CLADDING TECHNOLOGY SHANXI CO., LTDTube Sheet Cladding for Heat Exchanger Corrosion Protection
Technical Context and Application Scope
Tube sheet cladding is a critical process in the fabrication of heat exchangers, particularly those used in corrosive environments such as the chemical, petrochemical, and marine industries. The tube sheet is the component that separates the tube-side and shell-side fluids and provides the structural support for the tubes, and it is subjected to complex mechanical and thermal stresses in addition to the corrosive attack from the process fluids. The cladding of the tube sheet surface and the tube holes with a corrosion-resistant alloy provides a continuous protective layer that extends the service life of the heat exchanger and prevents the failure modes associated with corrosion of the base material.
The cladding process for tube sheets typically involves the overlay of austenitic stainless steel or nickel-based alloy on the carbon steel or low-alloy steel base plate, with the overlay applied to the tube sheet end face and the internal surface of the tube holes. The continuity of the overlay layer across the end face and the hole surface is critical, as any discontinuity can serve as a corrosion initiation site and lead to premature failure of the heat exchanger. The process must be designed to achieve a smooth, continuous transition between the end face overlay and the hole surface overlay, with no gaps, cracks, or lack of fusion at the transition zone.
Process Selection and Technical Parameters
The selection of the cladding process for tube sheets is influenced by the geometry of the tube sheet, the thickness of the overlay required, the material of the overlay, and the production volume. The following table compares the most commonly used processes for tube sheet cladding:
| Process | Typical Application | Overlay Thickness | Dilution Rate | Productivity | Surface Quality |
|---|---|---|---|---|---|
| GTAW (TIG) | Small tube sheets, precision applications | 1–5 mm | 10–30 percent | Low | Excellent |
| SAW (Submerged Arc) | Large tube sheets, high volume | 3–10 mm | 20–40 percent | High | Good |
| PTA (Plasma Transferred Arc) | Medium tube sheets, high quality | 1–3 mm | 5–15 percent | Medium | Excellent |
| Laser Cladding | Precision tube sheets, complex geometries | 0.5–2 mm | 5–15 percent | Medium | Excellent |
For tube sheets with hole diameters below 25 millimeters, GTAW is the preferred process due to its ability to produce clean, precise overlays in confined spaces. For larger tube sheets with hole diameters above 25 millimeters, SAW or PTA may be more appropriate due to their higher deposition rates. The choice of process must be made in consideration of the overall manufacturing strategy, including the cost of the process, the availability of equipment, and the required quality level.
The following table summarizes the typical process parameters for GTAW tube sheet cladding:
| Parameter | Typical Value | Notes |
|---|---|---|
| Welding current | 100–200 A | Depends on overlay thickness and pass number |
| Arc voltage | 14–18 V | DC electrode negative for stainless steel |
| Travel speed | 80–150 mm/min | Higher speed for lower dilution |
| Wire diameter | 1.6–2.4 mm | ER308L or ER316L for stainless steel overlay |
| Shielding gas | Argon | 15–20 L/min flow rate |
| Preheat temperature | 50–150 degrees C | Depends on base material thickness |
| Interpass temperature | Below 150 degrees C | To prevent excessive grain growth |
| Overlay thickness | 2–5 mm | Minimum 2 mm for corrosion protection |
Flatness and Bonding Requirements
The flatness of the tube sheet end face is a critical quality requirement, as it directly affects the sealing performance of the heat exchanger. The cladding process must be designed to maintain the flatness of the end face within the tolerance specified by the applicable standard, typically within 0.2 to 0.5 millimeters per meter of tube sheet diameter. The overlay passes must be planned to minimize the buildup of weld metal on the end face, and post-weld machining is typically required to achieve the final flatness tolerance.
The bonding strength of the overlay to the base material is another critical quality requirement, and it must be verified by destructive or non-destructive testing methods. The bond strength test, typically performed according to ASTM A263 or similar standards, involves the application of a tensile load to a coupon containing the overlay and the base material, with the test result expressed as the load per unit area required to separate the overlay from the base. The minimum bond strength requirement varies by application but is typically in the range of 50 to 100 megapascals.
Common Defects and Quality Control
The following table presents common defects encountered in tube sheet cladding and their countermeasures:
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Discontinuity at hole-end face transition | Inadequate overlap between hole and face passes | Plan pass sequence to ensure overlap; use PTA for transition |
| Lack of fusion at overlay-base interface | Excessive travel speed; insufficient current | Optimize parameters; verify first pass bonding |
| Porosity in overlay | Inadequate shielding; moisture in wire | Increase gas flow; use dry wire; clean substrate |
| Cracking in overlay | High residual stress; hydrogen embrittlement | Preheat; control interpass temperature; post-weld heat treatment |
| Excessive dilution | Low travel speed; high current | Increase travel speed; reduce current; use cold wire |
| Flatness deviation | Uneven overlay buildup | Plan pass sequence; use post-weld machining |
The discontinuity at the hole-end face transition is a particularly challenging defect, as it requires the overlay to be continuous across two surfaces that are at right angles to each other. The GTAW process can produce a continuous overlay at this transition, but it requires careful parameter control and a well-planned pass sequence. The PTA process is often preferred for this transition because it allows for more precise control of the weld pool and can produce a smoother transition.
Engineering Practice and Quality Assurance
In engineering practice, the quality assurance program for tube sheet cladding typically includes the following elements:
- Material verification: Verification of the base plate and filler metal chemistry and mechanical properties through mill certificates and independent testing.
- Procedure qualification: Qualification of the welding procedure according to NB/T 47014 or ASME IX, with attention to the dilution rate, bond strength, and corrosion performance of the qualified procedure.
- Welder qualification: Qualification of the welders performing the overlay, with specific qualification for the tube sheet geometry and the overlay process.
- In-process inspection: Monitoring of welding parameters, interpass temperature, and visual inspection of each pass to ensure compliance with the qualified procedure.
- Non-destructive testing: Application of magnetic particle testing or liquid penetrant testing to the overlay surface to detect surface and near-surface defects.
- Destructive testing: Bond strength testing, microstructural analysis, and corrosion testing on test coupons to verify the metallurgical quality of the overlay.
- Final inspection: Verification of the flatness, overlay thickness, and surface quality of the completed tube sheet.
The study of tube sheet cladding reveals the importance of process planning in achieving a continuous, high-quality overlay layer. The engineer must carefully consider the pass sequence, the transition between hole and face overlays, and the post-weld machining requirements to ensure that the final product meets the quality requirements of the application. The key insight is that the quality of the tube sheet cladding is determined not only by the welding process itself but also by the overall manufacturing strategy, including the material selection, the process planning, and the quality assurance program.