Microstructure and Erosive Wear Resistance of CrMoV Alloy Overlay

Literature Overview

This paper, authored by Zhao Jianhua, Zhao Zhanxi, Chen Xiaoliang, Lu Xiao, Zhou Xiang, and Ji Xiulin, was published in the journal Vibration and Shock (Volume 34, Issue 10, 2015, pages 111-114). The research was funded by the Hehai University Student Innovation Training Program (Project No. 201210294069). The study investigates the phase composition, microstructure, erosive wear resistance, and erosive wear mechanism of CrMoV alloy overlay layers deposited on 20 steel substrates using manual arc welding (SMAW). This work is directly relevant to engineers in the steel pipe industry who face erosive wear challenges in slurry transport, mining applications, and hydraulic systems.

Overlay Preparation and Dilution Effect

The authors prepared CrMoV alloy overlay layers on 20 steel substrates using shielded metal arc welding (SMAW) with multiple passes. The number of overlay passes was varied to study the effect of dilution on the overlay composition and microstructure. The dilution rate, defined as the fraction of base metal that mixes with the deposited metal during welding, decreases with increasing number of overlay passes. This is because each subsequent pass is deposited onto a previous overlay layer rather than directly onto the base metal, reducing the base metal contribution to the dilution.

The dilution effect is a critical parameter in overlay welding because it directly influences the final composition and microstructure of the overlay. High dilution reduces the alloy content of the overlay, potentially degrading its performance. Low dilution preserves the intended alloy composition but may compromise the metallurgical bonding with the base metal. Engineers must balance these competing requirements to achieve optimal overlay performance.

Microstructural Analysis and Hardness Distribution

The microstructure of the CrMoV alloy overlay layers consists primarily of martensite, retained austenite, and alloy carbides. The martensite phase provides high hardness and strength, while the retained austenite contributes toughness and wear resistance. The alloy carbides, formed by the interaction of chromium, molybdenum, and vanadium with carbon, provide additional hardening and wear resistance through their high hardness and stability.

The hardness distribution across the overlay layers reveals an interesting trend. The second and third overlay layers exhibit the highest average microhardness, reaching approximately 780 HV, which is more than four times the hardness of the 20 steel substrate. The first pass layer, despite being deposited directly on the substrate, has lower hardness due to the high dilution rate that reduces the alloy content. The subsequent passes, while having lower dilution, may exhibit slightly lower hardness due to the cumulative effect of multiple welding thermal cycles that can promote carbide coarsening and retained austenite decomposition.

The 780 HV hardness level is achieved through the combined effect of martensitic transformation, carbide precipitation, and solid solution strengthening. The chromium, molybdenum, and vanadium additions promote the formation of complex alloy carbides such as MC, M2C, and M6C type carbides, where M represents the alloying elements. These carbides are extremely hard and stable, providing excellent resistance to abrasive and erosive wear. The retained austenite in the microstructure provides a tough matrix that supports the hard carbides and prevents catastrophic failure under impact loading.

Erosive Wear Performance and Mechanism

The erosive wear testing was conducted at various impingement angles to simulate different service conditions. The CrMoV alloy overlay demonstrates superior erosive wear resistance compared to the 20 steel substrate across all tested impingement angles. This improvement is attributed to the high hardness, the presence of hard alloy carbides, and the tough martensitic matrix that can accommodate impact loading.

The erosive wear mechanism varies with the impingement angle, which is a critical finding for engineers selecting overlay materials for specific applications. At impingement angles less than 30 degrees, the erosive wear mechanism is dominated by micro-cutting, where the impacting particles plow through the surface material and remove it in the form of thin sheets or ridges. At angles greater than 30 degrees, the mechanism shifts to fatigue damage and localized plastic deformation, where the repeated impact loading causes subsurface crack initiation and propagation, leading to material removal in the form of flakes or chips.

The effect of particle size on erosive wear rate was also investigated. As the sand particle size increases, the erosive wear rate of both the CrMoV overlay and the 20 steel substrate increases. However, the increase is not proportional to the particle size increase, indicating that the wear rate does not follow a simple linear relationship with particle size. This non-linear behavior is attributed to the complex interaction between particle size, impact energy, and material response. Larger particles carry more kinetic energy, which increases the impact damage, but the stress distribution over a larger contact area reduces the peak stress, partially offsetting the increased energy.

Engineering Practice and Application Considerations

The CrMoV alloy overlay is particularly suitable for applications involving combined erosive and abrasive wear, such as slurry pipes, pump impellers, valve bodies, and mining equipment. The high hardness and the presence of hard alloy carbides provide excellent resistance to particle impact and abrasion, while the tough martensitic matrix prevents brittle failure under impact loading.

Engineers should consider several factors when applying CrMoV overlay in practice. First, the number of overlay passes should be optimized to achieve the desired dilution rate and hardness. For maximum hardness, two or three passes are recommended, as demonstrated in this study. Second, the welding parameters should be controlled to minimize porosity and ensure complete fusion between passes. Third, the surface preparation of the substrate is critical, as surface contamination or oxide layers can compromise the metallurgical bonding and create initiation sites for wear damage.

The erosive wear mechanism understanding provided in this study is valuable for engineers designing erosion-resistant systems. For applications with low impingement angles (less than 30 degrees), materials with high hardness and micro-cutting resistance are preferred. For applications with high impingement angles (greater than 30 degrees), materials with high toughness and fatigue resistance are more important. The CrMoV alloy overlay provides a good balance of hardness and toughness, making it suitable for a wide range of impingement angles.

Study Insights and Reflections

This research provides valuable insights into the microstructure-performance relationship of CrMoV alloy overlays and their erosive wear behavior. The systematic investigation of dilution effects, microstructural evolution, and wear mechanisms provides engineers with a comprehensive understanding of the factors governing overlay performance. The finding that the second and third overlay passes achieve the highest hardness is practically significant for optimizing overlay deposition procedures. The erosive wear mechanism analysis, distinguishing between micro-cutting and fatigue damage regimes, provides guidance for material selection based on the specific impingement conditions in service.

The study also highlights the importance of understanding the interaction between particle size and erosive wear rate. The non-linear relationship between particle size and wear rate indicates that simple extrapolation from small-scale laboratory tests to full-scale service conditions may not be reliable. Engineers should conduct wear testing under conditions that closely simulate the actual service environment, including particle size distribution, impingement velocity, and impingement angle. The CrMoV alloy overlay represents a cost-effective solution for erosive wear protection in many industrial applications, and the research provides the technical foundation for its rational application.