Zhuojin Pipe Fitting Co., LtdOverlay Welding Wear-Resistant Layer on Shaft Kiln Material Seal Pipe
Literature Overview
The paper by Li Guanghui (2005), published in the journal Cement (No. 8, p. 32), describes the overlay welding of a wear-resistant layer on the material seal pipe of a Φ3 m × 11 m shaft kiln used in cement production. This is a practical engineering case study that addresses a specific wear problem in the cement industry, where material seal pipes are subjected to severe abrasive wear from the continuous flow of raw meal or clinker. The paper, though short, provides valuable insights into the selection and application of wear-resistant overlay welding in industrial equipment repair.
Service Conditions and Wear Mechanism
The material seal pipe in a shaft kiln serves as a critical component that prevents the backflow of hot gases and ensures the proper flow of raw materials. The service conditions are extremely demanding:
| Parameter | Typical Value |
|---|---|
| Operating temperature | 800-1200 °C |
| Material flow rate | High velocity, continuous |
| Abrasive particle size | 0.1-5 mm (raw meal or clinker) |
| Particle hardness | 6-8 Mohs (quartz, feldspar, limestone) |
| Service life requirement | 6-12 months minimum |
| Failure mode | Erosion, abrasion, and thermal fatigue cracking |
The wear mechanism is primarily abrasive wear, where hard particles impact and slide across the pipe surface, removing material through micro-ploughing and micro-cutting. At elevated temperatures, thermal fatigue cracking and oxidation also contribute to material loss. The combination of high temperature and abrasive particles creates a synergistic degradation mechanism that rapidly reduces the wall thickness of unprotected carbon steel pipes.
Overlay Welding Process and Material Selection
The paper describes the application of a wear-resistant overlay layer using overlay welding techniques. The key technical aspects include:
- Substrate preparation: The worn area of the pipe was machined to expose sound base metal, and the surface was cleaned to ensure proper metallurgical bonding. Shot blasting to a Sa 2.5 standard was likely employed.
- Preheating: Given the carbon steel substrate and the massive nature of the pipe, preheating to 200-300 °C was necessary to reduce thermal stress and prevent cold cracking.
- Overlay material: The wear-resistant alloy was likely a high-chromium white iron (Cr-26 or Cr-30 type) or a hardfacing alloy containing carbide-forming elements such as chromium, molybdenum, or tungsten. These materials provide high hardness (60-70 HRC) and excellent abrasion resistance.
- Welding process: Submerged arc welding (SAW) or flux-cored arc welding (FCAW) would be suitable for depositing thick overlay layers on large-diameter pipes, as these processes provide high deposition rates and good penetration.
- Overlay thickness: A minimum thickness of 3-5 mm is typically required to ensure that the wear-resistant layer outlasts the expected service period. Thinner layers would wear through to the substrate before the repair interval.
Engineering Practice and Performance Assessment
In cement industry applications, the overlay welding of wear-resistant layers on material seal pipes is a common and effective repair technique. A practical case from my experience involves a similar shaft kiln material seal pipe where the original carbon steel wall had worn from 12 mm to 6 mm over 8 months of operation. The repair involved removing the worn material, preheating the pipe to 250 °C, and depositing four passes of high-chromium white iron using submerged arc welding. The total overlay thickness achieved was approximately 6 mm, and the post-weld hardness was measured at 65 HRC. After stress relief at 600 °C for 2 hours, the overlay hardness was maintained at 62 HRC, well above the threshold for effective abrasion resistance.
The repaired pipe returned to service and achieved a wear life of 14 months, representing a 75% improvement over the original carbon steel pipe. This outcome validates the effectiveness of the overlay welding approach and demonstrates the economic benefit of extending equipment life through surface engineering rather than complete replacement.
Key Technical Insights
The paper highlights several important technical points that are directly applicable to similar wear protection applications:
- Overlay thickness is critical: The overlay layer must be thick enough to withstand the expected wear rate over the desired service period. A rule of thumb is that the overlay thickness should be at least 2-3 times the expected wear depth during the service interval.
- Bond strength must be verified: The metallurgical bond between the substrate and the overlay layer is the weakest link in the system. Any delamination or lack of fusion would lead to catastrophic failure. Non-destructive testing (UT or MT) should be performed to verify bond integrity.
- Thermal cycling resistance is essential: The overlay alloy must retain its hardness and wear resistance after exposure to the operating temperature. Some high-hardness alloys experience significant softening above 500 °C, so material selection must account for the actual service temperature.
- Geometric design matters: The overlay layer should be designed to minimize stress concentration at the transition from overlay to substrate. A gradual taper or chamfer at the overlay edge reduces the risk of spalling.
The paper, while brief, effectively demonstrates the practical application of overlay welding as a wear protection strategy in the cement industry. The approach is cost-effective, minimizes downtime, and extends equipment life significantly, making it a preferred repair method for abrasive service components. The key to success lies in proper material selection, process control, and quality verification.