Microstructure and Wear Resistance of Nickel and Cobalt Based Alloy Overlays Deposited by Plasma Transferred Arc Welding

Literature Overview

This study investigates the microstructure and wear resistance of nickel-based and cobalt-based alloy overlays deposited using plasma transferred arc welding (PTA). These overlays are widely used in high-temperature and high-wear applications such as gas turbine components, petrochemical equipment, and mining machinery. PTA is a versatile and efficient process for depositing these overlays, offering high deposition rates, low dilution, and excellent control over the overlay microstructure. The research focuses on the relationship between microstructure, composition, and wear resistance, providing insights into the optimization of PTA parameters for achieving superior performance.

Core Technical Findings

The study evaluates several nickel-based alloys, including Inconel 625, Inconel 718, and Hastelloy C276, and cobalt-based alloys, including Stellite 6, Stellite 21, and Haynes 25. These alloys are known for their excellent combination of high-temperature strength, corrosion resistance, and wear resistance, making them ideal candidates for overlay applications in severe service environments. The study demonstrates that the microstructure of PTA deposits is strongly influenced by the process parameters, including current, voltage, travel speed, and wire feed speed, which affect the cooling rate, solidification mode, and phase formation.

Microstructural Characteristics

The microstructure of PTA deposits typically consists of a columnar dendritic structure with varying degrees of cellular or equiaxed grain formation depending on the cooling rate and solidification conditions. In nickel-based alloys, the primary phase is an austenitic gamma (γ) matrix, often with precipitation of carbides (e.g., MC, M7C3, M23C6) and intermetallic phases (e.g., Ni3Al, NiAl) depending on the composition and cooling rate. In cobalt-based alloys, the primary phase is an austenitic or martensitic matrix with precipitation of carbides (e.g., Cr7C3, WC, Co3W) that contribute significantly to the wear resistance.

Wear Mechanisms and Performance

The study identifies several wear mechanisms that govern the wear resistance of nickel-based and cobalt-based overlays, including adhesive wear, abrasive wear, oxidative wear, and erosion wear. The relative contribution of each mechanism depends on the service conditions, including temperature, load, sliding speed, and the presence of corrosive or abrasive media. At high temperatures, oxidative wear becomes dominant, while at lower temperatures, adhesive and abrasive wear are more significant.

Key Factors Influencing Wear Resistance

The wear resistance of PTA deposits is influenced by several factors, including:

• Hardness of the overlay material: Higher hardness generally improves abrasion resistance but may reduce toughness and increase susceptibility to cracking.
• Carbide distribution and morphology: Fine, uniformly distributed carbides provide the best combination of hardness and toughness, while coarse or segregated carbides can act as crack initiation sites.
• Matrix microstructure: The matrix phase (austenitic, martensitic, or duplex) influences the wear resistance by affecting the deformation behavior and fracture toughness.
• Dilution rate: Excessive dilution can alter the overlay composition and microstructure, reducing wear resistance.
• PTA process parameters: Current, voltage, travel speed, and wire feed speed affect the cooling rate and solidification mode, which in turn influence the microstructure and wear resistance.

The study demonstrates that the optimal PTA parameters for maximizing wear resistance while maintaining adequate toughness are:

• Current: 150-250 A for nickel-based alloys, 200-350 A for cobalt-based alloys
• Voltage: 25-35 V for nickel-based alloys, 30-40 V for cobalt-based alloys
• Travel speed: 100-200 mm/min for nickel-based alloys, 150-300 mm/min for cobalt-based alloys
• Wire feed speed: 1.0-2.0 m/min for nickel-based alloys, 1.5-3.0 m/min for cobalt-based alloys
• Shielding gas: Argon or argon-helium mixture with flow rate of 20-30 L/min

Process Parameters and Microstructural Control

The study emphasizes the importance of PTA process parameters in controlling the microstructure and, consequently, the wear resistance of the overlay. The heat input, which is determined by the current, voltage, and travel speed, affects the cooling rate and solidification mode. Higher heat inputs result in slower cooling rates, which promote the formation of coarser grains and larger carbides, potentially reducing wear resistance. Conversely, lower heat inputs result in faster cooling rates, which can promote the formation of finer grains and smaller carbides but may also increase the risk of cracking due to higher residual stresses.

Dilution Control

Dilution is a critical factor in PTA overlay application, as it affects the composition and microstructure of the overlay. Excessive dilution can reduce the concentration of alloying elements in the overlay, leading to lower hardness and reduced wear resistance. The study recommends maintaining a dilution rate below 15% for nickel-based alloys and below 10% for cobalt-based alloys to ensure adequate performance. Dilution can be controlled by adjusting the heat input, travel speed, and the geometry of the overlay bead.

Engineering Practice Integration

In engineering practice, nickel-based and cobalt-based PTA overlays are commonly applied to components subjected to severe wear and corrosion at high temperatures, such as gas turbine blades, combustion chamber components, petrochemical reactor internals, and mining equipment. The study provides practical guidance for the selection of overlay material, PTA parameters, and quality assurance procedures based on specific service conditions. Engineers must consider not only the wear resistance of the overlay but also its high-temperature strength, corrosion resistance, and compatibility with the substrate material.

The study also highlights the importance of post-weld heat treatment (PWHT) for PTA overlays. PWHT can relieve residual stresses, refine the microstructure, and improve the overall performance of the overlay. However, the PWHT cycle must be carefully tailored to the specific alloy system to avoid adverse effects such as carbide coarsening or phase transformation that could reduce wear resistance. For example, a typical PWHT cycle for Inconel 625 overlays involves heating to 1100-1200°C for 1-2 hours followed by air cooling, while Stellite 6 overlays may require solution treatment at 1150-1200°C followed by quenching and aging.

Key Questions and Reflections

The study raises several important questions regarding the long-term performance of PTA overlays under actual service conditions. First, how does the overlay perform under combined wear and corrosion conditions at high temperatures? While the study provides wear resistance data under controlled laboratory conditions, real-world applications often involve simultaneous exposure to abrasive particles, corrosive media, and thermal cycling, which can accelerate degradation through corrosion-wear interaction. Second, what is the effect of cyclic loading and thermal fatigue on the wear resistance of the overlay? Many applications involve cyclic loading and thermal cycling that can cause fatigue cracking in addition to wear, and the combined effect may be more severe than either mechanism alone.

Another area requiring further investigation is the effect of overlay thickness on wear resistance and residual stress. Thicker overlays may provide better protection against wear but may also experience higher residual stresses due to thermal contraction mismatch between the overlay and substrate. The optimal overlay thickness must be determined based on the specific service conditions and the expected wear rate.

Study Insights and Implications

The research provides valuable insights into the microstructure-wear resistance relationships in nickel-based and cobalt-based PTA overlays and offers practical guidance for engineers involved in the design, fabrication, and maintenance of components requiring wear-resistant surfaces at high temperatures. The key takeaway is that optimizing wear resistance requires a holistic approach that considers composition, process parameters, microstructure, and service conditions simultaneously. Engineers should not rely solely on maximizing hardness but must also evaluate the overlay's toughness, high-temperature strength, and compatibility with the substrate material.

The study also highlights the importance of quality assurance and inspection in ensuring the performance of PTA overlays. Non-destructive testing techniques such as ultrasonic testing (UT), magnetic particle testing (MT), and penetrant testing (PT) should be employed to detect defects that could compromise the overlay's performance. Regular inspection intervals should be established based on the service conditions and the expected wear rate.

Reference Value and Outlook

This study contributes to the understanding of microstructure and wear resistance in nickel-based and cobalt-based PTA overlays and provides practical recommendations for optimizing PTA parameters and overlay composition. Future research should focus on developing predictive models for wear life estimation that incorporate microstructural parameters, service conditions, and corrosion effects. Additionally, the development of new alloy compositions with improved wear resistance through advanced alloy design and processing techniques would be highly beneficial for extending the service life of components in severe wear and high-temperature applications.