CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Universal Rolling Mill Intermediate Frame - Technical Study Note
Literature Overview
This topic, originating from a 2011 publication in Shandong Metallurgy, addresses the weld overlay repair of the intermediate frame (牌坊) of a universal rolling mill at Laiwu Steel Group's shape steel plant. The intermediate frame is one of the most heavily loaded structural components in a rolling mill, bearing enormous compressive and tensile stresses from the rolling force, as well as vibration and thermal cycling. When cracks or excessive wear develop on critical load-bearing surfaces, complete replacement is economically prohibitive and often logistically impractical. The study by Peng Mingliang, Cheng Shoubao, Bi Yanping, Li Yuwen, and Gong Maoxiu represents a practical engineering solution that has been widely referenced in heavy equipment repair circles in China.
Core Technical Approach
The repair methodology centers on a multi-layer weld overlay strategy applied to the damaged regions of the intermediate frame. The process selection and parameter control are critical because the base material is typically a high-strength low-alloy steel (HSLA) or cast steel, and the overlay must restore not only geometric dimensions but also the mechanical integrity of the component.
The typical process parameters employed in such repairs include:
| Parameter | Typical Range | Notes |
|---|---|---|
| Base material | Cast steel / HSLA steel (e.g., Q345, ZG35CrMo) | Depends on original frame design |
| Overlay consumable | Low-hydrogen E7018 or equivalent SAW flux-cored wire | Match to base hardness |
| Preheating temperature | 200-250 °C | Prevent cold cracking in thick section |
| Interpass temperature | ≤ 250 °C | Maintain ductility, avoid excessive grain growth |
| Post-weld heat treatment | 550-600 °C, 2-4 h, furnace cool | Stress relief after overlay |
| Number of overlay layers | 2-4 passes | Depends on depth of damage |
| Required bond strength | ≥ 90% of base material UTS | Per NB/T 47014 qualification |
Process Analysis and Defect Control
The primary defects encountered in intermediate frame overlay repair include cold cracks in the heat-affected zone (HAZ), porosity in the overlay, lack of fusion at the interface, and residual stress-induced distortion. The 5W2H framework helps organize the countermeasures:
- What defects to prevent: Transverse cold cracks in the HAZ (hydrogen-induced), longitudinal cracks in the overlay (thermal stress), and porosity (gas inclusion from flux moisture).
- Why they occur: High carbon equivalent of the base material combined with hydrogen pickup and rapid cooling in thick sections; improper travel speed or arc length causing incomplete melting; inadequate flux drying.
- How to prevent: Strict preheating to 200-250 °C to slow the cooling rate below the critical threshold for hydrogen embrittlement; use of low-hydrogen consumables stored at 150 °C; controlled interpass temperature not exceeding 250 °C; back purging with argon if applicable to the geometry.
The FMEA approach identifies the highest risk failure mode as cold cracking in the HAZ at the interface between the overlay and the base. The risk priority number (RPN) for this mode is typically high because the severity is catastrophic (loss of structural integrity), the occurrence is moderate (common in thick sections with high carbon equivalent), and the detection difficulty is significant (cracks may be subsurface).
Engineering Practice Insights
From decades of experience in heavy equipment repair, several practical lessons emerge from this type of work:
- Surface preparation is paramount: The damaged area must be ground back to sound metal, with a minimum undercut of 1:6 to eliminate the crack root. Any residual oxide or contamination will cause lack of fusion.
- Weld sequence planning: For large repair areas, a symmetric welding sequence from the center outward minimizes residual stress and distortion. The first pass should be deposited at the lowest feasible current to avoid excessive heat input into the thick base section.
- Post-weld stress relief: Given the thick section and high restraint of the frame, a stress relief heat treatment at 550-600 °C is essential. The cooling rate should be controlled to avoid re-introducing residual stresses.
- Inspection protocol: After repair, the entire overlay and HAZ should be inspected by magnetic particle testing (MT) for surface-breaking defects and ultrasonic testing (UT) for subsurface cracks. A dye penetrant test (PT) provides an additional check on the surface quality.
Key Reflections
The significance of this case study lies in its demonstration that weld overlay repair can be a viable alternative to full component replacement for critical heavy machinery. However, the success of such repairs depends entirely on rigorous process qualification and quality control. The overlay does not merely "fill in" a damaged area; it creates a new metallurgical interface that must perform under the same severe loading conditions as the original material. Engineers must resist the temptation to shortcut the preheating, interpass temperature control, and post-weld heat treatment steps, as these are the primary safeguards against delayed cracking. The reference value of this work extends beyond the specific mill frame application to any thick-section structural repair where weld overlay is employed as a restoration technique.