Microstructure and Properties of Nitrogen-Carbon Alloyed Self-Shielded Hard-Facing Flux-Cored Wire Weld Overlay Layer

Literature Overview and Research Background

Hard-facing flux-cored wires are essential consumables for the repair and protection of equipment surfaces subjected to severe wear and corrosion. The incorporation of nitrogen and carbon into the weld metal promotes the formation of hard carbides and nitrides, significantly enhancing the hardness and wear resistance of the overlay layer. Self-shielded flux-cored wires offer the advantage of not requiring external shielding gas, making them suitable for field repair and outdoor applications. This study examines the microstructure and mechanical properties of a nitrogen-carbon alloyed self-shielded hard-facing flux-cored wire overlay layer, providing guidance for its application in wear-critical components.

Core Technical Approach and Methodology

The overlay layers are deposited on a 45 steel substrate using a self-shielded flux-cored wire with a composition optimized for nitrogen and carbon content. The welding is performed using gas metal arc welding (GMAW) equipment with a current range of 180–280 A. The deposited samples are analyzed using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness testing, and wear testing. The effect of welding current on the dilution ratio, microstructure, and properties is systematically evaluated.

Wire Composition and Welding Parameters

Interpretation of Key Findings

The microstructure of the overlay layer consists of a mixture of tempered martensite, retained austenite, and hard carbide and nitride precipitates. The primary carbides include Cr7C3, Mo2C, and M7C3, while the nitrides are primarily CrN and Cr2N. The hardness of the overlay layer ranges from 58–65 HRC, with the highest hardness achieved at medium current settings where the dilution ratio is moderate and the cooling rate is sufficient to promote fine carbide precipitation.

At low current, the dilution ratio is low, resulting in a high carbon and nitrogen content in the weld metal. This promotes the formation of a high volume fraction of hard carbides and nitrides, but the narrower weld bead may lead to incomplete coverage. At high current, the increased dilution reduces the effective carbon and nitrogen content, leading to coarser carbide morphology and lower hardness. The retained austenite content increases with increasing current due to the higher dilution and slower cooling rate, which provides some toughness but reduces the overall hardness.

Microstructure and Property Summary

Engineering Practice Integration

The study provides practical guidance for the selection of welding parameters when using nitrogen-carbon alloyed self-shielded flux-cored wires for hard-facing applications. For maximum hardness and wear resistance, medium current settings are recommended, as they provide an optimal balance between dilution control and carbide precipitation. The self-shielded nature of the wire makes it particularly suitable for field repair of equipment such as excavator buckets, crane hooks, and grinding mill liners, where external shielding gas is impractical.

From a quality assurance perspective, the study emphasizes the importance of controlling the welding current to ensure consistent dilution and microstructure. The dilution ratio can be monitored by analyzing the carbon and alloying element content of the weld metal using optical emission spectroscopy (OES) or X-ray fluorescence (XRF). Routine hardness testing of the overlay layer is also recommended to verify that the welding parameters are producing the desired properties.

Key Questions and Reflections

A notable question is the long-term stability of the retained austenite phase in the overlay layer under service conditions. The retained austenite can undergo strain-induced transformation to martensite during wear or impact loading, which may lead to increased hardness but also potential cracking due to volume expansion. This transformation behavior should be considered in the design of components subjected to high impact or cyclic loading.

Another consideration is the effect of the self-shielded flux composition on the weld metal properties. The flux in a self-shielded wire serves multiple functions: deoxidation, alloying, slag formation, and arc stability. The flux composition directly affects the weld metal chemistry and, consequently, the microstructure and properties. The study does not explicitly vary the flux composition, and a separate study on flux optimization would be valuable for further enhancing the overlay layer performance.

Study Insights and Implications

This literature demonstrates that nitrogen-carbon alloyed self-shielded flux-cored wires can produce hard-facing overlay layers with excellent wear resistance and hardness, suitable for demanding industrial applications. The key insight is that the welding current must be carefully controlled to optimize the dilution ratio and carbide precipitation, with medium current settings providing the best balance of hardness, toughness, and coverage.

The study also highlights the advantages of self-shielded flux-cored wires for field applications where external shielding gas is not available. The ability to produce high-hardness overlay layers without external gas makes these wires ideal for repair and maintenance of heavy equipment in remote locations. For engineers specifying hard-facing consumables, the study provides a solid basis for selecting wire compositions and welding parameters to achieve the desired overlay layer properties.