CLADDING TECHNOLOGY SHANXI CO., LTDEffects of Molybdenum and Nickel Content on Martensitic Stainless Steel Hardfacing Flux-Cored Wire Overlay
Introduction
Martensitic stainless steels are widely used in hardfacing applications where a combination of wear resistance, corrosion resistance, and moderate toughness is required. Flux-cored arc welding (FCAW) is a preferred method for depositing these overlay layers due to its high deposition rate and deep penetration. The chemical composition of the overlay material, particularly the content of molybdenum (Mo) and nickel (Ni), plays a decisive role in determining the microstructure, hardness, wear resistance, and corrosion performance of the deposited layer. This study note examines the systematic effects of Mo and Ni alloying on martensitic stainless steel hardfacing deposits produced by FCAW.
Metallurgical Background
Martensitic stainless steels derive their hardenability from the transformation of austenite to martensite upon cooling from the austenitizing temperature. The stability of the austenite phase and the characteristics of the martensitic transformation are strongly influenced by alloying elements. Molybdenum and nickel serve distinct but complementary roles in the alloy design.
| Element | Role in Martensitic Stainless Steel | Typical Range in Hardfacing Wire |
|---|---|---|
| Molybdenum (Mo) | Enhances hardenability, increases corrosion resistance, promotes secondary carbide precipitation | 2.0 – 6.0 wt% |
| Nickel (Ni) | Stabilizes austenite, reduces martensite start temperature (Ms), improves ductility and toughness | 1.0 – 4.0 wt% |
| Carbon (C) | Primary hardening element, promotes martensite formation | 0.3 – 0.8 wt% |
| Chromium (Cr) | Provides corrosion resistance, promotes carbide formation | 12.0 – 18.0 wt% |
Effect of Molybdenum Content
Molybdenum is a potent austenite stabilizer and hardenability enhancer in martensitic stainless steels. Its effects on the overlay microstructure and properties are multifaceted:
- Hardness enhancement: Increasing Mo content from 2.0 to 5.0 wt% typically increases the hardness of the as-deposited overlay from approximately 45 HRC to 55 HRC. This is attributed to the solid solution strengthening effect of Mo in the martensitic matrix and the formation of Mo-rich carbides (Mo₂C, Mo₄C₃) during cooling.
- Carbide morphology: Mo promotes the formation of secondary carbides, which precipitate during tempering or during slow cooling from the welding thermal cycle. These carbides contribute significantly to wear resistance. However, excessive Mo can lead to the formation of coarse, network-type carbides at grain boundaries, which may reduce toughness.
- Corrosion resistance: Mo improves the pitting and crevice corrosion resistance of the overlay layer by enhancing the stability of the passive film. This is particularly important in chloride-containing environments. The critical pitting temperature (CPT) increases with Mo content, indicating improved resistance to localized corrosion.
- Cracking susceptibility: Higher Mo content increases the hardenability of the overlay, which in turn increases the residual stress and the risk of cracking. The ductile-to-brittle transition temperature (DBTT) rises with increasing Mo, making the overlay more susceptible to low-temperature cracking.
Effect of Nickel Content
Nickel has a contrasting effect to molybdenum in many respects:
- Austenite stabilization: Ni is a strong austenite stabilizer that lowers the martensite start temperature (Ms). Increasing Ni content from 1.0 to 4.0 wt% can shift the Ms temperature from approximately 300°C to below 0°C, resulting in a higher fraction of retained austenite in the as-deposited overlay.
- Hardness reduction: The presence of retained austenite reduces the overall hardness of the overlay. A Ni content of 4.0 wt% can reduce the hardness by approximately 5–8 HRC compared to a Ni-free composition, due to the soft, face-centered cubic (FCC) nature of austenite.
- Toughness improvement: Retained austenite acts as a crack-arresting phase, improving the fracture toughness and fatigue resistance of the overlay. The combination of martensite and retained austenite provides a more ductile microstructure that can accommodate plastic deformation without catastrophic failure.
- Corrosion resistance: Ni enhances the general corrosion resistance of the overlay by improving the stability of the passive film. However, the effect of Ni on pitting resistance is less pronounced than that of Mo.
Microstructure Characterization
The microstructure of martensitic stainless steel hardfacing deposits is characterized by the following phases:
| Microstructural Feature | Description | Influence of Mo and Ni |
|---|---|---|
| Lath martensite | Primary phase, formed by diffusionless transformation | Mo increases lath spacing; Ni reduces lath volume fraction |
| Retained austenite | FCC phase, trapped during rapid cooling | Ni increases retained austenite fraction significantly |
| M₂₃C₆ carbides | Chromium-rich carbides, form at grain boundaries | Mo reduces M₂₃C₆ volume fraction |
| Mo₂C / Mo₄C₃ carbides | Molybdenum-rich carbides, hard and wear-resistant | Increase with Mo content |
| Cr₇C₃ carbides | Chromium-rich carbides, moderate hardness | Less affected by Mo and Ni |
Mechanical and Wear Performance
The mechanical properties of the overlay layer are directly linked to the Mo and Ni content. The following table summarizes typical performance data for different alloy compositions:
| Composition (wt%) | Hardness (HRC) | Wear Rate (mm³/N·m) | Impact Energy (J) | Pitting Resistance (CPT, °C) |
|---|---|---|---|---|
| C 0.5, Cr 13, Mo 2.0, Ni 1.0 | 48 | 0.85 | 45 | 42 |
| C 0.5, Cr 13, Mo 4.0, Ni 1.0 | 53 | 0.52 | 28 | 58 |
| C 0.5, Cr 13, Mo 2.0, Ni 3.0 | 44 | 1.10 | 62 | 44 |
| C 0.5, Cr 13, Mo 4.0, Ni 3.0 | 49 | 0.65 | 48 | 60 |
| C 0.5, Cr 13, Mo 6.0, Ni 2.0 | 56 | 0.40 | 18 | 62 |
The data clearly demonstrate the trade-off between hardness/wear resistance and toughness. High Mo content provides superior hardness and wear resistance but at the expense of toughness and increased cracking susceptibility. High Ni content improves toughness and reduces cracking risk but lowers hardness and wear resistance. The optimal composition depends on the specific service conditions.
Engineering Practice Implications
In selecting the appropriate flux-cored wire composition for a given hardfacing application, the following considerations should be applied:
- For severe abrasive wear environments with moderate corrosion exposure, a high-Mo composition (4.0–6.0 wt% Mo) with low Ni (1.0–2.0 wt%) is preferred. Preheating to 200–300°C and interpass temperature control are essential to prevent cracking.
- For applications requiring good toughness and resistance to impact loading, a balanced composition with moderate Mo (2.0–3.0 wt%) and higher Ni (3.0–4.0 wt%) is recommended. Post-weld tempering at 500–550°C can further improve toughness.
- For highly corrosive environments with moderate wear, a high-Mo, moderate-Ni composition provides the best balance. The Mo content should be at least 4.0 wt% to ensure adequate pitting resistance.
The interaction between Mo and Ni is synergistic in terms of corrosion resistance, as both elements contribute to passive film stability. However, they are antagonistic in terms of hardness, as Mo increases hardness while Ni decreases it through retained austenite formation. Engineers must carefully balance these competing effects based on the specific service requirements.
A critical practical consideration is the welding procedure qualification. FCAW of martensitic stainless steel hardfacing wires requires careful control of heat input, interpass temperature, and preheat. Excessive heat input can lead to grain coarsening and reduced hardness, while insufficient heat input can result in incomplete melting of previous passes and poor metallurgical bonding. The recommended heat input range is 1.5–3.5 kJ/mm, with an interpass temperature of 100–200°C for most compositions.