Effect of Molybdenum and Nickel Content on Microstructure and Properties of Martensitic Stainless Steel Hardfacing Flux-Cored Wire Overlay

Literature Overview

The study by Lv Yan and colleagues from Huazhong University of Science and Technology, published in Materials for Mechanical Engineering in 2013 (Vol. 37, No. 9, pp. 29-32), addresses a critical engineering problem in hardfacing technology: how to optimize alloy additions for martensitic stainless steel overlay deposits to achieve superior hardness and high-temperature wear resistance. The researchers developed a martensitic stainless steel hardfacing flux-cored wire with optimized molybdenum and nickel content, then applied submerged arc welding (SAW) overlay on Q235 carbon steel substrates. The systematic investigation of Mo and Ni content effects on microstructure, hardness, and high-temperature wear performance provides valuable guidance for engineers selecting hardfacing consumables for wear-critical applications.

Core Technical Findings

The researchers conducted a systematic study varying Mo and Ni mass fractions in the flux-cored wire composition. The key finding is that when both Mo and Ni content reach 2 wt%, the overlay deposit achieves the optimal combination of hardness (47.8 HRC) and high-temperature wear resistance (wear mass loss of 3.6 mg). The microstructure of all overlay deposits consists of three phases: martensite, retained austenite, and carbides. This three-phase microstructure is characteristic of martensitic stainless steel hardfacing deposits and is critical for understanding the mechanical behavior of these overlays.

Microstructural Analysis

The presence of retained austenite in the overlay microstructure is particularly significant. Retained austenite acts as a toughening phase that can transform to martensite under mechanical loading, providing strain-induced hardening. The carbide phase contributes to wear resistance through dispersion strengthening. The interaction between these three phases determines the overall performance of the overlay. The researchers' microstructural observations confirm that Mo and Ni additions promote carbide precipitation while simultaneously stabilizing some austenite in the microstructure, creating a beneficial phase balance.

Alloying Mechanism Interpretation

Role of Molybdenum

Molybdenum serves multiple functions in martensitic stainless steel hardfacing deposits. First, Mo forms stable M2C and MC-type carbides that provide excellent dispersion strengthening and contribute directly to wear resistance. Second, Mo increases the hardenability of the steel, promoting a more uniform martensitic transformation throughout the overlay deposit. Third, Mo raises the hardenability of the steel, promoting a more uniform martensitic transformation throughout the overlay deposit. Third, Mo increases the hardenability of the steel, promoting a more uniform martensitic transformation throughout the overlay deposit. Third, Mo improves the red hardness of the deposit, maintaining hardness at elevated temperatures where the application demands it.

Role of Nickel

Nickel plays a complementary role to molybdenum in the overlay system. Ni stabilizes the austenite phase, increasing the amount of retained austenite in the final microstructure. This retained austenite contributes to toughness and crack resistance, which is essential for hardfacing deposits that experience thermal cycling and mechanical impact. Additionally, Ni improves the weldability of the consumable by reducing the hardenability of the weld metal, thereby decreasing the susceptibility to cold cracking. The synergistic effect of Mo and Ni at 2 wt% each creates an optimal balance between hardness, toughness, and wear resistance.

Process and Quality Control Considerations

The submerged arc welding process used in this study requires careful control of several parameters to achieve the reported performance. The flux-cored wire design provides inherent shielding and deoxidation, which is advantageous for achieving consistent overlay properties. However, engineers must pay attention to the following quality control points:

The dilution between the overlay and the Q235 substrate is a critical factor that can significantly affect the final properties. Excessive dilution would reduce the alloy content in the effective overlay layer, diminishing the beneficial effects of Mo and Ni. Engineers should plan multi-pass overlay strategies to minimize dilution in subsequent passes, with the first pass accepting higher dilution and subsequent passes building up the full alloy composition.

Engineering Practice Integration

From an engineering application perspective, this research has direct relevance to several industrial scenarios. In the power generation industry, turbine components and pump impellers operating in abrasive slurries benefit from martensitic stainless steel overlays. In the mining and cement industries, crusher hammers, grinding media, and conveyor components face severe abrasive wear at elevated temperatures where the Mo-Ni optimized overlay would provide extended service life. The 47.8 HRC hardness level achieved in this study is well-suited for applications requiring both high wear resistance and moderate toughness, avoiding the brittleness associated with ultra-high hardness deposits.

When applying this technology in practice, engineers should consider the following implementation framework:

1. Pre-weld preparation: Ensure the substrate surface is clean, free of rust, scale, and oil contamination. Preheating at 150-250°C may be necessary depending on the substrate thickness and ambient conditions to prevent cold cracking at the overlay-substrate interface.
2. Process parameter selection: Optimize welding current, voltage, and travel speed to achieve adequate penetration without excessive dilution. The line energy should be controlled within a moderate range to promote complete melting of the flux-cored wire while limiting substrate melting.
3. Post-weld treatment: Stress relief annealing at 550-600°C for 2 hours can reduce residual stresses in the overlay and improve dimensional stability, particularly important for components with tight tolerance requirements.
4. Performance verification: Conduct hardness mapping, metallographic examination, and wear testing on production components to confirm that the overlay meets the design specifications before full-scale deployment.

Key Questions and Reflections

A notable question arising from this study is whether the optimal Mo and Ni content of 2 wt% each would remain applicable across different welding processes. The researchers used submerged arc welding, which has relatively high heat input and deep penetration. If the same wire composition were used in GTAW or FCAW with lower heat input, the resulting microstructure and properties might differ due to different cooling rates and solidification morphologies. Engineers should validate the wire composition for their specific welding process before committing to production use.

Another consideration is the long-term performance under thermal cycling conditions. While the study reports excellent high-temperature wear performance, repeated thermal cycling can cause fatigue cracking in hardfacing deposits, particularly at the overlay-substrate interface. The retained austenite in the microstructure may help accommodate thermal strain, but systematic thermal fatigue testing would provide more confidence in the long-term reliability of the overlay.

The study also raises the question of cost optimization. Molybdenum is a relatively expensive alloying element, and increasing its content beyond 2 wt% would raise consumable costs without proportional performance gains. The identified optimum at 2 wt% Mo and 2 wt% Ni represents a favorable cost-performance balance, which is highly relevant for large-scale industrial applications where consumable cost is a significant factor in the total cost of ownership.

Study Insights and Implications

This research demonstrates the power of systematic alloy optimization in hardfacing technology. The finding that a relatively modest addition of Mo and Ni (2 wt% each) can substantially improve both hardness and high-temperature wear resistance has broad implications for the design of hardfacing consumables. The three-phase microstructure of martensite, retained austenite, and carbides provides a robust framework for understanding and predicting overlay performance.

For fellow engineers, the key takeaway is that alloy optimization in hardfacing consumables should be approached as a multi-objective problem balancing hardness, toughness, wear resistance, and cost. The Mo-Ni combination studied here offers a practical solution for applications requiring high-temperature wear resistance with acceptable toughness. Future work should explore additional alloying elements such as chromium, titanium, and niobium to further extend the performance envelope of martensitic stainless steel hardfacing deposits.

The methodology employed in this study—systematic variation of alloying elements with comprehensive characterization of microstructure and properties—provides a replicable approach that engineers can apply to their own consumable development programs. This literature serves as a valuable reference point for hardfacing technology development in wear-critical industrial applications.