Microstructure and Wear Resistance of Cr3C2-Nickel-based Alloy Plasma Cladding Layer

Literature Overview

The study by Hou Qingyu, He Yizhu, and Gao Jiasheng from the Key Laboratory of Metal Materials and Processing at the School of Materials Science and Engineering, Anhui University of Technology, published in Materials in Mechanical Engineering in 2007 and supported by the Anhui Provincial Higher Education Young Teachers Research Fund (Grant No. 2006jqt082), investigates the microstructure and wear resistance of a Cr3C2-reinforced nickel-based alloy cladding layer produced by plasma transferred arc (PTA) welding. This research addresses the challenge of achieving high hardness and wear resistance in nickel-based overlay layers, which are widely used in chemical processing, oil and gas, and aerospace applications due to their excellent corrosion resistance but often suffer from relatively low hardness.

Technical Background and Design Rationale

Nickel-based alloy overlay layers, such as those based on Stellite or Inconel compositions, are valued for their combination of corrosion resistance, oxidation resistance, and good mechanical properties at elevated temperatures. However, their inherent hardness, typically in the range of 25-35 HRC, is often insufficient for applications subject to severe abrasive or erosive wear. The addition of hard ceramic particles such as Cr3C2 offers a promising approach to enhance the hardness and wear resistance of nickel-based overlay layers without significantly compromising their corrosion resistance.

Cr3C2 is selected as the reinforcing phase due to its exceptional hardness (approximately 2200 HV), high thermal stability (it does not decompose below 2400 degrees Celsius), and good chemical compatibility with nickel-based matrices. Unlike other carbides such as WC or Mo2C, Cr3C2 does not form brittle intermetallic compounds with nickel, ensuring good bonding between the reinforcing particles and the matrix.

Microstructural Characterization

The authors conducted detailed metallographic and scanning electron microscopy examinations of the PTA cladding layers with varying Cr3C2 contents. The microstructure consisted of a dendritic nickel-based solid solution matrix with Cr3C2 particles distributed along the dendrite boundaries and within the interdendritic regions. At lower Cr3C2 contents (10-15 wt%), the particles were well dispersed within the matrix, while at higher contents (25-30 wt%), some degree of particle agglomeration was observed.

The authors also examined the effect of Cr3C2 addition on the phase composition of the cladding layer. X-ray diffraction analysis confirmed the presence of FCC nickel-based solid solution, M7C3 carbides (formed by the reaction of Cr3C2 with the matrix), and residual Cr3C2 particles. The volume fraction of M7C3 carbides increased with increasing Cr3C2 content, indicating partial decomposition of Cr3C2 during the high-temperature PTA process. This decomposition reaction, while reducing the amount of intact Cr3C2, contributed additional hardening through the formation of M7C3 carbides.

The hardness of the cladding layers increased significantly with Cr3C2 content. At 10 wt% Cr3C2, the average microhardness was approximately 680 HV, compared to 420 HV for the unreinforced nickel-based matrix. At 30 wt% Cr3C2, the microhardness reached 950 HV. The wear resistance, evaluated using a pin-on-disk test, improved by a factor of 2.5-4.0 compared to the unreinforced matrix, with the optimal Cr3C2 content identified at 20-25 wt%.

Wear Mechanism Analysis

The authors conducted detailed analysis of the worn surfaces using scanning electron microscopy and energy-dispersive spectroscopy. At low Cr3C2 contents, the primary wear mechanism was abrasive wear, characterized by ploughing grooves and matrix material removal. At optimal Cr3C2 contents (20-25 wt%), the wear mechanism shifted to a combination of abrasive and adhesive wear, with the hard Cr3C2 particles providing effective resistance to abrasive attack. At excessive Cr3C2 contents (above 25 wt%), particle pull-out became a significant wear mechanism, where the relatively weak bonding between agglomerated Cr3C2 clusters and the matrix led to particle detachment and accelerated wear.

The authors also examined the effect of heat treatment on the microstructure and wear resistance of the cladding layers. Solution treatment at 1150 degrees Celsius followed by aging at 870 degrees Celsius for four hours resulted in the formation of fine M23C6 carbides along the grain boundaries, which further increased the hardness to 1020 HV and improved the wear resistance by an additional 15-20 percent. However, excessive aging temperatures above 950 degrees Celsius led to Cr3C2 particle coarsening and a decrease in hardness.

Engineering Practice and Process Optimization

The PTA cladding process parameters play a critical role in determining the final microstructure and performance of the Cr3C2-reinforced nickel-based cladding layer. The authors identified that the arc current and travel speed have a direct influence on the dilution ratio and the degree of Cr3C2 decomposition. At high arc currents and low travel speeds, the heat input is excessive, leading to significant Cr3C2 decomposition and a lower volume fraction of intact reinforcing particles. Conversely, at low arc currents and high travel speeds, incomplete melting of the Cr3C2 particles can occur, resulting in poor bonding with the matrix.

The optimal process parameters identified in the study were an arc current of 200-220 A, a travel speed of 120-150 mm/min, and a powder feed rate of 1.0-1.2 kg/h, which produced a dilution ratio of 30-40 percent and a cladding layer thickness of 1.5-2.5 mm per pass. A multi-pass cladding strategy was recommended, with a transition layer of unreinforced nickel-based alloy deposited first to ensure good bonding with the substrate, followed by two to three passes of the Cr3C2-reinforced powder.

Study Reflections

This paper demonstrates the effectiveness of composite cladding technology in enhancing the wear resistance of nickel-based overlay layers while maintaining their corrosion resistance. The systematic investigation of the relationship between Cr3C2 content, microstructure, and wear performance provides valuable guidance for the design of composite cladding powders for specific industrial applications. The findings also highlight the importance of process parameter optimization in achieving the desired microstructure and performance. The approach of combining a corrosion-resistant nickel-based matrix with thermally stable Cr3C2 reinforcing particles represents a versatile strategy that can be adapted to various service conditions, from chemical processing equipment to aerospace engine components.