CLADDING TECHNOLOGY SHANXI CO., LTDCladding Process Research in Crankshaft Repair A Literature Study Note
Literature Overview
This 2020 publication from Shenyang Ligong University, supported by the Liaoning Provincial Department of Education Key Project (LG201901) and the Shenyang Municipal Young and Middle-aged Science and Technology Innovation Talent Support Program, investigates the application of weld overlay (cladding) processes in the repair of automotive crankshafts. Authored by Lei Jinhong, Zhou Qiuzhong, Chen Zhuojun, Cui Liang, and Gu Xiaolong, this research addresses a practical industrial challenge: the economic and technically viable repair of worn or damaged crankshafts using advanced cladding techniques.
Research Background and Significance
Crankshafts are critical components in internal combustion engines, subjected to cyclic bending and torsional loading, friction wear at the bearing journals, and impact loading from the connecting rods. Over time, the bearing journals and thrust surfaces of the crankshaft experience wear, leading to dimensional deviation beyond the acceptable tolerance limits. Traditional repair methods include grinding and under-sizing, but this approach is limited by the minimum allowable journal diameter and can only be performed a limited number of times before the crankshaft must be replaced.
Weld overlay repair offers an alternative approach by depositing a layer of material on the worn surface, restoring the journal to its original dimensions or even oversizing it for subsequent grinding. This approach is particularly attractive for high-value crankshafts in heavy-duty applications such as diesel engines for trucks, construction equipment, and marine propulsion, where the cost of replacement is substantial.
Process Selection and Technical Approach
The study evaluates several cladding processes for crankshaft repair, with the following comparison:
| Process | Advantages | Limitations | Suitability for Crankshaft Repair |
|---|---|---|---|
| Submerged Arc Welding (SAW) | High deposition rate, low dilution | Limited to flat or large-radius surfaces | Limited |
| Gas Metal Arc Welding (GMAW) | Good deposition rate, flexible | Moderate dilution, requires skill | Good |
| Gas Tungsten Arc Welding (GTAW) | Low dilution, precise control | Low deposition rate | Good for small areas |
| Plasma Transferred Arc (PTA) | Very low dilution, excellent quality | High equipment cost | Excellent |
| Laser Cladding | Minimal heat input, excellent bonding | Limited to thin layers, high cost | Excellent for precision repair |
| Hot-wire TIG | Low dilution, moderate cost | Moderate deposition rate | Good |
For crankshaft repair, the study recommends a combination of GTAW for the first pass (to ensure good bonding with minimal dilution) followed by GMAW or hot-wire TIG for subsequent passes to build up the required material volume. This hybrid approach balances bonding quality with deposition efficiency.
Process Parameters and Welding Strategy
The welding parameters for crankshaft repair are carefully optimized to minimize distortion and ensure adequate bonding. The following parameters are typically employed:
| Parameter | GTAW Pass | GMAW Pass | Hot-wire TIG Pass |
|---|---|---|---|
| Arc voltage | 14-18 V | 20-25 V | 16-20 V |
| Welding current | 80-120 A | 150-250 A | 120-200 A |
| Travel speed | 200-400 mm/min | 300-600 mm/min | 250-500 mm/min |
| Shielding gas | Ar (99.99%) | Ar + 5% CO2 or Ar + 2% O2 | Ar (99.99%) |
| Wire diameter | 1.6 mm | 1.2 mm | 1.2-1.6 mm |
| Preheat temperature | 150-250°C | 150-250°C | 150-250°C |
The welding strategy involves the following sequence: (1) surface preparation by grinding the worn surface to bare metal, removing all oil and contamination; (2) application of a thin GTAW bonding pass to establish metallurgical bonding; (3) deposition of GMAW or hot-wire TIG passes to build up the material to the oversize dimension; (4) final grinding and finishing to the required dimensional tolerance and surface finish.
Metallurgical Considerations
The metallurgical compatibility between the crankshaft base metal and the overlay material is a critical consideration. Crankshafts are typically made from alloy steel such as 42CrMo, 40CrNiMo, or similar grades, which have a martensitic or bainitic microstructure after heat treatment. The overlay material should be selected to match or slightly exceed the hardness of the base metal to ensure that the repaired area is not a weak point in the crankshaft.
Common overlay materials for crankshaft repair include:
- Nickel-based alloys (e.g., Stellite 6, Inconel 625): Excellent wear resistance, good bonding with steel substrates, but high cost.
- Iron-based alloys (e.g., D2 tool steel equivalent, M2 tool steel equivalent): Good hardness and wear resistance, lower cost, but higher dilution sensitivity.
- Hardfacing alloys (e.g., carbide-containing alloys): Very high hardness, suitable for severe wear applications, but may be too brittle for impact loading.
The dilution from the base metal into the overlay layer can significantly affect the final hardness and microstructure of the overlay. For example, if a nickel-based overlay alloy is deposited on a 42CrMo crankshaft, the dilution of carbon and alloying elements from the base metal can promote the formation of martensite in the overlay, increasing hardness but potentially reducing toughness. The multi-pass welding strategy is essential to control the dilution ratio in the final overlay layer to within 10-15%.
Defect Analysis and Quality Control
The following defects are commonly encountered in crankshaft overlay repair and their countermeasures:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking in overlay | High cooling rate, high carbon equivalent | Increase preheat, use low-hydrogen filler, apply back-gassing |
| Lack of fusion | Surface contamination, insufficient penetration | Thorough surface preparation, increase current, use proper GTAW bonding pass |
| Excessive dilution | Too few passes, high travel speed | Increase number of passes, reduce travel speed, use appropriate wire diameter |
| Distortion | Excessive heat input, asymmetric welding | Use symmetric welding sequence, control interpass temperature, apply backing bar |
| Porosity | Contamination, inadequate shielding | Use high-purity shielding gas, ensure proper gas flow, clean surface thoroughly |
Quality control for crankshaft overlay repair includes the following inspections:
- Visual inspection: Check for surface defects, undercut, and weld geometry.
- Magnetic particle testing (MT): Detect surface and near-surface cracks, particularly in the HAZ and overlay layer.
- Ultrasonic testing (UT): Detect subsurface defects and measure overlay thickness.
- Hardness testing: Verify that the overlay hardness meets the specification and that the hardness gradient is acceptable.
- Dimensional inspection: Verify that the journal diameter, roundness, and taper are within tolerance.
Engineering Practice Integration
In practice, crankshaft overlay repair is performed in specialized repair shops equipped with the necessary welding equipment, preheating facilities, and inspection capabilities. The repair process typically involves the following steps:
- Assessment: Evaluate the extent of wear, dimensional deviation, and any existing defects (cracks, scuffing, etc.).
- Preparation: Remove oil, grease, and contamination from the repair area. Grind the surface to bare metal with a depth of 1-2 mm.
- Preheating: Heat the crankshaft uniformly to 150-250°C using an induction heater or gas torch. Maintain the preheat temperature throughout the welding process.
- Welding: Apply the overlay material using the selected process and parameters. For large journals, use a continuous welding sequence to minimize distortion.
- Post-weld treatment: Allow the crankshaft to cool slowly in the furnace or under insulation. Apply post-weld stress relief if required.
- Inspection: Perform MT and UT inspections. Measure hardness and overlay thickness.
- Finishing: Grind the journal to the required dimensions and surface finish. Perform final dimensional and surface quality inspection.
Key Questions and Reflections
Several important questions arise from this study. First, the long-term fatigue behavior of the repaired crankshaft under cyclic loading is a critical concern. The overlay layer and the HAZ may have different fatigue properties compared to the original base metal, and the interface between the overlay and the base metal could serve as a crack initiation site under cyclic loading.
Second, the effect of the repair process on the crankshaft's balance characteristics is an important consideration. The addition of material to the journal surface can change the mass distribution and, consequently, the balance quality of the crankshaft. This must be accounted for in the repair process by ensuring symmetric material deposition and performing final balancing after grinding.
Third, the economic viability of crankshaft overlay repair depends on the cost of the repair process compared to the cost of a new crankshaft. For high-value crankshafts in heavy-duty applications, overlay repair is typically economically justified. However, for smaller crankshafts in passenger vehicles, the cost of repair may exceed the cost of replacement, making overlay repair less attractive.
Study Insights and Broader Implications
This research contributes to the growing body of knowledge on the application of advanced welding and cladding technologies in component repair. The systematic evaluation of different cladding processes, combined with metallurgical analysis and quality control considerations, provides a practical framework for implementing crankshaft repair using weld overlay techniques.
The broader implication is that advanced cladding technologies can extend the service life of critical components, reducing waste and conserving resources. This is particularly relevant in the context of sustainable manufacturing and circular economy principles. For engineers involved in maintenance and repair, this research provides valuable guidance on process selection, parameter optimization, and quality control for crankshaft overlay repair.
The study also highlights the importance of metallurgical compatibility in repair welding. The selection of filler material, welding parameters, and post-weld treatment must be carefully tailored to the specific base metal and service conditions to ensure that the repaired component meets the required performance criteria.