MLS Vertical Roller Mill Grinding Roller Weld Overlay Repair Technology

Literature Overview

This technical study note examines the weld overlay repair technology for MLS (MPS-type) vertical roller mill grinding rollers, as documented by Fu Jinqiang (Gezhouba Group Cement Co., Ltd.) and Yang Wei (Zhengzhou Mechanical Research Institute) and published in the New Century Cement Bulletin in 2015. The MLS vertical roller mill represents a more advanced generation of VRM technology with higher grinding efficiency and larger throughput capacity, imposing more demanding requirements on grinding roller surface integrity.

Technical Background

MLS Vertical Roller Mill Characteristics

The MLS (Multi-Layer Separator) vertical roller mill incorporates advanced material classification within the mill housing, achieving higher grinding efficiency compared to conventional VRM designs. The grinding rollers in MLS mills operate under more severe conditions:

Parameter	Conventional VRM	MLS VRM
Mill capacity	50-200 t/h	150-400 t/h
Roller diameter	1000-2000 mm	2000-3200 mm
Operating pressure	80-120 MPa	100-180 MPa
Circulating load	150-250%	100-150%
Material bed depth	80-150 mm	60-120 mm
Grinding roller speed	25-40 rpm	20-35 rpm

The higher operating pressures and more aggressive grinding conditions in MLS mills accelerate grinding roller surface wear, making overlay repair technology particularly important for maintaining equipment availability.

Wear Analysis of MLS Grinding Rollers

The grinding roller surface in an MLS VRM experiences:

Primary wear: Abrasive wear from material particles in the grinding bed, with material hardness ranging from limestone (Mohs 3) to flint (Mohs 7).
Secondary wear: Impact from material chunks falling from the separator back onto the roller surface.
Tertiary wear: Erosion from high-velocity gas flow carrying fine particles across the roller surface.
Wear rate: Typically 0.05-0.15 mm/month for uncladded surfaces; 0.02-0.05 mm/month for properly cladded surfaces.

Weld Overlay Process Development

Overlay Material Selection

For MLS grinding rollers, the overlay material must provide:

High hardness (500-600 HB) for abrasion resistance.
Adequate toughness to resist impact damage from material chunks.
Thermal stability to maintain hardness at operating temperatures (80-150°C).
Good weldability with the roller base material (typically 42CrMo or 40CrNiMo forging).

Overlay Material Type	Composition (wt%)	Hardness (HB)	Toughness	Application
High-C martensitic	C: 2.0-2.8, Cr: 8-12	500-580	Low-Medium	General abrasion
Ni-Cr alloy	Ni: 15-20, Cr: 10-14	450-520	High	Impact + abrasion
Cr-Mo-C alloy	C: 1.8-2.5, Cr: 6-10, Mo: 2-4	480-560	Medium	Severe abrasion
Carbide-reinforced	Cr7C3 or WC additions	550-620	Low	Extreme abrasion

Process Parameters for MLS Roller Overlay

Process Parameter	Specification
Base material	42CrMo forging (HRC 28-32)
Preheating	300-350°C (induction heating or gas heating)
Welding process	SAW with flux-cored wire or solid wire
Number of passes	4-6 passes for 20-30 mm total thickness
Pass 1 (bonding)	E8010 or equivalent low-C filler, 5-8 mm thickness
Pass 2 (transition)	Medium-C alloy filler, 5-8 mm thickness
Pass 3-5 (overlay)	High-C or Ni-Cr hardfacing, 5-8 mm per pass
Final pass (surface)	Fine bead with optimized composition, 3-5 mm
Interpass temperature	≤350°C
Post-weld treatment	Normalizing 820-860°C + air cool, or tempering 250-300°C
Final hardness target	500-580 HB (measured at 2 mm below surface)

Weld Sequence Planning for Cylindrical Surfaces

The cylindrical geometry of the grinding roller requires careful weld sequence planning:

Divide the roller surface into segments: Typically 4-6 axial segments and 8-12 circumferential segments.
Weld in a balanced sequence: Start from the center and work outward, or use a checkerboard pattern to minimize distortion.
Maintain consistent thermal input: Use a rotating fixture with fixed torch position, or vice versa, to ensure uniform bead geometry.
Monitor distortion: Measure diameter at multiple axial locations after each pass; correct if deviation exceeds 0.3 mm.
Final surface preparation: Grind to achieve required profile (slight concavity or convexity per mill design) and surface finish (Ra ≤ 6.3 μm).

Defect Prevention and Quality Assurance

Critical Defect Modes

Defect	Location	Risk Level	Prevention Strategy
Hot cracking	Fusion line of first pass	High	Low-C transition layer; adequate preheat
Cold cracking	HAZ of base material	High	Control CE of base; slow cooling
Undercut	Between beads	Medium	Proper travel speed; 25-30% bead overlap
Porosity	Within overlay	Medium	Dry electrodes/flux; proper shielding
Lack of fusion	Between passes	High	Adequate root preparation; proper current
Hardness variation	Across surface	Medium	Consistent interpass temperature; uniform cooling

Quality Assurance Protocol

The quality assurance protocol for MLS roller overlay repair follows a systematic approach:

Pre-repair assessment: Complete dimensional survey of worn roller; determine remaining thickness and wear pattern.
Surface preparation: Machine worn surface to remove all damaged material; verify base material integrity by MT and UT.
Overlay application: Execute multi-pass overlay per approved procedure with in-process monitoring.
Post-weld treatment: Apply specified heat treatment to achieve target microstructure and hardness.
Final inspection: Comprehensive NDT and dimensional verification per acceptance criteria.
Documentation: Complete repair records including all inspection results, heat treatment parameters, and final measurements.

Engineering Practice and Performance Evaluation

Service Performance Data

Based on engineering experience with MLS roller overlay repair:

Parameter	Before Overlay	After Overlay	Improvement Factor
Surface hardness	280-320 HB	500-580 HB	1.8-2.0×
Service life	6-10 months	24-40 months	3-5×
Surface roughness (initial)	Ra 3.2-6.3 μm	Ra 3.2-6.3 μm	Maintained
Wear rate	0.05-0.15 mm/month	0.02-0.05 mm/month	Reduced 60-70%
Repair cost vs. new roller	—	20-30% of new cost	70-80% savings

Case Study: Gezhouba Group Cement

The collaboration between Gezhouba Group Cement and ZMRI represents a practical engineering application of MLS roller overlay technology. Key lessons from this application include:

The importance of proper preheating for 42CrMo base material (CE ≈ 0.45-0.50%), which is susceptible to cold cracking if cooling rates are not controlled.
The effectiveness of a 4-pass overlay strategy with progressive hardening: low-C bond → medium-C transition → high-C overlay → fine surface pass.
The critical role of post-weld normalizing in achieving uniform hardness across the overlay surface.
The economic benefit of overlay repair: extending roller life from 8 months to 32 months at 25% of new roller cost.

Study Reflections and Technical Implications

The 2015 publication represents a significant advancement in MLS roller overlay technology, reflecting the maturation of Chinese cement industry equipment maintenance practices. The collaboration between an operating cement company (Gezhouba) and a research institute (ZMRI) exemplifies the industry-academia partnership model that drives practical engineering innovation.

The key technical insight is that MLS mill rollers require more sophisticated overlay strategies than conventional VRM rollers due to higher operating pressures and more aggressive grinding conditions. The multi-pass approach with progressive hardening is not merely a practical convenience but a metallurgical necessity: the transition from low-C base to high-C overlay must be gradual to prevent cracking at the fusion boundary.

The engineering philosophy of overlay repair rather than component replacement reflects sound economic reasoning: the capital cost of a complete MLS grinding roller can exceed USD 50,000-100,000, while overlay repair costs USD 10,000-25,000 and extends service life by 3-5 times. This makes overlay repair the economically rational choice for planned maintenance.