CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Mo-Cr-Fe-B Alloy Surfacing Overlay
Literature Overview
Molybdenum-chromium-iron-boron (Mo-Cr-Fe-B) alloy surfacing overlays represent a class of advanced wear-resistant materials designed for severe abrasion and erosion-corrosion service. The addition of boron to the traditional Mo-Cr-Fe system introduces a unique metallurgical complexity that significantly influences the microstructure, hardness, and wear performance of the deposited layer. This literature provides a comprehensive investigation of the microstructural evolution and mechanical properties of Mo-Cr-Fe-B surfacing alloys, offering valuable insights for engineers designing overlays for applications such as coal handling equipment, cement mill liners, and mining machinery.
Core Technical Points
Compositional Design and Phase Formation
The Mo-Cr-Fe-B alloy system is characterized by the formation of multiple hard phases, including:
- Fe₂B and FeMoB: Iron-molybdenum borides that provide primary hardening
- Mo₂B and CrB: Molybdenum and chromium borides that form at higher boron concentrations
- M₇C₃ and M₂₃C₆: Molybdenum-chromium carbides that contribute to wear resistance
- B₂Fe and B₂Cr: Boron-rich phases that form at the highest boron levels
The phase assemblage is highly dependent on the boron content:
| Boron Content (wt%) | Dominant Phases | Microhardness (HV) | Wear Rate (mm³/N·m) |
|---|---|---|---|
| 0.5 | Fe₂B, M₇C₃ | 850 | 0.08 |
| 1.0 | Fe₂B, Mo₂B, M₇C₃ | 1050 | 0.05 |
| 1.5 | Fe₂B, Mo₂B, B₂Fe | 1200 | 0.03 |
| 2.0 | Mo₂B, B₂Fe, B₂Cr | 1350 | 0.02 |
| 2.5 | B₂Fe, B₂Cr, MoB₂ | 1400 | 0.025 |
| 3.0 | B₂Cr, MoB₂, CrB | 1380 | 0.04 |
The data reveals an optimal boron content of approximately 2.0 wt% where hardness peaks and wear rate is minimized. Beyond this point, the excessive formation of brittle boron-rich phases leads to increased susceptibility to spalling and chipping during wear.
Microstructural Characterization
The Mo-Cr-Fe-B surfacing deposits exhibit a complex dendritic microstructure with the following characteristics:
- Primary dendrites: BCC iron-based matrix with Mo and Cr in solid solution
- Interdendritic phases: Eutectic mixture of borides and carbides
- Carbide morphology: Rod-like M₇C₃ carbides distributed in the interdendritic regions
- Boride morphology: Blocky or irregular shapes of Fe₂B and Mo₂B phases
The cooling rate during surfacing significantly influences the microstructure. For submerged arc welding (SAW) with heat input of 1.5 to 2.5 kJ/mm, the cooling rate is approximately 5 to 15°C/s, producing a coarse dendritic structure with interdendritic spacing of 50 to 100 micrometers. For plasma transferred arc (PTA) cladding with lower heat input, the cooling rate increases to 20 to 50°C/s, resulting in a finer microstructure with interdendritic spacing of 20 to 40 micrometers.
Mechanical Properties and Wear Performance
Hardness Distribution
The hardness of Mo-Cr-Fe-B surfacing deposits varies across the cross-section due to microstructural gradients:
- Surface layer: Highest hardness (HV 1200 to HV 1400) due to fine microstructure and high concentration of hard phases
- Mid-section: Moderate hardness (HV 900 to HV 1100) with mixed microstructure
- Base metal interface: Lowest hardness (HV 400 to HV 600) due to dilution and tempering effects
This hardness gradient is beneficial for wear resistance but can lead to delamination failure if the bond strength is insufficient. The transition from hard overlay to softer base metal creates a stress concentration zone that must be carefully managed.
Wear Test Results
Abrasion wear tests conducted according to ASTM G99 (dry sand-rubber wheel) and ISO 9074 (wet sand) demonstrate the following performance:
| Test Condition | Wear Rate (mg/1000 cycles) | Specific Wear Rate (mm³/N·m) | Relative to 1045 Steel |
|---|---|---|---|
| Dry sand (SiC) | 12 | 0.025 | 1/45 |
| Wet sand | 18 | 0.038 | 1/30 |
| Abrasive slurry | 25 | 0.052 | 1/22 |
| Impact-abrasion | 35 | 0.072 | 1/16 |
The Mo-Cr-Fe-B overlay demonstrates exceptional wear resistance, particularly in dry sand conditions where the hard boride and carbide phases provide effective abrasion protection. However, in impact-abrasion conditions, the brittleness of the boron-rich phases leads to increased material removal through chipping and spalling.
Process Parameters and Their Influence
Welding Process Selection
The choice of surfacing process significantly affects the microstructure and properties of Mo-Cr-Fe-B overlays:
| Process | Heat Input (kJ/mm) | Cooling Rate (°C/s) | Dilution (%) | Microstructure | Hardness (HV) |
|---|---|---|---|---|---|
| SAW | 2.0-3.0 | 5-15 | 15-25 | Coarse dendritic | 900-1100 |
| GMAW | 1.5-2.5 | 10-25 | 10-20 | Medium dendritic | 1000-1200 |
| PTA | 1.0-2.0 | 20-50 | 5-15 | Fine dendritic | 1100-1350 |
| Laser cladding | 0.5-1.5 | 50-150 | 2-8 | Very fine dendritic | 1200-1400 |
PTA and laser cladding processes produce the finest microstructures and highest hardness values due to their lower heat input and reduced dilution. However, these processes are more expensive and have lower deposition rates than conventional arc welding methods.
Effect of Interpass Temperature
Interpass temperature control is critical for Mo-Cr-Fe-B surfacing to avoid excessive grain growth and phase coarsening:
- Below 100°C: Rapid cooling, fine microstructure, high hardness, but increased cracking risk
- 100-200°C: Moderate cooling, balanced microstructure, acceptable hardness, low cracking risk
- 200-300°C: Slow cooling, coarse microstructure, reduced hardness, risk of phase coarsening
- Above 300°C: Excessive grain growth, significant hardness reduction, potential for delamination
The recommended interpass temperature range is 100 to 200°C for optimal balance between microstructure refinement and crack resistance.
Defect Analysis and Countermeasures
Common Defects in Mo-Cr-Fe-B Surfacing
| Defect | Appearance | Root Cause | Countermeasure |
|---|---|---|---|
| Cracking | Transverse or longitudinal cracks | High brittleness, thermal stress | Reduce boron content, control interpass temperature |
| Porosity | Gas or shrinkage porosity | Inadequate shielding, high carbon | Improve gas shielding, reduce carbon content |
| Slag inclusion | Dark spots in cross-section | Poor slag removal, high slag viscosity | Increase slag fluidity, ensure complete slag removal |
| Delamination | Separation at overlay/base interface | High residual stress, poor bond | Stress relief, use transition layer |
| Ex |