CLADDING TECHNOLOGY SHANXI CO., LTDElectrode Strip Cladding Process for Q345R Tube Sheets
Introduction
Tube sheets are critical components in heat exchangers, where they provide structural support for the tubes and create a pressure boundary between the tube side and shell side. In applications requiring corrosion resistance, such as chemical processing, oil and gas, and marine service, the tube sheet surface must be clad with a corrosion-resistant material. The electrode strip cladding process, also known as electroslag welding (ESW) with electrode strip, is a widely used method for depositing thick overlay layers on large tube sheets. This study note examines the electrode strip cladding process for Q345R tube sheets, covering process parameters, quality control, and practical considerations.
Process Description
Electrode strip cladding is a variant of electroslag welding (ESW) in which a continuous strip of corrosion-resistant material is fed into the electroslag pool to build up a thick overlay layer. The process involves the following steps:
- Base metal preparation: The Q345R tube sheet surface is cleaned of mill scale, rust, and contamination. A groove is typically machined to create a keyway for mechanical interlock between the base metal and the overlay.
- Welding setup: The tube sheet is positioned horizontally, and the welding torch is mounted on a mechanized traverse system. The electrode strip is fed through the torch, and the flux is loaded into the slag well.
- Welding operation: The torch moves along the surface of the tube sheet, depositing the overlay layer in successive passes. Each pass is built upon the previous one, with the slag pool providing thermal insulation and protection from atmospheric contamination.
- Cooling and inspection: After welding, the overlay is cooled and inspected for defects using non-destructive testing methods.
Process Parameters
The following table summarizes the typical process parameters for electrode strip cladding of Q345R tube sheets with a 316L stainless steel overlay:
| Parameter | Value | Notes |
|---|---|---|
| Base material | Q345R | Low-alloy steel, minimum yield strength 345 MPa |
| Overlay material | 316L stainless steel strip | 1.5–3.0 mm thickness |
| Welding current | 800–1200 A | Depends on strip width and thickness |
| Welding voltage | 30–45 V | Electrode strip voltage |
| Welding speed | 50–150 mm/min | Higher speed for thinner strips |
| Slag ratio | 1.2–1.8 | Slag volume to metal volume |
| Flux type | Sinker or floating | Sinker flux for horizontal welding |
| Preheat temperature | 100–200°C | Reduces cracking susceptibility |
| Interpass temperature | 150–250°C | Maintains thermal balance |
| Overlay thickness | 3–10 mm | Multiple passes as required |
| Heat input | 3.0–6.0 kJ/mm | Controlled to prevent excessive dilution |
Quality Control
The quality of the electrode strip cladding is critical to the performance and safety of the heat exchanger tube sheet. The following quality control measures are essential:
Non-Destructive Testing
| NDT Method | Purpose | Standard Reference |
|---|---|---|
| Magnetic Particle Testing (MT) | Surface cracks, overlay defects | JB/T 4730.4 |
| Ultrasonic Testing (UT) | Overlay thickness, delamination | JB/T 4730.3 |
| Radiographic Testing (RT) | Internal defects, porosity | JB/T 4730.2 |
| Penetrant Testing (PT) | Surface cracks, porosity | JB/T 4730.5 |
Mechanical Testing
Mechanical testing of the overlay layer includes:
- Bond strength test: A tensile specimen is machined across the overlay-to-base metal interface to verify the metallurgical bond. The minimum bond strength for 316L on Q345R should be at least 250 MPa.
- Hardness test: The overlay hardness should be in the range of 180–250 HV for 316L stainless steel. Excessive hardness may indicate dilution with the base metal or improper welding parameters.
- Impact test: Charpy V-notch impact tests at 0°C and -20°C verify the toughness of the overlay layer. The minimum impact energy should be 27 J at -20°C for 316L overlay.
Chemical Analysis
The chemical composition of the overlay layer must be verified to ensure that the dilution from the base metal is within acceptable limits. For 316L overlay on Q345R, the maximum dilution is typically limited to 20–30%. The key elements to monitor are chromium (Cr) and nickel (Ni), which must remain above the minimum values for corrosion resistance.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Overlay delamination | Insufficient heat input, poor surface preparation | Increase heat input, improve cleaning |
| Surface cracking | High residual stress, hydrogen embrittlement | Preheat, reduce welding speed, use low-hydrogen flux |
| Excessive dilution | High welding current, low welding speed | Reduce current, increase speed, use thinner strip |
| Porosity | Flux moisture, contamination | Dry flux, clean base metal |
| Uneven overlay thickness | Torch misalignment, traverse inaccuracy | Calibrate traverse system, use thickness monitoring |
| Undercut | Excessive current, improper torch angle | Reduce current, adjust torch angle |
Engineering Practice Considerations
Welding Sequence
The welding sequence for a large tube sheet should be designed to minimize distortion and residual stress. A recommended approach is to weld in a symmetric pattern from the center outward, with the passes staggered to balance the thermal input. The tube holes should be drilled after the cladding is complete to avoid contamination of the overlay with drilling debris.
Post-Weld Heat Treatment
Post-weld heat treatment (PWHT) is typically required for Q345R tube sheets to relieve residual stresses. However, the PWHT temperature must be carefully controlled to avoid sensitization of the 316L overlay. A PWHT at 550–620°C for 1–2 hours is generally acceptable, but the temperature should not exceed 650°C to prevent chromium carbide precipitation at the grain boundaries.
Inspection Protocol
A comprehensive inspection protocol should be implemented at each stage of the cladding process:
- Pre-weld inspection: Verify base metal condition, surface cleanliness, and groove preparation.
- In-process monitoring: Monitor welding parameters, slag pool stability, and overlay thickness.
- Post-weld inspection: Perform NDT and mechanical testing to verify overlay quality.
- Final inspection: Verify overlay thickness, surface quality, and overall dimensions.
Performance and Cost Analysis
The electrode strip cladding process offers several advantages over alternative cladding methods for Q345R tube sheets:
| Method | Deposition Rate | Cost per m² | Overlay Quality | Suitability for Large Sheets |
|---|---|---|---|---|
| Electrode strip (ESW) | Very high | Low | Good | Excellent |
| Submerged arc welding (SAW) | High | Low | Good | Good |
| Gas metal arc welding (GMAW) | Moderate | Moderate | Good | Moderate |
| Plasma transferred arc (PTA) | Low | High | Excellent | Poor |
| Explosive cladding | Very high | Moderate | Excellent | Good |
The electrode strip cladding process is particularly well-suited for large tube sheets with overlay thicknesses of 3–10 mm. The high deposition rate reduces production time and cost, while the good overlay quality ensures reliable performance.
The key to successful electrode strip cladding is careful control of the welding parameters and thorough quality control. Engineers should ensure that the welding procedure is qualified in accordance with the applicable standard (e.g., NB/T 47014) and that all operators are trained and certified. Regular calibration of the welding equipment and periodic review of NDT results are essential for maintaining consistent overlay quality.