Development of Wear-Resistant Overlay Welding Electrodes for Roll Crushers

Literature Overview

The paper by Ying Pengzhan from China University of Mining and Technology in Xuzhou, published in 1998 in the journal Welding Technology, presents the development and characterization of a specialized overlay welding electrode designed for the wear-resistant protection of toothed surfaces on roll crushers used in coal preparation plants. The research addresses a critical maintenance challenge in the mining and mineral processing industry, where roll crushers are subjected to extreme abrasive and impact loading conditions that cause rapid wear of the crusher teeth. The author's systematic approach to electrode formulation, involving iterative adjustment of the flux composition to achieve the optimal alloy system, represents a practical and methodical approach to welding consumable development that is directly applicable to modern engineering practice.

Core Technical Viewpoints

The primary objective of this research is to develop an overlay welding electrode that produces a wear-resistant deposit with superior abrasion resistance compared to conventional materials such as 35CrMoTi steel, which was the standard material for roll crusher teeth at the time. The author achieved this by systematically adjusting the flux composition of the electrode to identify the optimal alloy system for the specific wear conditions encountered in roll crusher applications. The resulting electrode produces a weld deposit with a microstructure consisting of martensite, carbides, and retained austenite, achieving a hardness of HRC 64 to 65, which represents a substantial improvement over the base material.

The research methodology followed a logical progression from material design through to industrial validation. The author first identified the wear mechanisms operating in roll crusher teeth, which include abrasive wear from coal and rock particles, impact wear from the crushing action, and potentially some adhesive wear from material transfer. Based on this understanding of the service conditions, the author then designed an electrode formulation that would produce a deposit microstructure optimized for these specific wear modes. The martensitic matrix provides toughness and impact resistance, while the hard carbides provide abrasion resistance, and the retained austenite contributes to the overall toughness through transformation toughening.

Interpretation of Technical Points

The microstructure of the overlay weld deposit is the key determinant of its wear resistance. The three-phase microstructure of martensite plus carbides plus retained austenite is a classic design for wear-resistant overlay welds, and the specific balance of these phases is critical for achieving optimal performance. The martensite phase, typically of the lenticular or acicular morphology, provides a hard and tough matrix that resists plastic deformation and supports the hard carbide particles. The carbides, which may include M7C3, M23C6, or special carbides such as Cr7C3 or WC depending on the alloy composition, provide the primary abrasion resistance through their extreme hardness. The retained austenite phase, while softer than the other phases, contributes to the overall toughness of the deposit by transforming to martensite under impact or frictional loading, thereby providing a form of transformation toughening.

The hardness of HRC 64 to 65 achieved in this research is significant because it indicates that the deposit is composed primarily of hard carbides dispersed in a martensitic matrix. For comparison, the base material 35CrMoTi steel typically has a hardness of HRC 28 to 32 in its quenched and tempered condition, which is substantially lower than the overlay weld deposit. The reported wear resistance improvement of 8.6 times compared to 35CrMoTi steel and the 7-fold increase in service life of the crusher demonstrate the dramatic effectiveness of the overlay welding approach.

The role of the electrode flux composition in determining the final deposit microstructure is a critical aspect of this research. The flux serves multiple functions in the welding process: it provides a protective atmosphere to prevent oxidation and nitrogen pickup, it acts as a source of alloying elements that dissolve into the weld pool, and it influences the solidification behavior of the weld metal through its effects on nucleation and grain growth. The author's iterative approach to adjusting the flux composition reflects the complexity of the relationship between flux chemistry and deposit microstructure, which involves multiple interacting factors including alloy partitioning, cooling rate, and solidification sequence.

Process and Standards Analysis

The development and qualification of overlay welding electrodes for wear-resistant applications involves a comprehensive evaluation protocol that encompasses chemical composition analysis, microstructural characterization, mechanical property testing, and wear performance evaluation. The following table summarizes the key evaluation parameters and their significance:

The relevant standards for overlay welding consumables and their evaluation include ISO 14270 for classification and specification of overlay welding consumables, AWS A5.17 for welding consumables for surfacing, and various national standards for welding consumable qualification. For the specific application of roll crusher teeth, the performance requirements are typically defined by the equipment manufacturer based on the specific operating conditions of the crusher, including the type of material being crushed, the feed size, the crusher capacity, and the desired service life between maintenance intervals.

Integration with Engineering Practice

Roll crushers are widely used in coal preparation plants, mineral processing facilities, and aggregate production operations for the intermediate and fine crushing of hard and abrasive materials. The crusher teeth, which are the primary wear components, are subjected to extreme loading conditions that include:

• High impact forces from the crushing action, which can reach several hundred kilonewtons per tooth
• Continuous abrasive contact with coal and rock particles, which causes gradual material removal
• Varying temperatures due to frictional heating and the thermal properties of the material being crushed
• Potential corrosive effects from moisture and chemical constituents in the material

The overlay welding approach to protecting roll crusher teeth offers several practical advantages over alternative methods such as hardfacing with cast irons or replacement with solid wear-resistant materials. Overlay welding allows the repair and refurbishment of existing teeth without the need for complete replacement, which reduces maintenance costs and downtime. The welding process can be performed in the field with portable equipment, which is particularly advantageous for large crushers that are difficult to disassemble. The overlay weld deposit can be applied to specific wear zones on the tooth surface, which provides targeted protection where it is most needed.

The economic case for overlay welding of roll crusher teeth is compelling. The cost of the overlay welding consumable and labor is typically a small fraction of the cost of replacing the entire tooth or roller assembly. The extended service life of 7 times compared to unprotected teeth translates to significant reductions in maintenance frequency and associated downtime costs. For a large coal preparation plant operating multiple roll crushers, the cumulative savings from overlay welding can be substantial.

Key Questions and Reflections

One important question that arises from this research is the long-term stability of the overlay weld deposit under cyclic loading conditions. The retained austenite phase, while providing transformation toughening during the initial wear cycles, may undergo progressive transformation to martensite over time, which could lead to increased brittleness and potential cracking. The stability of the retained austenite phase depends on the carbon and alloy content of the austenite, the prior austenite grain size, and the temperature and stress conditions experienced during service. In the context of roll crusher teeth, where the operating temperatures are typically below the Mf temperature of the retained austenite, the phase stability is likely to be adequate for the expected service life.

Another consideration is the effect of dilution on the final deposit composition and microstructure. The dilution of the overlay weld deposit by the base material is inevitable in any fusion welding process, and it can significantly affect the hardness and microstructure of the deposit. For the electrode developed in this research, the dilution rate must be carefully controlled to ensure that the deposit retains sufficient alloy content to produce the desired microstructure and hardness. This can be achieved through the use of multiple weld layers, where the first layer has higher dilution and subsequent layers have progressively lower dilution, or through the use of a pre-welded transition layer that reduces the effective dilution of the functional layer.

The author's approach to electrode development, which involved iterative adjustment of the flux composition, is a practical and effective methodology that is still widely used in the welding consumable industry today. The key insight is that the flux composition is not merely a passive component that provides protection and basic alloying, but an active design variable that can be used to precisely control the deposit microstructure and properties. This understanding of the flux as a design tool is fundamental to the development of specialized welding consumables for demanding applications.

Study Insights and Implications

This paper demonstrates the power of systematic material design in solving practical engineering problems. The development of a specialized overlay welding electrode for roll crusher teeth involved a clear understanding of the service conditions, a rational selection of the target microstructure, and a methodical approach to consumable formulation. The resulting electrode, with its martensite plus carbides plus retained austenite microstructure and HRC 64 to 65 hardness, represents a significant improvement over conventional materials and provides a practical solution to a persistent maintenance challenge in the mining and mineral processing industry.

The broader implications of this research extend to the field of welding consumable development more generally. The paper illustrates that specialized welding consumables, designed for specific applications and service conditions, can provide dramatic improvements in component life and reliability. This approach is particularly valuable in industries where equipment downtime is costly and where the replacement of worn components is difficult or expensive. The methodology of iterative flux composition optimization, while straightforward in principle, requires careful experimental design and thorough characterization to achieve the desired results.

For practicing engineers, the key takeaway is that the selection and development of welding consumables should be driven by a detailed understanding of the service conditions and failure modes of the application. The generic welding consumables available from major manufacturers may be adequate for many applications, but for critical components subjected to severe wear conditions, specialized consumables with tailored compositions and microstructures can provide substantial performance improvements. The investment in consumable development and qualification is typically well justified by the resulting reductions in maintenance costs and equipment downtime.