Overlay Welding Repair Technology for MLS Vertical Mill Roller Segments

Literature Overview

The paper by Fu Jinqiang and Yang Wei, published in New Century Cement Herald in 2015, addresses the overlay welding repair of MLS-type vertical mill rollers. This work originates from Gezhouba Group Cement Co., Ltd. and Zhengzhou Machinery Research Institute, combining field experience with research institute expertise. The MLS vertical mill is a critical piece of grinding equipment in cement production, and its roller segments experience severe abrasive wear from continuous contact with raw meal. The authors focus on two repair scenarios: offline (shop-floor) overlay welding and online (in-situ) overlay welding, each requiring distinct tooling and process strategies.

Core Technical Content

The MLS roller design features a segmented structure where three rollers are lifted simultaneously by a hydraulic pressure frame. This segmented architecture has profound implications for repair methodology. Because the rollers can be removed from the mill housing, offline repair is feasible, but it requires specialized welding fixtures to hold the segments in proper alignment during the overlay pass. Conversely, the segmented design also facilitates online repair, as individual segments can be accessed while the mill remains partially assembled.

The authors developed two key pieces of equipment: a dedicated offline welding fixture that ensures geometric accuracy during shop repair, and a curved welding torch designed for online overlay welding that improves torch positioning efficiency and accelerates the overall repair schedule. The offline repair process involves cutting away damaged material followed by overlay welding, whereas online repair focuses primarily on building up worn surfaces without material removal.

Welding Consumables and Process Parameters

The study validates the ZD90X-O series wear-resistant overlay welding wire, which has been demonstrated through multiple field cases to provide excellent abrasion resistance suitable for cement mill service conditions.

The ZD90X-O wire is a high-carbon, high-chromium alloy system typically containing 12-14% Cr and 2.5-3.5% C, which produces a martensitic weld deposit with hardness exceeding 55 HRC after air cooling. This microstructure provides excellent resistance to sliding and grinding abrasion, which is the dominant wear mechanism in vertical mill roller service.

Engineering Practice Insights

From a metallurgical perspective, the repair of cast steel rollers requires careful attention to the base metal composition and the thermal cycling effects of overlay welding. MLS rollers are typically manufactured from medium-carbon cast steel or low-alloy steel with carbon equivalents in the range of 0.35-0.45%, which necessitates preheating to 150-250°C to prevent cold cracking in the heat-affected zone. The segmented design introduces additional challenges at the segment joints, where residual stresses from the original casting and subsequent machining may concentrate at weld boundaries.

The decision between offline and online repair involves a trade-off analysis. Offline repair offers superior geometric control and the ability to perform complete surface preparation, including grinding and cleaning of the entire roller surface. However, it requires significant downtime for roller removal, transport, and reinstallation. Online repair minimizes downtime but is constrained by limited access, the inability to control ambient conditions, and the challenge of maintaining weld quality in a confined space.

The curved torch design described in the paper is an elegant engineering solution to the access problem. In a vertical mill, the roller surface is concave, and standard straight torches cannot achieve adequate deposition angles on the lower portions of the roller. The curved torch allows the welder to maintain consistent arc length and travel speed across the entire surface contour, which is critical for achieving uniform weld bead geometry and avoiding undercut or excessive reinforcement.

Key Reflections and Technical Commentary

The paper demonstrates a practical, field-driven approach to solving a real industrial problem. The emphasis on fixture and tooling design rather than purely on welding parameters reflects the reality that in industrial repair welding, access and positioning are often the limiting factors, not arc stability or metallurgical compatibility. The validation of the ZD90X-O wire through repeated field use provides confidence in its reliability, although the paper would benefit from quantitative hardness profiling data across the weld deposit to characterize the hardenability gradient from the base metal interface to the free surface.

A critical consideration not fully addressed in the paper is the fatigue performance of the overlay weld. Vertical mill rollers experience cyclic loading from the hydraulic pressure system, and the overlay weld introduces a metallurgical discontinuity that could serve as a fatigue crack initiation site. Future work should incorporate fatigue testing of the overlay repair, particularly under the cyclic stress conditions representative of mill operation. The combination of segmented design, hydraulic loading, and overlay repair creates a complex stress state that warrants further investigation through finite element analysis and strain gauge measurement.

In summary, this paper provides valuable practical guidance for the repair of MLS vertical mill rollers, with particular emphasis on the tooling innovations that enable efficient online repair. The validation of the ZD90X-O consumable through field experience adds credibility to the recommended approach. Engineers working on similar repair challenges should carefully consider the segmented roller design when planning repair strategies, and should invest in purpose-built tooling to maximize repair quality and minimize downtime.