Microstructure and Hardness of Plasma Hardfacing Different Powders on Duplex Stainless Steel

Literature Overview and Context

This 2023 study by He Tao, Ma Jun, and colleagues from Wuzhong Instruments Co., Ltd. and Dalian University of Technology investigates the application of plasma hardfacing on F51 duplex stainless steel using three different powder consumables: Co106F, Co112F, and Ni55. The work is significant because duplex stainless steels (DSS) are widely used in high-performance valve bodies, pressure vessels, and chemical processing equipment where both strength and corrosion resistance are required. Surface hardfacing is often necessary to enhance wear resistance at sealing surfaces, valve seats, and other critical contact areas without compromising the bulk corrosion performance of the base material.

Experimental Design and Methodology

The authors employed plasma transferred arc (PTA) hardfacing to deposit two layers of each powder consumable onto F51 DSS substrates. The characterization suite included metallographic examination (optical microscopy), line-scan elemental analysis (EPMA or EDS), and Vickers microhardness testing. This multi-technique approach provides a comprehensive picture of the microstructure-hardness relationship and the dilution behavior at the fusion boundary.

Microstructural Analysis

The most striking finding is the similarity between Co106F and Co112F microstructures—both exhibit dendritic morphologies with no significant distinction. This suggests that the minor compositional differences between these two cobalt-based powders do not substantially alter the solidification behavior under plasma hardfacing conditions. The dendritic structure is characteristic of directional solidification under the high heat input and rapid cooling typical of PTA processes.

In contrast, the Ni55 powder produces a notably different microstructure. The layer is described as having a more compact distribution, with disordered columnar grains appearing at the interface between the first and second layers. The authors attribute this to supercooling effects at the remelt boundary—a phenomenon well-documented in multi-pass welding where the thermal cycle of the subsequent pass partially remelts the preceding layer, creating a distinct solidification morphology at the interface.

Hardness Distribution and Dilution Behavior

The hardness results reveal a clear hierarchy: Ni55 > Co112F > Co106F in terms of both layer hardness enhancement and surface hardness. The Ni55 layer achieves 1.62 times the base hardness, which is consistent with the known hardening mechanisms in nickel-based alloys—solid solution strengthening, precipitation of Ni3B and Ni3Mo phases, and the presence of fine carbide dispersions.

The surface hardness variation across the three consumables is particularly relevant for engineering applications. In valve seat applications, for instance, the surface hardness directly determines the wear life of the sealing interface. The Ni55 coating, with its highest surface hardness, would be preferred for applications requiring maximum wear resistance, while Co106F might be selected where moderate hardness with better toughness is desired.

Engineering Practice Integration

For valve manufacturers and process equipment engineers, this study provides actionable guidance on consumable selection. The key decision factors include:

1. Required surface hardness: Ni55 for maximum hardness, Co112F for balanced performance, Co106F for moderate enhancement.
2. Corrosion resistance retention: Cobalt-based coatings generally maintain better corrosion resistance in chloride-containing environments compared to nickel-based coatings, which is critical for DSS applications in marine and chemical environments.
3. Coating thickness and number of passes: The two-layer approach is standard practice to minimize dilution and achieve uniform composition.

The finding that Co106F and Co112F produce similar microstructures is practically useful—it suggests that cost considerations or supply availability can drive the choice between these two without significant performance trade-offs.

Key Questions and Reflections

Several aspects warrant further investigation. The study does not report corrosion resistance data for the hardfaced specimens, which is a critical gap given that DSS is selected specifically for its corrosion performance. Additionally, the elemental diffusion profiles at the fusion boundary—though measured via line scanning—are not discussed in terms of their impact on galvanic corrosion potential between the coating and the base material. The mechanical properties (tensile strength, impact toughness) of the hardfacing layer are also absent, which limits the assessment of coating reliability under cyclic or impact loading conditions.

The observation of columnar grains at the Ni55 layer interface raises concerns about potential cracking susceptibility. Columnar grain structures are more prone to transverse cracking than equiaxed structures, and this could be a concern in service applications involving thermal cycling.

Study Insights and Implications

This work demonstrates that PTA hardfacing is a viable and effective method for enhancing the surface properties of duplex stainless steels without compromising the bulk material. The ability to select from multiple powder consumables to achieve specific hardness targets provides valuable flexibility in engineering design. For practitioners working with DSS components in harsh environments, the key message is that surface hardfacing can significantly extend component life while maintaining the base material's corrosion resistance—provided that the consumable selection and process parameters are carefully matched to the application requirements. Future work should focus on integrating corrosion testing and mechanical property evaluation to provide a more complete engineering assessment of these hardfacing systems.