Microstructure and Properties of Iron-Based Amorphous-Nanocrystalline Composite Coatings by Arc Hardfacing

Literature Overview

Wang Bin and colleagues from Southwest Petroleum University, in collaboration with Shanghai Jianke Engineering Consulting and PetroChina Southwest Pipeline Company, published this research in Journal of Functional Materials (2014, Vol. 45, No. 19). The study investigates the preparation and characterization of iron-based amorphous/nanocrystalline composite hardfacing coatings using a low-hydrogen electrode with an Fe41Co7Cr15Mo14C15B6Y2 alloy core. This work is particularly relevant to pipeline operators seeking enhanced wear resistance for components subjected to erosive flow conditions.

Material Design and Alloy Composition

The alloy composition represents a carefully engineered system designed to maximize amorphous phase formation during the rapid solidification conditions of arc hardfacing:

Element	Content (at.%)	Role in Alloy Design
Fe	41	Base element, provides ferromagnetic properties
Co	7	Expands glass-forming range, stabilizes amorphous phase
Cr	15	Promotes amorphous formation, enhances corrosion resistance
Mo	14	Increases viscosity of melt, suppresses crystallization
C	15	Glass-former, contributes to hardness via amorphous carbon
B	6	Strong glass-former, lowers crystallization temperature
Y	2	Rare earth, refines microstructure, improves thermal stability

The inclusion of rare earth yttrium is notable. Rare earth elements are known to modify interfacial energy at grain boundaries and can influence the kinetics of crystallization. In this alloy system, the 2 at.% Y serves to refine the nanocrystalline phase and potentially enhance the thermal stability of the amorphous component.

Microstructural Evolution and Phase Analysis

The study prepared two different coatings on Q235 steel substrate by varying the welding current, resulting in different heat inputs:

Parameter	Sample A (150 A)	Sample B (160 A)
Welding current	150 A	160 A
Heat input (relative)	Lower	Higher
Amorphous phase content	47.44% (maximum)	Reduced
Nanocrystalline grain size	10–48 nm	Larger
Maximum hardness	1226 HV1	Lower
Wear resistance (vs. Q235)	8×	Lower than Sample A
Crystallization activation energy	107.476 kJ/mol	58.104 kJ/mol

The crystallization activation energy is a particularly important parameter. The significantly higher activation energy for Sample A (107.476 kJ/mol versus 58.104 kJ/mol for Sample B) indicates superior thermal stability. In service conditions where the coating may be subjected to thermal cycling—such as in pipeline components exposed to varying fluid temperatures—higher activation energy translates to greater resistance against devitrification and subsequent softening.

Phase Composition and Characterization

XRD, SEM, and TEM analyses confirmed that both coatings consist of an amorphous matrix with embedded nanocrystalline particles. The nanocrystalline phase is identified as bcc-Fe based on diffraction pattern analysis. The amorphous phase is characterized by broad halo peaks in XRD patterns, with no sharp diffraction reflections, confirming the absence of long-range atomic order.

The metallurgical bonding between the coating and Q235 substrate was confirmed to be excellent, with no interfacial defects such as porosity, lack of fusion, or cracking observed. This is attributed to the low-hydrogen electrode design, which minimizes hydrogen-induced porosity and provides adequate arc stability for uniform deposition.

Wear Mechanism Analysis

The 8× improvement in wear resistance relative to the base Q235 steel is attributed to the combined effect of high hardness (1226 HV1) and the unique amorphous/nanocrystalline microstructure. The amorphous phase provides uniform hardness distribution without the grain boundary softening typical of polycrystalline materials, while the nanocrystalline particles contribute to work hardening during wear. The wear mechanism is primarily micro-ploughing by hard asperities, with minimal material removal through adhesion or fatigue.

Engineering Implications for Pipeline Applications

For pipeline operators, particularly those managing pipelines in high-velocity flow conditions where erosion-corrosion is a concern, this technology offers a promising solution. The combination of high hardness, excellent metallurgical bonding, and good thermal stability makes these coatings suitable for application on pipe elbows, reducers, and tee fittings where impingement erosion is most severe. However, practical implementation requires consideration of the following factors:

The relatively low deposition rate of SMAW compared to automated processes such as PAW or SAW means that large surface areas may require significant welding time.
The maximum hardness of 1226 HV1 is achieved under specific parameter conditions; deviations in heat input can significantly reduce performance.
Long-term thermal stability in service must be verified through accelerated aging tests, as the crystallization activation energy, while promising, needs to be correlated with actual service temperatures.

Study Insights

This research demonstrates that the amorphous/nanocrystalline approach to hardfacing can achieve exceptional wear resistance through microstructural engineering rather than simply increasing hardness. The key insight is that the amorphous phase fraction must be maximized while maintaining adequate nanocrystalline content for toughness. The delicate balance between these two phases is controlled by heat input, which in turn is determined by welding parameters. This finding has broader implications for the design of other amorphous-based surface engineering systems, where the window for achieving optimal microstructure is narrow but the performance rewards are substantial.