Effect of Cr3C2 Addition on Plasma Cobalt-Based Overlay Microstructure and Performance

Literature Overview

Luo Yan, Xu Zhipeng, Li Fei, Wang Zhenxing, Ji Chunjiao, and Si Songhua from the School of Materials Science and Engineering at Anhui University of Technology published a study in the Journal of Anhui University of Technology (Natural Science Edition) (Volume 31, Issue 1, 2014, pages 34-38), funded by the Anhui Provincial Department of Education Key Research Project (KJ2007A106ZC). The study investigated the effect of Cr3C2 ceramic powder addition on the microstructure and wear resistance of plasma-welded cobalt-based (Co40) overlay layers deposited on low carbon steel. Three compositions were studied: pure Co40, Co40 with 20 wt% Cr3C2, and Co40 with 40 wt% Cr3C2. Characterization was performed using optical microscopy, SEM, XRD, and wear testing.

Base Overlay Material and Process

The Co40 cobalt-based alloy is a hardfacing alloy widely used in applications requiring high temperature wear resistance, such as valve seats, pump impellers, and hot section components of gas turbines. The base composition typically includes approximately 60-65% Co, 20-30% Cr, 5-10% W or Mo, and small amounts of Fe, Ni, and C. The matrix is typically austenitic (γ-Co) with dispersed chromium carbides (Cr23C6) that provide the primary wear resistance mechanism.

Plasma arc welding (PAW) is an advanced welding process that offers several advantages for overlay welding applications: (1) high energy density resulting in deep, narrow welds with low dilution; (2) precise control of heat input; (3) the ability to use a wide range of filler materials including ceramic powders; and (4) relatively low residual stresses compared to conventional arc welding processes. The low dilution ratio (typically 10-30%) is particularly advantageous when depositing expensive alloy overlays on carbon steel substrates.

Microstructural Evolution with Cr3C2 Addition

The microstructural evolution observed in this study follows a well-defined pattern that is consistent with the principles of hypereutectic solidification in metallic-matrix composites. In the pure Co40 overlay, the microstructure consists of a γ-Co matrix with Cr23C6 carbides distributed along grain boundaries and within grains. The solidification occurs by a subeutectic mechanism, where the liquid first solidifies as austenite and then undergoes eutectic transformation to produce the γ-Co + Cr23C6 microstructure.

Upon addition of 20 wt% Cr3C2, the microstructure undergoes significant refinement and homogenization while maintaining the subeutectic solidification mode. The Cr3C2 particles act as heterogeneous nucleation sites, promoting the formation of finer austenite grains and a more uniform distribution of Cr23C6 carbides. Some of the added Cr3C2 particles remain unmelted, serving as hard reinforcing phases within the matrix. Additionally, the Cr3C2 addition promotes the formation of Cr7C3 carbides, which are smaller and more numerous than the Cr23C6 carbides in the pure Co40 overlay.

At 40 wt% Cr3C2, the solidification mode transitions to hypereutectic, characterized by the formation of primary carbides during the initial stages of solidification, followed by dendritic growth of the austenitic matrix. The microstructure consists of large primary carbides (unmelted Cr3C2 and possibly Cr7C3) distributed within a dendritic γ-Co matrix with eutectic Cr23C6 in the interdendritic regions. This hypereutectic microstructure is inherently more brittle than the subeutectic structure due to the high volume fraction of hard, brittle carbide phases.

Mechanical Properties and Wear Resistance

The hardness and wear resistance of the overlay layers improved significantly with increasing Cr3C2 addition, following a trend that is well-established in the field of metallic-matrix composites. The pure Co40 overlay would typically exhibit a hardness of approximately 500-600 HV, while the Co40 + 40% Cr3C2 overlay could achieve hardness values exceeding 800-900 HV. The wear resistance improvement is attributed to several mechanisms: (1) the presence of hard ceramic particles (Cr3C2 and Cr7C3) that resist abrasive wear; (2) the refinement of the matrix microstructure, which improves the matrix's own wear resistance; and (3) the increased volume fraction of carbide phases, which provide a higher density of wear-resistant particles.

Engineering Practice and Process Optimization

From a practical engineering standpoint, the selection of Cr3C2 addition level requires a careful balance between wear resistance and toughness. The 20% Cr3C2 addition appears to offer an optimal compromise, providing significant improvements in hardness and wear resistance while maintaining the subeutectic microstructure that offers better toughness than the hypereutectic structure. The 40% Cr3C2 addition, while providing the highest wear resistance, may be susceptible to cracking and spalling under impact or cyclic loading due to the high brittleness of the hypereutectic microstructure.

The plasma arc welding process parameters must be carefully controlled to ensure proper melting and distribution of the Cr3C2 particles. Insufficient heat input may result in incomplete melting and poor bonding of the ceramic particles, while excessive heat input may cause excessive melting of the Cr3C2 particles, reducing their effectiveness as reinforcing phases and potentially causing porosity due to gas evolution. The optimal plasma arc current, arc voltage, travel speed, and powder feeding rate must be determined through systematic trial welding and characterization.

For industrial applications, the Co40 + 20% Cr3C2 overlay composition is likely the most practical choice for applications requiring a balance of wear resistance and toughness, such as valve seats in high-temperature service or pump impellers in slurry service. The Co40 + 40% Cr3C2 composition may be more suitable for applications where pure abrasion resistance is the primary requirement and impact loading is minimal, such as in certain mining equipment components.

Study Insights and Reflections

This study provides valuable insights into the microstructural design of cobalt-based overlay layers through the strategic addition of Cr3C2 ceramic particles. The clear correlation between Cr3C2 content, solidification mode, microstructure, and mechanical properties demonstrates the power of microstructural engineering in tailoring overlay performance for specific applications.

The transition from subeutectic to hypereutectic solidification at approximately 40% Cr3C2 addition is a critical threshold that engineers must be aware of. This transition fundamentally changes the microstructure and, consequently, the mechanical properties of the overlay. The practical implication is that the Cr3C2 addition should be carefully controlled to remain below this threshold if toughness is a design requirement.

The use of plasma arc welding for this application is well-justified, as the process provides the control necessary to achieve uniform distribution of the ceramic particles and to minimize dilution. However, the relatively high cost of plasma arc welding equipment and the specialized expertise required for operation may limit its widespread adoption in some industrial settings. Engineers should consider alternative processes such as HVOF (High Velocity Oxy-Fuel) spraying or laser cladding as potential alternatives, each with its own advantages and limitations.