Microstructure and Properties of Overlay Layer Using Ultra-Low Carbon Nitrogen-Strengthened Flux-Cored Wire

Literature Overview

This paper by Zheng Lijuan, Fu Yuming, Yu Guanying, and Zhang Jinsen from Yanshan University, published in the Journal of Plasticity Engineering in 2014 (Vol. 21, No. 1, pp. 131-134), presents the development and characterization of an ultra-low carbon nitrogen-strengthened flux-cored wire for overlay welding. The research was supported by the National Natural Science Foundation of China (Grant No. 51105325) and the Hebei Provincial Key Science and Technology Research Program (Grant No. ZH2012022). The study demonstrates the successful application of open-arc oscillating overlay welding technology to create a martensitic stainless steel overlay layer on 42CrMo substrate, achieving significant hardness improvement with excellent metallurgical integrity.

Core Technical Approach

The research addresses a critical challenge in overlay welding: the traditional trade-off between hardness and toughness. Conventional hardfacing materials achieve high hardness through carbide formation but often suffer from inadequate toughness, leading to cracking and spalling under impact loading. The nitrogen-strengthened approach offers an alternative strengthening mechanism that can enhance hardness without the brittleness associated with carbide-rich microstructures.

The flux-cored wire composition was specifically designed to achieve:

• Ultra-low carbon content to minimize carbide formation and reduce cracking susceptibility
• Controlled nitrogen addition to form fine, uniformly distributed nitrides that provide solid solution strengthening
• Martensitic stainless steel composition for excellent corrosion resistance and strength
• Self-shielding flux to ensure stable arc characteristics and consistent weld bead quality

Microstructure and Property Analysis

The metallurgical bond between the overlay layer and the 42CrMo substrate was confirmed to be complete, with no interfacial defects such as lack of fusion or microcracks. The single martensitic microstructure indicates that the cooling rate achieved during welding was sufficient to suppress the formation of other phases such as ferrite or austenite, while the ultra-low carbon content prevented the formation of carbides that would compromise toughness.

The hardness gradient from substrate to surface is a particularly important finding. This gradient indicates progressive dilution of the base metal composition with increasing distance from the substrate interface. The average hardness of 2.5 times the base metal represents a substantial improvement in wear resistance while maintaining the toughness advantages of the martensitic stainless steel system.

Process Technology and Welding Parameters

The open-arc oscillating overlay welding technique employed in this study offers several advantages over conventional welding methods:

• Oscillation control: The lateral oscillation of the welding arc ensures uniform heat distribution and consistent weld bead geometry, reducing the risk of localized overheating and cracking.
• Open-arc configuration: This configuration allows for better visual monitoring of the weld pool and easier adjustment of process parameters during production.
• Flux-cored wire advantages: The flux provides self-shielding, deoxidation, and alloying effects, resulting in cleaner welds with fewer inclusions and better mechanical properties.

Optimal welding parameters were determined through systematic trial welding, considering factors such as current density, travel speed, oscillation frequency and amplitude, and layer thickness. The key parameter windows for achieving defect-free overlay layers included:

• Current density: Moderate to avoid excessive dilution while ensuring adequate penetration
• Travel speed: Controlled to maintain optimal cooling rate for martensitic transformation
• Oscillation parameters: Tuned to ensure uniform heat input and consistent bead width
• Layer thickness: Optimized to balance hardness improvement with residual stress management

Engineering Practice Integration

For applications in the piping and fitting industry, this technology offers significant advantages for components requiring both wear resistance and corrosion resistance. The martensitic stainless steel overlay layer provides:

• Excellent resistance to oxidative and aqueous corrosion
• High hardness for abrasion resistance
• Adequate toughness to resist impact damage
• Metallurgical compatibility with common carbon and alloy steel substrates

Potential applications include valve seat overlays, pump impeller hardfacing, and erosion-resistant linings for pipeline components in corrosive-abrasive environments. The uniform nitride distribution and absence of cracks make this material system particularly suitable for cyclic loading conditions where fatigue resistance is critical.

Summary and Reflections

This research demonstrates the effectiveness of nitrogen strengthening as an alternative to carbide-based hardening in overlay welding applications. The development of an ultra-low carbon flux-cored wire with controlled nitrogen content achieves a favorable balance between hardness and toughness, addressing a long-standing limitation in traditional hardfacing materials. The successful application of open-arc oscillating welding technology ensures process consistency and defect-free results. For engineers working in piping and fitting manufacturing, this technology represents a viable option for enhancing component durability in corrosive-abrasive service conditions. The key innovation lies in leveraging nitrogen as a strengthening element without the brittleness penalties associated with carbide-rich microstructures.