Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair of ZGM113G Medium-Speed Roller Mill Stationary Ring
Literature Overview
The technical case study by Luan Jiangtao and Luan Chenchen (2021), published in Cement (Issue 2, pp. 53–55), documents the overlay welding repair of a stationary ring in a ZGM113G medium-speed roller coal mill used in a cement plant. The study describes a practical field repair scenario where the stationary ring of the nozzle ring assembly experienced severe and uneven wear, necessitating overlay welding as an alternative to replacement due to parts unavailability and long manufacturing lead times. This is a representative case of industrial equipment remanufacturing in the cement industry.
Problem Description and Diagnostic Analysis
The ZGM113G medium-speed roller coal mill processes coal for kiln firing in cement production. The nozzle ring assembly consists of a rotating ring (dynamic ring) and a stationary ring, with a design clearance of 5–10 mm between them. During the January 2014 maintenance shutdown, inspection revealed that the stationary ring had experienced significant and non-uniform wear, with the clearance between the dynamic and stationary rings increasing to 25–40 mm. This excessive clearance caused reduced gas velocity at the nozzle, resulting in increased coal mill slag discharge and degraded mill efficiency.
| Parameter | Design Condition | Actual Condition (Jan 2014) | Deviation |
|---|---|---|---|
| Ring clearance | 5–10 mm | 25–40 mm | 3–4× design value |
| Wear pattern | Uniform | Non-uniform | Severe asymmetry |
| Mill efficiency | Normal | Degraded | Increased slag discharge |
| Parts availability | Standard | Unavailable | Long lead time |
Repair Strategy and Process Selection
Given that the manufacturer had no stock of replacement stationary rings and the manufacturing lead time was unacceptably long, overlay welding repair was selected as the most practical solution. The repair strategy involved:
- Surface preparation: The worn surface was ground to remove loose material and establish a sound base for welding.
- Preheating: The component was preheated to prevent cracking in the base material, which is typically a cast iron or low-alloy steel.
- Overlay welding: Multiple passes of wear-resistant overlay welding were applied to build up the worn surface to the required geometry.
- Post-weld machining: The overlay layer was machined to restore the precise geometry and clearance specifications.
The selection of overlay welding consumable is critical. For coal mill applications, the wear mechanism is primarily abrasive, involving solid particles (coal, slag, and mineral dust) impacting and abrading the ring surfaces. Therefore, a hardfacing consumable with high carbide content—such as a high-carbon, high-chromium type (similar to Stellite or D2 hardfacing)—would be appropriate. The overlay layer must provide sufficient hardness (HRC 50–60) and abrasion resistance while maintaining adequate toughness to resist impact loading.
Process Parameters and Quality Control
The overlay welding process parameters must be carefully controlled to achieve a sound, crack-free overlay layer. Key process considerations include:
| Process Parameter | Recommended Range | Rationale |
|---|---|---|
| Preheat temperature | 150–250°C | Prevent cold cracking in base metal |
| Interpass temperature | ≤ 300°C | Control thermal cycle |
| Current type | DCEN or DCEP | Dependent on consumable type |
| Current | 180–250 A | Adequate penetration and deposition |
| Travel speed | 50–80 mm/min | Balance deposition rate and cooling rate |
| Shielding gas | Ar or Ar/CO₂ mix | Protect molten pool from oxidation |
| Number of passes | 3–5 | Achieve required build-up height |
Quality control measures should include visual inspection for cracks and porosity, magnetic particle testing (MT) for surface and near-surface defects, and dimensional verification of the machined overlay layer. The hardness of the overlay layer should be verified to ensure it meets the minimum specification for the intended service conditions.
Engineering Practice Implications
This case study illustrates several important principles of industrial equipment repair:
- Economic justification: Overlay welding repair can be significantly more cost-effective than replacement, particularly when spare parts are unavailable or have long lead times. The decision to repair should be based on a comprehensive cost-benefit analysis considering downtime costs, parts cost, labor cost, and expected remaining service life.
- Design considerations for repairability: Equipment designers should consider repairability in the original design. Features such as accessible surfaces, sufficient material thickness for overlay welding, and provision for post-weld machining can facilitate future repairs.
- Root cause analysis: The non-uniform wear pattern suggests potential issues with alignment, material flow distribution, or operating conditions. A thorough root cause analysis should be conducted to address the underlying causes and prevent recurrence.
- Maintenance scheduling: The repair should be integrated into a predictive maintenance program that monitors wear progression and schedules repairs before critical clearance limits are reached.
Key Questions and Reflections
The case study, while practically valuable, lacks detailed metallurgical characterization of the overlay layer. Without microstructural analysis, hardness profiles, and wear testing data, it is difficult to assess the long-term durability of the repair. In professional practice, overlay welding repairs should be supported by metallurgical evaluation to ensure the overlay layer meets performance requirements.
Additionally, the study does not address the residual stress state of the repaired component. Overlay welding introduces significant residual stresses that can affect the component's dimensional stability and fatigue performance. Post-weld stress relief or controlled cooling may be necessary for critical applications. The non-uniform wear pattern also raises concerns about whether the overlay welding repair alone will address the root cause, or whether additional corrective actions (alignment adjustment, operating parameter optimization) are required.
This case demonstrates the practical application of overlay welding in industrial equipment remanufacturing. While the technical details are limited, the fundamental approach—diagnosis, repair strategy selection, process execution, and quality verification—provides a useful framework for similar repair scenarios in the cement and mineral processing industries.