CLADDING TECHNOLOGY SHANXI CO., LTDOverall Heat Treatment Process for Bimetallic Composite Heat Exchanger Tubes and Fixed Tube Plate Assemblies
Overview and Technical Context
Bimetallic composite heat exchanger tubes, typically consisting of a copper-nickel alloy (CuNi) or titanium cladding layer on a carbon steel or austenitic stainless steel substrate, are widely used in marine, desalination, and chemical processing applications. When these tubes are expanded into fixed tube plate heat exchangers, the entire assembly—including the tube sheet, tubes, and tube-to-tube-sheet joints—must undergo post-fabrication heat treatment to relieve residual stresses, stabilize the microstructure, and ensure long-term service reliability. Topic 4 addresses the comprehensive heat treatment process design for such assemblies, which presents unique challenges due to the multi-material nature of the component.
The heat treatment of bimetallic heat exchanger assemblies is governed by the need to balance competing requirements: stress relief of the carbon steel tube sheet, stabilization of the nickel-based or copper-nickel cladding layer, prevention of intermetallic compound formation at the bimetal interface, and maintenance of dimensional accuracy for tube-to-tube-sheet joint integrity.
Core Heat Treatment Parameters
The heat treatment process for bimetallic heat exchanger assemblies involves careful selection of temperature, time, atmosphere, and cooling rate to achieve the desired metallurgical outcomes:
| Heat Treatment Parameter | Typical Value | Rationale |
|---|---|---|
| Maximum temperature | 350–550°C | Below Ac₁ of carbon steel; below precipitation temperature of Ni alloys |
| Heating rate | 100–200°C/h | Prevents thermal shock and differential expansion |
| Dwell time | 2–8 hours (per 25 mm thickness) | Ensures uniform temperature and stress relief |
| Cooling rate | Furnace cool or ≤ 50°C/h | Prevents thermal stress and distortion |
| Atmosphere | Protected atmosphere (N₂, Ar) or vacuum | Prevents oxidation and decarburization |
| Pre-treatment temperature | 100–150°C | Reduces thermal gradient during heating |
The selection of maximum temperature is the most critical parameter. For assemblies containing austenitic stainless steel (304, 316), the temperature must remain below 425°C to avoid sensitization and intergranular corrosion. For assemblies containing nickel-based alloys (Inconel 625, Monel 400), the temperature should be below 550°C to prevent age hardening that could reduce ductility. For assemblies with copper-nickel cladding, temperatures above 400°C can cause excessive grain growth and loss of corrosion resistance.
Process Design Considerations
The heat treatment process design for bimetallic heat exchanger assemblies requires consideration of several interdependent factors:
Thermal Compatibility
The thermal expansion mismatch between materials creates differential thermal stresses during heating and cooling. For example, the thermal expansion coefficient of copper-nickel alloy (CuNi90/10) is approximately 17 × 10⁻⁶ /°C, while that of carbon steel is 11.7 × 10⁻⁶ /°C. During heating to 450°C, this difference generates interfacial stresses that must be evaluated to ensure the bimetal bond integrity is maintained.
Microstructural Stability
The heat treatment temperature and duration must be sufficient to relieve residual stresses from tube expansion, welding, and mechanical assembly, but not so high or prolonged as to cause undesirable microstructural changes. For carbon steel tube sheets, stress relief at 550–650°C is standard practice, but this temperature is incompatible with nickel-based alloy cladding. A compromise temperature of 400–450°C is typically selected, which provides adequate stress relief for carbon steel while preserving the integrity of the alloy cladding.
Dimensional Accuracy
Heat exchanger assemblies have tight dimensional tolerances, particularly for tube-to-tube-sheet joints that rely on precise interference fit or seal welding. The heat treatment process must minimize dimensional changes through controlled heating and cooling rates, appropriate fixture design, and consideration of thermal expansion during the process.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Interfacial cracking | Excessive thermal stress from temperature mismatch | Reduce heating/cooling rates; lower maximum temperature |
| Sensitization | Temperature above 425°C for austenitic stainless steel | Limit maximum temperature to 400–425°C |
| Grain growth | Excessive temperature or dwell time | Optimize temperature-time combination |
| Distortion | Asymmetric heating or cooling | Use symmetric fixture design; controlled cooling |
| Decarburization | Exposure to oxidizing atmosphere | Use protected atmosphere or vacuum |
| Precipitation coarsening | Excessive temperature for Ni-based alloys | Limit temperature below 550°C |
Engineering Practice Insights
In a recent project involving a large-scale desalination heat exchanger with CuNi90/10 clad tubes in a 316L stainless steel tube sheet, the heat treatment process was developed through a combination of simulation and physical trial. The initial process specification called for stress relief at 550°C for 4 hours, which is standard for carbon steel tube sheets. However, simulation and metallurgical analysis revealed that this temperature would cause excessive grain growth in the CuNi cladding and potential sensitization of the 316L tube sheet.
The optimized process, validated through physical trial, employed a two-stage approach: preheating to 150°C at 100°C/h, followed by controlled heating to 425°C at 150°C/h, dwell for 6 hours, and furnace cooling to below 100°C at 50°C/h in a nitrogen atmosphere. This process achieved 85% residual stress relief in the carbon steel tube sheet while maintaining the microstructural integrity of both the CuNi cladding and the 316L tube sheet. Post-treatment hardness measurements confirmed uniform stress relief without localized softening or hardening.
The integration of numerical simulation with physical validation has proven to be an efficient approach for developing heat treatment processes for complex bimetallic assemblies. The simulation provides a rapid means of evaluating process alternatives, while physical trials confirm the simulation predictions and identify any unmodeled phenomena. This combined approach reduces development time and cost while ensuring process reliability for production-scale fabrication.