CLADDING TECHNOLOGY SHANXI CO., LTDManual Arc Cladding and Heat Treatment of Heat Exchanger Tube Sheets
Overview of the Research Topic
This study note examines the application of manual arc cladding (SMAW) combined with post-weld heat treatment for repairing and cladding heat exchanger tube sheets. Tube sheets are critical structural components in shell-and-tube heat exchangers that provide mechanical support for tubes and maintain pressure boundaries. They are frequently exposed to corrosive environments and require corrosion-resistant overlay layers to ensure long-term service life. The literature documents the process parameters, metallurgical considerations, heat treatment procedures, and quality assurance practices for this application.
Tube Sheet Requirements and Cladding Challenges
Heat exchanger tube sheets typically serve multiple demanding functions simultaneously:
- Structural support: They must withstand the combined loads of tube bundle weight, pressure differential across the exchanger, and thermal expansion forces.
- Pressure boundary: The tube sheet forms part of the pressure-containing boundary and must maintain leak-tight integrity.
- Corrosion resistance: The tube side and shell side surfaces are exposed to corrosive process fluids, requiring overlay layers with superior corrosion resistance.
- Tube hole integrity: The tube holes (typically 0.5–1.0 mm interference fit) must maintain precise dimensional accuracy after cladding and heat treatment.
Common Cladding Materials for Tube Sheets
| Overlay Material | Base Material | Application | Key Properties |
|---|---|---|---|
| 304L stainless steel | Carbon steel (SA-266 Gr.2) | Mild corrosive environments | Good general corrosion resistance |
| 316L stainless steel | Carbon steel (SA-266 Gr.2) | Chloride-containing environments | Superior pitting resistance |
| 321 stainless steel | Carbon steel (SA-266 Gr.2) | High-temperature oxidizing environments | Excellent creep strength |
| Hastelloy C-276 | Nickel alloy base | Highly aggressive chemical environments | Outstanding corrosion resistance |
| Inconel 625 | Nickel alloy base | Severe corrosion + high temperature | Excellent strength and corrosion resistance |
The selection of overlay material depends on the process fluid composition, operating temperature, and pressure. For example, 316L is commonly used for tube sheets exposed to mildly corrosive aqueous environments, while Hastelloy C-276 or Inconel 625 is required for highly aggressive chemical processing applications.
Manual Arc Cladding Process Parameters
Manual arc cladding using shielded metal arc welding (SMAW) is the most commonly employed method for tube sheet cladding due to its flexibility, equipment simplicity, and suitability for large, flat surfaces. The literature provides the following process parameters:
Typical SMAW Cladding Parameters
| Parameter | Value | Notes |
|---|---|---|
| Electrode type | E309L, E316L, or specialized nickel-based | Selected based on overlay material |
| Electrode diameter | 2.5–4.0 mm | 2.5 mm for thin layers, 4.0 mm for thick layers |
| Current | 80–180 A (DCEN) | Depends on electrode diameter |
| Travel speed | 150–300 mm/min | Controls heat input and bead width |
| Bead width | 20–35 mm | Overlap of 30–50% between adjacent beads |
| Layer thickness per pass | 2–3 mm | Multiple passes for total thickness |
| Inter-pass temperature | < 200°C | Prevents excessive grain growth |
| Preheat temperature | 100–200°C | Reduces cracking risk |
The use of E309L or E316L electrodes provides a dilution-resistant overlay with a two-phase microstructure (austenite + ferrite) that offers good resistance to solidification cracking. The carbon equivalent of the electrode must be controlled to minimize the risk of cold cracking, particularly when cladding onto high-carbon steel substrates.
Cladding Sequence and Pattern
The cladding sequence is critical for minimizing residual stresses and ensuring uniform overlay thickness. The literature recommends the following sequence for large tube sheets:
- Divide the tube sheet into rectangular zones of approximately 300–500 mm in width.
- Clad each zone using a back-step sequence: Start at the center of the zone and weld outward in both directions, completing one direction before switching to the other.
- Overlap adjacent zones by 20–30 mm to ensure complete coverage at the boundaries.
- Apply multiple layers with each subsequent layer offset by 50% of the bead width from the previous layer.
This sequence minimizes the accumulation of longitudinal residual stresses and reduces the risk of distortion. For tube sheets with a thickness of 50–100 mm, the total cladding thickness is typically 3–5 mm on each side, achieved in 2–3 layers.
Post-Weld Heat Treatment
Post-weld heat treatment (PWHT) is essential for tube sheet cladding to relieve residual stresses, improve the ductility of the overlay layer, and reduce the risk of stress corrosion cracking (SCC). The literature documents the following heat treatment procedures:
Heat Treatment Procedures
| Overlay Material | PWHT Temperature | Holding Time | Cooling Rate | Purpose |
|---|---|---|---|---|
| 304L / 316L | 1050–1100°C | 1 h per 25 mm thickness | Furnace cool to 800°C, then air cool | Solution treatment, grain refinement |
| 321 | 1050–1100°C | 1 h per 25 mm thickness | Furnace cool | Solution treatment |
| 347 | 1050–1100°C | 1 h per 25 mm thickness | Furnace cool | Solution treatment |
| Hastelloy C-276 | 1050–1100°C | 1 h per 25 mm thickness | Furnace cool to 800°C, then air cool | Solution treatment |
| Inconel 625 | 1050–1100°C | 1 h per 25 mm thickness | Furnace cool | Solution treatment |
For austenitic stainless steel overlays, solution heat treatment dissolves carbide precipitates and homogenizes the microstructure, restoring full corrosion resistance. The cooling rate from the solution treatment temperature is critical: rapid cooling (air cooling) promotes a fully austenitic microstructure, while slow cooling (furnace cooling) can promote carbide precipitation and sensitization.
A critical consideration for tube sheets is the effect of PWHT on the tube holes. The thermal expansion and contraction during heat treatment can cause distortion of the tube holes, affecting the tube-to-tubesheet joint quality. The literature recommends that tube holes be reamed after PWHT to restore dimensional accuracy. Additionally, the heat treatment temperature must be carefully controlled to avoid excessive softening of the base material, which could compromise the structural integrity of the tube sheet.
Quality Assurance and Inspection
The quality assurance protocol for tube sheet cladding includes the following inspection stages:
- Visual inspection (VT): After each layer, the overlay surface is visually inspected for cracks, porosity, undercut, and incomplete fusion. Any defects are repaired before proceeding to the next layer.
- Magnetic particle testing (MT): Applied after the final cladding layer to detect surface and near-surface cracks in the overlay and the overlay-substrate interface.
- Ultrasonic testing (UT): Applied to verify the bond between the overlay layer and the base material. The acceptance criteria typically require no indications exceeding 20% of the reference block signal amplitude.
- Hardness testing: The overlay layer hardness is measured to verify that the heat treatment has been effective. For solution-treated 304L or 316L, the hardness should be below 250 HV.
- Intergranular corrosion testing: Performed on coupons taken from the overlay layer to verify that the heat treatment has eliminated sensitization. The ASTM A262 Practice E or Practice A test is commonly used.
- Dimensional inspection: After grinding and reaming, the tube sheet dimensions and tube hole sizes are verified to meet the required tolerances.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Solidification cracking | High dilution, rapid cooling, unfavorable microstructure | Use E309L electrodes, control inter-pass temperature, optimize travel speed |
| Lack of fusion | Insufficient heat input, poor surface preparation | Increase current, clean base metal surface, ensure adequate overlap |
| Porosity | Gas shielding deficiency, electrode contamination | Use properly dried electrodes, ensure clean work area |
| Excessive sensitization | Insufficient PWHT temperature or holding time | Verify PWHT parameters, ensure uniform heating |
| Tube hole distortion | Excessive thermal gradient during PWHT | Control heating and cooling rates, use gradual temperature ramps |
Engineering Practice Insights
From a practical standpoint, the cladding of heat exchanger tube sheets presents several unique challenges that distinguish it from other cladding applications:
The presence of numerous tube holes creates a complex stress field that can concentrate residual stresses at the hole edges, potentially initiating cracks or accelerating corrosion. The literature emphasizes the importance of cladding sequence optimization to minimize stress concentrations at tube holes. One effective strategy is to clad the areas around tube holes first, allowing the holes to be reamed after cladding and heat treatment to remove any distortion.
Another practical consideration is the handling and support of large tube sheets during cladding. Tube sheets can weigh several tons, and the cladding process must be performed with the tube sheet in a position that allows access to all areas while maintaining dimensional stability. The use of temporary stiffeners or backing plates can help minimize distortion during cladding and heat treatment.
The literature also highlights the importance of documenting the entire cladding and heat treatment process for traceability and quality assurance purposes. This includes recording the electrode lot numbers, welding parameters for each pass, inter-pass temperatures, PWHT temperature-time curves, and all NDT results. Such documentation is essential for meeting the requirements of pressure vessel codes such as ASME VIII Div.1 or GB/T 150.
Study Reflections and Conclusions
This literature review underscores the importance of careful process planning, parameter control, and quality assurance in the cladding of heat exchanger tube sheets. The combination of manual arc cladding with post-weld heat treatment offers a practical and cost-effective solution for providing corrosion-resistant overlay layers on large, complex tube sheet geometries. The key to success lies in the careful selection of overlay materials, optimization of the cladding sequence to minimize residual stresses, and rigorous quality assurance to ensure metallurgical integrity and dimensional accuracy. For engineers involved in heat exchanger fabrication and repair, a thorough understanding of the metallurgical interactions between the overlay layer and base material, the effects of heat treatment on microstructure and properties, and the requirements of applicable codes and standards is essential for delivering reliable, code-compliant components that will perform effectively throughout their service life.