CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Online Wear-Resistant Overlay Welding on Roller Press
Overview of Roller Press Technology in Cement Grinding
The roller press has become the dominant comminution device in modern cement grinding circuits, particularly in High Efficiency Separator (HES) systems and combined roller press–ball mill configurations. The two counter-rotating rolls compress clinker, coal, or slag under extremely high specific pressure, typically in the range of 100–200 MPa. This intense contact loading, combined with abrasive particles in the feed material, causes rapid wear of the roll surface. Overlay welding provides an economical and effective means to restore or enhance the surface hardness and wear resistance of the rolls.
The concept of "online" overlay welding refers to performing the weld overlay operation on the roller press while it remains installed in the mill housing, without full disassembly. This approach significantly reduces downtime compared to traditional offline repair, where the entire roll assembly must be removed, transported to a workshop, welded, and reinstalled.
Key Technical Challenges of Online Overlay Welding
Online welding on roller press rolls introduces several unique challenges that differ from conventional offline cladding operations. The primary difficulties include:
- Geometric constraint: The roll is a large-diameter cylinder (typically 1000–1500 mm diameter, 3000–5000 mm length) positioned inside a confined mill housing with limited access for welding equipment and personnel.
- Positional welding: Welding positions include flat, horizontal, overhead, and all-position welds on a curved surface, requiring flexible process selection.
- Heat input management: The massive base metal (usually low-carbon or low-alloy steel) acts as a heat sink, but the confined space limits cooling control, creating a risk of uneven heat distribution and residual stress accumulation.
- Spatter and slag control: In a confined space, spatter can damage adjacent components such as hydraulic systems, electrical motors, and the separator housing.
Process Selection and Parameters
For online roller press roll overlay welding, submerged arc welding (SAW) with multiple layers remains the most widely used process due to its high deposition rate and excellent penetration characteristics. However, gas metal arc welding (GMAW) with flux-cored wire (FCAW) has gained increasing acceptance for its flexibility in positional welding and reduced equipment requirements.
| Parameter | SAW (Submerged Arc) | FCAW (Flux-Cored Arc) | GMAW (Solid Wire) |
|---|---|---|---|
| Deposition rate | 15–25 kg/h | 8–15 kg/h | 5–10 kg/h |
| Travel speed | 200–400 mm/min | 150–300 mm/min | 100–250 mm/min |
| Current | 400–800 A | 200–450 A | 150–350 A |
| Voltage | 25–35 V | 22–32 V | 18–28 V |
| Positional flexibility | Limited (flat/horizontal) | Good (all positions) | Good (all positions) |
| Typical wire diameter | 2.0–3.2 mm | 1.2–1.6 mm | 1.2–1.6 mm |
The overlay material selection for roller press applications typically targets surface hardness in the range of HRC 50–65, depending on the severity of the abrasive environment. Common overlay materials include:
- Cr-Cr2C6 type: Chromium carbide-based alloys providing excellent abrasion resistance against hard particles, typical hardness HRC 55–62.
- W-C type: Tungsten carbide-based alloys for extreme abrasion conditions, hardness HRC 60–70.
- Ni-Cr-Mo type: Nickel-chromium-molybdenum alloys offering good toughness with moderate abrasion resistance, hardness HRC 45–55.
- Fe-Cr-B type: Iron-chromium-boron alloys for severe abrasive wear, hardness HRC 55–65.
Weld Procedure and Layer Design
A typical overlay weld procedure for roller press rolls involves the following sequence:
- Surface preparation: The existing worn surface is ground back to sound metal using a portable grinder or milling cutter. Surface roughness should be controlled to Ra ≤ 6.3 μm to ensure good bond strength. Any cracks, porosity, or loose material must be removed.
- Preheating: Preheat the base metal to 150–250°C using portable gas torches or electric heating bands. The preheat temperature must be maintained uniformly across the entire weld zone to minimize thermal gradients.
- Binder layer deposition: A transition layer (typically a low-carbon nickel-based or iron-based alloy) is deposited to prevent cracking in the overlay layer. This layer is usually 2–3 mm thick.
- Overlay layer deposition: Multiple passes of the wear-resistant overlay material are deposited, typically 4–8 mm total thickness. Each pass should overlap the previous pass by 50% to ensure full coverage and avoid undercutting.
- Post-weld heat treatment: After welding, the roll is cooled slowly in a controlled manner. If the residual hardness exceeds HRC 60, a stress-relief annealing treatment at 600–700°C for 2–4 hours may be required.
Defect Analysis and Countermeasures
Based on field experience, the following defects are most commonly encountered in online roller press overlay welding:
| Defect Type | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Bond-line cracking | Excessive carbon content in overlay, high residual stress | MT, UT | Use low-carbon binder layer, reduce welding current |
| Surface porosity | Moisture in flux, inadequate shielding | PT, MT | Dry flux thoroughly, improve gas shielding |
| Undercutting | Excessive travel speed, improper electrode angle | Visual, UT | Reduce travel speed, adjust electrode angle to 10–15° |
| Excessive hardness | Too many overlay layers, no stress relief | Hardness test | Reduce layer count, apply post-weld annealing |
| Incomplete fusion | Insufficient preheat, excessive travel speed | UT, MT | Increase preheat temperature, reduce travel speed |
Engineering Practice Insights
From a practical standpoint, the success of online overlay welding on roller presses depends heavily on pre-job planning and crew coordination. The following factors should be considered:
- Downtime optimization: Online welding should be scheduled during planned maintenance windows. A typical 4000-ton-per-day cement plant with a roller press of 1200 mm diameter can expect 2–3 days of online welding time per roll, compared to 5–7 days for offline repair including disassembly and reassembly.
- Quality verification: After welding, the overlay surface should be tested for hardness (minimum 3 locations per roll), bond strength (per ASTM G143 or equivalent), and surface profile. The total overlay thickness should be verified by ultrasonic measurement at intervals of 500 mm along the roll length.
- Operational monitoring: After returning the roll to service, the wear rate should be monitored by measuring the roll diameter at regular intervals (typically every 2000–5000 operating hours). A well-executed overlay weld should restore the roll to its original grinding profile within ±0.5 mm tolerance.
The economic benefit of online overlay welding is substantial. For a typical roller press, the cost of online welding is approximately 40–60% of the cost of replacing the roll with a new one, and the downtime reduction alone can save 200,000–500,000 RMB in lost production value for a large cement plant.
Study Reflection and Key Takeaways
The study of online wear-resistant overlay welding on roller presses reveals that the technology is highly sensitive to process discipline and operator skill. The confined working environment demands meticulous preparation, including thorough surface cleaning, controlled preheating, and systematic layer deposition. The selection of overlay material must balance hardness against toughness to avoid brittle fracture under impact loading from hard particles. Engineers should pay particular attention to the transition between the base metal and the overlay layer, as this is the most critical zone for crack initiation. Future improvements may focus on developing new low-dilution overlay consumables that can be applied in fewer passes while maintaining adequate bond strength, and on the development of automated welding systems that can navigate the complex geometry of the roll surface with greater precision.