Effect of Welding Parameters on Microstructure and Microhardness of Ni60 Alloy Plasma Surfacing Layer

Literature Overview

This paper by Ge Yanliu, Deng Dewei, Tian Xin, and Guan Meng (Dalian University of Technology and Shenyang Blower Works Group, 2011, published in China Surface Engineering, Vol. 24, No. 5, pp. 26-31) investigates the influence of welding current and powder feeding rate on the microstructure and microhardness of Ni60 alloy plasma surfacing layers deposited onto Z2CN18-10 stainless steel substrates. The study is supported by the National Natural Science Foundation of China (Grant No. 11072045). The authors employed scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), and microhardness testing to characterize the deposited layers systematically.

Core Technical Findings

The most striking result is that the microhardness of the Ni60 surfacing layer is substantially higher than that of the Z2CN18-10 substrate, with a peak value of 630 HV achieved at a welding current of 110 A. The authors document a clear hardness gradient in the fusion zone, accompanied by an element diffusion region, which is critical for understanding the mechanical integrity of the joint. As welding current increases or powder feeding rate decreases, both the overall surfacing layer hardness and the hardness gradient in the fusion zone decrease noticeably. This observation is counterintuitive at first glance—higher current typically implies deeper penetration and greater dilution, which would reduce the effective Ni60 composition in the weld metal and thus lower hardness. The powder feeding rate reduction leads to a thinner deposit with more substrate dilution per pass, similarly reducing the Ni60 content and hardening effect.

The microstructure exhibits a pronounced gradient from the fusion line to the surface. The fusion zone shows a dendritic structure with needle-like, strip-like, and small-flower-like (rosette) morphologies. The near-surface region displays equiaxed grain structures with blocky and plate-like features. These microstructural gradients are directly linked to the cooling rate distribution: the fusion zone experiences the slowest cooling (due to substrate heat sink effects), while the near-surface region cools more rapidly, promoting finer and more equiaxed structures.

Interpretation of Key Technical Points

The formation of borides is a critical aspect of Ni60 alloy hardening. The Ni60 powder (UNS W90600) typically contains 56-62% Ni, 57-67% Cr, 5.5-6.5% B, and 0.35-0.55% C. The boron and carbon content drives the formation of hard borides (such as CrB, Cr2B, and CrB2) and carbides (such as M7C3 and M23C6), which are the primary hardening phases responsible for the elevated microhardness values. The study reports that when the powder feeding rate is reduced to 6 g/min, directional long-strip and discrete blocky borides appear in the mid-region of the surfacing layer. This indicates that at low feeding rates, the thermal input per unit volume of deposited material is relatively high, allowing sufficient time for boride nucleation and growth into coarse morphologies.

The small-flower-like (rosette) eutectic structure observed in the near-surface region is characteristic of high-alloy austenitic-ferritic systems. The volume fraction of this rosette structure increases with increasing welding current but does not disappear entirely. This suggests that the eutectic reaction between austenite and ferrite (or between austenite and carbide/boride phases) is thermodynamically favored in the Ni60 system regardless of thermal input, but the kinetics of growth are modified by the cooling rate.

Process Parameters and Their Effects

The optimal welding current of 110 A represents a balance between adequate powder melting and deposition efficiency, and the avoidance of excessive dilution. At this current level, the powder feeding rate should be maintained at a value that ensures sufficient material deposition without allowing excessive heat concentration per unit volume. The practical implication is that for Ni60 plasma surfacing applications, a moderate thermal input with controlled powder delivery is essential to achieve both high hardness and a controlled microstructure.

Connection with Engineering Practice

In engineering applications involving Ni60 plasma surfacing—such as the hardfacing of pump impellers, valve seats, and wear-resistant pipe sections—the control of welding current and powder feeding rate is paramount. The hardness gradient in the fusion zone is a double-edged sword: while it ensures a gradual transition in mechanical properties, an excessively steep gradient can lead to residual stress concentration and potential cracking. The element diffusion region identified in this study highlights the need for careful consideration of interdiffusion between the Ni60 deposit and the Z2CN18-10 substrate, particularly when the substrate is a low-alloy or austenitic stainless steel with a significantly different composition.

From a process design perspective, the findings suggest that multi-pass surfacing with interpass temperature control is advisable to manage the thermal history and prevent excessive coarsening of borides. The presence of directional long-strip borides at low powder feeding rates (6 g/min) is particularly concerning from a toughness perspective, as elongated brittle phases can act as crack initiation sites under impact or cyclic loading.

Key Questions and Reflections

One question that arises from this study is the interaction between welding current and powder feeding rate as coupled parameters. The authors vary these parameters independently, but in practice, their ratio (thermal input per unit mass of deposited material) is the more physically meaningful variable. A future study that systematically varies this ratio would provide more actionable process guidance. Additionally, the study does not address the effect of travel speed, which is another critical parameter in plasma surfacing that influences the cooling rate and dilution rate.

Another point of reflection is the absence of toughness or fracture toughness data. High microhardness is desirable for wear resistance, but it often comes at the expense of ductility and toughness. In applications where the Ni60 surfacing layer is subjected to impact loading (such as in mining equipment or oil and gas well components), the balance between hardness and toughness must be carefully managed.

Study Insights and Implications

This paper provides valuable quantitative data on the microstructure-hardness relationship in Ni60 plasma surfacing. The identification of distinct microstructural zones—fusion zone, near-fusion dendritic zone, and near-surface equiaxed zone—offers a framework for interpreting metallographic observations in practical quality control. Engineers can use the microstructural features described here as reference benchmarks when evaluating surfacing deposits in production environments.

The observation that boride morphology is sensitive to powder feeding rate has direct implications for powder delivery system design. Ensuring stable and consistent powder feeding is not merely a matter of deposition efficiency but also a critical factor in controlling the final microstructure and mechanical properties of the surfacing layer. Process monitoring systems that track powder flow rate in real time can contribute to the consistent quality of Ni60 surfacing deposits.

Reference Value and Outlook

This study, though published in 2011, remains highly relevant to current practice in hardfacing and surface engineering. The Ni60 alloy continues to be one of the most widely used hardfacing alloys in the oil and gas, mining, and power generation industries. The systematic approach adopted by the authors—combining SEM, EPMA, XRD, and microhardness testing—sets a methodological standard that should be followed in similar investigations. Future research should extend these studies to include multi-pass surfacing, different substrate materials, and the effects of post-weld heat treatment on the microstructure and properties of Ni60 deposits.