Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair Technology for Roller Surface Spalling in Roller Presses
Literature Overview
This paper by Gui Xujun from Jidong Heidelberg (Jingyang) Cement Co., Ltd. (published in China Cement, 2006, No. 1) addresses the overlay welding repair of roller surface spalling in a roller press—a critical grinding equipment used in cement production. The roller press is used to reduce the particle size of clinker and other cementitious materials, and the rollers are subject to severe wear and impact loading. The paper discusses the causes of roller surface spalling, the overlay welding repair technique, and the practical considerations for restoring the rollers to service.
Core Technical Points
Failure Analysis of Roller Surface Spalling
Roller surface spalling is a complex failure mode that involves multiple contributing factors:
| Factor | Description |
|---|---|
| Material fatigue | Cyclic contact stresses exceed the material's fatigue limit |
| Surface hardening | Excessive hardness from prior hardfacing reduces toughness |
| Thermal effects | Friction heating during operation causes thermal cracking |
| Contamination | Foreign material (e.g., tramp metal) trapped between rollers causes impact damage |
| Lubrication failure | Inadequate lubrication increases friction and wear |
The spalling typically initiates at subsurface cracks that propagate to the surface under cyclic loading, resulting in the detachment of material chunks. The affected areas can range from small localized pits to large areas of surface loss, as indicated by the paper's discussion of the spalling area and its effect on throughput.
Overlay Welding Repair Process
The repair process involves the following steps:
- Surface assessment: Measuring the spalling area, depth, and remaining roller diameter to determine the repair scope.
- Surface preparation: Grinding the spalled area to expose sound base metal, with a bevel angle of 45–60° for proper weld fusion.
- Preheating: Applying preheat to 200–300°C to minimize cracking risk, particularly for high-carbon or pre-hardened rollers.
- Overlay welding: Multi-pass hardfacing using a suitable alloy electrode or wire, with controlled heat input to minimize dilution and maintain hardness.
- Post-weld treatment: Stress-relieving at 550–620°C, followed by machining to restore the roller profile.
- Hardness verification: Measuring the hardness of the overlay weld and HAZ to ensure compliance with specifications.
Welding Consumable Selection
The selection of hardfacing consumables depends on the service conditions and the desired hardness:
| Consumable Type | Composition | Hardness (HRC) | Application |
|---|---|---|---|
| Type 1 (Cr-C) | 20–30% Cr, 1.5–3% C | 55–62 | General wear resistance |
| Type 2 (Cr-Mo) | 15–20% Cr, 2–3% Mo | 58–65 | High wear resistance |
| Type 3 (Ni-based) | Ni-20Cr-15Mo | 45–55 | Impact resistance |
| Type 4 (Co-based) | Co-25Cr-5W | 55–60 | High-temperature wear |
For cement roller applications, Type 2 (Cr-Mo) consumables are typically preferred due to their excellent wear resistance and moderate cost. However, the high hardness must be balanced against the risk of cracking, particularly in thick sections or in the presence of pre-existing defects.
Engineering Practice Integration
The roller press is a high-value asset in cement production, and downtime due to roller failure can result in significant production losses. The overlay welding repair must therefore be performed efficiently and reliably, with minimal disruption to the production schedule.
Practical Considerations
- Roller removal: The roller must be removed from the press and transported to a welding facility, which requires careful planning to minimize downtime.
- Thermal management: The large mass of the roller (typically 5–10 tons) provides thermal inertia, but the high heat input required for overlay welding can still cause significant thermal distortion.
- Machining allowance: The overlay weld must be deposited with sufficient excess to allow for machining to the final profile, typically 3–5 mm per side.
- Balance verification: After machining, the roller must be dynamically balanced to ensure smooth operation and prevent vibration-induced fatigue.
Quality Assurance
The repair must be accompanied by a rigorous quality assurance program:
| Inspection Step | Method | Acceptance Criteria |
|---|---|---|
| Surface preparation | Visual + MT | No cracks, pits, or contamination |
| Overlay weld | UT (each pass) | No lack of fusion, slag, or porosity > 2 mm |
| Final surface | MT + PT | No surface cracks or inclusions |
| Hardness | Vickers or Rockwell | Within specified range (e.g., 55–65 HRC) |
| Profile | CMM or laser scanner | Within ±0.5 mm of nominal |
| Balance | Dynamic balancing | G2.5 grade or better |
Key Questions and Reflections
A critical question is whether the overlay repair can fully restore the original fatigue life of the roller. The overlay weld and its HAZ represent a region of microstructural heterogeneity that may be susceptible to fatigue cracking under cyclic contact loading. The hardness gradient between the overlay and the base metal can also create stress concentrations that promote crack initiation. Engineers should consider implementing a monitoring program that includes periodic UT or MT inspections of the roller surface to detect early signs of fatigue cracking.
Another consideration is the effect of the repair on the roller's contact mechanics. The overlay weld may have different elastic properties and surface roughness compared to the original roller surface, which can affect the contact stress distribution and wear rate. Engineers should consider performing contact stress analysis after repair to ensure that the modified roller surface does not introduce new failure modes.
Study Insights and Implications
This paper highlights the practical challenges of overlay welding repair in heavy industrial applications, where the balance between wear resistance, toughness, and processability is critical. The successful repair of roller surface spalling requires not only proper welding technique but also careful attention to thermal management, quality assurance, and post-repair verification. Engineers should adopt a systematic approach to repair that incorporates failure analysis, process optimization, and long-term monitoring to ensure the durability and reliability of the repaired component. The experience gained from this repair can be applied to similar roller failures in other grinding and crushing equipment, providing a valuable reference for maintenance engineers in the cement, mining, and mineral processing industries.