F.L.S Company Surface Wear-Resistant Overlay Technology Introduction

Literature Overview and Background

F.L.S (now part of FLSmidth) is one of the world's leading suppliers of cement and mineral processing equipment, and their surface wear-resistant overlay technology represents a mature and widely deployed solution for extending the service life of equipment components subjected to severe abrasion. This technology encompasses a comprehensive system of overlay design, consumable selection, welding procedure qualification, application methodology, and field service support. The study of F.L.S overlay technology provides valuable insights into the industrial-scale implementation of weld overlay solutions, covering everything from the metallurgical design of overlay consumables to the practical aspects of field application and maintenance management. Understanding the F.L.S approach is particularly instructive for engineers who are evaluating overlay solutions for their own applications, as it demonstrates how a systematic, integrated approach to overlay technology can deliver significant improvements in equipment availability and lifecycle cost.

Overview of F.L.S Overlay Technology System

The F.L.S overlay technology system is built on several key pillars. First is the consumable portfolio, which includes a range of overlay wires, electrodes, and powders designed for specific wear environments. These consumables are classified according to the predominant wear mechanism: abrasive wear (for grinding and crushing applications), adhesive wear (for sliding contact applications), and erosive wear (for slurry and particulate impact applications). Each consumable type is formulated with a specific combination of alloying elements, including chromium, molybdenum, tungsten, cobalt, and carbon, to optimize the microstructure for the intended wear mechanism.

Second is the welding procedure qualification program, which ensures that each overlay application is performed according to a qualified welding procedure specification (WPS) that has been validated through mechanical testing, hardness profiling, and wear testing. The F.L.S system places strong emphasis on the importance of proper surface preparation, preheat control, interpass temperature management, and post-weld treatment to achieve consistent overlay performance.

Third is the application methodology, which includes detailed procedures for on-site and shop application of overlay layers. The methodology covers substrate assessment, overlay thickness selection, weld pass sequence design, and quality verification procedures.

Key Technical Features and Engineering Principles

The F.L.S overlay technology is distinguished by several key technical features that reflect deep metallurgical understanding and extensive field experience. One of the most important features is the use of multi-pass overlay sequences designed to achieve a gradient in hardness and composition from the base material to the overlay surface. The first pass, known as the transition layer, is formulated to metallurgically bond the overlay to the base material without introducing excessive dilution of the overlay composition. The subsequent overlay passes progressively increase the hardness and wear resistance of the surface layer. This gradient approach minimizes the risk of cracking at the overlay-base interface and ensures that the full hardness and wear resistance of the overlay consumable is achieved at the surface.

Another key feature is the emphasis on overlay geometry optimization. The F.L.S system recognizes that the shape and pattern of the overlay weld beads have a significant effect on the wear performance. For example, a chevron or herringbone bead pattern is used for applications involving sliding abrasion to deflect wear particles and reduce ploughing depth. A flat, smooth overlay surface is used for applications involving impact abrasion to minimize stress concentration at surface asperities. The bead width, height, and spacing are all specified according to the wear mechanism and the geometry of the component.

The F.L.S technology also places significant emphasis on the quality assurance and verification procedures. Each overlay application is subject to visual inspection, magnetic particle testing (MT) for surface crack detection, and hardness measurement at multiple locations across the overlay surface. The hardness profile is measured through the thickness of the overlay to verify that the transition layer is functioning correctly and that the surface hardness meets the specification.

Field Application Experience and Lessons Learned

The F.L.S field experience provides several important lessons for engineers implementing overlay technology. First, the base material condition is critical. Surface contamination, scale, rust, and existing cracks must be completely removed before overlay application, as even minor contamination can lead to lack of fusion or porosity defects that compromise the overlay bond strength. The recommended surface preparation method is mechanical grinding to bare metal followed by solvent cleaning, with a surface roughness of Ra 25 to 50 micrometers to promote good weld fusion.

Second, the preheat and interpass temperature control is essential for preventing cracking in the overlay layer. For martensitic overlay consumables on carbon steel or low-alloy steel substrates, a preheat temperature of 200 to 300 degrees Celsius is recommended, and the interpass temperature should be maintained below 250 degrees Celsius to prevent excessive grain growth and softening of the previously deposited layers. The cooling rate after welding must be controlled to avoid the formation of untempered martensite in the overlay layer, which is susceptible to cracking during subsequent cooling or service.

Third, the overlay thickness must be sufficient to accommodate the expected wear depth over the service interval. A minimum overlay thickness of 3 to 5 mm is recommended for most applications, with the understanding that the top 1 to 2 mm of the overlay will be worn away during service and must be replaced at the next maintenance interval. The F.L.S system recommends planning for overlay thickness to be 50% greater than the maximum expected wear depth to provide a safety margin.

Study Insights and Practical Recommendations

The F.L.S overlay technology demonstrates that the successful implementation of weld overlay solutions requires an integrated approach that encompasses consumable metallurgy, welding procedure qualification, application methodology, and quality assurance. The key insight is that overlay technology is not simply a matter of applying a hard weld metal to a worn surface; it is a systematic engineering discipline that requires careful consideration of the wear mechanism, the base material condition, the welding process parameters, and the service environment. Engineers evaluating overlay solutions for their own applications should adopt the F.L.S philosophy of comprehensive planning and rigorous quality control, and should invest in the training and certification of welding personnel to ensure consistent overlay performance. The demonstrated wear life improvements of 3 to 15 times compared to unprotected base materials make overlay technology an economically attractive solution for extending equipment service intervals and reducing total maintenance costs, provided that the implementation is executed with the level of technical rigor that the F.L.S system exemplifies.