CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Molybdenum and Nickel Content on Martensitic Stainless Steel Hardfacing Flux-Cored Wire Cladding Layer
Literature Overview
This study investigates the influence of molybdenum (Mo) and nickel (Ni) content on the microstructure and mechanical properties of hardfacing cladding layers produced using martensitic stainless steel flux-cored wire (FCAW). Martensitic stainless steel hardfacing alloys are widely used in applications requiring high wear resistance combined with moderate corrosion resistance, such as mining equipment, cement mill liners, pump impellers, and valve seats. The addition of Mo and Ni to the base martensitic composition (typically Cr12–Cr18 type) is a well-established strategy for enhancing toughness and corrosion resistance, but the optimal balance between these elements and their effect on hardfacing performance is not always straightforward.
Alloy Design and Experimental Matrix
The study examined a series of flux-cored wires with varying Mo and Ni contents while maintaining a relatively constant chromium content of approximately 12–14 wt%. The experimental matrix is summarized below:
| Wire Designation | Cr (%) | Mo (%) | Ni (%) | C (%) | Base Composition |
|---|---|---|---|---|---|
| A | 12.5 | 0 | 0 | 0.35 | Cr12 martensitic |
| B | 12.5 | 1.0 | 0 | 0.35 | Cr12 + Mo |
| C | 12.5 | 2.0 | 0 | 0.35 | Cr12 + Mo |
| D | 12.5 | 0 | 2.0 | 0.35 | Cr12 + Ni |
| E | 12.5 | 0 | 4.0 | 0.35 | Cr12 + Ni |
| F | 12.5 | 1.5 | 2.0 | 0.35 | Cr12 + Mo + Ni |
| G | 12.5 | 2.0 | 3.0 | 0.35 | Cr12 + Mo + Ni |
The cladding layers were deposited on low-carbon steel substrates using FCAW with a flux-cored wire diameter of 1.2 mm, under a shielding atmosphere of CO2 or Ar + 2% CO2. The welding parameters were optimized to achieve a consistent heat input range of 0.8–1.5 kJ/mm.
Microstructural Evolution
The microstructural response to Mo and Ni addition was analyzed using optical microscopy, SEM, and X-ray diffraction (XRD). The following trends were observed:
- In the base Cr12 composition (Wire A), the cladding layer consists primarily of martensite with some retained austenite and M7C3 carbides. The hardness is high but the toughness is limited.
- The addition of Mo (Wires B and C) promotes the formation of M2C and Mo-rich carbides, which increase the hardness slightly but also refine the martensitic microstructure. Mo acts as a strong ferrite stabilizer and increases the hardenability of the alloy.
- The addition of Ni (Wires D and E) increases the retained austenite fraction and suppresses martensitic transformation. Higher Ni content leads to a softer, more ductile microstructure with increased toughness but reduced hardness.
- The combined addition of Mo and Ni (Wires F and G) achieves a balanced microstructure with moderate hardness and significantly improved toughness compared to the base Cr12 composition.
| Wire | Hardness (HV) | Retained Austenite (%) | Impact Energy (J) | Wear Rate (mg/1000 rev) |
|---|---|---|---|---|
| A | 520 | 5 | 8 | 45 |
| B | 540 | 3 | 12 | 40 |
| C | 550 | 2 | 10 | 38 |
| D | 480 | 18 | 22 | 52 |
| E | 440 | 28 | 30 | 58 |
| F | 510 | 12 | 18 | 42 |
| G | 500 | 15 | 22 | 39 |
Mechanical Property Analysis
The hardness results show that Mo addition increases hardness by 20–30 HV, while Ni addition decreases hardness by 40–80 HV. This is consistent with the known metallurgical behavior of these elements: Mo promotes carbide formation and increases solid solution strengthening, while Ni stabilizes the austenite phase and reduces martensite volume fraction.
The impact energy results reveal a more significant difference. The base Cr12 composition exhibits very low impact energy (8 J), indicating brittle behavior. Mo addition provides only marginal improvement in toughness (10–12 J), while Ni addition substantially increases impact energy (22–30 J). The combined Mo + Ni composition achieves a favorable balance with impact energies of 18–22 J, representing a 125–175% improvement over the base composition.
Wear resistance testing, conducted using a pin-on-disc tribometer under dry sliding conditions, shows that the Mo + Ni composition (Wire G) achieves the best wear resistance (39 mg/1000 rev) among all compositions tested, despite having a lower hardness than the Mo-only compositions. This counterintuitive result is explained by the improved toughness of the Mo + Ni alloy, which reduces the propensity for microcracking and material removal during sliding wear.
Corrosion Resistance Evaluation
Potentiodynamic polarization testing in 3.5% NaCl solution revealed that the corrosion resistance of the cladding layer is primarily governed by the chromium content and the retained austenite fraction. The Ni addition improves corrosion resistance by increasing the retained austenite, which is more corrosion-resistant than martensite due to its face-centered cubic structure. The Mo addition provides additional corrosion resistance through the formation of Mo-rich passive films.
| Wire | Corrosion Potential (mV vs. SCE) | Corrosion Current Density (μA/cm²) |
|---|---|---|
| A | -320 | 18.5 |
| B | -305 | 15.2 |
| C | -298 | 14.0 |
| D | -285 | 11.8 |
| E | -270 | 9.5 |
| F | -275 | 10.2 |
| G | -265 | 8.8 |
Engineering Practice Implications
The selection of Mo and Ni content in martensitic hardfacing alloys should be guided by the specific service conditions. For applications dominated by abrasive wear (such as mining equipment and cement mill liners), Mo-rich compositions with 1.5–2.0% Mo provide excellent wear resistance with acceptable toughness. For applications involving impact loading or thermal cycling (such as valve seats and pump impellers), Ni-rich compositions with 2.0–4.0% Ni offer superior toughness and thermal fatigue resistance. For combined wear and corrosion service, the balanced Mo + Ni composition (1.5% Mo, 2.0% Ni) provides the best overall performance.
From a welding process perspective, the flux-cored wire cladding process offers several advantages over solid wire processes, including higher deposition rates, lower dilution ratios, and reduced spatter. The flux coating provides additional alloying elements and acts as a slag former, which helps to refine the grain structure and remove impurities from the weld pool.
Key Reflections
This study demonstrates that the optimization of hardfacing alloy composition is a multi-objective problem that cannot be solved by maximizing any single property. The traditional approach of maximizing hardness through carbide-forming element addition often results in unacceptable brittleness and poor wear resistance under impact or sliding conditions. The balanced Mo + Ni composition identified in this study represents a more rational design approach that considers the synergistic interaction between hardness, toughness, and corrosion resistance.
The retained austenite fraction emerges as a critical microstructural parameter that governs the overall performance of martensitic hardfacing alloys. Engineers should monitor the retained austenite content during procedure qualification and consider post-weld heat treatment strategies to optimize this fraction for the specific application.
This study provides valuable guidance for engineers developing custom hardfacing alloys for specialized applications. The systematic approach to alloy design, combined with comprehensive property evaluation, offers a template for future research and development in the hardfacing field.