CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding Process for Waste Heat Boiler Tube Sheets
Literature Overview
This 2009 publication by Liu Baoxiang from Qingdao Softcontrol Heavy Industry Co., Ltd. addresses the overlay welding process for tube sheets in waste heat boilers. Published in "China Chemical Equipment" (中国化工装备), this work represents a practical engineering solution to a critical challenge in waste heat recovery systems. Waste heat boilers are integral components in industrial processes such as cement manufacturing, steel production, chemical processing, and power generation, where they recover thermal energy from exhaust gases to generate steam. The tube sheet is one of the most critical components of a waste heat boiler because it simultaneously serves as the pressure boundary, the tube support structure, and the interface between the gas side and the water/steam side.
Service Conditions and Design Requirements
Waste heat boiler tube sheets operate under a combination of severe conditions that demand careful material selection and fabrication:
| Parameter | Typical Range | Design Challenge |
|---|---|---|
| Operating temperature | 250-450°C | Thermal cycling fatigue |
| Operating pressure | 1.0-4.0 MPa | Pressure boundary integrity |
| Gas composition | Flue gas with SO₂, HCl, dust | Corrosion and erosion |
| Tube hole diameter | φ22-φ51 mm | High hole density, tight tolerances |
| Tube sheet thickness | 20-80 mm | Deep weld penetration required |
| Tube layout | Square, triangular, or rotated | Complex hole pattern |
The tube sheet is typically fabricated from carbon steel or low-alloy steel for structural strength, but the gas-side surface requires overlay welding with a corrosion-resistant alloy to withstand the aggressive flue gas environment. The water-side surface may also require overlay welding if the boiler water chemistry is aggressive or if high-temperature corrosion is a concern.
Overlay Material Selection
The selection of overlay material depends on the specific composition of the waste gas and the operating temperature. Common overlay materials for waste heat boiler tube sheets include:
| Overlay Material | Application Scenario | Corrosion Resistance | Temperature Limit (°C) |
|---|---|---|---|
| 309Cb (25Cr-35Ni) | High-temperature gas with moderate corrosion | Good | 1100 |
| 310Cb (25Cr-35Ni-3Nb) | High-temperature with thermal cycling | Excellent | 1200 |
| Inconel 625 (62Ni-22Cr-9Mo) | Severe corrosion (HCl, SO₂) | Excellent | 1000 |
| Hastelloy C-276 (58Ni-16Cr-16Mo) | Very severe corrosion | Excellent | 900 |
| 321Cb (18Cr-10Ni-1Ti) | Moderate temperature, low corrosion | Moderate | 900 |
| 347Cb (18Cr-10Ni-1Nb) | Moderate temperature, moderate corrosion | Moderate | 900 |
For cement kiln waste heat boilers, where the flue gas contains high concentrations of alkali chlorides and sulfates, nickel-based alloys such as Inconel 625 or Hastelloy C-276 are typically required for the overlay layer. For steel mill waste heat boilers, austenitic stainless steels with higher chromium and nickel content (309Cb or 310Cb) are often sufficient.
Welding Process Selection
The choice of welding process for tube sheet overlay welding depends on the tube sheet thickness, the number of tube holes, and the required overlay thickness.
Process Comparison for Tube Sheet Overlay
| Process | Advantages | Disadvantages | Typical Application |
|---|---|---|---|
| Submerged arc welding (SAW) | High deposition rate, low spatter | High heat input, limited positional flexibility | Thick tube sheets, large areas |
| Gas metal arc welding (GMAW) | Good control, moderate deposition rate | Higher consumable cost, shielding gas required | Medium thickness, complex geometry |
| Gas tungsten arc welding (GTAW) | Excellent quality, low dilution | Low deposition rate, high labor cost | Thin tube sheets, repair |
| Flux-cored arc welding (FCAW) | High deposition rate, good penetration | Higher porosity risk, slag removal needed | Thick overlay layers |
| Plasma transferred arc (PTA) | Very low dilution, excellent quality | High equipment cost, limited wire diameter | Critical applications, thin overlay |
For waste heat boiler tube sheets, SAW is often the preferred process for the main overlay layers due to its high deposition rate and ability to handle thick tube sheets. However, the tube holes present a significant challenge because the overlay weld metal must not bridge or partially fill the tube holes, which would prevent proper tube insertion and tube-to-tube-sheet welding.
Tube Hole Protection Strategy
Protecting tube holes during overlay welding is one of the most critical aspects of tube sheet fabrication. The strategies include:
| Method | Description | Effectiveness |
|---|---|---|
| Plug insertion | Insert steel plugs into tube holes before welding | Excellent - prevents weld metal entry |
| Hole sealing with putty | Apply refractory putty over tube holes | Good - requires removal after welding |
| Temporary cover plate | Weld a temporary plate over tube holes | Moderate - requires removal |
| Post-weld hole cleaning | Drill or ream tube holes after overlay welding | Common - but risks damaging overlay |
| Localized overlay | Weld overlay only on areas away from tube holes | Limited - complex pattern |
The plug insertion method is the most reliable approach. Steel plugs should be slightly smaller than the tube hole diameter to allow easy removal after welding. The plugs should be made of a material compatible with the overlay weld metal to avoid galvanic coupling issues during the welding process. After the overlay welding is complete, the plugs are removed and the tube holes are inspected for any residual weld metal or damage to the overlay layer.
Welding Procedure Specification
A typical welding procedure specification (WPS) for waste heat boiler tube sheet overlay welding includes the following elements:
Base Metal and Consumable Data
| Parameter | Specification |
|---|---|
| Base metal | Q345R or 16MnR (carbon steel) |
| Transition layer | 309Cb (25Cr-35Ni) |
| Overlay layer | 310Cb or Inconel 625 |
| Welding process | SAW with flux-cored wire or solid wire |
| Shielding gas | None (flux-covered) or Ar + CO₂ mixture |
Process Parameters
| Layer | Current (A) | Voltage (V) | Travel Speed (mm/min) | Wire Feed Speed (m/min) |
|---|---|---|---|---|
| Transition layer | 500-700 | 32-38 | 150-250 | 8-12 |
| Overlay layer 1 | 450-650 | 30-36 | 180-280 | 7-10 |
| Overlay layer 2 | 400-600 | 28-34 | 200-300 | 6-9 |
Heat Treatment Requirements
Post-weld heat treatment (PWHT) is typically required for tube sheets to relieve residual stresses and to stabilize the microstructure of the overlay layer. The PWHT parameters depend on the base metal and overlay material:
| Material | PWHT Temperature (°C) | Soak Time (h) | Cooling Rate |
|---|---|---|---|
| Carbon steel base | 550-620 | 1 h per 25 mm thickness | Controlled (< 100°C/h) |
| Austenitic overlay | 850-950 | 1-2 h | Air cool or furnace cool |
| Nickel-based overlay | 980-1050 | 1-2 h | Air cool |
A challenge arises when the PWHT temperature required for the base metal (stress relief at 550-620°C) is significantly lower than the solution treatment temperature required for the overlay layer (850-1050°C). In such cases, the overlay layer may experience sensitization or intergranular corrosion susceptibility if the PWHT temperature is too high. The solution is to either accept the PWHT temperature for the base metal and rely on the inherent corrosion resistance of the overlay material, or to perform a separate solution treatment of the overlay layer after the base metal PWHT.
Quality Control and Inspection
The quality of the overlay weld on a waste heat boiler tube sheet is critical for the long-term reliability of the boiler. The following inspection procedures are essential:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection (VT) | Surface defects, undercut, porosity | No cracks, no undercut > 0.5 mm |
| Penetrant testing (PT) | Surface-breaking cracks | No indications |
| Magnetic particle testing (MT) | Sub-surface cracks in ferromagnetic materials | No indications |
| Ultrasonic testing (UT) | Lack of fusion, internal defects | Per ASME V or GB/T 11345 |
| Dye penetrant on tube holes | Weld metal in tube holes | No weld metal in holes |
| Hardness testing | Verify overlay composition | Within specified range |
| Metallographic examination | Microstructure, dilution | No detrimental phases |
| Intergranular corrosion test | Sensitization of overlay | Per ASTM A263 or A264 |
The intergranular corrosion test is particularly important for austenitic stainless steel overlay layers because the welding process can cause chromium carbide precipitation at grain boundaries, leading to sensitization and intergranular corrosion. The ASTM A263 test (65% boiling HNO₃) and ASTM A264 test (50% boiling H₂SO₄) are commonly used to evaluate the sensitization resistance of the overlay weld metal.
Engineering Practice Considerations
The fabrication of waste heat boiler tube sheets with overlay welding presents several practical challenges that must be addressed in the engineering process:
- Thermal distortion: The large heat input from overlay welding can cause significant thermal distortion of the tube sheet. Pre-bending or backing plates may be required to control distortion within acceptable limits.
- Tube hole alignment: After overlay welding and any subsequent machining, the tube holes must maintain their original position and size tolerances. Any distortion or deformation must be corrected before tube insertion.
- Overlay thickness uniformity: The overlay layer thickness should be uniform across the tube sheet surface to ensure consistent corrosion protection. Variations in thickness can lead to localized thinning and premature failure.
- Tube-to-tube-sheet welding: After the overlay layer is applied, the tubes must be welded to the tube sheet through the overlay layer. This requires a compatible welding procedure that accounts for the dissimilar metal joint between the overlay alloy and the tube material.
- Cost optimization: Nickel-based overlay materials are expensive. The overlay thickness should be optimized to provide adequate corrosion protection without excessive material usage. A typical overlay thickness of 3-5 mm provides sufficient protection for most waste heat boiler applications.
Study Insights and Reflections
This publication highlights the importance of overlay welding as a cost-effective solution for protecting critical pressure vessel components from corrosion in harsh industrial environments. The tube sheet of a waste heat boiler is an excellent example of how overlay welding can extend the service life of a component by providing a corrosion-resistant surface while maintaining the structural integrity of the base material. The practical challenges of tube hole protection, thermal distortion control, and overlay thickness optimization are directly relevant to engineers working on similar applications.
The work also underscores the need for careful material selection based on the specific service conditions. The choice between austenitic stainless steel and nickel-based alloy overlays depends on the severity of the corrosion environment, the operating temperature, and the economic constraints of the project. Engineers should always perform a detailed corrosion assessment before selecting the overlay material to avoid under-specification (leading to premature failure) or over-specification (leading to unnecessary cost).