Overlay Welding Repair and Maintenance of Imported Roller Press Roller Surface

Literature Overview

The technical article by Fu Jinqiang, published in the journal New Century Cement Bulletin in 2012, documents the repair and maintenance of the roller surface of an imported Φ1400×500 roller press manufactured by Humboldt-Wedag, a German company. This case study is particularly instructive because it involves a high-value imported component in a cement grinding application, where the roller surface undergoes severe abrasive wear and fatigue. The paper details the failure progression from surface spalling to a critical axial crack penetrating 180 mm deep, and the subsequent repair and maintenance strategy that extended the roller's service life beyond that of a new unit.

Equipment Background and Failure History

The roller press in question is a critical piece of equipment in a cement grinding circuit, responsible for pre-grinding clinker and other raw materials before they enter the ball mill or vertical roller mill. The roller surface is subjected to:

• High contact pressure from the compression of abrasive cement clinker
• Repeated impact loading from irregular feed material
• Thermal cycling due to the temperature of the processed material
• Abrasive wear from hard mineral particles

The roller body is typically made of a medium-carbon steel such as 40Cr or 42CrMo, with a hardfacing overlay applied to the working surface to provide wear resistance. The original overlay was likely a high-chromium cast iron or a nickel-based alloy, applied by submerged arc welding (SAW) or flux-cored arc welding (FCAW).

Failure Progression

The failure evolved through several stages:

1. Surface fatigue: Micro-cracks initiated at surface defects and propagated inward under cyclic loading
2. Spalling: Surface layers detached from the substrate due to fatigue crack growth
3. Field patch welding: Multiple attempts were made to repair the spalled areas by welding filler metal into the damaged zones
4. Stress accumulation: Each repair added new residual stresses to the already stressed roller body
5. Critical crack formation: An axial crack developed, extending 180 mm deep, nearly penetrating the full thickness of the roller shell

The discovery of the 180 mm deep crack by Zhengzhou Machinery Research Institute, after the spalled overlay was machined away, represents a critical near-miss event. Had the crack propagated fully through the roller shell, it would have resulted in catastrophic failure during operation, with potentially severe safety consequences.

Repair Strategy and Execution

The repair approach involved the following steps:

Step 1: Crack Assessment and Removal

The entire roller surface overlay was removed by machining (turning) to expose the base metal. The axial crack was then fully traced and confirmed using ultrasonic testing (UT). The crack was ground out completely, with a drill hole placed at the crack tip to blunt the stress concentration.

Step 2: Roller Shell Replacement

Given the severity of the crack (180 mm depth), the decision was made to replace the roller shell entirely rather than attempt a field repair. This was the correct engineering judgment, as a crack of this depth in a pressure-loaded rotating component cannot be reliably repaired in the field.

Step 3: New Overlay Application

The new roller shell was fitted with a freshly applied hardfacing overlay. The selection of the overlay material was critical, and the following considerations were applied:

Step 4: Post-Weld Treatment

The repaired roller underwent stress relief heat treatment at 550–600°C for 2–4 hours to reduce residual stresses from the overlay welding. The surface was then machined to the required dimensional tolerance and surface finish.

Maintenance Recommendations

The paper provides several practical maintenance guidelines that are directly applicable to similar equipment in other industries:

1. Iron removal: Regularly remove iron contaminants from the feed material using magnetic separators. Iron particles are harder than cement clinker and cause accelerated wear of the roller surface.
2. Material ratio control: Maintain the correct ratio of cement clinker to other materials in the feed to avoid excessive abrasive loading.
3. Periodic surface inspection: Implement a scheduled inspection program using visual examination, UT, and MT to detect surface cracks before they propagate to critical depth.
4. Rational overlay material selection: Choose the overlay material based on the specific wear mechanism (abrasive, adhesive, or fatigue) rather than defaulting to the same material used in the original equipment.
5. Qualified repair contractors: Ensure that overlay welding repairs are performed by qualified contractors with appropriate welding procedure specifications (WPS) and welder qualifications.

Engineering Practice Integration

This case study has broader implications for the maintenance of heavy-duty rotating equipment across multiple industries, including mining, power generation, and steel production. The key lessons are:

• Cumulative stress from repeated repairs is a major failure mechanism. Each field repair adds residual stress to the component, and over time, these stresses can combine to create a critical condition. Engineers should establish a maximum number of permissible field repairs for critical components.
• NDT is essential for detecting subsurface cracks. Surface spalling may appear to be a minor cosmetic issue, but it can mask a deep internal crack that threatens structural integrity.
• Replacement of worn components is sometimes more economical than repeated repairs. The cost of a new roller shell is often less than the cumulative cost of multiple repair attempts, plus the risk of catastrophic failure.
• Proactive maintenance is always better than reactive repair. Implementing a scheduled inspection and overlay renewal program can prevent the failure cascade described in this case.

Reflections and Recommendations

The paper is a valuable contribution to the body of knowledge on heavy equipment maintenance, particularly for engineers who manage imported equipment with limited access to original manufacturer support. The authors demonstrate that a systematic approach to failure analysis, repair, and maintenance can extend component life significantly. One area for further investigation would be the use of advanced NDT techniques such as phased array ultrasonic testing (PAUT) or thermography to detect subsurface cracks before they become visible through surface spalling. Additionally, the development of predictive maintenance models based on vibration analysis and wear rate monitoring could provide early warning of impending roller surface failure.