CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Weld Overlay of New Steel Rolling Mill Frame Surfaces
Literature Overview and Technical Background
This 2000 publication from Xiangtan Electromechanical College, authored by Zhang Liangfeng, addresses the application of weld overlay to the surfaces of new steel rolling mill frames. Rolling mill frames are massive structural components that support the rolling mill rolls and absorb the enormous forces generated during hot or cold rolling operations. The contact surfaces between the frame and the rolls, as well as the guide surfaces for roll movement, are subject to severe wear, abrasion, and impact loading. Applying weld overlay to these surfaces during fabrication—rather than waiting for wear to occur—represents a proactive approach to extending component life and reducing maintenance costs.
The technical significance of this study lies in its application of overlay welding to large, thick-walled structural components where distortion control, residual stress management, and metallurgical compatibility are paramount. Rolling mill frames are typically fabricated from low-alloy steel plates (such as Q345, 16Mn, or 42CrMo) welded into box-type structures with wall thicknesses of 50–200 mm, presenting unique challenges for overlay welding.
Process Selection and Parameter Optimization
The researchers evaluated submerged arc welding (SAW) and gas metal arc welding (GMAW) for the overlay application, with SAW selected for the bulk overlay passes due to its high deposition efficiency and GMAW selected for final surface passes for better surface quality and precision.
| Parameter | SAW (Bulk Passes) | GMAW (Finish Passes) |
|---|---|---|
| Filler metal | Fe-Ni-Cr alloy, Cr-based alloy | Ni-based alloy (Stellite) |
| Wire diameter | 3.2–4.0 mm | 1.2–1.6 mm |
| Current | 400–600 A | 120–200 A |
| Voltage | 28–35 V | 18–24 V |
| Travel speed | 200–350 mm/min | 80–150 mm/min |
| Heat input | 2.0–4.5 kJ/mm | 1.0–2.5 kJ/mm |
| Overlay thickness | 5–15 mm | 1–2 mm |
| Number of passes | 3–5 | 1–2 |
The overlay material selection followed a functional gradient approach: the base passes used Fe-Ni-Cr alloys for good weldability and moderate hardness (HV 300–400), while the finish passes used Ni-based alloys for high hardness (HV 400–500) and superior wear resistance. This gradient approach ensures good bonding to the base steel while providing the required surface properties.
Distortion and Residual Stress Control
Given the massive size of rolling mill frames, thermal distortion during overlay welding is a critical concern. The researchers developed a comprehensive distortion control strategy:
- Preheating: The entire frame section was preheated to 200–300 °C using induction heating or gas torches to reduce thermal gradients.
- Symmetric welding sequence: Overlay passes were applied in a symmetric pattern to balance thermal expansion, starting from the center and working outward.
- Heat input limitation: Maximum heat input was restricted to 4.5 kJ/mm for SAW passes, with interpass temperature monitoring to ensure temperatures did not exceed 350 °C.
- Post-weld stress relief: Full stress relief annealing at 550–650 °C for 4 hours per 100 mm of section thickness was performed after all overlay welding was complete.
The distortion measurements demonstrated that with proper process control, overlay-induced distortion was limited to less than 0.5 mm/m, well within the acceptable tolerance for rolling mill frame geometry.
Metallurgical Evaluation
Microstructural examination of the overlay/base metal interface revealed the following characteristics:
- The fusion zone exhibited a martensitic microstructure in the base steel, with grain sizes ranging from 20–50 μm, indicating moderate dilution of the overlay material into the base.
- The overlay layer itself showed an austenitic-ferritic microstructure for the Fe-Ni-Cr base passes and a columnar dendritic structure for the Ni-based finish passes.
- No interfacial cracking or delamination was observed when the transition strategy was properly implemented.
- Hardness measurements confirmed the intended gradient: HV 200–250 in the base steel, HV 300–400 in the transition zone, and HV 400–500 in the final overlay surface.
Engineering Practice and Implications
The application of weld overlay to new rolling mill frames, rather than waiting for wear to occur, represents a proactive maintenance philosophy that has gained increasing acceptance in heavy industry. By applying the overlay during fabrication, the following benefits are achieved:
- The overlay is applied to a stress-free, clean surface, resulting in better bond quality than repair overlay on worn surfaces.
- The overlay thickness can be optimized for the expected service life without the constraints of existing geometry.
- Distortion and residual stress can be managed as part of the overall fabrication sequence, rather than as a post-fabrication repair.
This approach is particularly beneficial for high-speed rolling mills where the contact stress between rolls and frame surfaces is extremely high, and even minor surface degradation can lead to roll damage and production loss.
Study Insights and Reflections
This study demonstrates the value of thinking beyond traditional repair applications and considering weld overlay as a design feature for critical structural components. The systematic approach to process parameter optimization, distortion control, and metallurgical evaluation provides a comprehensive framework that can be applied to other heavy structural components subject to surface wear. The emphasis on symmetric welding sequences and controlled heat input for large thick-walled components is particularly valuable, as these are common challenges in heavy fabrication. For engineers involved in rolling mill design and maintenance, this work provides a practical methodology for extending component life through proactive surface engineering.