CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair of Universal Joint Yoke on 1350 Roughing Mill
Literature Overview
This 1994 technical paper by Liu Chengmin, published in a welding technology journal, documents the cladding repair of a universal joint yoke on a 1350 roughing mill at Baosteel. The paper is classified under "cladding (weld overlay)" with the technical description "Baosteel technology," indicating that it represents practical industrial experience from one of China's largest steel producers. The 1350 roughing mill refers to a rolling mill with a maximum rolling width of 1350 mm, a critical piece of equipment in the hot rolling production line.
Core Technical Challenge
The universal joint yoke is a critical drivetrain component that transmits torque from the motor to the rolling mill rolls. It is subjected to severe cyclic loading, impact, and wear at the bearing surfaces and the cross-shaped inner raceway. When the yoke wears beyond acceptable limits, the options are typically replacement (expensive and time-consuming) or repair by cladding (economical and faster). The challenge in cladding repair is to restore the dimensional accuracy and surface quality of the worn surfaces while maintaining the mechanical integrity of the base material.
Component Specifications and Wear Characteristics
| Parameter | Typical Value |
|---|---|
| Mill type | 1350 hot rolling mill (roughing stand) |
| Yoke material | 42CrMo or 40CrNiMo (quenched and tempered) |
| Original surface hardness | 28-32 HRC |
| Wear limit | 1-2 mm material loss on bearing surfaces |
| Required repair thickness | 1-3 mm |
| Service life requirement | 6-12 months between repairs |
Process Selection and Implementation
The repair of a universal joint yoke requires a process that can achieve:
- Good metallurgical bonding with the quenched and tempered base metal
- Low dilution to maintain the hardness of the overlay
- Minimal distortion to preserve dimensional accuracy
- Adequate hardness and wear resistance for the operating conditions
Recommended Cladding Processes for Yoke Repair
| Process | Advantages | Limitations | Suitability |
|---|---|---|---|
| SMAW | Portable, low cost, suitable for field | Low deposition rate, higher dilution | Good for small areas |
| GTAW (TIG) | Low dilution, precise control | Low deposition rate, requires skill | Excellent for critical surfaces |
| SAW | High deposition rate, consistent quality | Requires specialized equipment | Good for flat surfaces |
| PTA | Very low dilution, high quality | Expensive equipment, limited portability | Excellent for high-value repairs |
| Laser cladding | Lowest dilution, excellent bonding | Limited coverage, high cost | Best for precision repair |
For the 1994 Baosteel application, SMAW or GTAW was most likely used, given the equipment availability and the nature of the repair. The process parameters would have been carefully selected to minimize heat input and prevent cracking of the quenched and tempered base metal.
Key Process Parameters
| Parameter | Value | Rationale |
|---|---|---|
| Preheat temperature | 200-300 °C | Prevent cracking of quenched base metal |
| Interpass temperature | <300 °C | Control thermal cycling |
| Post-weld heat treatment | 550-600 °C for 2-4 hours | Stress relief, soften overlay |
| Welding current | 100-180 A (SMAW) or 80-150 A (GTAW) | Low heat input |
| Travel speed | 200-400 mm/min | Limit penetration |
| Number of passes | 2-3 | Build up required thickness |
Metallurgical Considerations
The cladding of a quenched and tempered steel component presents unique metallurgical challenges:
- Base metal microstructure: The 42CrMo or 40CrNiMo yoke is typically quenched and tempered to achieve a tempered martensite structure with a hardness of 28-32 HRC. The welding heat input can cause local tempering or even re-austenitization of the base metal near the weld, reducing the hardness and potentially causing cracking.
- Dilution: The base metal dilution in the overlay can introduce carbon and alloy elements that affect the overlay microstructure. For a wear-resistant overlay, dilution should be kept below 20% to maintain the required hardness.
- Residual stress: The thermal cycling during cladding can introduce significant residual stress, which can cause distortion or cracking. Post-weld stress relief is essential.
- Overlay microstructure: The overlay should have a fine-grained, homogeneous microstructure with adequate toughness to resist impact loading. A martensitic or martensitic-carbide structure is preferred for wear resistance.
Quality Control and Inspection
The repaired yoke must undergo thorough inspection before returning to service:
| Inspection | Method | Acceptance Criteria |
|---|---|---|
| Surface hardness | Rockwell C hardness tester | Meets specified range |
| Dilution rate | Macrograph examination | <20% |
| Bond strength | Peel test or tensile test | Meets NB/T 47014 or equivalent |
| Cracks | MT or PT | No indications |
| Dimensional accuracy | Coordinate measuring machine (CMM) | Within tolerance |
| Residual stress | X-ray diffraction | Within acceptable limits |
Key Questions and Reflections
Several important questions arise from this repair case:
- When is cladding repair preferable to replacement? The decision depends on the cost of the replacement part, the availability of spare parts, the criticality of the equipment, and the extent of the wear. For a 1350 roughing mill yoke, the replacement cost is likely very high (several hundred thousand RMB), and the lead time may be 6-12 months. Cladding repair, costing a fraction of the replacement cost, is clearly the preferred option.
- What is the maximum number of times a yoke can be repaired? Each repair removes some base metal material and introduces additional residual stress. After several repairs, the base metal thickness may be insufficient, or the accumulated residual stress may cause cracking. A conservative limit of 2-3 repairs is generally recommended.
- How does the repair affect the fatigue life of the yoke? The cladding repair can introduce stress concentrations at the weld toe, which may reduce the fatigue life. A smooth transition from the overlay to the base metal, achieved by grinding and polishing, is essential to minimize stress concentration.
Study Insights and Implications
This 1994 paper represents an early example of cladding repair technology being applied to critical heavy industrial equipment in China. The Baosteel context is significant: as one of China's largest steel producers, Baosteel had a strong incentive to minimize equipment downtime and spare parts costs. The cladding repair of the universal joint yoke is a textbook example of how welding overlay technology can extend the service life of expensive components and improve the overall availability of production equipment.
The paper's publication in a welding technology journal indicates that the repair technique was considered novel or noteworthy enough to be shared with the broader welding community. For today's engineers, the lessons are clear: cladding repair is a viable and often economical alternative to component replacement, but it requires careful process selection, metallurgical understanding, and thorough quality control. The principles of low heat input, controlled dilution, and post-weld stress relief remain as relevant today as they were in 1994.