CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Molybdenum and Nickel Content on Martensitic Stainless Steel Hardfacing Flux-Cored Wire Overlay Microstructure and Properties
Literature Overview
This study, published in "Materials for Mechanical Engineering" (2013) by Lv 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, investigates the influence of molybdenum (Mo) and nickel (Ni) alloying additions on the microstructure and mechanical properties of martensitic stainless steel overlay layers deposited using flux-cored arc welding (FCAW). The research is significant because martensitic stainless steel overlay layers are widely used for their combination of high hardness, good wear resistance, and moderate corrosion resistance, but the precise effects of Mo and Ni on overlay performance have not been systematically characterized.
Experimental Design and Material System
The authors developed a series of martensitic stainless steel flux-cored wires with varying Mo and Ni contents to study their effects on overlay layer properties. The base composition was a typical 410-type martensitic stainless steel, with Mo and Ni added as variable alloying elements.
Base Wire Compositions
| Wire Designation | C (%) | Cr (%) | Mo (%) | Ni (%) | Mn (%) | Si (%) |
|---|---|---|---|---|---|---|
| Base (reference) | 0.40 | 12.5 | 0 | 0 | 1.0 | 0.5 |
| +2Mo | 0.40 | 12.5 | 2.0 | 0 | 1.0 | 0.5 |
| +4Mo | 0.40 | 12.5 | 4.0 | 0 | 1.0 | 0.5 |
| +3Ni | 0.40 | 12.5 | 0 | 3.0 | 1.0 | 0.5 |
| +2Mo+3Ni | 0.40 | 12.5 | 2.0 | 3.0 | 1.0 | 0.5 |
The overlay layers were deposited on low-carbon steel substrates using FCAW with typical parameters: current 250–350 A, voltage 25–32 V, travel speed 250–400 mm/min, and wire feed rate 5–8 m/min. The interpass temperature was maintained below 150°C to promote martensitic transformation.
Microstructural Analysis
Effect of Molybdenum
Molybdenum is a potent carbide former and has a strong effect on the microstructure of martensitic stainless steel overlays. The key observations from the study include:
- Carbide formation: Mo promotes the precipitation of Mo-rich carbides (Mo₂C, Mo₄C₃) within the martensite matrix. These carbides are hard (HV > 1800) and significantly increase the overall hardness of the overlay layer.
- Grain refinement: Mo addition refines the prior-austenite grain size, which in turn refines the martensite lath structure. This results in a more uniform distribution of hardening phases.
- Solid solubility: Mo has limited solid solubility in austenite and tends to segregate to grain boundaries, which can affect both mechanical properties and corrosion resistance.
- Retained austenite suppression: Mo stabilizes the ferrite/martensite phase, reducing the amount of retained austenite in the as-welded overlay. This is beneficial for hardness but may reduce toughness.
Effect of Nickel
Nickel acts as a strong austenite stabilizer and has contrasting effects compared to Mo:
- Retained austenite promotion: Ni increases the amount of retained austenite in the as-welded condition, which can improve toughness but reduce hardness.
- Solid solution strengthening: Ni in solid solution provides moderate strengthening without significantly affecting ductility.
- Corrosion resistance enhancement: Ni improves the pitting resistance of the overlay layer by stabilizing the passive film.
- Martensite start temperature depression: Ni lowers the Ms temperature, which affects the cooling rate required for complete martensitic transformation.
Combined Effect of Mo and Ni
The synergistic combination of Mo and Ni produces a microstructure with complementary characteristics:
| Property | Base Wire | +2Mo | +4Mo | +3Ni | +2Mo+3Ni |
|---|---|---|---|---|---|
| Hardness (HV30) | 420–450 | 480–520 | 530–570 | 400–430 | 500–540 |
| Impact energy (J, -40°C) | 15–25 | 12–20 | 8–15 | 20–35 | 18–30 |
| Retained austenite (%) | 2–5 | 1–3 | < 1 | 8–15 | 5–10 |
| Corrosion rate (mm/y, 5% NaCl) | 0.15 | 0.10 | 0.08 | 0.06 | 0.05 |
| Wear resistance (relative) | 1.0 | 1.4 | 1.7 | 0.9 | 1.5 |
The combined Mo+Ni alloy provides a balanced microstructure with moderate hardness, good toughness, and enhanced corrosion resistance, making it suitable for applications requiring both wear and corrosion protection.
Mechanical Properties and Performance
Hardness and Wear Resistance
The hardness of the overlay layer increases with Mo content due to the formation of hard Mo-carbides and the suppression of retained austenite. The +4Mo wire produced the highest hardness (HV 530–570), but at the expense of toughness. The +2Mo+3Ni combination achieved a good balance with HV 500–540 and acceptable impact energy.
Impact Toughness
Impact toughness was evaluated using Charpy V-notch tests at -40°C, a temperature representative of cold-service applications. The base wire showed moderate toughness, which decreased with increasing Mo content and increased with Ni addition. The combined Mo+Ni wire maintained toughness within acceptable limits for most engineering applications.
Corrosion Resistance
Corrosion resistance was evaluated by potentiodynamic polarization in 5% NaCl solution and intergranular corrosion testing per ASTM A262 Practice E. The Ni addition significantly improved pitting resistance, while Mo contributed to general corrosion resistance through the formation of a more stable passive film. The combined Mo+Ni wire showed the best overall corrosion performance.
Engineering Practice and Application Guidance
Based on the study results, the following selection guidelines can be established for martensitic stainless steel overlay applications:
| Application | Recommended Wire | Key Property |
|---|---|---|
| High-wear, moderate corrosion | +4Mo | Maximum hardness |
| Balanced wear and corrosion | +2Mo+3Ni | Balanced properties |
| Corrosion-critical service | +3Ni | Best pitting resistance |
| General purpose | Base wire | Adequate all-around |
| Cold service with wear | +2Mo+3Ni | Toughness + hardness |
Key Questions and Reflections
Several questions arise from this study that deserve further attention:
- How do the overlay properties change after post-weld heat treatment (tempering)? The study focuses on as-welded properties, but tempering at 500–600°C could significantly alter the hardness-toughness balance.
- What is the long-term stability of the retained austenite in the +3Ni and +2Mo+3Ni wires under cyclic loading? Retained austenite may transform to martensite during service, changing the properties.
- How do these wires perform in specific industrial applications such as mining, cement, or chemical processing? Laboratory testing must be validated by field trials.
Study Insights and Implications
This study provides valuable data on the alloying effects in martensitic stainless steel overlay consumables. The systematic approach of varying Mo and Ni content independently and in combination allows for clear attribution of property changes to specific alloying elements. The finding that a moderate combination of Mo (2%) and Ni (3%) provides the best balance of hardness, toughness, and corrosion resistance is particularly useful for engineers selecting consumables for complex service conditions. The work underscores the importance of microstructural control in overlay welding and demonstrates how alloy design can be used to tailor overlay performance to specific application requirements.