Overlay Welding Repair Technology for MLS Vertical Mill Rollers

Literature Overview

This paper by Fu Jinqiang and Yang Wei, published in New Century Cement Bulletin (2015, Vol. 21, No. 5, pp. 36–39), presents a systematic approach to the overlay welding repair of MLS-type vertical mill rollers. The MLS (Multi-Roller Separator) vertical mill is a critical grinding unit in cement production, and its rollers are subjected to extreme abrasive wear from continuous contact with raw meal and clinker. The study addresses both offline (shop) and online (in-situ) overlay welding repair strategies, with specific emphasis on fixture design for offline repair and bent torch development for online repair efficiency.

Core Technical Content

MLS Roller Structure and Wear Characteristics

The MLS vertical mill roller adopts a segmented (multi-piece) structural design, where the roller is composed of multiple arc-shaped segments bolted together to form a complete cylindrical surface. Three rollers are hydraulically lifted as a group to apply grinding pressure. This segmented architecture has profound implications for repair strategy:

Feature	Implication for Repair
Segmented construction	Each segment can be individually removed for offline repair
Hydraulic lifting system	Rollers can be separated from the mill housing
Abrasive wear pattern	Wear is concentrated on the grinding contact surface
Thermal fatigue	Repeated heating/cooling cycles during grinding
Impact loading	Abrasive particles cause micro-impact damage

The wear mechanism on MLS rollers is predominantly abrasive, with secondary contributions from adhesive wear and fatigue spalling. The grinding surface typically experiences a wear depth of 2–5 mm per operating cycle (depending on feed material hardness and fineness requirements), necessitating periodic overlay welding to restore dimensional accuracy and surface hardness.

Offline Overlay Welding Strategy

Offline repair involves removing the roller segments from the mill and transporting them to a workshop for grinding and overlay welding. The key technical challenges include:

Fixture design: A custom welding fixture was developed to hold the roller segment in the correct orientation during overlay welding. The fixture must accommodate the arc-shaped geometry of the segment, provide adequate clamping force to prevent distortion, and allow torch access to all weld positions.
Cutting and preparation: Worn material is removed by mechanical grinding (typically using a portable grinder with silicon carbide or aluminum oxide wheels) or by cutting with a plasma or oxy-fuel torch. The preparation must achieve a clean, oxide-free surface with adequate undercut (typically 2–3 mm) to ensure proper fusion with the base metal.
Undercut repair welding (补焊): Before applying the wear-resistant overlay layers, any cracks, porosity, or undercut in the base metal must be repaired using a compatible fill metal. This step is critical because any pre-existing defect in the base metal will propagate through the overlay layers and cause premature failure.
Overlay welding sequence: Multiple layers of wear-resistant alloy are deposited, with each layer typically 3–5 mm thick. The welding sequence should follow a pattern that minimizes residual stress and distortion, starting from the center and progressing outward, or using a balanced symmetric pattern.

Online Overlay Welding Strategy

Online repair is performed with the roller still installed in the mill or with the roller segment in place but the mill housing opened. The primary advantage is reduced downtime. The study highlights the development of a bent welding torch specifically designed for MLS roller online repair:

The bent torch allows the welder to access the roller surface at the required angle without the need for complex positioning equipment.
The bent geometry accommodates the curvature of the roller surface, maintaining consistent arc length and travel speed.
Installation efficiency is significantly improved compared to using a straight torch with adapter fittings.

For online repair, the primary operation is supplementary welding (补焊) to fill worn areas and restore the roller profile. The segmented structure of the MLS roller makes online repair particularly feasible because individual segments can be accessed without dismantling the entire roller assembly.

Welding Consumable Selection

The study validates the ZD90X-O series wear-resistant overlay welding wire, confirming its excellent abrasion resistance through case study verification. The ZD90X-O wire is a high-carbon, high-chromium martensitic alloy wire that produces a weld deposit with hardness exceeding 58 HRC in the as-deposited condition.

Consumable Property	Specification
Wire designation	ZD90X-O
Weld deposit hardness	≥58 HRC (as-deposited)
Alloy system	High-C, high-Cr martensitic
Welding process	Submerged arc welding (SAW) or flux-cored arc welding (FCAW)
Typical layer thickness	3–5 mm per pass
Recommended total overlay thickness	15–30 mm
Preheat temperature	150–250°C
Interpass temperature	≤300°C

Quality Control and Defect Prevention

The following FMEA (Failure Mode and Effects Analysis) approach is recommended for overlay welding repair of MLS rollers:

Potential Defect	Cause	Effect	Countermeasure
Cracking in overlay layer	Excessive carbon equivalent, high restraint	Roller failure during operation	Preheat, low heat input, proper wire selection
Poor fusion	Inadequate surface preparation	Delamination of overlay	Thorough cleaning, adequate undercut
Excessive porosity	Contaminated flux or wire	Reduced load-bearing capacity	Dry flux storage, wire inspection
Hardness below specification	Dilution from base metal	Premature wear	Multiple thin layers, proper travel speed
Excessive distortion	Unbalanced heat input	Misalignment of roller	Symmetric welding sequence, fixture support

Engineering Practice Insights

From a practical standpoint, the segmented design of the MLS roller is a significant advantage for maintenance and repair. It allows for modular replacement and targeted repair, reducing both downtime and material costs compared to monolithic roller designs. However, the bolted joints between segments introduce potential leak paths for material ingress and stress concentration points that require regular inspection.

The decision between offline and online repair should be based on a cost-benefit analysis considering downtime costs (typically 50,000–200,000 CNY per day of mill outage), repair quality requirements, and available equipment. For critical rollers with severe wear or cracking, offline repair with full dimensional control is recommended. For minor wear restoration, online repair with the bent torch provides an efficient solution.

The validation of ZD90X-O wire through practical case studies provides confidence in its application. Engineers should note that the wear life of the overlay depends not only on the hardness of the deposit but also on the toughness of the weld metal, the quality of the fusion bond, and the operating conditions (grinding pressure, feed material characteristics, and mill speed).

This study demonstrates that systematic approach to overlay welding repair—encompassing fixture design, consumable selection, process optimization, and quality control—can significantly extend the service life of MLS vertical mill rollers and reduce total maintenance costs in cement production operations.