Optimization of Tungsten Carbide Nickel-Based Composite Coating Deposited by Plasma Transferred Arc Hardfacing

Literature Overview

This study by Ying Wei, Xian-shun Wei, Bo Chen, Jian-yong Zuo, Tian-cai Ma, and Jun Shen from Tongji University, published in the Transactions of Nonferrous Metals Society of China in 2018 (Vol. 28, No. 12, pp. 2511-2519), reports on the parameter optimization of tungsten carbide (WC) nickel-based composite coatings deposited by plasma transferred arc hardfacing. The coating material is Stelcar65, a commercial composite powder with a high tungsten carbide content. The welding parameters were optimized using orthogonal experimental design, and the microstructure and microhardness of the optimal coating were analyzed. The research was supported by multiple national and local funding programs.

Core Technical Content

The WC/Ni-based composite coating system is widely used for applications requiring high wear resistance, particularly in sliding and abrasive wear conditions. The tungsten carbide particles provide exceptional hardness (approximately 2400 HV), while the nickel-based matrix offers good bonding strength, thermal shock resistance, and corrosion resistance. The key challenge in PTA hardfacing of WC/Ni-based composites is minimizing the degradation of tungsten carbide during the welding process.

Tungsten carbide degradation occurs when the WC particles react with the molten nickel matrix to form tungsten carbides with lower carbon-to-tungsten ratios, such as W2C, WCx (x<1), and tungsten carbide-nickel intermetallic compounds. These degraded phases have significantly lower hardness than intact WC, reducing the overall wear resistance of the coating.

Orthogonal Experimental Design

The authors used orthogonal experimental design to systematically evaluate the influence of three welding parameters on tungsten carbide degradation:

The orthogonal test results identified welding current as the most significant factor affecting WC degradation, followed by powder feed rate and welding speed. The optimal parameters were determined to be:

At these optimal parameters, the produced WC/Ni-based composite coatings were free of cracks and WC degradation. The microstructure consisted of intact WC particles embedded in a nickel-based matrix with good interfacial bonding.

Microstructure and Microhardness Analysis

The microstructure of the optimal coating exhibits a two-phase structure: hard tungsten carbide particles distributed in a ductile nickel-based matrix. The WC particles are predominantly cubic in shape with sizes ranging from 5 to 50 micrometers. The nickel matrix consists of austenite and possibly some delta ferrite, depending on the specific alloy composition.

The microhardness of the optimal coating is significantly higher than that of the base material, with the hardness of the WC particles exceeding 2000 HV and the matrix hardness ranging from 200 to 300 HV. The composite hardness of the coating, which represents the effective hardness of the two-phase structure, is approximately 800-1000 HV, providing excellent wear resistance.

The absence of WC degradation is confirmed by the retention of the cubic WC crystal structure and the high microhardness of the carbide particles. Degraded WC particles would exhibit lower microhardness and may show evidence of phase transformation to W2C or other lower-carbon tungsten carbides.

Engineering Practice Implications

The parameter optimization study provides a practical process window for PTA hardfacing of WC/Ni-based composite coatings. The optimal parameters of 100 A current, 25 g/min powder feed rate, and 40 mm/min welding speed represent a balance between deposit thickness, dilution rate, and WC degradation.

For industrial applications, the following considerations are important:

1. Powder feed system calibration: The powder feed rate must be precisely controlled to ensure uniform deposit composition. Variations in powder feed rate can lead to variations in WC content and deposit quality.
2. Arc stability: The PTA process requires stable arc conditions to ensure consistent melting and mixing of the powder and base metal. Arc instability can lead to porosity, incomplete melting, or excessive dilution.
3. Multi-pass deposition: For thick coatings, multiple passes are required. The interpass temperature should be controlled to avoid excessive thermal cycling of the previously deposited layers.
4. Post-deposition inspection: The coating should be inspected for cracks, porosity, and spallation using visual examination, magnetic particle testing, or ultrasonic testing.

Study Insights and Reflections

The research by Wei and colleagues provides a systematic approach to process optimization for PTA hardfacing of WC/Ni-based composite coatings. The use of orthogonal experimental design is an efficient methodology for identifying the optimal parameter combination with a minimum number of experiments.

One important insight is the identification of welding current as the most significant factor affecting WC degradation. This finding is consistent with the understanding that higher welding current increases the heat input, which promotes the reaction between WC and the molten nickel matrix. The optimal current of 100 A represents the minimum current required to achieve adequate melting and bonding while minimizing WC degradation.

The study also highlights the importance of powder feed rate and welding speed in controlling the heat input per unit length. A higher powder feed rate increases the amount of material deposited per unit time, which dilutes the heat input and reduces WC degradation. A higher welding speed also reduces the heat input per unit length, further minimizing degradation. However, excessive welding speed can lead to incomplete melting and poor bonding, so a balance must be struck.

For engineers working on wear-resistant coatings for pipelines, pressure vessels, and rotating equipment, this study provides valuable guidance for selecting and optimizing PTA hardfacing parameters. The key principle is to minimize the thermal exposure of the WC particles while ensuring adequate melting and bonding of the coating to the base material.

The economic implications of this work are significant. The ability to produce crack-free, degradation-free WC/Ni-based composite coatings with optimized parameters reduces the need for rework and extends the service life of coated components. This translates to reduced maintenance costs and improved operational reliability.

Conclusion

The five studies reviewed in this document collectively address critical aspects of hardfacing technology, including alloy design, process optimization, microstructure-property relationships, and post-weld treatment. The common thread linking these works is the pursuit of optimal balance between competing properties—hardness and toughness, wear resistance and thermal stability, cost and performance. Engineers working in the pipe, fitting, and welding industry can draw practical lessons from these studies to improve the reliability and cost-effectiveness of hardfacing operations in their respective applications.