Zhuojin Pipe Fitting Co., LtdApplication of Surface Overlay Welding Technology on Skidding Rolls
Literature Overview
The paper by Zhang Dongming and Wang Jiebing (Welding, Issue 7, 2006, pp. 40–42) presents a practical engineering solution for the restoration of scrap skidding rolls (夹送辊) using surface overlay welding. Skidding rolls are critical components in hot rolling mills, responsible for transporting and positioning workpieces between rolling stands. They are subjected to severe abrasive wear, thermal cycling, and mechanical impact during service. The traditional approach to handling worn or damaged rolls is replacement, which is economically costly, particularly when the rolls contain alloyed or exotic materials. This study demonstrates that strategic overlay welding can restore functional integrity, improve mechanical properties, and significantly reduce manufacturing costs while eliminating dependence on imported replacements.
Technical Approach and Process Design
Material Selection and Electrode Design
The core of the overlay welding repair strategy lies in the careful selection of electrode composition. The authors determined the optimal wire and flux compositions through systematic experimentation, balancing the following requirements:
- Wear resistance: The overlay must exhibit hardness ≥ 400 HV to resist abrasive wear from hot steel workpieces.
- Crack resistance: The electrode must tolerate the thermal stresses induced by the rolling mill environment without developing cracks.
- Bond strength: The overlay must maintain strong metallurgical bonding to the base roll material, which is typically a low-alloy steel (e.g., 42CrMo, 35CrMo).
- Weldability: The process must be compatible with the production schedule, requiring minimal preheating and post-weld heat treatment.
| Parameter | Specification | Notes |
|---|---|---|
| Overlay hardness | ≥ 400 HV (as-welded) | Target for wear resistance |
| Electrode type | Low-hydrogen stick electrode or flux-cored wire | Depends on production facility |
| Preheating temperature | 150–250°C | Based on base roll carbon equivalent |
| Interpass temperature | ≤ 250°C | Prevents overheating and grain coarsening |
| Post-weld heat treatment | Optional: 550–600°C tempering | Reduces residual stress if required |
| Overlay thickness | 5–15 mm per side | Depends on wear allowance |
Welding Process Parameters
The overlay welding was performed using shielded metal arc welding (SMAW) or flux-cored arc welding (FCAW), depending on the accessibility of the roll surface and the production requirements. The process parameters were optimized to achieve the following:
- Deposition rate: 3–5 kg/h to ensure adequate productivity.
- Weld bead profile: Convex bead with a leg length of 25–30 mm to maximize surface contact area.
- Travel speed: 150–200 mm/min to balance deposition rate and penetration.
- Arc length: 2–3 mm (SMAW) or 5–8 mm (FCAW) to maintain stable arc and consistent bead geometry.
Non-Destructive Testing (NDT) Protocol
The quality assurance protocol included the following inspection steps:
- Visual inspection (VT): Every weld bead was visually inspected for surface defects such as porosity, undercut, and spatter.
- Magnetic particle testing (MT): Applied to all overlay welds to detect surface and near-surface cracks. This is critical because skidding rolls operate under high cyclic loading, and undetected surface cracks can lead to catastrophic failure.
- Ultrasonic testing (UT): Performed on critical welds to detect subsurface inclusions and lack of fusion at the overlay-base metal interface.
- Hardness testing: Vickers hardness measurements were taken across the overlay thickness to verify uniform hardness distribution and to assess the hardness gradient at the dilution zone.
Performance Evaluation and Engineering Outcomes
The restored skidding rolls were subjected to field trials in a hot rolling mill, and the following outcomes were observed:
- Service life: The overlay-welded rolls achieved a service life comparable to new rolls, with wear rates within 10% of the original specification.
- Cost savings: The repair cost was approximately 30–40% of the cost of new replacement rolls, representing a significant economic benefit.
- Production continuity: The repair process eliminated the need to import replacement rolls, reducing lead times from months to days.
- Mechanical properties: The overlay layer exhibited superior wear resistance compared to the original roll surface, attributed to the higher carbon and alloy content of the electrode composition.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Surface cracks | Excessive preheat or interpass temperature | Control interpass temperature ≤ 250°C |
| Lack of fusion | Inadequate base metal preparation | Grind base metal to bare metal with 30–45° chamfer |
| Excessive dilution | High heat input | Reduce current and increase travel speed |
| Porosity | Contamination of base metal surface | Thoroughly clean base metal before welding |
| Hardness variation | Inconsistent welding parameters | Monitor and log all parameters in real time |
Study Insights and Implications
This paper exemplifies the practical engineering philosophy of "repair over replacement," which is particularly relevant in resource-constrained environments. The authors demonstrate that overlay welding is not merely a repair technique but can be used to enhance the performance of existing components beyond their original design specification. This is a powerful concept for asset management in heavy industry, where the lifecycle cost of critical components can be significantly reduced through strategic surface engineering.
One area where I believe further investigation would be valuable is the long-term fatigue behavior of the overlay-welded interface under cyclic thermal and mechanical loading. While the short-term wear performance is satisfactory, the fatigue crack initiation threshold at the overlay-base metal interface may be reduced due to residual stresses and microstructural heterogeneity. Future work should include fatigue testing of overlay-welded specimens under conditions simulating rolling mill service to establish a reliable service life prediction model.
The broader lesson is that surface engineering through overlay welding is a versatile tool that can address not only repair needs but also performance enhancement, and it should be considered as a standard option in the maintenance strategy for critical industrial components.