CLADDING TECHNOLOGY SHANXI CO., LTDCladding Process for a Large Carbonization Tower
Literature Overview
This 2007 publication by Lin Zhusheng of Shandong Qilu Petrochemical Construction Company addresses the weld overlay cladding of a large carbonization tower used in coal gasification or coking processes. Carbonization towers operate under severe corrosive and erosive conditions involving high-temperature gas streams containing hydrogen sulfide, carbon monoxide, and particulate matter. The interior surface must be protected by a corrosion- and erosion-resistant overlay, typically a nickel-based alloy or austenitic stainless steel.
The scale of the carbonization tower presents unique challenges: large diameter (often 3–8 meters), significant height (20–50 meters), and the need for continuous internal coverage. The cladding must be performed either during fabrication (on horizontal sections before erection) or in-situ (on erected vertical sections), each approach carrying distinct technical implications.
Core Technical Points
Material Selection
The overlay material selection depends on the specific corrosive environment within the carbonization tower:
| Operating Condition | Recommended Overlay Material | Typical Alloy |
|---|---|---|
| High H₂S + CO environment | Nickel-based alloy | Inconel 625 or Alloy 625 |
| Moderate corrosion, high erosion | Austenitic stainless steel | 310S or 309 |
| High-temperature carburization | Nickel-chromium alloy | Hastelloy C-276 |
| General internal corrosion | Austenitic stainless steel | 304L or 316L |
The base material for a carbonization tower is typically carbon steel (Q345R or 16MnR per GB/T 150), chosen for structural economy, with the overlay providing the corrosion barrier.
Process Parameters
For large-diameter tower sections, submerged arc welding (SAW) is the most economical and efficient process for applying thick overlay layers:
| Process Parameter | Specification |
|---|---|
| Process | SAW (single or dual wire) |
| Wire diameter | 2.0–3.2 mm |
| Flux type | Rutile or basic flux, low-hydrogen |
| Current | 400–600 A (DC) |
| Voltage | 28–35 V |
| Travel speed | 150–250 mm/min |
| Layer thickness per pass | 3–5 mm |
| Total overlay thickness | 6–12 mm |
| Preheat temperature | 100–150 °C |
| Interpass temperature | ≤ 200 °C |
Vertical Cladding Challenges
When cladding is performed on erected vertical tower sections, gravity effects on the molten pool become critical. The weld pool tends to sag, producing irregular bead profiles and potentially incomplete fusion at the bottom of the bead. Countermeasures include:
- Reducing current by 10–20% compared to horizontal-position parameters.
- Using a slightly faster travel speed to limit pool size.
- Employing a dual-wire SAW configuration for better pool control.
- Welding in short segments with frequent inspection.
- Using a slightly upward or downward weave pattern to control metal flow.
Standards and Quality Control
The cladding operation must comply with applicable pressure vessel codes. For a carbonization tower classified as a pressure vessel, GB/T 150 and NB/T 47002 govern the fabrication requirements. The overlay weld procedure must be qualified per NB/T 47014, and the welder must hold a valid qualification for the specific process and material combination.
| Inspection Method | Application | Acceptance Criteria |
|---|---|---|
| Dye penetrant (PT) | Surface of overlay | No linear indications ≥ 0.5 mm |
| Magnetic particle (MT) | Surface of overlay (ferromagnetic base) | No cracks or linear indications |
| Ultrasonic (UT) | Bond line between overlay and base | No separation > 25% of area |
| Hardness test | Overlay surface | Within specified range (e.g., 180–250 HV for Inconel 625) |
| Chemical analysis | Overlay composition | Within ASTM specification limits |
| Macrograph | Cross-section | Full penetration, no lack of fusion |
Engineering Practice Considerations
In the context of a large carbonization tower, the cladding schedule must be integrated with the overall project timeline. Typical sequence involves:
- Fabrication of tower sections (shell, heads, nozzles) from carbon steel plate.
- Assembly and fitting of internal sections on horizontal fixtures.
- SAW cladding of internal surfaces while in horizontal position (preferred for quality).
- Erection of cladded sections into vertical position.
- In-situ cladding of remaining areas (top head, vertical transitions) using vertical SAW or GTAW.
- Non-destructive examination of all overlay welds.
- Hydrostatic pressure testing after complete assembly.
The critical quality concern is the bond line between the carbon steel base and the nickel-based or stainless overlay. Incomplete fusion at this interface creates a corrosion pathway that can lead to catastrophic failure through intergranular corrosion or blistering. The macrograph examination is therefore mandatory for a representative sample of the cladding, typically 5–10% of total overlay area.
Study Insights
This document highlights the practical engineering approach to large-scale cladding in petrochemical construction. The emphasis on SAW as the primary process reflects the economic reality of applying thick overlay layers over large areas. The challenges of vertical-position welding and bond-line integrity are perennial concerns that require rigorous procedure qualification and workmanship control. The document underscores that material selection alone is insufficient; process parameters, joint design, and inspection protocols must all be aligned to achieve a reliable corrosion barrier.