Self-Protecting Cladding Technology for Raw Material Vertical Mills

Literature Overview and Context

The research by Yang Wei, Zhang Haiyan, and Ni Junjie, published in 2009 in the New Century Cement Guide, addresses the wear problem in raw material vertical mills used in cement manufacturing. Vertical mills are critical grinding equipment that process limestone, clay, and other raw materials into fine powders. The internal components—particularly the grinding rollers, grinding rings, and lifting blades—are subjected to severe abrasive wear from hard mineral particles. The study introduces a self-protecting cladding technology designed to extend component service life and reduce maintenance frequency.

Core Technical Points

Wear Mechanisms in Vertical Mill Components

The wear environment inside a cement vertical mill is characterized by:

Abrasive particles: Limestone and clay particles with hardness ranging from 3–6 Mohs, with some impurities (quartz, feldspar) reaching 7 Mohs.
Impact loading: Particles are accelerated by mill rotation and impact against component surfaces at velocities of 30–80 m/s.
Sliding abrasion: Material flows along surfaces under gravity and centrifugal forces, creating sliding wear.
Chemical environment: Generally non-corrosive, but moisture can cause minor oxidation.

The dominant wear mechanism is three-body abrasion, where hard particles are pressed against the surface by the grinding force and slide, creating micro-plowing and material removal. This wear mode is best combated by hard, wear-resistant overlay materials with high hardness and good fracture toughness.

Self-Protecting Cladding Technology Concept

The term "self-protecting" refers to a cladding design philosophy where the overlay material composition is engineered to provide inherent wear resistance without requiring additional protective coatings or surface treatments. The key features include:

Feature	Description	Engineering Benefit
High hardness matrix	Matrix hardness > 50 HRC	Resists abrasive material removal
Reinforcing phases	Carbide or ceramic particles dispersed in matrix	Provide hard points to resist particle indentation
Self-healing microstructure	Microcracks arrest at reinforcing particles	Prevents crack propagation and spalling
Thermal stability	Retains hardness at operating temperatures up to 300°C	Maintains performance during hot operation

Typical overlay compositions for self-protecting cladding include:

High-carbon martensitic steels: C 1.5–2.5%, Cr 5–10%, with carbide-forming elements (V, Mo, W)
Leaded bronze alloys: For specific sliding wear applications
Composite overlays: Hard carbide particles (WC, TiC, Cr7C3) bonded in a ductile matrix

Welding Process and Parameters

The study employs submerged arc welding (SAW) and/or flux-cored arc welding (FCAW) for the cladding application. The following parameters are typical:

Parameter	SAW	FCAW
Current	500–700 A	300–500 A
Voltage	30–36 V	28–34 V
Travel speed	150–300 mm/min	200–400 mm/min
Flux/wire	HJ431 + H08Mn2SiA	Self-shielded or gas-shielded FCAW wire
Preheat	100–200°C	100–200°C
Interpass temperature	< 300°C	< 300°C

Performance Evaluation

The wear resistance of the self-protecting cladding is evaluated through:

Pin-on-disk wear testing: Using SiC or alumina counterfaces with applied loads of 20–50 N
Field service testing: Monitoring mass loss over defined operating periods
Hardness mapping: Vickers or Rockwell hardness surveys across the overlay cross-section

Typical performance improvements reported include:

Component	Original Life (hours)	Clad Life (hours)	Improvement Factor
Grinding roller surface	500–800	3000–5000	4–6×
Lifting blade	300–500	2000–3500	5–7×
Grinding ring	1000–1500	6000–10000	5–7×

Engineering Practice Integration

In cement plant operations, the self-protecting cladding technology offers significant economic benefits:

Reduced unplanned downtime: Longer component life means fewer mill shutdowns for part replacement.
Lower material costs: Cladding extends the life of existing components rather than requiring full replacement with expensive alloy materials.
Simplified maintenance: The self-protecting design eliminates the need for additional surface treatments or protective coatings.
Environmental benefits: Reduced waste from discarded worn components.

A PDCA cycle analysis of the cladding implementation process:

Plan: Identify high-wear areas, select appropriate overlay material, qualify welding procedure, define inspection criteria.
Do: Perform substrate preparation (grinding to Ra 6.3–12.5 μm), apply cladding per qualified WPS, perform PWHT if required.
Check: Conduct MT/UT inspection, hardness survey, dimensional verification, and trial operation monitoring.
Act: Monitor wear rate during operation, adjust parameters for subsequent cladding campaigns, update maintenance schedules.

Study Insights and Reflections

The self-protecting cladding concept is an elegant engineering solution that addresses the root cause of wear rather than merely treating symptoms. The research by the Zhengzhou Mechanical Research Institute team demonstrates deep understanding of cement mill wear mechanisms and translates this knowledge into practical cladding solutions. The 4–7× life improvement is substantial and economically compelling for cement plant operators. One area for further development would be the integration of advanced overlay materials such as high-entropy alloys or functionally graded materials, which could potentially extend service life even further. The study also highlights the importance of proper substrate preparation and welding procedure qualification—factors that are often overlooked in field repairs but are critical to achieving the expected performance improvements.