resistance.
- Stress relief (500-600 °C): Primarily reduces residual stresses without significantly altering the microstructure. This is a practical treatment for in-service repair applications where full solution treatment is not feasible.
Property Assessment
The mechanical and corrosion properties of the Ni/316L overlay are significantly influenced by the heat treatment:
| Property | As-Deposited | After Solution Treatment | After Solution + Aging |
|---|---|---|---|
| Hardness (HV) | 200-250 | 180-220 | 220-280 |
| Tensile strength (MPa) | 450-550 | 500-600 | 550-650 |
| Elongation (%) | 15-25 | 20-30 | 15-25 |
| Corrosion potential (mV) | -150 to -50 | -100 to +50 | -120 to -20 |
| Corrosion current density (μA/cm²) | 0.5-2.0 | 0.2-1.0 | 0.3-1.5 |
The solution treatment generally improves ductility and corrosion resistance by dissolving the δ-ferrite and homogenizing the composition. The subsequent aging improves strength and hardness through precipitation hardening but may slightly reduce ductility. The optimal heat treatment condition depends on the specific application requirements: for maximum corrosion resistance, solution treatment alone may be preferred; for maximum strength, solution plus aging is recommended.
Engineering Practice Considerations
For engineers implementing Ni/316L layered laser cladding in drilling applications, the following considerations are important:
- Thermal management: The high energy density of laser cladding can cause excessive heat accumulation in the substrate, particularly when depositing multiple layers. Inter-pass cooling or a reduced number of layers per session may be necessary to prevent base material degradation.
- Interface bonding: The Ni/316L interface is a critical region for corrosion resistance. If the interface is not fully melted and metallurgically bonded, it can serve as a preferential path for corrosion attack. The laser cladding process should be optimized to ensure complete interlayer melting.
- Heat treatment feasibility: For large components or in-situ repairs, full solution treatment may be impractical due to distortion concerns. In such cases, stress relief treatment at 500-600 °C provides a practical compromise, reducing residual stresses without significantly altering the microstructure.
- Qualification per standards: The overlay procedure should be qualified per ASME IX or NB/T 47014, with bond strength testing per ASTM A263/A264 and non-destructive examination per JB/T 4730 or ASME V.
Key Reflections
The Ni/316L layered laser cladding approach offers a compelling strategy for combining the superior corrosion resistance of nickel with the economic and mechanical advantages of 316L stainless steel. The study demonstrates that post-weld heat treatment is not merely an optional step but a critical process variable that can dramatically improve the performance of the overlay. The most significant finding is that the solution treatment effectively eliminates the detrimental effects of the as-deposited microstructure, including δ-ferrite segregation and residual stress, while the subsequent aging provides additional strengthening. Engineers should carefully select the heat treatment condition based on the specific service environment and mechanical loading requirements, recognizing that there is often a trade-off between strength, ductility, and corrosion resistance. The reference value of this work extends to the broader field of functionally graded overlays, where the strategic combination of dissimilar materials and controlled heat treatment can produce tailored performance characteristics.