Self-Shielded Cladding Technology for Raw Material Vertical Mill

Literature Overview

This 2009 study by Yang Wei, Zhang Haiyan, and Ni Junjie from Zhengzhou Machinery Research Institute investigates the application of self-shielded cladding technology for raw material vertical mills in cement production. Published in the journal of New Century Cement Guide, this work addresses the critical challenge of extending the service life of vertical mill components subjected to severe abrasive and impact wear conditions.

Raw material vertical mills are essential equipment in cement production, grinding raw materials to the required fineness for clinker production. These mills operate under extremely harsh conditions with high temperatures, abrasive materials, and continuous vibration. Key components such as grinding rings, rollers, and mill liners experience rapid wear and require frequent replacement or repair.

Core Technical Viewpoints

The study focuses on the development and implementation of self-shielded cladding technology, which eliminates the need for external shielding gas by using flux-coated wires or self-shielded flux-cored wires. This approach offers significant advantages for field repair applications where shielding gas equipment may be unavailable or impractical.

Self-Shielded Cladding Process Characteristics

Feature	Self-Shielded	Gas-Shielded
Shielding method	Flux in wire coating	External shielding gas
Equipment complexity	Low	High
Field applicability	Excellent	Limited
Deposition rate	High	High
Spatter	Moderate	Low
Fume generation	High	Low
Wind sensitivity	Low	High
Cost per kg deposited	Lower	Higher

Material Selection for Vertical Mill Cladding

The study evaluates several overlay material systems for vertical mill applications:

High chromium white iron (HCWI): Excellent abrasion resistance, suitable for severe sliding abrasion.
Medium chromium white iron (MCWI): Good balance of abrasion resistance and toughness.
Manganese steel: Excellent impact resistance, suitable for high-impact applications.
Nickel-hard iron: Good combination of hardness and toughness.
Maraging steel: High strength, good fatigue resistance.

For raw material vertical mills, high chromium white iron is typically the preferred overlay material due to its excellent resistance to sliding abrasion, which is the dominant wear mechanism in these applications.

Process Analysis and Implementation

Self-Shielded Flux-Cored Wire Composition

The recommended self-shielded flux-cored wire composition for vertical mill cladding is:

Base alloy: High chromium white iron (Cr: 18-25%, C: 2.0-3.5%)
Flux coating: Contains deoxidizers, alloying elements, and gas-forming agents
Wire diameter: 1.2-1.6 mm for typical applications
Wire length: 1-2 meters for practical handling

Welding Process Parameters

Parameter	Typical Range
Current	150-250 A
Voltage	22-30 V
Travel speed	150-300 mm/min
Wire feed speed	3-6 m/min
Electrode angle	5-15° forward
Preheat temperature	150-250°C
Interpass temperature	<250°C

Microstructural Characteristics

The self-shielded cladding deposits exhibit the following microstructural features:

Primary cementite (Fe3C) and chromium carbides (Cr7C3, Cr23C6) in a pearlitic-ferritic matrix.
High hardness (800-900 HV) due to the high chromium and carbon content.
Good resistance to sliding abrasion but limited impact toughness.
Potential for cracking if preheat and interpass temperatures are not controlled.

Engineering Practice Integration

The self-shielded cladding technology has been successfully applied in numerous cement plant applications:

Vertical mill grinding rings: Cladding of worn grinding rings to restore original dimensions and extend service life.
Roller shells: Repair of worn roller shells on vertical roller mills.
Mill liners: Overlay of internal mill liners to improve wear resistance.
Chutes and hoppers: Cladding of material handling components subject to abrasive wear.

A representative industrial case involves the repair of a vertical mill grinding ring that had worn through after 8 months of service. By applying self-shielded high chromium white iron cladding with a total thickness of 15-20 mm, the service life was extended to over 18 months, representing a 125% improvement in operational availability.

Quality Control Protocol

The following quality assurance steps are recommended for self-shielded cladding of vertical mill components:

Pre-weld inspection: Assess wear extent, verify component integrity, plan repair strategy.
Surface preparation: Grind to remove worn material, ensure clean surface for bonding.
Preheating: Apply uniform preheat to reduce thermal stresses and prevent cracking.
Welding execution: Maintain consistent parameters, monitor bead quality.
Post-weld inspection: Visual examination, magnetic particle testing for cracks.
Hardness verification: Confirm overlay hardness meets specifications (800-900 HV).
Dimensional check: Ensure all critical dimensions are restored to original specifications.

Defect Analysis and Countermeasures

Defect	Root Cause	Countermeasure
Cracking	Insufficient preheat, high carbon content	Increase preheat, reduce carbon in wire
Poor bonding	Surface contamination	Thorough cleaning, proper surface preparation
Excessive spatter	High current, improper wire angle	Reduce current, optimize wire angle
Porosity	Flux contamination	Use quality wire, ensure proper storage
Hardness variation	Uneven heat input	Maintain consistent travel speed and current

Study Insights and Implications

This research demonstrates that self-shielded cladding technology is a highly effective solution for extending the service life of vertical mill components in cement production. The key technical insight is that the elimination of external shielding gas significantly improves field applicability and reduces equipment requirements, making it ideal for maintenance and repair operations.

The study also highlights the importance of material selection. While high chromium white iron provides excellent abrasion resistance, engineers must consider the specific wear mechanism in their application. For applications involving significant impact loading, a more ductile material such as manganese steel or maraging steel may be more appropriate.

For cement manufacturers considering self-shielded cladding for vertical mill maintenance, the following recommendations are provided:

Conduct a detailed wear analysis to determine the dominant wear mechanism and select the appropriate overlay material.
Invest in proper preheating equipment to ensure consistent thermal management during welding.
Implement a comprehensive quality control program that includes hardness testing and dimensional verification.
Train maintenance personnel in proper self-shielded welding techniques and safety practices.
Maintain detailed records of all cladding operations for performance tracking and continuous improvement.

In summary, self-shielded cladding technology offers a practical and cost-effective solution for maintaining vertical mill components in cement production. The technology's field applicability, combined with the excellent wear resistance of high chromium white iron overlays, makes it an ideal choice for extending equipment life and reducing maintenance costs. Engineers should adopt a systematic approach that combines material selection expertise with process control to achieve optimal results in this demanding industrial application.