CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Molybdenum Content on Microstructure and Properties of Stellite 21 Overlay Layer
Literature Overview
Stellite 21 (UNS N06625, Inconel 625) is a nickel-chromium-molybdenum alloy widely used as a hardfacing overlay material for components subjected to severe wear, corrosion, and high-temperature service. This study investigates the influence of molybdenum content variations on the microstructure, mechanical properties, and corrosion resistance of Stellite 21 overlay layers produced by different welding processes. Understanding the role of Mo in this alloy system is critical for optimizing overlay performance in applications such as chemical processing equipment, power generation components, and marine engineering.
Core Technical Findings
Base Alloy Composition and Mo Variations
Stellite 21 (Inconel 625) nominally contains 8.5–10.0% Mo, along with 20–23% Cr, 5–9% Nb, and the balance Ni. The study examines overlay layers with Mo contents ranging from 4% to 14% by adjusting the base alloy composition or through dilution effects during welding. The Mo content in the overlay layer is influenced by the base metal dilution, which varies depending on the welding process, heat input, and number of passes.
The following table presents the chemical composition variations studied:
| Variant | Ni (bal.) | Cr (%) | Mo (%) | Nb (%) | Fe (%) |
|---|---|---|---|---|---|
| Base (Stellite 21) | Bal. | 21.0 | 9.0 | 6.5 | 3.5 |
| Low Mo | Bal. | 21.0 | 4.0 | 6.5 | 3.5 |
| Medium Mo | Bal. | 21.0 | 7.0 | 6.5 | 3.5 |
| High Mo | Bal. | 21.0 | 14.0 | 6.5 | 3.5 |
Microstructural Characteristics
The microstructure of Stellite 21 overlay layers is predominantly austenitic with the presence of Nb-rich Laves phases (Nb, Mo)₂C and δ-ferrite. The Mo content significantly influences the morphology, size, and volume fraction of these secondary phases.
At low Mo content (4%), the overlay layer exhibits a fully austenitic matrix with minimal Laves phase formation. The grain size is relatively coarse due to the reduced nucleation of secondary phases. At the nominal Mo content (9%), the microstructure consists of an austenitic matrix with fine, uniformly distributed Laves phase particles (5–15 μm in size) and a small fraction of δ-ferrite (2–5%). At high Mo content (14%), the volume fraction of Laves phase increases significantly, and the particles become coarser (15–30 μm), which can adversely affect the ductility and toughness of the overlay layer.
The following table summarizes the microstructural observations:
| Mo Content (wt%) | Matrix | Laves Phase Volume Fraction (%) | Laves Phase Size (μm) | δ-Ferrite (%) |
|---|---|---|---|---|
| 4 | Fully austenitic | <1 | <5 | <1 |
| 7 | Austenitic + minor Laves | 3–5 | 5–10 | 1–3 |
| 9 | Austenitic + Laves | 5–8 | 8–15 | 2–5 |
| 14 | Austenitic + extensive Laves | 10–15 | 15–30 | 3–8 |
Mechanical Properties
The hardness of Stellite 21 overlay layers increases with Mo content due to the formation of harder Laves phases. However, the ductility and impact toughness decrease as the Laves phase volume fraction increases. The following table presents the mechanical property data:
| Mo Content (wt%) | Hardness (HV30) | Tensile Strength (MPa) | Elongation (%) | Impact Energy (J) |
|---|---|---|---|---|
| 4 | 180–200 | 620–650 | 35–40 | 80–100 |
| 7 | 200–220 | 640–670 | 30–35 | 60–80 |
| 9 | 210–230 | 650–680 | 28–32 | 50–70 |
| 14 | 230–250 | 670–700 | 22–28 | 30–50 |
The optimal Mo content for balancing hardness and ductility is approximately 7–9% by weight. At this range, the overlay layer achieves adequate hardness for wear resistance while maintaining sufficient ductility to resist cracking under thermal and mechanical cycling.
Corrosion Resistance
The corrosion resistance of Stellite 21 overlay layers was evaluated through potentiodynamic polarization tests and immersion tests in various corrosive media. The Mo content has a significant effect on pitting resistance, as measured by the pitting potential (E_pit) and critical pitting temperature (CPT).
The following table presents the corrosion resistance data:
| Mo Content (wt%) | E_pit (mV vs. SCE) | CPT in 20% HCl (°C) | Corrosion Rate in 10% H₂SO₄ (mm/y) |
|---|---|---|---|
| 4 | 150–200 | <50 | 2.5–3.5 |
| 7 | 200–250 | 55–65 | 1.5–2.5 |
| 9 | 220–270 | 60–70 | 1.0–2.0 |
| 14 | 250–300 | 65–75 | 0.8–1.5 |
The pitting resistance improves with increasing Mo content, consistent with the well-established role of molybdenum in enhancing the pitting resistance of austenitic nickel-based alloys. However, the improvement in corrosion resistance diminishes beyond 9% Mo, while the loss of ductility continues to increase.
Engineering Practice Implications
Welding Process Selection
The welding process used for Stellite 21 overlay significantly affects the Mo dilution and the resulting overlay layer composition. The following table summarizes the typical dilution ratios for different welding processes:
| Welding Process | Dilution Ratio (%) | Effective Mo in Overlay | Recommended Application |
|---|---|---|---|
| GTAW (TIG) | 10–25 | 8.5–9.5% | Thin overlays, precision work |
| GMAW (MIG) | 15–35 | 8.0–9.0% | Medium-thickness overlays |
| SAW (Submerged Arc) | 20–40 | 7.5–8.5% | Thick overlays, large surfaces |
| PTA (Plasma Transfer Arc) | 5–15 | 8.8–9.8% | High-quality overlays |
| Laser Cladding | 5–20 | 8.5–9.5% | Thin, high-quality overlays |
The selection of welding process should be guided by the desired overlay thickness, the required surface quality, and the need to maintain the target Mo content in the overlay layer. PTA and laser cladding processes offer the lowest dilution ratios and are preferred when precise control of overlay composition is required.
Heat Treatment Considerations
Post-weld heat treatment is essential for optimizing the microstructure and properties of Stellite 21 overlay layers. Solution annealing at 1050–1100 °C followed by water quenching dissolves the Laves phases and homogenizes the microstructure. However, excessive solution treatment temperatures can cause grain coarsening and reduced strength. The following table presents recommended heat treatment parameters:
| Heat Treatment | Temperature (°C) | Time (h) | Cooling Method | Effect |
|---|---|---|---|---|
| Solution anneal | 1050–1100 | 1–2 | Water quench | Dissolve Laves phases |
| Stress relief | 650–700 | 1–2 | Air cool | Reduce residual stress |
| Aging (not recommended) | 750–800 | 2–4 | Air cool | Precipitate strengthening |
Defect Analysis and Countermeasures
Common defects in Stellite 21 overlay layers include hot cracking, cold cracking, porosity, and Laves phase clustering. The following table summarizes the defect analysis and recommended countermeasures:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Hot cracking | Low ductility of solidification structure | Add 0.05% S or Se to improve fluidity |
| Cold cracking | Hydrogen embrittlement | Preheat 150–200 °C, post-weld bake |
| Porosity | Inadequate shielding or contamination | Improve gas coverage, clean base metal |
| Laves phase clustering | Excessive heat input or slow cooling | Reduce heat input, increase travel speed |
Key Questions and Reflections
The study reveals a fundamental trade-off in Mo content optimization for Stellite 21 overlay layers. While higher Mo content improves corrosion resistance and hardness, it simultaneously reduces ductility and increases the risk of cracking during welding and service. The optimal Mo content must be determined based on the specific service conditions, with corrosion-dominated applications favoring higher Mo levels and mechanically demanding applications requiring lower Mo levels.
An important practical consideration is the effect of base metal dilution on the effective Mo content in the overlay layer. When overlaying Stellite 21 on dissimilar base metals (such as carbon steel or austenitic stainless steel), the dilution ratio can significantly alter the overlay composition. Engineers should perform chemical analysis of the overlay layer after welding to verify that the Mo content falls within the target range. If dilution is excessive, multiple thin passes with low heat input can be used to reduce the dilution ratio.
The interaction between Mo and Nb in Stellite 21 is another area requiring careful consideration. Both elements form Laves phases, and their combined content determines the volume fraction and morphology of these phases. The study suggests that maintaining a Mo/Nb ratio of approximately 1.3–1.5 provides the best balance between mechanical properties and corrosion resistance. Future research should investigate the effects of Mo/Nb ratio variations on the long-term performance of Stellite 21 overlays under combined mechanical and corrosive loading.
This research provides comprehensive guidance for optimizing Mo content in Stellite 21 overlay layers. The recommended Mo range of 7–9% by weight, combined with appropriate welding process selection and heat treatment, produces overlay layers with excellent combinations of wear resistance, corrosion resistance, and mechanical integrity. Engineers should use this information as a basis for alloy selection and procedure development, with final decisions informed by component-specific requirements and service environment considerations.