Effect of Plasma Welding Parameters on Microstructure and Microhardness of Ni60 Cladding Layer

Literature Overview

This study, published in 2011 in the journal China Surface Engineering, was conducted by researchers from Dalian University of Technology and Shenyang Blower Works Group, funded by the National Natural Science Foundation of China (Grant No. 11072045). The work investigates how plasma transferred arc (PTA) welding parameters influence the microstructural evolution and microhardness distribution of Ni60 alloy overlay deposits applied onto carbon steel substrates. Ni60, a nickel-cobalt-chromium alloy, is widely employed in the chemical, petrochemical, and mining industries for its exceptional resistance to thermal fatigue, thermal shock, and abrasive wear. The study adopts a systematic experimental approach, varying key PTA process parameters and correlating them with metallurgical outcomes through optical microscopy, scanning electron microscopy, and Vickers microhardness testing.

Core Technical Content

PTA Process Parameters Investigated

The study examined the influence of several critical PTA parameters on the cladding layer quality. The key parameters and their typical ranges are summarized below.

Microstructural Evolution

The microstructure of the Ni60 cladding layer is predominantly austenitic, with the austenite matrix containing dispersed carbides and intermetallic phases. The primary microstructural features include:

• Austenite matrix (γ phase): The dominant phase, providing ductility and toughness to the cladding layer.
• MC-type carbides: Typically Cr₇C₃ and Cr₃C, which contribute to wear resistance but can reduce toughness when excessively coarse.
• M₇C₃ carbides: Formed at higher cooling rates or when dilution from the carbon steel substrate introduces excess carbon into the deposit.
• Boride phases (Ni₃B, Ni₄B₃): Present when boron is added to the Ni60 powder, enhancing hardness but potentially embrittling the microstructure.

The dilution rate, which represents the proportion of base metal melted into the cladding layer, was found to be the most critical factor governing microstructure. At dilution rates below 15%, the Ni60 layer retains its designed composition and microstructure. However, when dilution exceeds 25%, the carbon content in the deposit increases significantly due to the carbon steel substrate, leading to excessive carbide precipitation and a shift from austenite to martensite in some regions.

Microhardness Distribution

The microhardness of the Ni60 cladding layer was measured using Vickers hardness testing (HV10 load). The results revealed a clear trend:

The study demonstrated that an optimal dilution rate of 10–15% provides the best balance between hardness and toughness. Below this range, the hardness is insufficient for wear applications; above it, the microstructure becomes embrittled by coarse carbides and martensite.

Process Optimization Insights

Parameter Interaction Effects

The study highlighted that welding parameters do not act independently but interact in complex ways:

• Current-to-feed-rate ratio (I/F): This ratio determines the heat input per unit of deposited metal. A higher I/F ratio increases dilution and promotes carbide coarsening.
• Travel speed effect: Increasing travel speed reduces the heat input per unit length, resulting in finer microstructures and lower dilution, but excessive travel speed leads to incomplete powder melting and porosity.
• Voltage effect: Higher arc voltage widens the bead and increases the interaction zone between the arc and substrate, potentially increasing dilution.

Multi-Pass Cladding Considerations

For thick cladding layers, multi-pass deposition is required. The study noted that:

1. The first pass experiences the highest dilution due to direct contact with the carbon steel substrate.
2. Subsequent passes show progressively lower dilution as the previous Ni60 layer acts as the substrate.
3. Interpass temperature control is critical; excessive interpass heating promotes grain growth and carbide coarsening in previously deposited layers.
4. A recommended interpass temperature of below 200°C was suggested to maintain microstructural integrity.

Engineering Practice Implications

Application to Bimetal Pressure Vessel Fabrication

The findings of this study have direct relevance to the fabrication of bimetal pressure vessels, particularly hydrogenation reactors and heat exchangers where Ni60 overlay is applied to carbon steel or low-alloy steel shells. In pressure vessel applications governed by standards such as NB/T 47002 and ASME VIII Div.1, the following considerations arise:

• Bond strength requirements: The overlay layer must achieve a bond strength exceeding 4.5 MPa (as specified in ASME IX and relevant Chinese standards) to ensure reliable adhesion to the base metal.
• Crack sensitivity: High dilution rates increase the risk of hot cracking and cold cracking in the overlay layer, particularly at the overlay-base metal interface.
• Residual stress management: The thermal cycling inherent in PTA cladding generates residual stresses that must be relieved through post-weld heat treatment (PWHT) in accordance with applicable codes.

Quality Control Recommendations

Based on the study's findings, the following quality control measures are recommended for Ni60 PTA cladding in engineering practice:

Defect Analysis and Countermeasures

Study Insights and Reflections

The most significant insight from this study is the recognition that dilution rate, rather than any single welding parameter, serves as the master variable controlling the microstructure and properties of Ni60 PTA cladding layers. This finding has profound implications for process development: rather than optimizing individual parameters in isolation, engineers should focus on controlling the dilution rate through coordinated adjustment of current, feed rate, and travel speed.

Furthermore, the study underscores the importance of understanding the metallurgical consequences of dilution. When carbon from the substrate enters the Ni60 deposit, it fundamentally alters the phase balance, potentially transforming a ductile austenitic microstructure into a brittle carbide-rich or martensitic one. This metallurgical perspective is essential for engineers who must balance wear resistance requirements against the need for toughness and crack resistance in pressure vessel applications.

One area that warrants further investigation is the long-term performance of Ni60 cladding layers under cyclic thermal loading conditions, such as those experienced in hydrogenation reactors. The study provides static microstructural and hardness data but does not address fatigue behavior or thermal cycling resistance, which are critical for pressure vessel service.

Conclusion

This study provides a valuable foundation for understanding the relationship between PTA welding parameters and the microstructural properties of Ni60 cladding layers. The identification of dilution rate as the primary controlling variable offers a clear process optimization strategy for engineers. For bimetal pressure vessel fabrication, the recommended dilution range of 10–20% with interpass temperature control below 200°C provides a practical process window that balances hardness, toughness, and metallurgical integrity. The findings should be integrated into welding procedure qualification (WPQ) programs in accordance with NB/T 47014 and ASME IX to ensure consistent production quality.