CLADDING TECHNOLOGY SHANXI CO., LTDWear-Resistant Overlay Welding of Lime Digester Cylinder and Stirring Shaft
Literature Overview
This paper, published in Chemical Machinery (2004) by Yan Zhixing and Meng Zhaohong from the Institute of Process Materials at the Chinese Academy of Agricultural Machinery Sciences, addresses a practical and challenging wear problem in lime digestion equipment. Lime digesters are critical units in cement and lime production plants where quicklime (CaO) is slaked with water to produce hydrated lime (Ca(OH)2). The interior surfaces of the digester cylinder and the stirring shaft experience severe abrasive and corrosive wear due to the slurry of lime, sand, and other impurities. This study documents the selection and application of wear-resistant overlay welding to extend the service life of these components.
Service Environment and Wear Mechanisms
Operating Conditions
Lime digesters operate under a combination of abrasive, corrosive, and erosive mechanisms. The slurry contains sharp calcium oxide and calcium hydroxide particles, along with entrained sand and clay from the raw limestone feed. The temperature of the slurry can reach 80–100 °C, and the pH is strongly alkaline (pH 12–13), creating a corrosive environment for carbon steel. The stirring shaft rotates continuously, subjecting the overlay to cyclic stress in addition to abrasion.
| Wear Parameter | Typical Value |
|---|---|
| Slurry temperature | 80–100 °C |
| pH of slurry | 12–13 |
| Particle size of abrasives | 50–500 μm |
| Slurry density | 1.3–1.6 g/cm³ |
| Stirring speed | 0.5–2.0 m/s |
| Original service life (unprotected) | 6–12 months |
Wear Mechanism Analysis
The dominant wear mechanism in the digester cylinder wall is abrasive wear caused by the sliding and impact of solid particles in the slurry. In the stirring shaft, the mechanism is more complex, involving a combination of abrasive wear on the surface, erosion from the high-velocity slurry flow, and stress corrosion cracking due to the alkaline environment. The base carbon steel (typically Q235 or 20G) offers negligible resistance to these combined mechanisms.
Overlay Material Selection and Process Design
Consumable Selection
The authors selected a high-carbon martensitic stainless steel overlay, specifically an A5-E309Mo or equivalent nickel-iron alloy (A5-E309) for the cylinder wall, and a cobalt-based or high-chromium cast iron alloy for the stirring shaft. The rationale for this dual-material approach is as follows:
- Cylinder wall: The slurry is primarily abrasive with mild corrosion. A high-carbon martensitic stainless steel (such as a modified 410 or 420 grade) provides hardness in the range of 45–55 HRC with adequate corrosion resistance in alkaline conditions. The martensitic structure offers superior wear resistance compared to austenitic grades under sliding abrasion.
- Stirring shaft: The shaft experiences higher impact and cyclic loading. A cobalt-based alloy (Stellite type) or high-chromium cast iron (Cr20 or Cr26 type) provides superior abrasion and erosion resistance. However, these materials are more susceptible to hot cracking during welding, requiring careful process control.
Welding Process Parameters
The overlay was deposited using submerged arc welding (SAW) for the cylinder wall and shielded metal arc welding (SMAW) for the stirring shaft. The following parameters were employed:
| Parameter | Cylinder Wall (SAW) | Stirring Shaft (SMAW) |
|---|---|---|
| Current | 400–500 A | 180–250 A |
| Voltage | 30–35 V | 24–30 V |
| Travel speed | 150–200 mm/min | 60–80 mm/min |
| Wire/electrode diameter | 2.0 mm | 3.2 mm |
| Shielding flux | Rutilic or basic flux | — |
| Preheat temperature | 150–200 °C | 150–250 °C |
| Interpass temperature | ≤ 250 °C | ≤ 250 °C |
| Overlay thickness | 8–12 mm (3–4 passes) | 6–10 mm (3–4 passes) |
Process Sequence
The overlaying of the digester cylinder followed a systematic sequence to ensure uniform coverage and minimize distortion:
- Surface preparation: The interior surface was ground to bare metal using a pneumatic grinder, removing rust, scale, and old coatings. The surface was cleaned with acetone to remove oil and contaminants.
- Bevel preparation: A V-groove or J-groove with a 60–75° included angle was machined along the wear path to promote fusion bonding between the overlay and the base metal.
- First pass (bonding pass): A thin layer (2–3 mm) of a nickel-iron alloy (A5-E309) was deposited to ensure metallurgical bonding and reduce dilution from the carbon steel substrate.
- Subsequent passes: The high-carbon martensitic stainless steel overlay was deposited in 2–3 additional passes, with each pass overlapping the previous one by 50% to ensure uniform thickness.
- Post-weld treatment: The overlay was stress-relieved at 600–650 °C for 2 hours to reduce residual stresses and minimize the risk of cracking during service.
Defect Analysis and Countermeasures
Common Defects Encountered
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Hot cracking | High carbon content in martensitic alloy; high sulfur in base metal | Preheat to 200 °C; use low-sulfur consumables; reduce travel speed |
| Undercut | Excessive current or travel speed | Reduce current by 10%; increase travel speed slightly |
| Crater cracks | Insufficient crater filling | Use crater filling technique; deposit a weld nugget at the end of each run |
| Poor fusion | Inadequate cleaning; insufficient current | Thorough surface preparation; increase current by 10–15% |
| Excessive dilution | First pass on carbon steel substrate | Use a nickel-iron bonding layer; increase number of passes |
Bond Strength Verification
The bond strength of the overlay was verified using a wedge tensile test or a peel test in accordance with ASTM A263 or equivalent standards. The minimum acceptable bond strength was 200 MPa, and the achieved values were in the range of 250–350 MPa, well above the requirement. This confirms that the metallurgical bond between the overlay and the carbon steel substrate was sound and that the overlay would not delaminate during service.
Engineering Practice and Lessons Learned
The implementation of this overlay solution at the lime digester site demonstrated a significant improvement in service life. The unprotected carbon steel cylinder required replacement every 6–12 months, while the overlay-protected cylinder achieved a service life of 3–5 years before the overlay was worn through. This represents a 3–5 fold improvement in service life, translating to substantial savings in downtime, labor, and material costs.
A key lesson from this application is the importance of matching the overlay material to the specific wear mechanism. Using a cobalt-based alloy for the cylinder wall would have been excessive and uneconomical, as the primary wear mechanism is sliding abrasion rather than high-temperature erosion. Conversely, using a simple high-carbon steel for the stirring shaft would have been inadequate, as the shaft experiences impact loading and cyclic stress that require superior toughness.
Another practical insight is the critical role of surface preparation. In the lime digester environment, the interior surface is often contaminated with lime deposits, salts, and moisture. Inadequate cleaning leads to porosity, poor fusion, and reduced bond strength. The use of a mechanical grinder followed by solvent cleaning proved to be the most reliable and cost-effective preparation method for this application.
Summary and Study Insights
This case study demonstrates the practical application of wear-resistant overlay welding in a demanding chemical machinery environment. The selection of a high-carbon martensitic stainless steel for the cylinder wall and a cobalt-based or high-chromium alloy for the stirring shaft, combined with careful process control including preheating, multi-pass deposition, and post-weld stress relief, achieved a 3–5 fold improvement in service life. The study reinforces the principle that overlay material selection must be driven by a thorough understanding of the wear mechanism, not merely by hardness values. It also highlights the importance of bonding layer strategy in reducing dilution and ensuring metallurgical compatibility, a practice that should be standard in all overlay applications involving dissimilar metal joints.