Microstructure and Wear Resistance of Cobalt-Based Alloy Plasma Surfacing Layers

Literature Overview

This 2014 study by Zhang Xi and Gao Wei from China University of Petroleum (Beijing), published in "Hot Working Technology" (热加工工艺), investigates the microstructure evolution and tribological performance of cobalt-based alloy overlays produced by plasma transfer arc surfacing (PTAW) on Q235 carbon steel. Funded by the National Science and Technology Support Program, this research addresses the practical need for durable wear-resistant surfaces in oil and gas equipment where cobalt-based alloys are the industry standard for high-temperature and high-wear applications.

Core Technical Content

Cobalt-based alloys, particularly those in the Stellite family (Stellite 6, Stellite 21, etc.), are widely used in oilfield equipment, turbine blades, and valve components due to their exceptional combination of high-temperature strength, wear resistance, and corrosion resistance. The plasma arc surfacing process is preferred for these alloys because it provides precise thermal input control, minimal dilution with the base metal, and the ability to produce dense, crack-free overlays with fine microstructure.

The study employs a ring-on-block friction wear tester to evaluate dry friction wear behavior under varying normal loads. The microstructural characterization combines optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and microhardness profiling to establish the relationship between microstructure and wear performance.

Microstructural Analysis

The overlay microstructure is dominated by dendritic γ-Co solid solution matrix with interdendritic eutectic regions composed of γ-Co solid solution and M7C3 carbides (where M = Cr, W, Fe). This microstructure is characteristic of rapidly solidified cobalt-base alloys and provides the foundation for their excellent wear resistance through two mechanisms: the hard carbide particles resist abrasive wear, while the ductile γ-Co matrix accommodates deformation without catastrophic fracture.

The hardness gradient from the overlay surface to the base metal interface is a critical finding. The overlay surface typically achieves 400–500 HV, while the dilution zone transitions to approximately 200–250 HV, approaching the base metal hardness of Q235 steel (~120–150 HV). This gradient means that the effective wear resistance is concentrated in the upper portion of the overlay, and sufficient overlay thickness (typically 1.5–3.0 mm) is necessary to ensure the functional surface remains within the high-hardness zone throughout service life.

Wear Behavior Analysis

The friction coefficient decreases with increasing normal load, which is attributed to the progressive ploughing of carbide particles into the softer matrix, creating a protective tribofilm. The dominant wear mechanism is identified as delamination wear, where subsurface cracks initiate beneath the wear surface, propagate laterally, and eventually cause material removal in plate-like fragments. This wear mode is consistent with the layered microstructure of the overlay, where the interface between the dendritic matrix and the eutectic regions serves as preferential crack initiation sites.

The load-dependent friction behavior can be explained through the Archard wear model framework. At lower loads, the asperity contact area is small and the friction coefficient is higher due to adhesion-dominated interactions. As the load increases, the real contact area grows, the carbide particles begin to embed, and the friction coefficient drops. However, beyond a critical load, the delamination mechanism becomes more aggressive, and the wear rate increases non-linearly.

Engineering Practice Implications

For oilfield applications, the selection of cobalt-based overlay systems must balance wear resistance with cost considerations. Plasma arc surfacing with wire electrodes is generally more economical than thermal spray or cladding methods for large components such as drill collars, pump shafts, and valve seats. The dilution zone identified in this study has practical implications: if the overlay is too thin, the dilution zone may be exposed during service, resulting in premature wear. Industry practice typically specifies a minimum overlay thickness of 2.0 mm for critical wear applications, with 3.0–5.0 mm for severe service conditions.

The delamination wear mechanism identified in this study suggests that overlay thickness should be designed with a safety factor to ensure that even after significant wear, the remaining material maintains adequate hardness and structural integrity. In practice, this means designing the overlay thickness to be at least 3–5 times the expected wear depth over the component's service life.

Study Reflections

This research provides valuable quantitative data on the microstructure-property relationships of cobalt-based overlays, but its scope is limited to dry sliding wear under laboratory conditions. Real-world oilfield environments involve wet sliding, erosion-corrosion, and impact loading that are not captured by the ring-on-block test configuration. Nevertheless, the fundamental microstructural findings—the dendritic γ-Co matrix with M7C3 carbides, the hardness gradient, and the delamination wear mechanism—remain applicable across various service conditions. The paper's methodology of combining multiple characterization techniques to build a comprehensive microstructure map is a model approach that should be adopted in any overlay qualification program.