CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding of Roller Press Squeeze Rollers for Final Grinding Slag
Overview of Slag Grinding Technology
Slag grinding is a critical process in cement manufacturing, where ground granulated blast furnace slag (GGBFS) is ground to a fine powder for use as a supplementary cementitious material. Modern cement plants use roller presses combined with vertical mills or ball mills for slag grinding, achieving high efficiency and low energy consumption. The roller press squeeze rollers are subjected to extreme abrasion from hard slag particles, requiring robust overlay welding for wear protection.
Slag grinding presents unique challenges compared to cement or raw material grinding:
- Extreme abrasivity: Slag particles contain hard glassy phases and crystalline inclusions that are highly abrasive.
- High compressive stress: The specific pressure in slag grinding roller presses is typically 150–250 MPa, higher than cement grinding.
- Impact loading: Slag particles can be irregularly shaped and may impact the roller surface with significant force.
- Moisture sensitivity: Slag can absorb moisture, which may affect the welding consumables and overlay integrity.
Technical Requirements for Slag Grinding Overlay
The overlay welding requirements for slag grinding roller press squeeze rollers are the most demanding among cement grinding applications:
- Surface hardness: Minimum HRC 60, preferably HRC 60–70, to resist the extreme abrasion of slag particles.
- Toughness: Adequate impact resistance to prevent chipping and spalling from hard particle impact.
- Bond strength: Minimum 200 MPa to ensure the overlay layer remains bonded to the base metal under high compressive stress.
- Wear life: The overlay should provide 20,000–30,000 hours of service life before re-welding is required.
Overlay Material Selection for Slag Grinding
The selection of overlay material for slag grinding is critical and must balance hardness against toughness:
| Material Type | Composition | Hardness (HRC) | Abrasion Resistance | Impact Resistance | Suitability for Slag |
|---|---|---|---|---|---|
| W-C type | Fe-W-C | 60–70 | Excellent | Low | Good for severe abrasion |
| Fe-Cr-B type | Fe-Cr-B | 55–65 | Excellent | Low | Good for high abrasion |
| Cr-Cr2C6 type | Fe-Cr-C | 55–62 | Very good | Moderate | Acceptable for moderate abrasion |
| Ni-Cr-Mo type | Ni-Cr-Mo | 45–55 | Good | High | Not recommended for severe abrasion |
For slag grinding applications, the W-C type overlay material is generally preferred due to its excellent abrasion resistance and ability to withstand high compressive stress. However, the low toughness of W-C materials requires careful control of welding parameters to prevent cracking.
Welding Procedure for Slag Grinding Rollers
The overlay welding procedure for slag grinding roller press squeeze rollers involves the following steps:
Step 1: Surface Preparation
- Remove the existing worn overlay by grinding or milling, exposing sound base metal.
- Clean the surface using wire brushing and solvent cleaning to remove scale, rust, and contamination.
- Preheat the base metal to 250–350°C using portable heating elements or gas torches.
- Inspect the prepared surface using magnetic particle testing (MT) to detect cracks or defects.
Step 2: Binder Layer Deposition
| Parameter | Value |
|---|---|
| Welding process | SAW (Submerged Arc Welding) |
| Welding wire | Low-carbon nickel-based alloy, 2.5 mm diameter |
| Flux | Low-hydrogen flux, pre-dried at 300°C for 2 hours |
| Current | 450–600 A |
| Voltage | 26–32 V |
| Travel speed | 200–300 mm/min |
| Layer thickness | 2–3 mm total (2 passes) |
Step 3: Overlay Layer Deposition
| Parameter | Value |
|---|---|
| Welding process | SAW (Submerged Arc Welding) |
| Welding wire | W-C type, 2.5 mm diameter |
| Flux | Specialized flux for W-C alloy, pre-dried at 300°C for 2 hours |
| Current | 500–700 A |
| Voltage | 28–34 V |
| Travel speed | 200–350 mm/min |
| Layer thickness | 3–5 mm per pass |
| Total overlay thickness | 8–12 mm |
| Interpass temperature | 150–250°C |
Step 4: Post-Weld Heat Treatment
- Cool the welded roller slowly under an insulated blanket at a rate of less than 50°C per hour.
- After cooling to ambient temperature, apply stress-relief annealing at 600–650°C for 4–6 hours.
- Cool again under insulated blanket at a rate of less than 50°C per hour.
Defect Analysis and Countermeasures for Slag Grinding Overlay
The following defects are commonly encountered in slag grinding overlay welding and should be prevented:
| Defect | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking | High carbon content, excessive heat input, rapid cooling | MT, UT | Use low-carbon binder layer, maintain interpass temperature, slow cooling |
| Porosity | Moisture in flux, inadequate shielding | PT, MT | Dry flux thoroughly, ensure proper gas shielding |
| Incomplete fusion | Insufficient preheat, excessive travel speed | UT, MT | Increase preheat to 300°C, reduce travel speed by 20% |
| Excessive hardness | Too many overlay layers, no stress relief | Hardness test | Limit overlay thickness to 10–12 mm, apply stress relief |
| Surface irregularity | Inconsistent travel speed, poor technique | Visual, CMM | Use welding positioner with constant speed, train operators |
Quality Control and Inspection Protocol
A rigorous quality control protocol is essential for slag grinding overlay welding:
- Pre-weld inspection: Visual examination and MT of the prepared surface to detect cracks or defects.
- In-process monitoring: Real-time tracking of welding parameters (current, voltage, travel speed) and visual inspection of each pass.
- Post-weld testing:
- Hardness measurement at 7 locations per roll: minimum HRC 60
- Bond strength test per ASTM G143: minimum 200 MPa
- Ultrasonic thickness measurement at 12 points per roll
- Surface profile verification using a CMM or laser scanner: within ±0.5 mm of nominal
- Impact test (Charpy V-notch) on test coupons: minimum 25 J at room temperature
Engineering Practice Considerations
The implementation of overlay welding for slag grinding roller press squeeze rollers requires careful attention to several practical factors:
- Wear monitoring: The wear rate of the overlay should be monitored by measuring the roller diameter at regular intervals (every 1000–3000 operating hours). A well-executed overlay weld should provide 20,000–30,000 hours of service life before re-welding is required.
- Economic analysis: The cost of overlay welding is approximately 40–60% of the cost of replacing the roller with a new one. For a large cement plant with multiple slag grinding roller presses, the annual savings from overlay welding can exceed 1,500,000 RMB.
- Scheduling: Overlay welding should be scheduled during planned maintenance windows to minimize production disruption. A typical slag grinding roller press roller overlay takes 3–5 days.
- Operator qualification: Operators should be qualified per AWS D10.15 or equivalent standards for hardfacing welding. Regular certification testing should be performed to maintain competency.
Study Reflection and Key Takeaways
The study of overlay welding for roller press squeeze rollers in final grinding slag applications reveals that the technology demands the highest level of process discipline and quality control among cement grinding applications. The W-C type overlay material offers the best abrasion resistance for slag grinding but requires careful control of welding parameters to prevent cracking. The binder layer is critical for preventing cracking at the base metal-overlay interface, and its composition must be carefully selected to match the thermal expansion coefficient of both the base metal and the overlay material. Engineers should establish a systematic approach to wear monitoring and overlay maintenance scheduling to maximize equipment availability and minimize unplanned downtime. The integration of overlay welding with predictive maintenance strategies can further optimize the performance and economics of slag grinding systems. Future developments may include the use of advanced powder metallurgy consumables with tailored microstructures for specific slag grinding applications, and the integration of robotic systems for fully automated roller repair.