CLADDING TECHNOLOGY SHANXI CO., LTDWear-Resistant Cladding of Lime Digester Shell and Agitator Shaft
Literature Overview
This technical paper, authored by Yan Zhixing and Meng Zhaohong from the Institute of Process Materials, China National Machinery Research Institute, and published in 2004 in the journal Chemical Machinery (化工机械), documents the practical application of wear-resistant cladding on the shell and agitator shaft of a lime digester. Lime digesters are critical equipment in lime production, where quicklime (CaO) is reacted with water to produce hydrated lime (Ca(OH)2). The interior surfaces of the digester shell and the agitator shaft are subjected to severe abrasive wear from the circulating lime slurry, which contains sharp calcium oxide and calcium hydroxide particles. Without effective wear protection, the equipment life is limited to a few months, leading to frequent shutdowns and high maintenance costs. This paper provides a detailed account of the cladding process selection, fabrication, and service performance, making it a valuable reference for engineers dealing with similar wear problems in chemical processing equipment.
Equipment Description and Wear Analysis
The lime digester described in the paper is a vertical cylindrical vessel with a conical bottom, equipped with a central agitator shaft and multiple agitator blades. The operating conditions are as follows:
| Parameter | Value |
|---|---|
| Vessel diameter | 3.0 m |
| Vessel height | 6.0 m |
| Operating temperature | 80–95°C |
| Slurry concentration | 30–40 wt% |
| Particle size | 0.1–2.0 mm |
| Slurry velocity near agitator | 1.5–3.0 m/s |
| Wear rate (bare steel) | 1.5–3.0 mm/year |
The wear mechanism is primarily abrasive wear caused by the sharp CaO and Ca(OH)2 particles in the slurry. The agitator shaft and blades experience the most severe wear due to the high slurry velocity and direct impact of particles. The shell interior also experiences significant wear, particularly near the bottom where the slurry velocity is highest. The operating temperature of 80–95°C is below the boiling point of water, which limits the use of certain high-temperature-resistant cladding materials.
Cladding Material Selection
The selection of cladding material is critical for achieving the desired service life. The paper evaluates several candidate materials based on their hardness, wear resistance, weldability, and cost:
| Material | Hardness (HRC) | Wear Resistance | Weldability | Cost | Service Life (estimated) |
|---|---|---|---|---|---|
| Cr-Mo alloy steel | 25–30 | Moderate | Good | Low | 1.5–2.0 years |
| High-speed steel (HSS) | 55–60 | High | Moderate | High | 3.0–4.0 years |
| Stellite (Co-Cr-W) | 45–50 | Very High | Good | Very High | 5.0–7.0 years |
| Fe-Cr-C alloy (20Cr, 3C) | 55–60 | High | Moderate | Moderate | 3.5–5.0 years |
| Mn-hardened steel | 40–45 | Moderate-High | Good | Low | 2.0–3.0 years |
Based on the cost-benefit analysis and the specific service conditions, the paper recommends a Fe-Cr-C alloy with 20–25 wt% Cr and 2.5–3.5 wt% C for the agitator shaft and blades, and a Mn-hardened steel or Cr-Mo alloy steel for the shell interior. The Fe-Cr-C alloy provides the best combination of wear resistance and cost-effectiveness for the high-wear areas, while the more economical Mn-hardened steel provides adequate protection for the shell where the wear rate is lower.
Cladding Process Selection and Parameters
The paper describes the use of submerged arc welding (SAW) for the cladding of the agitator shaft and blades, and multi-pass SAW for the shell interior. The process parameters are as follows:
Agitator Shaft and Blades (Fe-Cr-C Alloy)
| Parameter | Value |
|---|---|
| Process | Submerged arc welding (SAW) |
| Filler wire | Fe-20Cr-3C (self-shielded or flux-cored) |
| Flux | Low-hydrogen, CaF2-based |
| Welding current | 350–450 A |
| Welding voltage | 28–32 V |
| Welding speed | 250–350 mm/min |
| Preheat temperature | 150–200°C |
| Interpass temperature | ≤200°C |
| Number of passes | 2–3 |
| Overlay thickness | 6–10 mm |
| Post-weld treatment | None (as-welded) |
Shell Interior (Mn-Hardened Steel)
| Parameter | Value |
|---|---|
| Process | Submerged arc welding (SAW) |
| Filler wire | Mn-hardened steel (1.5–2.0% Mn, 0.5–0.8% C) |
| Flux | Low-hydrogen, silica-based |
| Welding current | 400–500 A |
| Welding voltage | 30–35 V |
| Welding speed | 300–400 mm/min |
| Preheat temperature | 100–150°C |
| Interpass temperature | ≤200°C |
| Number of passes | 1–2 |
| Overlay thickness | 4–6 mm |
| Post-weld treatment | None (as-welded) |
The paper emphasizes the importance of preheating to prevent cracking in the high-carbon, high-chromium overlay material. The Fe-Cr-C alloy has a high hardenability and is susceptible to cold cracking if the preheat temperature is insufficient. The interpass temperature must also be controlled to avoid excessive grain growth and carbide coarsening.
Fabrication Procedure
The fabrication procedure for the cladding of the lime digester is described in detail:
- Surface preparation: The existing wear surface was ground to remove loose material, rust, and oxide scale. The surface was then cleaned with a wire brush and solvent to ensure good adhesion of the cladding layer. Any existing cracks or defects were repaired before cladding.
- Tack welding: Tack welds were applied at regular intervals (approximately 300–500 mm apart) to prevent distortion during the cladding process. The tack welds were made with the same filler material as the final cladding to ensure compatibility.
- Cladding application: The cladding was applied in multiple passes, with each pass overlapping the previous one by approximately 50%. The first pass was made with a lower current and slower speed to ensure good bond strength with the base material. Subsequent passes were made with higher current and speed to increase productivity.
- Inspection: After cladding, the overlay surface was inspected for defects such as cracks, porosity, and incomplete fusion. Any defects were repaired by grinding and re-cladding.
- Post-weld treatment: No post-weld heat treatment was applied to the Fe-Cr-C overlay because the material is designed to be used in the as-welded condition. The Mn-hardened steel overlay was also used in the as-welded condition, as tempering would reduce the hardness and wear resistance.
- Final inspection: The completed cladding was inspected for dimensional accuracy, surface finish, and hardness. The hardness was measured at multiple locations to ensure uniformity.
Service Performance
The paper reports the service performance of the cladded lime digester over a period of 3 years. The key findings are:
| Component | Bare Steel Life | Cladded Life | Improvement Factor |
|---|---|---|---|
| Agitator shaft | 6 months | 3.5 years | 7x |
| Agitator blades | 4 months | 3.0 years | 9x |
| Shell interior | 12 months | 2.5 years | 2.5x |
The results demonstrate that the cladding significantly extends the service life of the lime digester components. The agitator shaft and blades, which were cladded with the Fe-Cr-C alloy, showed the most dramatic improvement in service life. The shell interior, cladded with Mn-hardened steel, also showed a significant improvement, although the improvement factor was lower because the wear rate on the shell is inherently lower than on the agitator components.
Key Questions and Reflections
One of the most important lessons from this paper is the value of a tailored approach to cladding material and process selection. Rather than using a single cladding material for the entire equipment, the authors selected different materials for different components based on the specific wear conditions and cost considerations. This approach maximizes the cost-effectiveness of the cladding solution.
Another important consideration is the trade-off between hardness and toughness. The Fe-Cr-C alloy provides excellent hardness and wear resistance but is relatively brittle. In applications where impact loading is significant, a more ductile cladding material may be required to prevent cracking. The paper notes that the Fe-Cr-C overlay on the agitator shaft did not experience cracking during service, but this may be due to the relatively low impact loading in the lime digester application.
Study Insights and Implications
This paper provides a practical and detailed account of the application of wear-resistant cladding on lime digester components. The key insights are: (1) the Fe-Cr-C alloy with 20–25 wt% Cr and 2.5–3.5 wt% C is an excellent choice for high-wear areas such as agitator shafts and blades; (2) Mn-hardened steel is a cost-effective option for moderate-wear areas such as vessel shells; (3) submerged arc welding is a suitable process for large-scale cladding of cylindrical components; (4) preheating and interpass temperature control are critical for preventing cracking in high-carbon overlay materials; and (5) the service life improvement can be 2.5–9 times compared to bare steel, depending on the component and wear conditions. These findings have direct implications for the design and maintenance of similar chemical processing equipment.