Overlay Repair of Static Ring in ZGM113G Medium-Speed Roller Coal Mill

Literature Overview

The paper by Luan Jiangtao and Luan Chenchen, published in Cement (2021, No. 2, pp. 53–55), documents a practical overlay welding repair case for the static ring of a ZGM113G medium-speed roller coal mill at Nanyang Zhonglian Cement Co., Ltd. This is a straightforward engineering case study that illustrates the application of overlay welding technology in the cement industry, where coal mills operate under severe abrasive wear conditions.

Problem Description and Technical Context

The ZGM113G medium-speed roller coal mill is a widely used grinding equipment in cement plants, designed to grind raw coal into fine particles for kiln fuel preparation. The nozzle ring assembly consists of a dynamic ring (rotating) and a static ring (fixed), with a designed operating clearance of 5–10 mm between them.

During a January 2014 maintenance inspection, the static ring was found to be severely and unevenly worn, with the clearance between the dynamic and static rings having increased to 25–40 mm. This excessive clearance caused a reduction in air velocity at the nozzle, leading to increased slag discharge from the coal mill and degraded grinding efficiency.

Operational Impact of Wear

Parameter	Design Specification	Condition Found	Impact
Ring clearance	5–10 mm	25–40 mm	Air velocity reduction
Coal mill output	Normal	Reduced	Increased slag discharge
Grinding efficiency	Design value	Degraded	Higher energy consumption
Replacement lead time	N/A	No stock available	Extended downtime

The critical constraint was that the equipment manufacturer had no spare static rings in stock, and the machining lead time was excessively long. This forced the decision to pursue overlay welding repair rather than waiting for a replacement part.

Overlay Repair Strategy

The repair approach involved the following steps:

Surface preparation — Grinding of the worn surface to remove loose material and establish a uniform base for welding.
Welding consumable selection — Selection of a wear-resistant overlay wire suitable for the coal mill application (likely a high-carbon martensitic or austenitic type).
Welding sequence planning — Multi-pass overlay to build up the material and restore the required dimensions.
Post-weld machining — Precision turning to restore the final geometric dimensions and surface finish.

Welding Process Considerations

For this application, the following process parameters and considerations are critical:

Preheating — The carbon steel static ring likely requires 100–150°C preheating to prevent cracking, particularly if a high-carbon overlay alloy is used.
Interpass temperature control — Maintain below 200°C to prevent softening of the overlay alloy.
Weld bead design — Overlapping beads with 50–70% overlap to ensure uniform composition and minimize porosity.
Welding direction — Alternating directions to minimize distortion in the ring geometry.
Deposition rate — Balance between productivity and dilution control.

Quality Assurance Considerations

For a rotating component operating under dynamic loading and abrasive wear, the following quality checks are essential:

Dimensional accuracy — The machined surface must meet the original geometric tolerances to ensure proper clearance with the dynamic ring.
Surface hardness — Vickers hardness testing to confirm the overlay meets wear resistance requirements (typically 45–55 HRC for coal mill applications).
Bond strength — Peel testing or cross-sectional examination to verify metallurgical bonding between overlay and substrate.
Crack inspection — Magnetic particle testing (MT) or liquid penetrant testing (PT) to detect any surface or near-surface cracks.
Balance — Dynamic balancing after repair to prevent vibration issues during operation.

Engineering Practice Insights

This case study highlights several important lessons for maintenance engineers:

Supply chain risk management — The unavailability of spare parts is a common risk in heavy industry. Having overlay welding capability in-house or through a qualified contractor provides a critical contingency option.
Cost-benefit analysis — Overlay repair is typically 30–50% of the cost of a new part replacement, with significantly shorter turnaround time.
Skill requirement — Overlay welding of wear-resistant alloys requires experienced welders and qualified welding procedures. The success of the repair depends heavily on welder skill and process control.
Documentation — Complete records of the repair (welding parameters, consumables, inspection results, post-weld machining data) should be maintained for future reference and asset management.

Study Insights and Reflections

While this paper is relatively brief and does not include extensive metallurgical analysis, its practical value is significant. It demonstrates that overlay welding is not merely a laboratory technology but a proven field repair method that can save critical downtime in industrial operations.

The key engineering insight is that the decision to repair versus replace should be based on a comprehensive assessment that includes: remaining fatigue life of the substrate, availability and cost of replacement, production impact of downtime, and technical feasibility of the repair. In this case, the overlay repair was the only viable option given the supply chain constraints.

For future repairs of similar components, I would recommend establishing a qualified welding procedure specification (WPS) specifically for the static ring repair, including documented parameters, consumable specifications, and acceptance criteria. This would enable consistent repair quality and facilitate regulatory compliance where applicable.