Surfacing Material Selection and Welding Process for Roller Press Roller Surface in Cement Grinding

Literature Overview

This paper by Gu Weiguo, published in Cement (1998, No. 6), addresses the practical problem of rapid roller surface wear in a roller press used in a cement grinding system at Jinling Cement Plant in Jiangsu Province. The roller press model RPV1000-400, manufactured by Luoyang Mining Machinery Factory based on KHD (Germany) technology, is paired with a Φ3.0m × 6.5m circulating mill. While the system has demonstrated production enhancement and energy savings, the roller surface wears rapidly, requiring frequent shutdown for repair welding, which severely impacts equipment availability. The paper focuses on the selection of appropriate surfacing materials and the arrangement of the welding process to extend roller surface life.

Core Technical Points

Wear Mechanism Analysis

The wear of the roller press roller surface in a cement grinding system is primarily caused by the combined action of abrasion, impact, and adhesion. The cement clinker and raw materials being ground contain hard particles of quartz, feldspar, and other minerals that abrade the roller surface through micro-cutting and micro-plowing mechanisms. The rolling contact between the rollers also generates high contact stresses that cause plastic deformation and material displacement on the roller surface. Over time, the surface profile degrades, reducing the grinding efficiency and increasing the specific energy consumption of the grinding process.

The wear rate is influenced by several factors:

• Material properties of the ground material: Harder minerals (such as quartz) cause more severe wear than softer materials (such as limestone).
• Roller surface hardness: A harder roller surface resists abrasion more effectively, but excessive hardness can reduce toughness and lead to spalling.
• Roller surface profile: The corrugation pattern on the roller surface affects material flow and contact stress distribution. A worn profile changes the contact geometry and can accelerate further wear.
• Operating conditions: Roll pressure, roller speed, and feed rate all influence the contact stress and wear rate.

Surfacing Material Selection

The paper discusses the selection of surfacing materials based on the specific wear conditions in the cement grinding system. The key criteria for material selection include:

1. Hardness: The surfacing material should have a hardness significantly higher than the base metal and higher than the abrasive particles in the cement material. A minimum hardness of 55 HRC is typically required.
2. Toughness: The surfacing material must have adequate fracture toughness to resist spalling under impact loading. Pure carbide-based alloys are too brittle and should be avoided for roller press applications.
3. Weldability: The surfacing material must be compatible with the base metal (typically a medium-carbon alloy steel such as 42CrMo or similar) and should not be prone to cracking during welding.
4. Cost-effectiveness: The material cost, welding consumable cost, and expected service life must be balanced to achieve the best overall economic performance.

Common surfacing materials for roller press roller surfaces include:

For the RPV1000-400 roller press described in the paper, the recommended surfacing material is a high-carbon martensitic alloy or a Ni-Cr-Mo hardfacing alloy, which provides a good balance of hardness and toughness for the cement grinding application.

Welding Process Arrangement

The welding process for the roller surface must be carefully designed to ensure uniform coverage, adequate bond strength, and minimal distortion. The key process considerations include:

• Preheating: The roller surface should be preheated to 200-300 °C to reduce the cooling rate of the weld and prevent cold cracking. The preheating temperature should be maintained throughout the welding operation.
• Welding process: Submerged arc welding (SAW) is preferred for large roller surfaces because it provides high deposition rates, deep penetration, and a clean weld surface with minimal spatter. If the roller surface is complex or has limited access, SMAW may be used for the initial passes, followed by SAW for the build-up passes.
• Welding sequence: The roller surface should be welded in a spiral or helical pattern, starting from one end and progressing to the other. Each pass should overlap the previous one by 30-50% of the bead width. The skip-welding technique can be applied within each helical pass to control heat accumulation.
• Interpass temperature control: The interpass temperature should be maintained between 150-250 °C to prevent excessive cooling between passes, which could lead to cracking.
• Post-weld heat treatment: If the base metal is a high-strength alloy steel, a post-weld stress relief treatment may be required to reduce residual stresses and prevent delayed cracking.

Engineering Practice Integration

Case Study: RPV1000-400 Roller Press

The RPV1000-400 roller press at Jinling Cement Plant is a single-pair roller press designed for the pre-grinding stage of the cement grinding system. The roller diameter is 1000 mm and the roller length is 400 mm. The roll pressure is approximately 400 tons, and the roller surface speed is in the range of 1.5-2.5 m/s, depending on the operating conditions.

The original roller surface was made of a medium-carbon alloy steel with a hardness of approximately 35-40 HRC. After approximately 500-800 operating hours, the roller surface showed significant wear, with the corrugation depth reduced by more than 50% and the surface hardness reduced due to work hardening and material displacement. The frequent shutdowns for repair welding resulted in significant production losses, estimated at several hundred thousand yuan per year.

After implementing the surfacing repair procedure described in the paper, the roller surface life was extended to approximately 2000-3000 operating hours, a three- to six-fold improvement. The surfacing material used was a high-carbon martensitic alloy with a hardness of approximately 58 HRC, applied in two passes with a total overlay thickness of 3-4 mm.

Quality Control Protocol

The quality of the roller surface surfacing must be verified through the following protocol:

1. Pre-weld inspection: The roller surface must be thoroughly cleaned of rust, scale, and previous weld material. Any existing cracks or defects must be repaired before surfacing. The base metal hardness should be measured to confirm that it is within the expected range.
2. In-process monitoring: The welding parameters (current, voltage, travel speed, arc length) must be recorded and monitored throughout the operation. The interpass temperature should be measured at regular intervals using an infrared thermometer or contact thermometer.
3. Post-weld inspection: The surfacing layer must be inspected for visual defects (cracks, porosity, lack of fusion, undercut). The hardness must be measured at multiple locations across the roller surface, with a minimum of 5 measurements per meter of roller length. The overlay thickness must be measured using an ultrasonic thickness gauge or by sectioning a test coupon.
4. Dimensional verification: The roller surface profile must be measured using a profilometer or a coordinate measuring machine to confirm that the corrugation pattern is within tolerance of the original design.

Study Insights and Reflections

This paper is a practical example of how a systematic approach to surfacing material selection and welding process design can dramatically improve the service life of a critical industrial component. The key insight is that the wear resistance of the roller surface is not solely a function of the overlay hardness but is also influenced by the welding process parameters, the overlay thickness, and the post-weld condition. A harder overlay does not necessarily mean a longer service life if the overlay is prone to cracking or spalling under the high contact stresses of the roller press.

The paper also highlights the importance of understanding the wear mechanism before selecting a surfacing material. In the cement grinding application, the wear is primarily abrasive, but there is also a significant component of impact loading from the material being fed into the roller gap. A material that is hard but brittle may fail prematurely due to spalling, while a material that is tough but soft may wear rapidly due to abrasion. The optimal material is one that provides a balanced combination of hardness and toughness, which is typically achieved through a martensitic or austenitic microstructure with appropriate alloying.

Another important consideration is the effect of the surfacing layer on the roller dynamics. The addition of a surfacing layer changes the mass distribution of the roller, which can affect the balance and vibration characteristics of the roller press. If the surfacing layer is not applied uniformly, it can introduce an imbalance that leads to increased vibration and premature bearing failure. Therefore, the welding sequence and the overlay thickness must be carefully controlled to ensure that the mass distribution is symmetric about the roller axis.

The paper also raises the question of whether the surfacing repair should be performed on-site or in a workshop. On-site repair is more convenient but may be limited by access, equipment availability, and environmental conditions. Workshop repair allows for more controlled conditions and the use of advanced welding equipment, but requires the removal and transportation of the roller, which can be expensive and time-consuming. The choice depends on the specific operating conditions and the cost-benefit analysis of each option.

Summary

The surfacing of the roller press roller surface in a cement grinding system is a well-established practice that, when properly executed, can extend the roller surface life by a factor of three to six. The key to success lies in the careful selection of the surfacing material based on the specific wear conditions, the design of a welding process that ensures uniform coverage and adequate bond strength, and the implementation of a rigorous quality control protocol. Engineers should adopt a systematic approach that integrates wear mechanism analysis, materials selection, process optimization, and quality verification, and should always consider the economic implications of the repair strategy in the context of the overall equipment life-cycle cost.