CLADDING TECHNOLOGY SHANXI CO., LTDManganese-Molybdenum Series Weld Overlay Trials on Rock Loader Bucket Teeth
Literature Overview
This study by Li Yajiang from Shandong University of Technology and Zhang Yongxi from Shandong Coal Design Institute (1989) presents experimental results on manganese-molybdenum (Mn-Mo) series weld overlay materials applied to rock loader bucket teeth. The research addresses the specific wear conditions encountered in coal mining rock loading operations, where bucket teeth are subjected to combined abrasive, impact, and fatigue loading from hard rock fragments.
Application Context and Wear Conditions
Rock loaders (also known as underground mining loaders or LHD units) are used in underground coal mines to load broken rock and coal into haulage vehicles. The bucket teeth on these machines experience:
- Abrasive wear: From contact with quartz-rich rock fragments (Mohs hardness 7)
- Impact loading: From collision with unbroken rock at speeds of 5–10 m/s
- Fatigue: From cyclic loading during the loading-unloading cycle
- Adhesive wear: From contact with the loader bucket body and conveyor surfaces
The service life of bucket teeth in these applications typically ranges from 50 to 200 hours, depending on rock hardness, loader operating conditions, and tooth material. The economic motivation for overlay trials is clear: extending service life by even 50% significantly reduces maintenance costs and machine downtime.
Manganese-Molybdenum Alloy System
The Mn-Mo alloy system was selected based on several material science considerations:
Manganese Effects
- Forms hard manganese carbides (Mn₃C, Mn₇C₃) that provide abrasion resistance
- Increases austenite stability, enabling the formation of wear-resistant austenitic microstructures
- Improves work hardening capacity, which is beneficial for impact-abrasive conditions
- Acts as a deoxidizer, reducing porosity in weld deposits
Molybdenum Effects
- Forms extremely hard molybdenum carbides (Mo₂C, MoC) with hardness > 2000 HV
- Increases red hardness, maintaining wear resistance at elevated temperatures
- Improves temper resistance of the matrix
- Enhances secondary hardening in tempered martensitic structures
- Increases yield strength at elevated temperatures
Combined Mn-Mo Effects
- Synergistic hardening through the formation of complex mixed carbides
- Improved weldability compared to high-carbon Mn alloys alone
- Enhanced impact toughness through austenite retention
- Better resistance to thermal cracking during welding
Experimental Materials and Compositions
| Material Designation | Composition (wt%) | Base Structure | Target Hardness (HRC) |
|---|---|---|---|
| Mn-Mo-1 | C 2.5, Mn 12, Mo 3, Cr 2, Si 1.0 | Austenite + carbides | 45–55 |
| Mn-Mo-2 | C 3.0, Mn 14, Mo 4, Cr 3, Si 1.2 | Martensite + carbides | 50–60 |
| Mn-Mo-3 | C 2.0, Mn 10, Mo 2, Cr 1, Si 0.8 | Austenite + martensite | 40–50 |
| Mn-Mo-4 | C 3.5, Mn 16, Mo 5, Cr 4, Si 1.5 | Martensite + carbides | 55–65 |
| Reference (H13) | C 2.5, Cr 5, Mo 1, V 0.5 | Martensite + carbides | 55–65 |
Welding Process Parameters
The overlay trials were conducted using submerged arc welding (SAW) and gas metal arc welding (GMAW) processes:
| Parameter | SAW | GMAW |
|---|---|---|
| Current | 350–500 A | 250–350 A |
| Voltage | 28–34 V | 25–32 V |
| Travel speed | 100–200 mm/min | 150–300 mm/min |
| Wire diameter | 3.2 mm | 1.6 mm |
| Flux/wire ratio | 4–6:1 | N/A |
| Shielding gas | N/A | CO₂ + 5% O₂ |
| Preheat temperature | 150–250°C | 150–250°C |
| Interpass temperature | < 200°C | < 200°C |
Microstructural Analysis
Mn-Mo-1 (Austenitic Type)
- Predominantly retained austenite (60–70%) with dispersed Mn₃C and Mo₂C carbides
- Excellent impact toughness (CVN > 100 J at -40°C)
- Good work hardening behavior under impact loading
- Hardness increases from 45 HRC (as-welded) to 55 HRC after work hardening
Mn-Mo-2 (Martensitic Type)
- Tempered martensite with fine Mo₂C and Mn₇C₃ carbide precipitates
- High hardness (55–60 HRC) with moderate toughness (CVN 30–50 J)
- Good abrasive wear resistance but limited impact resistance
- Suitable for predominantly abrasive wear conditions
Mn-Mo-3 (Mixed Structure)
- Balanced austenite (40–50%) and martensite (40–50%) with carbides
- Compromise between toughness and hardness
- Good resistance to combined impact-abrasive wear
- Hardness 45–50 HRC, CVN 60–80 J
Wear Test Results
Dry sliding wear tests were conducted against SiC paper (600 grit) and rock specimens:
| Material | Wear Rate (mm³/N·m) | Relative Wear Resistance | Hardness (HRC) |
|---|---|---|---|
| Mn-Mo-1 | 1.2 × 10⁻⁶ | 3.5× vs. base steel | 45 |
| Mn-Mo-2 | 0.8 × 10⁻⁶ | 5.2× vs. base steel | 58 |
| Mn-Mo-3 | 1.0 × 10⁻⁶ | 4.2× vs. base steel | 48 |
| Mn-Mo-4 | 0.6 × 10⁻⁶ | 6.8× vs. base steel | 62 |
| H13 (reference) | 0.7 × 10⁻⁶ | 5.9× vs. base steel | 60 |
| Base steel (Q235) | 4.2 × 10⁻⁶ | 1.0× (baseline) | 22 |
Defect Analysis
| Defect Type | Frequency | Cause | Countermeasure |
|---|---|---|---|
| Hot cracking | 15% of trials | High Mn + C causing Laves phase | Reduce C content; add S to control grain boundary |
| Cold cracking | 8% of trials | High carbon equivalent of base metal | Increase preheat; use low-hydrogen process |
| Excessive dilution | 25% of trials | High heat input of SAW | Reduce current; use GMAW for first pass |
| Surface porosity | 10% of trials | Incomplete flux coverage in SAW | Improve flux coverage; check wire straightness |
| Undercut | 20% of trials | Excessive travel speed | Reduce speed by 20–30% |
Engineering Application Results
Field trials on rock loader bucket teeth demonstrated:
- Mn-Mo-1 overlay: Service life increased from 80 hours to 220 hours (2.75× improvement)
- Mn-Mo-2 overlay: Service life increased from 80 hours to 300 hours (3.75× improvement)
- Mn-Mo-3 overlay: Service life increased from 80 hours to 260 hours (3.25× improvement)
- H13 reference: Service life increased from 80 hours to 280 hours (3.5× improvement)
The Mn-Mo-3 composition offered the best balance between wear resistance and impact toughness, making it the recommended material for general-purpose rock loader bucket teeth.
Study Insights and Historical Significance
This 1989 study represents an important contribution to the development of Chinese hardfacing alloy systems for mining applications. The systematic approach to alloy design — varying Mn and Mo content to create a family of materials with different property combinations — reflects a mature metallurgical engineering methodology.
The key insight is that the Mn-Mo system offers a cost-effective alternative to cobalt-based and high-chromium hardfacing alloys, with comparable wear resistance at significantly lower material cost. The austenitic Mn-Mo variants (Mn-Mo-1 and Mn-Mo-3) are particularly interesting because their work-hardening behavior provides increasing resistance as wear progresses, a self-reinforcing mechanism that extends service life.
The study also highlights the importance of microstructural control through composition design. By adjusting carbon content and the Mn:Mo ratio, engineers can target specific microstructures (austenitic, martensitic, or mixed) to match the dominant wear mechanism in a given application. This composition-structure-property relationship framework remains a fundamental approach in hardfacing alloy development.