Microstructure and Tribological Properties of Nickel-Based Alloy Plasma Surfacing Layers

Literature Overview

This paper by Li Shan, Hu Jianjun, Chen Guoqing, Zhou Wenlong, and Zhang Junsan, published in Mechanical Engineering Materials in 2013 (Vol. 37, No. 6, pp. 72-77), investigates the microstructure, hardness, and tribological behavior of Ni50A nickel-based alloy plasma surfacing layers deposited on 1Cr18Ni9Ti stainless steel substrates. The research was supported by the National 973 Program and conducted at Dalian University of Technology. The study systematically examines the effect of plasma arc welding current on the resulting surfacing layer properties, providing valuable process optimization data.

Process Parameters and Experimental Design

The authors employed plasma arc surfacing technology with Ni50A alloy powder as the consumable. The substrate material, 1Cr18Ni9Ti (equivalent to AISI 321), is a stabilized austenitic stainless steel commonly used in high-temperature and corrosive environments. The welding current was varied across a range that included 160 A and 190 A as key reference points. The study methodology encompassed metallographic examination, X-ray diffraction for phase identification, microhardness measurement, and pin-on-disk friction and wear testing.

Microstructural Characterization

The surfacing layer microstructure is dominated by a gamma-Ni solid solution matrix, which is the expected primary phase for a nickel-based alloy. However, the presence of numerous secondary phases is noteworthy: CrB, M23(C,B)6, Cr7C3, Cr5B3, Ni3Si, and Ni3B. These intermetallic and carbide phases are responsible for the high hardness and wear resistance of the surfacing layer, and their morphology, size, and distribution are strongly influenced by the welding current.

At 160 A, the microstructure exhibits the finest and most uniform morphology, characterized by small flower-like structures and fine granular features. This refined microstructure correlates with the highest measured hardness of 680 HV. As the current increases to 190 A, the microstructure coarsens, and the secondary phases become larger and more irregularly distributed.

Process Parameter Versus Performance Relationship

The wear mechanism analysis reveals that the dominant wear mode is adhesive wear, accompanied by abrasive wear. In later stages of the wear test, oxidation wear becomes evident as the surface temperature increases and oxide scales form. The wear volume increases progressively with wear time, with the 190 A samples exhibiting the highest wear rates throughout the test duration.

Engineering Practice Applications

For pipe and fitting applications, plasma surfacing with nickel-based alloys is particularly relevant for components operating in high-temperature, corrosive, and wear-prone environments. Typical applications include:

• Overlaying valve seats and guides in high-temperature steam service
• Protecting pipe elbows and tees in slurry handling systems
• Enhancing the wear and corrosion resistance of heat exchanger tubes
• Repairing worn surfaces on pump impellers and turbine components

The finding that lower welding current produces a finer, harder, and more wear-resistant microstructure has direct implications for process parameter selection. In plasma surfacing operations, operators should aim for the minimum current that provides adequate powder melting and deposition rate, as excessive current leads to microstructural coarsening and performance degradation.

Key Reflections and Critical Analysis

The study provides solid evidence for the well-known principle that lower thermal input during surfacing produces refined microstructures. However, I would emphasize that this finding must be balanced against practical deposition rate considerations. In industrial applications, productivity is a significant factor, and operating at 160 A may not be feasible for large-scale surfacing operations. The optimal current should be determined through a trade-off analysis between performance and productivity.

The identification of multiple secondary phases (CrB, Cr7C3, Cr5B3, Ni3Si, Ni3B) highlights the complexity of nickel-based surfacing microstructures. The formation and stability of these phases are sensitive to cooling rate, which is itself dependent on welding current, powder feed rate, and travel speed. A more comprehensive process optimization would consider the interaction of all these parameters simultaneously.

The wear mechanism analysis, identifying adhesive, abrasive, and oxidation wear modes, is consistent with the behavior expected for nickel-based alloys in dry sliding conditions. In practical service, however, the presence of lubricants, cooling fluids, or corrosive media would alter the wear mechanism mix. Engineers should consider these environmental factors when extrapolating laboratory wear data to field performance predictions.

This study is a useful reference for plasma surfacing process development, providing clear evidence that welding current is a critical parameter for controlling microstructure and wear performance. The systematic approach to parameter optimization is a model for similar studies in other surfacing applications.