Effect of Plasma Welding Current on Microstructure and Properties of Nickel-Based Alloy Plasma Overlay Deposits

Literature Overview

This paper by Cui Wendong and colleagues from Shenyang Blower Works Nuclear Pump Industry Co. and Shenyang University of Technology investigates the influence of plasma welding current on the microstructure, phase composition, microhardness, and wear resistance of nickel-based alloy overlay deposits applied to Z2CN18-10 austenitic stainless steel. The research was supported by the National Key R&D Program (2016YFB1100204) and Shenyang Municipal Science and Technology Bureau key R&D programs. The work was published in the journal "Welding" (焊接) in 2017, Volume 12, pages 36-40.

The engineering motivation is clear: Z2CN18-10 austenitic stainless steel, commonly used in nuclear pump impellers and casing components, suffers from inadequate wear resistance in highly abrasive and corrosive flow environments. Plasma arc overlay welding (PAOW) provides a dilution-controlled, low-heat-input method to deposit wear-resistant nickel-based alloy cladding layers, making it an attractive surface engineering solution for critical nuclear-grade pump components.

Core Technical Findings

Phase Composition and Microstructure Evolution

The nickel-based overlay deposits consist of a γ-Ni solid solution matrix with eutectic phases including FeNi₃, Cr₂₃C₆, Cr₇C₃, and CrB. The carbide and boride phases serve as the primary hardening precipitates responsible for the enhanced wear resistance of the overlay layer.

A critical finding is the microstructural evolution with increasing welding current. At lower currents, the microstructure exhibits a clustered petal-like morphology. As current increases, the morphology transitions through a water-plant (dendritic-branch) pattern toward elongated strip-like structures. This evolution is directly related to the heat input and cooling rate: higher current increases the thermal gradient and modifies the solidification front morphology, promoting directional solidification patterns.

Wear Mechanism Analysis

The wear mechanism at the optimal current of 110 A is a mixed mechanism involving adhesive wear and abrasive wear in the early stage, transitioning to oxidative wear in the later stage. This is significant because it indicates that the nickel-based overlay layer does not simply resist wear through hardness alone but also forms protective oxide films during sliding contact.

Process Parameter Optimization and Engineering Implications

Optimal Current Selection

The identification of 110 A as the optimal plasma welding current represents a critical process window for industrial application. At this current:

• The microhardness reaches its peak value of 898 HV, which is substantially higher than the base material hardness (typically 180-220 HV for Z2CN18-10).
• The relative wear resistance of 13.8 indicates that the overlay layer can withstand approximately 13.8 times the material loss of the bare base material under identical test conditions.
• The microstructure achieves a favorable balance between fine carbide/boride dispersion and matrix toughness.

Heat Input and Dilution Control

Plasma arc overlay welding operates on the principle of gas shielding with a separate arc, which allows the arc to be focused onto the base material surface without direct contact of the electrode with the workpiece. This fundamentally reduces base material dilution compared to conventional arc welding processes. However, increasing the current increases the heat input, which can lead to:

• Greater base material dilution if the current exceeds the optimal window
• Coarsening of the microstructure due to slower cooling rates
• Potential formation of excessive intermetallic compounds at the overlay-base interface

Integration with Nuclear Pump Engineering Practice

For nuclear-grade pump impellers and wear rings, the selection of overlay parameters must balance wear resistance with several additional requirements:

1. Neutron irradiation stability: The overlay microstructure must maintain integrity under prolonged neutron flux exposure, which can cause embrittlement and phase instability.
2. Corrosion resistance in reactor coolant: The overlay must not create galvanic couples that accelerate localized corrosion in the primary coolant environment.
3. Welding residual stress management: The thermal cycling during multi-pass overlay can generate significant residual stresses that may affect fatigue life.
4. Non-destructive testing compatibility: The overlay layer must be inspectable by UT or RT methods to detect subsurface defects.

The 898 HV hardness achieved at 110 A is particularly noteworthy because it approaches the hardness range required for cavitation erosion resistance in pump applications. However, engineers should note that hardness alone does not guarantee superior cavitation resistance; the toughness of the matrix and the ability to accommodate plastic deformation under high-frequency pressure cycling are equally important.

Key Questions and Study Insights

The transition from petal-like to strip-like microstructure with increasing current raises an important metallurgical question: at what point does the elongated morphology become detrimental to mechanical properties? The paper indicates that beyond 110 A, both hardness and wear resistance decrease, suggesting that excessively elongated structures may have reduced load-bearing capacity in the transverse direction.

From a quality control perspective, the following FMEA considerations apply to plasma overlay welding of nuclear pump components:

• Failure mode: Incomplete melting or lack of fusion at the overlay-base interface due to insufficient current
• Detection method: Cross-section metallographic examination, dye penetrant testing of the overlay surface
• Preventive action: Strict current monitoring with real-time feedback control, prequalification of welding procedures

The wear mechanism transition from adhesive-abrasive to oxidative wear suggests that the Cr₂₃C₆ and Cr₇C₃ carbides play a dual role: they provide abrasive resistance through their high hardness, while the Cr-rich matrix promotes the formation of protective Cr₂O₃ oxide films during sliding. This dual mechanism is particularly valuable in pump applications where the overlay surface is continuously exposed to liquid media.

Study Conclusions

This research provides a well-defined process window for plasma overlay welding of nickel-based alloy cladding on austenitic stainless steel pump components. The optimal current of 110 A yielding 898 HV hardness and 13.8 relative wear resistance represents a significant improvement over the base material. The identified mixed wear mechanism and the microstructural evolution patterns offer valuable guidance for process optimization in nuclear pump surface engineering applications. Engineers implementing this technology should pay particular attention to maintaining the current within the optimal range and performing thorough NDE on the overlay layer to ensure metallurgical soundness.