Microstructure and Friction Wear Properties of Nickel-Based Alloy Plasma Overlay Layer

Literature Overview

This research by Li Shan, Hu Jianjun, Chen Guoqing, Zhou Wenlong, and Zhang Junshan, published in Materials in Mechanical Engineering in 2013 (Vol. 37, No. 6, pp. 72-77), investigates the microstructure, hardness, and tribological performance of Ni50A nickel-based alloy overlay layers deposited by plasma arc welding onto 1Cr18Ni9Ti stainless steel substrates. The study was funded by the National 973 Program, the New Century Excellent Talents Support Program, and the Jiangxi Provincial Natural Science Foundation. The work addresses the critical challenge of optimizing plasma overlay parameters for maximum wear performance, a concern of significant relevance to engineers working with corrosion-resistant and wear-resistant pipe systems operating in aggressive environments.

Core Technical Findings

Microstructural Composition of the Overlay Layer

The overlay layer is composed primarily of a γ-Ni matrix with various secondary phases including CrB, M₂₃(C,B)₆, Cr₇C₃, Cr₅B₃, Ni₃Si, and Ni₃B. These secondary phases play a crucial role in determining the wear resistance of the overlay, as they provide hard reinforcing particles within the ductile nickel matrix. The presence of multiple carbide and boride phases indicates a complex solidification and diffusion process during plasma arc welding, where the high energy density input creates a molten pool with significant alloying element redistribution.

Current-Dependent Microstructure Evolution

The study reveals that welding current is the dominant process parameter governing overlay microstructure. At 160 A, the microstructure exhibits the finest and most uniform morphology, characterized by small flower-like dendritic patterns and fine granular particles. This fine microstructure correlates with the highest hardness of 680 HV and the best wear resistance. As the welding current increases to 190 A, the molten pool becomes larger and deeper, resulting in slower cooling rates, coarser grain structures, and reduced hardness. The relationship between current and microstructure can be explained through classical solidification theory: higher current input increases the thermal energy per unit volume, which reduces the cooling rate and promotes grain coarsening and phase coarsening.

Wear Mechanism Characterization

The wear testing results indicate that all overlay layers exhibit increasing wear volume with extended wear time, with the 190 A layer showing the maximum wear volume. The primary wear mechanism is identified as adhesive wear, accompanied by abrasive wear throughout the testing duration, with oxidative wear appearing in the later stages. The adhesive wear mechanism dominates because the nickel-based overlay surface maintains intimate contact with the counterface material, leading to material transfer and localized plastic deformation. The presence of hard secondary phases (CrB, Cr₇C₃, etc.) provides resistance to abrasive penetration, but the adhesive bonding between the overlay and counterface remains the primary failure mode.

Integration with Engineering Practice

Ni50A-type nickel-based overlays are widely used in the oil and gas industry for protecting critical components such as pump shafts, valve stems, and pipe spools operating in corrosive and abrasive environments. The findings of this study directly inform welding procedure specification: a welding current of 160 A should be targeted for plasma arc overlay of Ni50A on stainless steel pipe surfaces to achieve the optimal combination of hardness and wear resistance. In the context of pipe manufacturing, this parameter optimization is essential when applying overlay cladding to carbon steel or stainless steel pipe sections that will be exposed to solid particle erosion in slurry service or high-velocity flow conditions.

The process window for optimal performance is relatively narrow, emphasizing the need for precise current control during production welding. Engineers should also consider the dilution effect of the stainless steel substrate on the overlay composition, as chromium diffusion from the base metal can alter the secondary phase composition and potentially improve or degrade performance depending on the specific service environment.

Key Questions and Reflections

An important practical consideration not fully addressed in this study is the effect of multi-pass overlay welding on the final microstructure and properties. In industrial applications, overlay thickness requirements often necessitate multiple passes, and the interpass temperature and total thermal cycles can significantly affect the final grain structure and phase distribution. Additionally, the study focuses on dry wear conditions, but in many pipeline applications, the overlay is subjected to wet or corrosive wear environments where the wear mechanism may shift from adhesive to erosive or cavitation-dominated. The identification of oxidative wear in the later stages of testing suggests that environmental factors could play a more significant role in long-duration service than in short-term laboratory testing. This study provides a solid foundation for plasma overlay parameter optimization, but practical implementation requires consideration of additional factors including dilution, multi-pass effects, and environmental exposure.