New Method for Online Cladding of MLS3726 Vertical Mill Grinding Rollers

Literature Overview

The reference by Zhao Yunfeng (2013), published in the journal Cement and associated with Feixian Yizhou Cement Co., Ltd., documents a novel approach to online cladding repair of MLS3726 vertical mill grinding rollers. The MLS3726 is a widely used mid-sized vertical roller mill in Chinese cement plants, with grinding rollers typically of 400–500 mm diameter. Unlike roller press rolls, vertical mill grinding rollers experience a different wear pattern—primarily radial abrasion from grinding media and feed material, with periodic thermal cycling from hot feed. The paper presents an improved online cladding methodology that addresses the specific challenges of vertical mill roller geometry and operating conditions.

Core Technical Content

The MLS3726 vertical mill roller presents unique challenges for cladding repair: the roller is relatively small in diameter but mounted in a position with limited access; the operating conditions involve high sliding contact stress; and the repair must be completed within a narrow maintenance window. The new method described in the reference introduces several innovations in process design, equipment configuration, and quality control.

Roller Geometry and Wear Characteristics

Parameter	MLS3726 Specification
Roller diameter	400–500 mm
Roller length	300–400 mm
Base material	40Cr or 42CrMo (quenched and tempered)
Original overlay	Cr-Cr₂C₃ hardfacing, 6–8 mm thick
Operating pressure	60–80 MPa
Feed temperature	200–350 °C
Typical service life	6–10 months
Primary wear mode	Abrasive + adhesive

The wear pattern on MLS3726 rollers is typically uniform across the working surface, with accelerated wear at the edges where material exits the grinding zone. This uniform wear pattern makes the repair process more predictable than roller press applications, where wear distribution is more complex.

Novel Process Features

The new method introduces several process innovations:

Segmented approach: The roller circumference is divided into 8 segments, each repaired sequentially. This allows systematic quality control at each segment and prevents the accumulation of thermal distortion.
In-situ preheating with embedded thermocouples: Thermocouples are embedded in the substrate at multiple depths to monitor thermal history in real time, enabling precise control of heating and cooling rates.
Dual-process approach: Transition layers are deposited by SMAW for flexibility, while working layers are applied by GMAW with flux-cored wire for high deposition rates and consistent bead geometry.
Post-weld in-situ stress relief: A portable induction coil is used to heat the roller to stress relief temperature without removal from the mill housing.

Process Parameters for MLS3726 Roller Cladding

Parameter	Value	Notes
Preheat temperature	250 °C	Verified by embedded thermocouples
Transition layer	E309L, 2 passes, 3 mm total	SMAW, 4.0 mm electrode
Working layer	E51700 (Cr-Cr₂C₃), 4–5 passes	GMAW-F, 1.2 mm wire
Wire feed speed	4.0–5.5 m/min	Adjusted for bead profile
Travel speed	200–250 mm/min	Maintained by positioner
Shielding gas	Pure CO₂ at 15–20 L/min	For FCAW process
Interpass temperature	≤ 200 °C	Monitored by thermocouple
Stress relief	550 °C × 1.5 h	Induction heating
Final overlay thickness	8–10 mm	Beyond original specification
Final grinding	To specified profile	Ra ≤ 6.3 μm

FMEA Analysis of Critical Process Steps

A failure mode and effects analysis (FMEA) was conducted for the critical process steps:

Process Step	Potential Failure Mode	Effect	Severity	Occurrence	Detection	RPN	Mitigation
Preheating	Incomplete heating	Cold cracking	10	3	4	120	Embedded thermocouples; minimum 30 min soak
Transition layer	Insufficient penetration	Delamination	8	3	5	120	Verify weld toe geometry; UT if accessible
Working layer	Excessive dilution	Low hardness	7	4	3	84	Monitor bead geometry; hardness test each segment
Working layer	Porosity	Reduced strength	6	4	4	96	Proper gas flow; clean substrate
Stress relief	Insufficient temperature	Residual stress	8	3	3	72	Thermocouple verification; minimum soak time
Final grinding	Over-grinding	Insufficient thickness	7	2	2	28	Measure thickness before grinding

Quality Verification and Performance Results

The paper reports the following quality verification results for the new method:

Test Parameter	Specification	Measured Result	Acceptance
Overlay hardness	≥ 600 HV	635–672 HV	Pass
Bond strength	≥ 220 MPa	245–268 MPa	Pass
Surface roughness	Ra ≤ 6.3 μm	Ra 4.2–5.8 μm	Pass
Runout	≤ 0.03 mm	0.01–0.02 mm	Pass
MT inspection	No linear indications	Clean	Pass
Dimensional accuracy	± 0.05 mm	± 0.02 mm	Pass

Post-repair service life data:

Repair Batch	Service Life	Wear Rate	Notes
1st (new method)	12 months	0.8 mm/month	Uniform wear
2nd (new method)	14 months	0.7 mm/month	Improved process
3rd (new method)	16 months	0.6 mm/month	Optimized parameters

The progressive improvement in service life demonstrates the effectiveness of the iterative process refinement approach. The wear rate of 0.6–0.8 mm/month is comparable to the original equipment manufacturer's specification of 0.7 mm/month, confirming that the repair achieves equivalent performance.

Engineering Practice Integration

The implementation of this new method required several organizational changes:

Welder training: Operators were trained specifically on the segmented approach and thermocouple monitoring techniques.
Equipment investment: Portable induction heater, embedded thermocouple system, and GMAW-F equipment were procured.
Documentation: Each repair was documented with thermal history data, hardness maps, and dimensional measurements, creating a database for continuous improvement.
Maintenance scheduling: The predictable 12–16 month service life allows proactive scheduling of repairs, eliminating emergency stoppages.

Study Insights and Implications

This reference demonstrates that even well-established repair processes can be significantly improved through systematic engineering analysis and innovation. The introduction of embedded thermocouples for real-time thermal monitoring represents a quantum leap in process control for field welding applications. The segmented approach to welding, while seemingly simple, addresses the fundamental challenge of thermal distortion management in constrained geometries. The FMEA analysis provides a structured framework for identifying and mitigating process risks, which is often absent in field repair operations. The progressive improvement in service life over multiple repair cycles validates the continuous improvement philosophy and provides quantitative evidence of the method's effectiveness. For engineers managing vertical mill maintenance, this reference provides a complete, implementable methodology that balances quality, cost, and schedule objectives.