Microstructure and Properties of Arc Weld Overlay Iron-Based Amorphous-Nanocrystalline Composite Coatings

Research Background and Motivation

Iron-based amorphous and nanocrystalline alloys have attracted significant attention in the field of wear-resistant and corrosion-resistant coatings due to their unique atomic structure and exceptional mechanical properties. The arc weld overlay process offers a practical route to deposit these advanced coatings onto engineering components, combining the high deposition rates of arc welding with the superior properties of amorphous and nanocrystalline microstructures. This study examines the microstructural evolution, phase composition, and mechanical performance of iron-based amorphous-nanocrystalline composite coatings produced by arc welding, providing valuable insights for engineers considering this technology for industrial applications.

Microstructural Characteristics and Phase Analysis

The microstructure of the arc weld overlay coating exhibited a characteristic composite morphology consisting of an amorphous matrix with embedded nanocrystalline regions. The amorphous phase, identified as Fe-B-Si-C based, formed during rapid solidification at the surface of the deposited layer where cooling rates exceeded 10^4 K/s. Beneath the fully amorphous surface region, a nanocrystalline zone with grain sizes of 20-80 nm was observed, formed through controlled crystallization during the slower cooling in the subsurface region. Further below, the dilution zone showed a conventional crystalline microstructure with ferrite and martensite phases, indicating the transition from the coating to the substrate.

X-ray diffraction analysis confirmed the presence of broad amorphous halos superimposed on sharp crystalline peaks from the substrate, indicating a partial amorphization of the coating surface. Transmission electron microscopy revealed the nanocrystalline regions as equiaxed grains dispersed within the amorphous matrix, with preferential orientations corresponding to the Fe-B-Si system. The presence of these nanocrystals within the amorphous matrix provided a synergistic combination of hardness from the amorphous phase and toughness from the nanocrystalline regions.

Mechanical and Tribological Performance

The mechanical properties of the composite coating demonstrated remarkable improvements over conventional weld overlay coatings. The Vickers hardness reached 850-950 HV in the fully amorphous surface zone, representing a threefold improvement over the base carbon steel substrate. The wear resistance, evaluated through pin-on-disk testing, showed a specific wear rate of 1.2 × 10^-6 mm³/(N·m), which was approximately 8 times lower than that of conventional 316L overlay coatings under similar test conditions. This exceptional wear resistance is attributed to the combination of high hardness from the amorphous phase and the absence of grain boundaries that would otherwise serve as crack initiation sites.

The corrosion resistance of the coating was evaluated through potentiodynamic polarization testing in 3.5% NaCl solution. The corrosion potential shifted positively by approximately 120 mV compared to the bare substrate, and the corrosion current density decreased by two orders of magnitude to 0.5 μA/cm². The amorphous phase, being free of grain boundaries, segregation, and other microstructural defects that typically serve as corrosion initiation sites, provides a homogeneous barrier against corrosive attack. This homogeneity is a fundamental advantage of amorphous coatings over their crystalline counterparts in aggressive chemical environments.

Process Parameters and Crystallization Control

The key to achieving a high fraction of amorphous and nanocrystalline phases in the arc weld overlay coating lies in controlling the cooling rate through process parameter optimization. Higher welding speeds increase the cooling rate and promote amorphization, but excessive speeds reduce the deposition thickness and may lead to incomplete melting. The optimal parameter window identified was a current of 180-220 A, voltage of 22-28 V, and travel speed of 300-500 mm/min for single-pass deposition. Multi-pass deposition required careful management of interpass temperature to prevent recrystallization of previously deposited amorphous layers; interpass temperatures below 150°C were maintained to preserve the amorphous fraction.

The dilution rate was a critical factor affecting both the microstructure and the properties of the coating. With a dilution rate of 20-35%, sufficient base metal alloying elements were incorporated to promote amorphization without excessively reducing the coating thickness. Higher dilution rates above 40% led to increased crystallinity due to the higher carbon and manganese content from the substrate, which acted as nucleation sites for crystallization. The alloy composition of the filler wire, enriched with boron (1.5-2.5 wt%), silicon (1.0-2.0 wt%), and carbon (0.3-0.6 wt%), was specifically designed to lower the glass transition temperature and promote amorphous formation during rapid solidification.

Engineering Implications and Study Insights

This research demonstrates that arc weld overlay can produce coatings with properties comparable to those achieved by more expensive processes such as plasma spray or laser cladding, while maintaining the advantages of arc welding in terms of equipment accessibility and deposition rate. The composite microstructure of amorphous matrix with nanocrystalline reinforcement offers a superior combination of hardness, wear resistance, and corrosion resistance that is difficult to achieve with single-phase coatings. For engineers, the key takeaway is that the amorphous fraction can be maximized through careful control of cooling rates and dilution, and that the resulting properties are highly dependent on the balance between these two factors.

The practical limitations of this technology include the relatively thin deposit thickness achievable with significant amorphous content, typically limited to 1-3 mm per pass, and the potential for devitrification during subsequent heat treatments or prolonged service at elevated temperatures. The glass transition temperature of the coating was measured at approximately 520°C, meaning that service temperatures above this threshold would lead to crystallization and loss of the beneficial amorphous properties. Engineers must therefore carefully evaluate the service temperature range before selecting this technology for a specific application. The research provides a solid foundation for further development of iron-based amorphous-nanocrystalline coatings for industrial wear and corrosion protection applications, particularly in mining, chemical processing, and energy infrastructure where cost-effective high-performance coatings are needed.