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

Literature Overview

The 2013 publication by Lü Yan, Yu Shengfu, Xing Shule, Liu Yulong, and Wang Lehu from the School of Materials Science and Engineering at Huazhong University of Science and Technology provides a systematic metallurgical investigation of martensitic stainless steel hardfacing flux-cored wires. This study is particularly relevant for engineers designing hardfacing consumables for applications requiring a combination of wear resistance, corrosion resistance, and toughness, such as mining equipment, cement mill liners, and chemical processing components. The focus on Mo and Ni as alloying variables addresses two of the most critical elements for controlling the microstructure and mechanical properties of martensitic overlay layers.

Metallurgical Fundamentals

Martensitic stainless steels in the hardfacing context typically belong to the Cr13-Mo-Ni family, with compositions ranging from approximately 12–14% Cr, 0.4–0.8% C, and variable Mo and Ni contents. The microstructure of these overlay layers is primarily martensitic, with the potential for retained austenite, carbide precipitation, and secondary phases depending on the cooling rate and alloy composition.

Role of Molybdenum

Molybdenum is a strong carbide former that promotes the formation of Mo2C and MoC carbides, which contribute to wear resistance through dispersion strengthening. The study examines Mo contents ranging from 0% to 5.0%, with the following observed effects:

The data clearly demonstrate that increasing Mo content raises hardness and corrosion resistance but significantly reduces impact toughness. The optimal Mo content for most engineering applications appears to be in the range of 1.5–3.0%, where a reasonable balance between hardness, toughness, and corrosion resistance is achieved.

Role of Nickel

Nickel is an austenite-stabilizing element that reduces the martensite start temperature (Ms) and increases the amount of retained austenite in the as-welded microstructure. The study examines Ni contents from 0% to 8.0%, with the following key observations:

Nickel addition significantly improves toughness and corrosion resistance at the expense of hardness. The retained austenite acts as a toughening phase that absorbs energy during impact loading and provides additional corrosion resistance through its austenitic character. However, excessive Ni content (>6%) leads to a substantial reduction in hardness, which is detrimental for wear-resistant applications.

Combined Effect of Mo and Ni

The study's most valuable contribution is the examination of the combined effect of Mo and Ni. The following matrix shows the hardness and impact toughness for selected Mo-Ni combinations:

The optimal composition for most general-purpose hardfacing applications appears to be approximately 2.0% Mo and 3.0% Ni, which provides a hardness of approximately 530 HV (equivalent to about 52 HRC) with an impact toughness of about 75 J. This combination offers a good balance between wear resistance, toughness, and corrosion resistance.

Welding Process Considerations

The study also addresses the welding process parameters for flux-cored wire hardfacing, noting that the dilution rate and cooling rate significantly affect the final microstructure and properties. For flux-cored wire applications, the typical process parameters include:

• Shielding gas: CO2 or Ar/CO2 mixture (80/20)
• Current: 200–350 A depending on wire diameter (1.2–1.6 mm)
• Voltage: 28–35 V
• Travel speed: 100–250 mm/min
• Wire feed speed: 3–6 m/min
• Dilution rate: 5–15% for single-pass, 2–5% for multi-pass

The cooling rate after welding is critical for martensitic hardfacing alloys. A slow cooling rate promotes the formation of carbides and reduces retained austenite, increasing hardness but decreasing toughness. A fast cooling rate preserves more retained austenite, improving toughness but potentially reducing hardness. In practice, the cooling rate is controlled by the substrate material, ambient temperature, and the use of thermal control plates or preheating.

Study Insights and Implications

This study provides a comprehensive metallurgical foundation for the design of martensitic stainless steel hardfacing flux-cored wires. The systematic examination of Mo and Ni effects, both individually and in combination, offers clear guidance for consumable designers and welding engineers. The key insight is that there is no single "optimal" composition; rather, the composition must be tailored to the specific service conditions, with hardness-critical applications favoring higher Mo and lower Ni, and toughness-critical applications favoring lower Mo and higher Ni. For engineers specifying hardfacing consumables, this literature underscores the importance of understanding the underlying metallurgy rather than relying solely on datasheet hardness values. The study also highlights the need for multi-pass overlay strategies to minimize dilution and achieve the target composition in the final overlay layer.