Effect of Welding Current on Microstructure and Properties of Ni60 Cr3C2 Plasma Surfacing Layer

Literature Overview

This paper by Zhang Yu et al., published in Journal of Mechanical Engineering Materials (2025, Vol. 49, No. 3, pp. 74-80), investigates the influence of welding current on the microstructure, hardness, and wear resistance of a single-pass Ni60/Cr3C2 plasma surfacing layer deposited on Q235 low-carbon steel. The authors employed a Ni60 alloy powder blended with 20 wt% Cr3C2 powder as the surfacing consumable and systematically varied the welding current from 110 A to 140 A. The work is funded by the Heilongjiang Provincial Department of Education Basic Scientific Research Business Fee Project (2023-KYYWF-0561). This study is particularly relevant to engineers dealing with wear-resistant surfacing on low-alloy steel substrates, a common requirement in mining, cement, and pipeline industries.

Core Technical Findings

The study reveals a clear non-monotonic relationship between welding current and the resulting coating performance. The optimal welding current was identified as 130 A, where the surfacing layer exhibited the finest microstructure, the highest carbide density, the maximum hardness, and the minimum abrasive wear mass loss.

Microstructural Evolution with Welding Current

At 110 A, the thermal input is insufficient to fully dissolve the Cr3C2 addition, resulting in fewer carbide phases. As the current increases to 130 A, the enhanced heat input promotes complete dissolution and subsequent controlled solidification, generating a richer carbide assemblage including Cr7C3, Cr23C6, and retained Cr3C2. The flower-like morphology observed at 130 A is a distinctive feature where Cr carbides nucleate and grow in a radial pattern, indicating a favorable solidification gradient.

Dilution Rate and Geometry Parameters

The dilution rate increases monotonically with welding current, as expected from fundamental arc welding thermodynamics. However, excessive dilution at 140 A not only degrades the coating composition but also introduces surface oxidation burn-off, where the high thermal energy causes preferential evaporation of alloying elements such as Cr and Ni from the molten pool surface.

Interpretation of Key Technical Points

The non-monotonic behavior of hardness and wear resistance with increasing current can be explained through two competing mechanisms. First, increasing the current refines the dendritic structure and promotes carbide nucleation, which enhances hardness through Hall-Petch strengthening and Orowan bypass strengthening. Second, beyond an optimal point, excessive thermal input causes grain coarsening and carbide coarsening (Ostwald ripening), which reduces the density of strengthening particles and degrades wear resistance.

The appearance of the flower-like morphology at 130 A is particularly significant. This microstructure suggests a localized supersaturation of Cr and C at the dendrite tips during solidification, leading to radial growth of Cr carbides from a central nucleus. Such a morphology maximizes the interfacial area between the hard carbide phase and the tough Ni matrix, providing superior resistance to abrasive wear through a combination of micro-indentation resistance and crack deflection.

Process Window and Engineering Implications

For practical plasma surfacing of Ni60/Cr3C2 coatings on Q235 steel, the following process parameters should be considered:

In engineering practice, this study provides a clear guideline for optimizing plasma surfacing parameters for wear-resistant coatings. The finding that 130 A produces the best combination of hardness and wear resistance is directly applicable to the surfacing of pump impellers, valve seats, and pipeline components subject to solid particle erosion. The flower-like carbide morphology at 130 A is a useful metallographic indicator for quality control purposes; its presence can be used as a non-destructive screening criterion during production inspection.

Study Insights and Reflections

This work demonstrates the importance of thermal input control in plasma surfacing beyond simple geometric optimization. The identification of a specific optimal current that balances microstructural refinement against grain coarsening is a valuable contribution to the field. One notable aspect is the systematic increase in carbide phase diversity with current, which suggests that the dissolution behavior of Cr3C2 in the Ni60 matrix is thermally activated. This has implications for the design of multi-component surfacing powders where controlled dissolution and re-precipitation are desired.

For engineers working with plasma surfacing, this study reinforces the principle that there is always a process window beyond which increasing energy input becomes detrimental. The practical recommendation is to conduct a small current sweep test on a coupon before production surfacing, examining both the macroscopic appearance (smoothness, absence of burn-off) and the microstructural characteristics (carbide density, dendrite fineness) to confirm the optimal parameter.