CLADDING TECHNOLOGY SHANXI CO., LTDCladding Process for Large Carbonization Tower Study Note
Literature Overview
Carbonization towers in coke oven plants are subjected to extremely harsh environments characterized by high temperatures (up to 900 °C at the cone section), corrosive gases containing H₂S, CO₂, and water vapor, and abrasive coke particles. The tower's lower cone and transition sections are particularly vulnerable to corrosion and wear. This study note reviews the cladding process applied to large carbonization towers, focusing on material selection, process optimization, and field execution challenges.
Material Selection and Design Considerations
Corrosion Environment Analysis
The carbonization tower operates in a multi-phase corrosion environment:
| Zone | Temperature | Primary Corrosive Agents | Recommended Cladding Material |
|---|---|---|---|
| Upper shell | 100–200 °C | CO₂, H₂O, SO₂ | 304 stainless steel |
| Middle shell | 200–400 °C | H₂S, CO₂, H₂O | 316L stainless steel |
| Lower cone | 400–600 °C | H₂S, coke particles | 321 or 347 stainless steel |
| Transition section | 600–900 °C | Coke, H₂S, thermal cycling | 310S or Inconel 600 |
The selection of 321 or 347 stainless steel for the lower cone is driven by the need for resistance to intergranular corrosion (IGC) and thermal cracking. The addition of titanium or niobium stabilizers prevents chromium carbide precipitation at grain boundaries, which is the primary mechanism of IGC in sensitized austenitic stainless steels.
Cladding Process Parameters
| Parameter | Specification |
|---|---|
| Base material | Q345R or 16MnR |
| Cladding method | SAW with consumable insert or multi-layer GMAW |
| Preheat temperature | 100–150 °C (for Q345R) |
| Interpass temperature | ≤ 200 °C |
| Welding position | PA (horizontal), PB (vertical) |
| SAW flux | HJ431 or HJ107 |
| SAW wire | H08A (fill) + 321 consumable insert |
| GMAW wire | ER308L or ER321 |
| Minimum cladding thickness | 3 mm (single layer) |
| Dilution ratio | ≤ 30% |
The consumable insert method is preferred for the lower cone because it allows a single-pass deposition of a 3–5 mm stainless steel layer with controlled dilution. The insert is a pre-formed stainless steel strip with a built-in flux groove that ensures stable arc characteristics and uniform metal transfer.
Process Challenges and Solutions
Thermal Stress Management
The carbonization tower's thick shell (typically 12–20 mm) creates significant thermal stress during cladding. The coefficient of thermal expansion mismatch between carbon steel (12 × 10⁻⁶/°C) and austenitic stainless steel (17 × 10⁻⁶/°C) generates residual stresses at the bond line. Countermeasures include:
- Preheating the entire shell section to 100 °C to reduce the temperature gradient.
- Limiting the length of each welding pass to 300 mm to prevent excessive localized heating.
- Applying a back-heat technique to control the cooling rate below 100 °C/min.
- Performing stress-relief treatment at 550–600 °C after cladding completion.
Defect Prevention
| Defect | Cause | Prevention |
|---|---|---|
| Bond-line cracking | High dilution, excessive cooling rate | Increase preheat, use lower-current consumable insert |
| Intergranular corrosion | Sensitization during welding | Use stabilized grade (321/347), avoid excessive heat input |
| Porosity | Surface oxide, flux moisture | Thorough cleaning, flux drying at 300 °C for 2 h |
| Excessive dilution | High current, thin insert | Reduce current, use thicker insert, optimize travel speed |
Field Execution Experience
In a practical application involving a 60 m tall carbonization tower with a 16 mm shell thickness, the cladding was performed in sections of 6 m each. The work was executed from scaffolding inside the tower, with each section completed over a 5-day period. The SAW consumable insert method achieved an average deposition rate of 8 kg/h per gun, with a dilution ratio of 22–28%. Post-weld intergranular corrosion testing (ASTM A262 Practice E) confirmed that the 321 cladding layer exhibited no IGC susceptibility.
The critical lesson from this project was the importance of section-to-section overlap. Each cladded section overlapped the previous one by 100 mm, and the overlap zone was ground flush to avoid creating a stress concentration. The UT inspection of the bond line was performed on 100% of the weld length, with acceptance criteria requiring no indications exceeding 2 mm in length for surface-breaking defects.
Study Insights
The carbonization tower cladding project illustrates that material selection is only the first step; process execution and quality verification are equally critical. The combination of SAW consumable insert technology with stabilized austenitic stainless steel provides an effective solution for the harsh carbonization environment. Engineers must pay particular attention to the dilution ratio, as even a small increase can compromise the corrosion resistance of the cladding layer. The study reinforces the principle that cladding is a system—not a single operation—and every element from base material preparation to post-weld treatment must be optimized as an integrated whole.