Weld Overlay Repair of Novel Rolling Mill Cast Frames

Component Description and Failure Modes

Rolling mill cast frames are massive structural components, typically weighing several tons to tens of tons, that support the mill rolls and absorb the enormous forces generated during hot or cold rolling operations. These frames are usually fabricated from cast steel grades such as ZG270-500 or ZG35CrMo, and they are subjected to cyclic loading, thermal cycling, and occasional impact damage from mill upsets. Surface damage in the form of spalling, gouging, and wear on bearing seats and guide surfaces is common and requires periodic weld overlay repair to restore dimensional accuracy and structural integrity.

This study note examines the unique challenges of weld overlay repair on cast steel frames, including the effects of cast microstructure on weldability, the management of residual stresses in thick sections, and the selection of appropriate overlay materials and processes for different damage types.

Weldability Challenges of Cast Steel Substrates

Cast steel substrates present distinct weldability challenges compared to wrought steel. The cast microstructure typically contains a higher concentration of impurities, including sulfur, phosphorus, and inclusions, which increase the susceptibility to hot cracking and cold cracking. Additionally, the grain structure of cast steel is often coarse and irregular, leading to non-uniform dilution and potential cracking at the fusion line.

The following table compares the weldability characteristics of common cast steel grades used in rolling mill frames.

For ZG35CrMo and higher carbon grades, the carbon equivalent (CE) typically exceeds 0.5%, placing them in the high cracking susceptibility category. Preheating to 200–300°C is essential to reduce the cooling rate in the heat-affected zone and minimize the risk of hydrogen-induced cracking. The interpass temperature should be maintained at or above the preheat temperature throughout the repair operation.

Process Selection for Cast Frame Overlay

The geometry and accessibility of damaged areas on rolling mill cast frames dictate the choice of welding process. Large flat surfaces such as bearing seats and guide rails are amenable to submerged arc welding (SAW) or flux-cored arc welding (FCAW), while complex geometries and confined areas may require gas metal arc welding (GMAW) or gas tungsten arc welding (GTAW).

For large-area surface restoration, SAW with a self-shielded or gas-shielded hardfacing wire is the most productive option, offering deposition rates of 5–8 kg/h. The overlay material should be selected based on the service condition of the repaired area:

• Bearing seats and guide surfaces: High-carbon martensitic hardfacing (58–62 HRC) for wear resistance
• Impact-loaded surfaces: Medium-carbon austenitic or duplex overlay (35–45 HRC) for toughness
• General surface restoration: Low-carbon steel filler matching the base composition (25–30 HRC)

The multi-pass welding sequence for deep gouges or spalled areas typically begins with a low-carbon, high-toughness first pass to ensure a crack-free bond to the cast substrate, followed by medium-carbon intermediate passes, and concluding with the high-hardness overlay pass. This graded approach minimizes the risk of cracking at the fusion line while achieving the required surface hardness.

Residual Stress Management and Post-Weld Treatment

Given the massive dimensions of rolling mill cast frames, residual stress management is a critical aspect of the repair process. The high heat input associated with welding thick sections generates significant residual stresses that can lead to distortion, dimensional inaccuracy, or even cracking during subsequent service. The following strategies are recommended:

1. Sequencing: Weld in a sequence that promotes uniform heat distribution, such as welding from the center outward or using a symmetric pattern around the damaged area.
2. Interpass Temperature Control: Maintain interpass temperature at 200–250°C for high-carbon cast steels to reduce thermal gradients.
3. Post-Weld Stress Relief: For repairs exceeding 50 mm in length or depth, a full stress relief heat treatment at 580–620°C for 2–4 hours per 25 mm of section thickness is recommended.
4. Mechanical Peening: For areas where heat treatment is impractical, mechanical peening of the weld surface can introduce compressive residual stresses that counteract the tensile welding stresses.

Defect Analysis and NDT Requirements

The inspection of weld overlay repairs on cast frames requires a combination of volumetric and surface NDT methods. Given the thick section sizes, phased array ultrasonic testing (PAUT) is preferred over conventional UT for its superior capability in detecting planar defects in thick sections. Radiographic testing (RT) may be employed for critical repairs but is limited by the thickness and geometry of the frame.

Study Insights and Engineering Practice

The literature highlights that the most significant challenge in cast frame repair is not the welding operation itself but the integration of the repair into the overall structural integrity of the frame. A locally excellent weld that introduces significant residual stresses or causes distortion in adjacent areas can be more detrimental than the original damage. Therefore, the repair strategy must consider the global structural response of the frame, not just the local weld quality. The study reinforces the importance of pre-repair assessment, including measurement of existing residual stresses and evaluation of the remaining structural capacity, before commencing any welding operation on these critical mill components.