Weld Overlay Repair of Universal Rolling Mill Intermediate Frame

Literature Overview and Core Research Question

Universal rolling mills are the backbone of heavy plate and wide-strip production, and their intermediate frames bear enormous alternating loads during the rolling process. Over time, the bearing surfaces of the frame—particularly at the chock and screw interfaces—suffer from severe abrasive and adhesive wear, leading to dimensional inaccuracy, increased rolling force, and eventual structural failure. This study documents the weld overlay repair of a universal rolling mill intermediate frame at a major steel plant, addressing the metallurgical challenges of restoring a large carbon steel structural component with high-performance overlay layers.

Damage Assessment and Repair Strategy

The intermediate frame, fabricated from Q345R low-carbon structural steel, exhibited wear depth ranging from 3 to 8 mm at the bearing contact surfaces. The repair strategy involved complete removal of the worn material to a sound base, followed by multi-layer weld overlay deposition to restore the original dimensional profile and exceed the original hardness requirement.

The selection of D2-type overlay material was based on its excellent wear resistance, high hardness after proper heat treatment, and compatibility with the carbon steel substrate in terms of thermal expansion and weldability.

Welding Process Parameters and Technique

The repair was executed using submerged arc welding (SAW) with a single wire and flux, selected for its high deposition rate, deep penetration, and excellent weld quality on thick sections. The process parameters were optimized through coupon testing prior to production repair.

The first pass was critical for achieving full fusion with the Q345R substrate. A higher current (480 A) and slower travel speed (160 mm/min) were used for the first pass to ensure adequate penetration and bond strength. Subsequent passes employed slightly lower current and faster travel to control heat input and minimize dilution.

Defect Control and Quality Assurance

The primary concerns in this repair were hydrogen-induced cracking (HIC) in the heat-affected zone and cracking at the weld root due to the high carbon equivalent of the D2 overlay material. The study implemented a comprehensive countermeasure package:

• Thorough flux baking at 300°C for 2 hours to minimize hydrogen pickup
• Preheating to 150-200°C to slow the cooling rate and reduce HAZ hardness
• Interpass temperature control below 200°C to prevent excessive grain growth
• Post-weld stress relief at 200°C for 2 hours per 25 mm of thickness
• Magnetic particle inspection (MT) of all weld surfaces per JB/T 4730

The results confirmed complete bond integrity with no detectable cracks or lack-of-fusion defects. Hardness testing across the overlay layers showed values of 58-62 HRC after stress relief, meeting the design requirement. Wear testing on coupon samples demonstrated a 3.5× improvement in wear life compared to the original Q345R surface.

Study Insights and Engineering Implications

This repair case demonstrates that large structural components in heavy industry can be effectively restored through weld overlay technology, significantly extending service life and avoiding costly replacement. The key success factors were meticulous damage assessment, appropriate material selection, rigorous process qualification, and comprehensive quality control. For similar applications in the future, the study recommends incorporating ultrasonic testing (UT) for subsurface defect detection in addition to surface MT, and considering hot-wire TIG overlay for areas with complex geometry where SAW access is limited. The economic benefit of this repair approach—estimated at 85% cost savings compared to frame replacement—makes it a compelling option for plant maintenance strategies.