Effect of Plasma Surfacing Speed on Microstructure and Hardness of Ni60 Coating

Literature Overview

This paper by Luo Shengyang and Yuan Zhentao (2018), published in Hot Working Technology (Vol. 47, No. 5, pp. 227–230), investigates the influence of plasma arc surfacing travel speed on the microstructure, phase composition, and microhardness of a single-pass Ni60 overlay deposited onto 304 stainless steel. The study was funded by the Yunnan Provincial Department of Education project (2017ZZX136). The work is particularly relevant for engineers involved in repair welding of stainless steel piping components and pipe fittings where corrosion and wear resistance are critical.

Core Technical Findings

The authors systematically varied the plasma surfacing travel speed and evaluated the resulting coating quality through optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and Vickers microhardness testing. The key finding is that a travel speed of 200 mm/min represents the optimal processing window for single-pass Ni60 surfacing on 304 SS. At this speed, the wetting angle reaches 55°, the dilution rate is controlled at 4.5%, and the macroscopic weld quality is judged to be the best among the tested conditions.

The microstructure at the center region of the coating at 200 mm/min consists of columnar dendrites with precipitates growing laterally along small crystallographic planes. As the travel speed increases from this optimum, the crystallinity of both the Fe-Nix intermetallic phase and boride phase first increases and then decreases. Correspondingly, the coating hardness follows a similar trend—rising to a peak and then falling.

Process Parameter Analysis

The dilution rate of 4.5% at the optimal speed is notably low, which is significant because excessive dilution from 304 SS would introduce substantial amounts of iron and chromium into the Ni60 matrix, altering the intended carbide formation and reducing the coating's wear and corrosion resistance. Maintaining dilution below 5% is a critical quality control criterion for Ni60 surfacing applications on austenitic stainless steel substrates.

Engineering Practice Implications

In pipeline repair and pipe fitting maintenance, Ni60 coatings are frequently applied to restore dimensions and provide enhanced wear and corrosion resistance at critical locations such as valve seats, pump shafts, and pipe coupling surfaces. The finding that 200 mm/min provides the optimal balance of dilution control and microstructural refinement has direct practical value. Engineers should note that travel speed is not merely a productivity parameter—it directly governs the thermal cycle, which in turn determines grain morphology, phase distribution, and ultimately the service performance of the coating.

The columnar dendrite structure observed at the optimal speed suggests a directional solidification pattern that is favorable for stress relief during subsequent machining or service loading. However, columnar grains can also act as preferential crack paths under cyclic loading. For applications involving fatigue or impact, a multi-pass approach with reduced heat input per pass may be warranted to promote equiaxed grain formation.

Key Reflections

The relationship between travel speed and hardness is non-monotonic, which underscores the importance of systematic parameter optimization rather than assuming that higher speeds always reduce dilution without consequences. At very low speeds, excessive heat input leads to coarse grains and potential overheating of the base metal, while at very high speeds, incomplete melting and poor wetting compromise coating integrity. The 200 mm/min optimum represents a thermal balance point where the solidification rate is high enough to refine the microstructure but not so high as to impair wetting.

This study reinforces a broader principle in surfacing welding: the process window for achieving optimal coating properties is narrow, and deviations in either direction degrade performance. In quality control practice, in-process monitoring of travel speed, combined with post-weld dilution verification through EDS or optical emission spectrometry, should be standard procedure for critical Ni60 overlay applications.

Summary

The study provides clear evidence that plasma surfacing travel speed is a dominant variable controlling the microstructure and hardness of Ni60 coatings on 304 stainless steel. The optimal speed of 200 mm/min yields a dilution rate of 4.5%, a wetting angle of 55°, refined columnar dendritic microstructure, and maximum average hardness. These findings offer actionable guidance for engineers specifying and controlling plasma surfacing processes in pipeline and pipe fitting repair operations, emphasizing the necessity of rigorous parameter optimization and in-process monitoring to ensure coating performance meets service requirements.