Performance Study of Ni-Based Alloy Powder Plasma Hardfacing Layers

Literature Overview

The paper by Yuwen Li, Liu Xiuli, and Li Weihua, published in Heat Treatment of Metals (Vol. 31, No. 10, 2006, pp. 41–43), investigates the microstructure and wear resistance of two Ni-based alloy powder hardfacing layers produced by plasma hardfacing on Q235 carbon steel. The research was conducted at the Department of Architectural Engineering, Shijiazhuang Vocational Technical Institute. The study compares Ni-W-C alloy powder and Ni-Cr-W-Mn composite powder, analyzing their microstructures through metallography, SEM, XRD, and tribological testing.

Technical Background

Plasma hardfacing (also known as plasma transferred arc welding or PTAW) is a highly efficient process for depositing hardfacing layers with low dilution, high deposition rates, and excellent metallurgical bonding. The process uses a high-temperature plasma arc to simultaneously melt the base material and the delivered powder, creating a well-mixed molten pool with controlled composition.

The study focuses on two powder formulations:

1. Ni-W-C alloy powder: a conventional Ni-based hardfacing powder with tungsten carbide reinforcement
2. Ni-Cr-W-Mn composite powder: a multi-element composite powder incorporating chromium, tungsten, and manganese for enhanced properties

Microstructural Analysis

Ni-W-C Alloy Powder Deposit

The XRD and SEM analysis of the Ni-W-C deposit revealed the following phases:

The microstructure consists of a γ-Ni matrix with dispersed hard carbide particles. The WC particles, with a hardness of approximately 2500 HV, provide the primary wear resistance mechanism through abrasion resistance. The Cr₇C₃ phase, while softer than WC, contributes to overall hardness and may form during the interaction between chromium and carbon during solidification.

Ni-Cr-W-Mn Composite Powder Deposit

The composite powder deposit exhibits a more complex phase composition:

The composite powder deposit contains a significantly richer phase composition, with multiple types of hard carbides and intermetallic compounds distributed throughout the matrix. This multiphase architecture provides synergistic wear resistance through multiple mechanisms.

Wear Resistance Comparison

The tribological testing revealed a dramatic difference in wear resistance between the two powder formulations:

Mechanisms of Enhanced Wear Resistance

The approximately tenfold improvement in wear resistance achieved with the Ni-Cr-W-Mn composite powder can be attributed to the following mechanisms:

1. Solid solution strengthening: The incorporation of Cr, Mn, and W atoms into the γ-Ni matrix creates lattice distortion, increasing the yield strength of the matrix phase.
2. Carbide dispersion hardening: The presence of multiple carbide types (WC, W₂C, Cr₂₃C₆, Cr₇C₃) provides a dense network of hard particles that resist abrasive wear through ploughing and fracture resistance.
3. Intermetallic compound formation: The formation of NiB and Ni₂B phases, which are hard and brittle intermetallics, contributes to overall hardness through volume fraction effects.
4. Phase boundary strengthening: The high number of phase boundaries in the multiphase microstructure impedes dislocation motion, increasing the flow stress of the material.

Process Parameters and Engineering Considerations

Plasma Hardfacing Process Parameters

Dilution Control

Plasma hardfacing typically achieves lower dilution rates than conventional arc welding processes, often in the range of 5–15% for Ni-based powders on carbon steel substrates. This low dilution is critical for maintaining the corrosion resistance and hardness of the deposit. The composite powder's enhanced properties are more readily achieved at lower dilution levels because the beneficial alloying elements remain at higher concentrations in the deposit.

Engineering Practice Applications

Application Selection Guide

Quality Control for Plasma Hardfacing

Study Insights and Reflections

This research demonstrates the significant potential of composite powder formulations in plasma hardfacing. The tenfold improvement in wear resistance achieved through the addition of Cr and Mn to the Ni-W-C system is remarkable and has important implications for the design of hardfacing powders.

The key insight from this study is that wear resistance in Ni-based hardfacing deposits is not solely determined by the presence of hard carbides but also by the matrix properties and the overall phase architecture. The composite powder approach leverages multiple strengthening mechanisms simultaneously, creating a synergistic effect that cannot be achieved by simply increasing the carbide content of a conventional powder.

From a process engineering perspective, the plasma hardfacing process offers excellent control over deposit composition and microstructure. The low dilution rates and high deposition rates make it suitable for industrial applications where thick, high-performance overlays are required. However, the process requires careful control of powder feed rate and plasma parameters to ensure consistent powder melting and proper deposit quality.

The research also highlights the importance of powder characterization in hardfacing applications. Engineers should not assume that all Ni-based powders are equivalent; the specific composition, particle size distribution, and powder morphology significantly affect the deposit properties. Powder suppliers should provide detailed composition analysis and performance data to enable informed selection.

For industrial implementation, the Ni-Cr-W-Mn composite powder represents a premium option that should be reserved for applications where maximum wear resistance is required and the cost premium can be justified by extended service life. For less demanding applications, the conventional Ni-W-C powder provides adequate performance at lower cost.

The multiphase microstructure achieved with composite powders also has implications for inspection and quality control. Engineers should develop appropriate metallographic examination procedures to verify the expected phase composition and distribution, as the presence of multiple hard phases provides a unique microstructural signature that can be used for quality verification.

Summary

The five studies reviewed in this document collectively address critical aspects of hardfacing technology across different materials, processes, and applications. From the microstructural refinement achieved through rare earth oxide modification, to the practical repair of nuclear equipment using OVERLAY techniques, to the statistical optimization of GTAW parameters for Inconel 625, to the field-proven repair of mining hydraulic cylinders, and to the advanced composite powder formulations for plasma hardfacing, these studies represent the breadth and depth of modern hardfacing engineering. The common thread connecting all five studies is the fundamental relationship between process parameters, microstructure, and mechanical performance, which remains the cornerstone of hardfacing technology development and application. Engineers working in this field should embrace both the metallurgical principles and the practical process considerations that these studies illustrate, recognizing that successful hardfacing requires a holistic understanding of materials, processes, and service conditions.