Online Weld Overlay Repair of Roller Press Rollers and Process Adjustment

Literature Overview

This technical report by Liu Minghong from Hunan Chenxi Huazhong Cement Co., published in 2014, documents the practical experience of performing online weld overlay repair on roller press rollers in a cement production environment. Roller presses are critical equipment in the cement grinding process, where they reduce the particle size of raw materials or clinker through compression. The rollers are subjected to severe abrasion, impact, and thermal cycling, leading to progressive surface degradation that necessitates periodic repair. This case study provides valuable insights into the challenges and solutions associated with field repair of large rotating components.

Technical Context and Challenges

Service Conditions of Roller Press Rollers

Roller press rollers operate under extreme conditions:

The combination of high contact pressure, abrasive materials, and thermal cycling creates a complex damage mechanism that includes abrasive wear, adhesive wear, and fatigue spalling. The roller surface must therefore possess high hardness, good toughness, and resistance to thermal shock.

Repair Challenges

Online repair of roller press rollers presents several unique challenges:

1. Limited access: The rollers are installed within the roller press housing, making it difficult to approach with welding equipment.
2. Large diameter: Rollers typically have diameters of 1000–2000 mm, requiring extensive overlay coverage.
3. Residual stress: The existing roller may contain residual stresses from prior welding, grinding, or service, which can interact with new welding stresses.
4. Surface condition: The worn surface may be contaminated with cement dust, moisture, or oxide scale, requiring thorough cleaning before welding.
5. Production pressure: Cement plants operate continuously, and any downtime for roller repair represents significant production loss, creating pressure to complete repairs quickly.

Weld Overlay Process and Materials

Overlay Material Selection

The selection of overlay material for roller press rollers depends on the specific service conditions and the desired balance between hardness and toughness. Common overlay materials include:

For cement roller press applications, high-carbon steel or carbide-containing hardfacing alloys are most commonly used due to their favorable balance of wear resistance, cost, and weldability.

Welding Process Parameters

The overlay welding process typically employs submerged arc welding (SAW) or shielded metal arc welding (SMAW) for field repair operations. The following process parameters are critical:

• Preheat temperature: 200–300 °C to reduce cooling rate and prevent cold cracking in the base metal HAZ.
• Interpass temperature: 250–350 °C to maintain thermal continuity and minimize thermal cycling.
• Weld layer thickness: 2–4 mm per pass to control dilution and thermal input.
• Weld bead profile: Flush or slightly convex to ensure uniform surface geometry after grinding.
• Weld sequence: Symmetrical, multi-pass sequence to minimize distortion and residual stress.

Process Adjustment for Online Conditions

The key contribution of this study is the process adjustment made to accommodate the constraints of online repair. In a cement plant environment, the following adjustments are typically necessary:

1. Partial roller removal: In some cases, the roller can be partially extracted from the housing to provide access for welding, while in others, welding must be performed in situ.
2. Portable welding equipment: The welding setup must be compact and easily maneuverable within the confined space of the roller press housing.
3. Heat input control: Lower heat input is often required to minimize distortion, which is particularly critical for large-diameter rollers where even small angular deviations can affect press performance.
4. Post-weld treatment: Localized post-weld heat treatment using induction heating or portable gas flames to relieve residual stresses in the weld zone.

Quality Control and Inspection

Non-Destructive Testing

The overlay repair must be inspected to ensure weld quality and detect any defects that could lead to premature failure. The following NDT methods are typically employed:

Mechanical Property Verification

Post-repair mechanical testing may include:

• Hardness survey across the overlay and HAZ to verify hardness gradient and ensure no excessive hardening in the base metal.
• Peel test or bend test on a coupon weld to verify bond strength between the overlay and the base metal.
• Dimensional check after grinding to ensure the roller diameter and runout are within tolerance.

Engineering Practice and Lessons Learned

Case Study: Roller Press Roller Repair

In a typical cement plant scenario, a roller press roller with a diameter of 1400 mm and a length of 1200 mm has experienced progressive wear after 18 months of continuous operation. The original hardfacing layer has been worn through in high-contact zones, exposing the base metal to severe abrasive attack. The decision is made to perform an online repair rather than replacing the entire roller, due to the high cost of replacement and the extended lead time for procurement.

The repair procedure involves the following steps:

1. Surface preparation: The worn surface is ground back to sound metal, removing all contaminated and oxide-affected material. The grinding depth is typically 3–5 mm, depending on the extent of wear and the condition of the underlying material.
2. Preheating: The roller is preheated to 250 °C using portable gas flames or induction heating coils. The preheat temperature is verified using temperature indicators or infrared pyrometers.
3. Weld overlay: Multiple passes of hardfacing alloy are deposited using SAW or SMAW, following a symmetrical sequence to minimize distortion. Each pass is kept within the specified thickness and width to control dilution and thermal input.
4. Post-weld heat treatment: The weld zone is heated to 600–650 °C and held for 1–2 hours to relieve residual stresses and temper any hard phases.
5. Grinding and finishing: The overlay surface is ground to the specified profile, ensuring uniform diameter and smooth surface finish.
6. Inspection: The repaired roller is inspected using MT and UT to detect any surface or internal defects. Hardness surveys are performed to verify the overlay hardness and HAZ condition.

Process Adjustment Insights

The study highlights several process adjustments that were critical to the success of the online repair:

• Sequence optimization: The welding sequence was carefully planned to ensure that the thermal input was distributed evenly around the roller circumference, minimizing distortion. A symmetrical, multi-pass sequence was used, with welds placed at diametrically opposite positions to balance thermal effects.
• Heat input management: Lower heat input was used compared to a shop repair, to minimize the risk of distortion and to reduce the cooling time. This was achieved by using smaller diameter electrodes or wires and by maintaining a consistent travel speed.
• Interpass temperature control: The interpass temperature was maintained at 250–300 °C to ensure proper fusion between passes and to minimize the thermal cycling that could lead to cracking. Temperature monitoring was performed using infrared pyrometers at multiple locations on the roller surface.
• Post-weld treatment timing: The post-weld heat treatment was performed immediately after welding, before the roller had cooled significantly, to minimize the thermal gradient between the weld zone and the base metal.

Study Insights and Reflections

This case study provides a practical demonstration of the challenges and solutions associated with online weld overlay repair of large rotating components. The most significant lesson is that process flexibility is essential in field repair operations. Unlike shop fabrication, where conditions can be carefully controlled, field repairs must accommodate a wide range of constraints, including limited access, ambient conditions, and production schedules.

The emphasis on process adjustment is particularly instructive. In many industrial settings, there is a tendency to apply standardized welding procedures without considering the specific conditions of the repair operation. This study demonstrates that even small adjustments to the welding sequence, heat input, and interpass temperature can have a significant impact on weld quality and repair success.

The case also highlights the importance of thorough surface preparation. In cement plant environments, the roller surface is often contaminated with cement dust, moisture, and oxide scale, which can lead to porosity, lack of fusion, and other defects if not properly removed. The study emphasizes that grinding back to sound metal is not optional but essential for achieving a reliable overlay bond.

From a quality assurance perspective, the study reinforces the need for comprehensive inspection of repaired components. Even when the welding procedure is qualified and the welder is certified, the unique conditions of a field repair operation introduce additional risks that must be addressed through rigorous inspection. The combination of MT, UT, and hardness surveys provides a comprehensive assessment of weld quality and is essential for ensuring long-term service reliability.

In summary, this study provides valuable practical insights into the online repair of roller press rollers in cement production. The emphasis on process adjustment, surface preparation, and quality control is particularly relevant for engineers working in industrial maintenance and repair. The findings can be directly applied to improve repair procedures for similar rotating components in other industries, including mining, power generation, and bulk materials handling.