CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Molybdenum Content on Microstructure and Properties of Carbon Arc Weld Overlay Fe-Cr-C-Mo-B Wear-Resistant Layer
Literature Overview
This 2015 study published in Materials Protection by researchers from the School of Mining Technology at Liaoning Technical University examines how molybdenum content influences the microstructure and wear resistance of Fe-Cr-C-Mo-B system overlay layers deposited by carbon arc welding (oxy-fuel carbon arc gouging/cladding). The research addresses a practical industrial need: developing cost-effective, high-performance wear-resistant overlay consumables using readily available welding processes and materials. Carbon arc welding, while less sophisticated than plasma or laser methods, offers the advantage of equipment simplicity and low operational costs, making it attractive for field repair and maintenance applications.
Core Technical Content
Metallurgical Significance of Molybdenum
Molybdenum is a potent alloying element in wear-resistant steels and overlay alloys, contributing to performance through multiple mechanisms:
- Solid solution strengthening: Mo atoms in solution in the ferrite or martensite matrix create significant lattice distortion due to their large atomic radius
- Carbide formation: Mo2C (approximately 1800 HV) and MoC (approximately 1600 HV) are hard, stable carbides that resist dissolution during tempering
- Temper stability: Mo-containing martensite resists softening during tempering, maintaining hardness at elevated service temperatures
- Phase stability: Mo stabilizes the austenite phase and promotes the formation of retained austenite, which contributes to work-hardening capacity
Systematic Molybdenum Content Study
The study systematically varied Mo content in the Fe-Cr-C-Mo-B overlay composition:
| Mo Content (wt%) | Matrix Hardness (HV) | Overall Hardness (HV) | Wear Resistance Index | Microstructure |
|---|---|---|---|---|
| 0.0 | 380–420 | 500–550 | 1.0 (baseline) | Martensite + M7C3 |
| 0.5 | 400–440 | 550–600 | 1.3 | Martensite + M7C3 + Mo2C |
| 1.0 | 420–460 | 580–630 | 1.6 | Fine martensite + M7C3 + Mo2C |
| 1.5 | 440–480 | 610–660 | 1.8 | Martensite + M7C3 + Mo2C |
| 2.0 | 460–500 | 630–680 | 1.9 | Martensite + M7C3 + Mo2C |
| 2.5 | 480–520 | 650–700 | 1.8 | Coarse Mo2C + martensite |
| 3.0 | 500–540 | 660–710 | 1.7 | Coarse Mo2C network |
The results demonstrate that increasing Mo content from 0% to 2.0% progressively improves both hardness and wear resistance. However, beyond 2.0%, the benefit plateaus and may decline due to the formation of coarse Mo2C particles and potential brittleness.
Carbon Arc Welding Process Characteristics
Carbon arc welding (also known as carbon arc gouging or oxy-carbon arc cladding) uses a carbon electrode and oxy-fuel flame to create a molten pool into which cladding powder or wire is fed. Key process characteristics include:
| Parameter | Typical Value | Effect |
|---|---|---|
| Oxygen pressure | 0.3–0.5 MPa | Controls arc stability |
| Acetylene pressure | 0.05–0.1 MPa | Flame temperature adjustment |
| Powder feed rate | 100–200 g/min | Layer thickness per pass |
| Travel speed | 5–12 cm/min | Cooling rate control |
| Dilution rate | 15–25% | Higher than PTA or SAW |
The higher dilution rate of carbon arc welding compared to PTA or laser cladding is a significant consideration. The dilution introduces substrate carbon and alloying elements into the overlay, potentially altering the designed composition. This must be accounted for in consumable design and process qualification.
Process and Standards Analysis
Quality Control for Carbon Arc Cladding
Carbon arc welding overlay presents specific quality challenges:
- Carbon pickup: The carbon electrode can introduce excess carbon into the overlay, leading to hard, brittle carbide networks
- Inclusion formation: Oxy-fuel processes can introduce oxide and nitride inclusions
- Porosity: Gas entrapment from the powder feed or substrate contamination
- Cracking: High carbon equivalent in the overlay increases cracking susceptibility
The following inspection protocol is recommended:
- Visual inspection: Surface quality, bead overlap, and uniformity
- Magnetic particle testing (MT): Detection of surface and near-surface cracks
- Hardness mapping: Grid pattern to identify soft spots or hard brittle zones
- Microstructural examination: Cross-section to verify dilution and phase distribution
Standards and Specifications
For carbon arc welding overlay applications, the following standards provide guidance:
- NB/T 47014: Welding procedure qualification requirements
- GB/T 150: Pressure vessel design and fabrication
- JB/T 4730: NDT methods for pressure equipment
- API 934: Welding of pressure-containing equipment (for overlay applications)
- ASTM A263/A264/A265: Clad plate specifications (if used for clad plate production)
Engineering Practice Integration
The Fe-Cr-C-Mo-B overlay system deposited by carbon arc welding is particularly suited for:
- Field repair of worn equipment where sophisticated cladding equipment is unavailable
- Mining equipment including shovels, buckets, and conveyor components
- Cement industry components including kiln liners, mill liners, and slide plates
- Coal handling equipment exposed to abrasive wear
The optimal Mo content of 1.0–2.0 wt% provides the best balance of hardness (600–680 HV), wear resistance, and workability. In practice, the carbon arc welding process requires skilled operators to maintain consistent bead quality and control dilution.
Key Technical Reflections
The molybdenum content study in the Fe-Cr-C-Mo-B system reveals important insights for overlay alloy design. Mo2C carbides are notably resistant to dissolution during tempering, making them effective for applications involving elevated temperature wear. However, the coarse Mo2C particles that form at high Mo concentrations can serve as crack initiation sites, particularly under impact loading.
The carbon arc welding process, while simple and cost-effective, introduces challenges related to dilution control and inclusion formation. The higher dilution rate (15–25%) compared to advanced cladding methods means that the actual overlay composition deviates from the nominal consumable composition. This must be accounted for in process qualification and performance prediction.
Study Insights and Implications
This research demonstrates that molybdenum is an effective alloying addition for improving the wear resistance of Fe-Cr-C-Mo-B overlay systems, with an optimal content window of 1.0–2.0 wt%. The combination of Mo2C hard particles with the martensitic matrix provides a synergistic strengthening effect. Engineers should note that the carbon arc welding process, while economically attractive, requires careful process control to maintain overlay quality. The dilution rate must be monitored and controlled, and the consumable composition should be adjusted to compensate for substrate dilution. For applications requiring higher performance or tighter quality control, upgrading to PTA or laser cladding may be justified despite the higher equipment cost.