Overlay Welding Repair of Universal Rolling Mill Intermediate Frame

Literature Overview

This paper, published in Shandong Metallurgy (Vol. 33, No. 4, 2011, pp. 72–73) by authors from Laiwu Steel Group's Section Mill Plant, Laiwu Shangde Industry and Trade Co., Ltd., and Shandong Institute of Metallurgical Science, documents the overlay welding repair of the intermediate frame (牌坊) of a universal rolling mill on a medium-sized production line. The intermediate frame, which had been in service for over 10 years, exhibited severe wear on all working surfaces, negatively impacting the quality of rolled products. The repair methodology involved removing the fatigue layer from worn surfaces, applying overlay welding, machining to drawing dimensions, and repairing failed bolt holes. This case study provides valuable practical insights into the repair of heavy-duty steel mill structural components.

Universal Rolling Mill Intermediate Frame Function and Wear Analysis

The intermediate frame (牌坊) of a universal rolling mill is a critical structural component that houses the mill rolls and provides the reaction forces for the rolling process. It is subjected to extreme cyclic loading, thermal cycling, and mechanical wear during operation.

Service Conditions

Wear Mechanisms

The wear on the intermediate frame working surfaces is attributed to:

• Contact fatigue: Repeated cyclic loading at the roll housing and bearing seats leads to subsurface crack initiation and surface spalling.
• Abrasive wear: Contact with mill rolls, guides, and other components causes material removal.
• Thermal fatigue: Temperature cycling between hot rolling and cooling leads to thermal stress cracking.
• Impact wear: Occasional impact loading from mill upset or material misalignment.

Repair Methodology

The repair process described in the paper follows a systematic approach:

Step 1: Fatigue Layer Removal

The worn working surfaces were machined or ground to remove the fatigue layer, which typically includes:

• Surface oxidation scale and contamination
• Work-hardened and cold-worked material
• Fatigue-damaged subsurface layer with micro-cracks
• Spalled or delaminated material

The depth of material removal depends on the extent of damage and is typically 2-10 mm, depending on the severity of wear and the remaining structural thickness.

Step 2: Overlay Welding

After surface preparation, overlay welding was applied to the repaired surfaces using appropriate electrode materials. The welding process considerations include:

• Electrode selection: Low-hydrogen shielded metal arc welding (SMAW) electrodes were used, selected for compatibility with the base material and the desired overlay properties. The electrode composition was chosen to provide adequate strength, toughness, and wear resistance.
• Welding sequence: A careful welding sequence was designed to minimize residual stresses and distortion, particularly important for a large structural component like the intermediate frame.
• Preheating: Preheating was applied to reduce the cooling rate and minimize cracking susceptibility, especially in areas with high constraint.
• Interpass temperature control: Maintaining interpass temperature within the recommended range (typically 150-250°C) to avoid excessive grain growth and cracking.

Step 3: Machining to Drawing Dimensions

After overlay welding, the repaired surfaces were machined on a large boring and milling machine to restore the original drawing dimensions and tolerances. This step is critical because:

• The overlay welding process introduces dimensional changes and surface irregularities.
• The machined surface removes any surface defects and ensures proper fit with mating components.
• Dimensional accuracy is essential for the proper alignment and operation of the rolling mill.

Step 4: Bolt Hole Repair

Failed bolt holes were also repaired as part of the comprehensive restoration. Bolt hole repair typically involves:

• Drilling out the damaged hole to a larger diameter
• Filling with appropriate weld metal or a bushing
• Reaming or machining to the original hole diameter and tolerance

Quality Control and Verification

The repair was verified through the following quality control measures:

Engineering Practice Considerations

Residual Stress Management

The overlay welding of a large structural component like the intermediate frame introduces significant residual stresses, which can lead to distortion, cracking, or reduced fatigue life. The paper's approach of machining after welding helps to relieve some surface residual stresses, but the bulk residual stress field remains. In practice, the following measures should be considered:

• Controlled welding sequence: Welding from the center outward, or in a balanced pattern, to minimize distortion.
• Post-weld stress relief: Low-temperature stress relief (e.g., 550-650°C for 2-4 hours) can be applied if the component design allows, but this must be coordinated with the overall repair schedule.
• Mechanical peening: Shot peening or hammer peening of the weld surface can introduce beneficial compressive residual stresses.

Metallurgical Compatibility

The selection of welding consumables must ensure metallurgical compatibility with the base material. For high-strength structural steels like Q345, the welding electrode should have a carbon equivalent (CE) and dilated carbon equivalent (Pcm) below critical thresholds to minimize cold cracking susceptibility. Typical requirements include:

• CE (IIW) < 0.40% for unalloyed or low-alloy steels
• Pcm < 0.25% for weldability assessment
• Hydrogen content in the weld metal < 5 mL/100g to minimize hydrogen-induced cracking

Study Insights and Implications

This case study demonstrates the practical application of overlay welding repair for heavy-duty steel mill structural components, which are often too large or complex for complete replacement. The systematic approach—fatigue layer removal, overlay welding, machining, and bolt hole repair—provides a comprehensive methodology for restoring worn structural components to serviceable condition. The successful restoration of the intermediate frame to original design dimensions and precision highlights the effectiveness of this repair strategy when properly executed.

For maintenance engineers in steel mills, this work reinforces several key principles: first, that overlay welding repair is a viable alternative to complete component replacement for large structural components, offering significant cost and time savings; second, that the quality of the repair depends critically on proper surface preparation and fatigue layer removal; third, that post-weld machining is essential for restoring dimensional accuracy and removing surface defects; and fourth, that comprehensive quality control including NDT and dimensional inspection is necessary to ensure the repair meets design requirements. The 10-year service life of the original frame and the successful repair demonstrate that proper maintenance and repair strategies can significantly extend the service life of critical mill infrastructure, providing substantial economic benefits.