Cladding Repair of New Rolling Mill Casting Frame

Literature Overview

This 2000 publication by Zhang Liangfeng from Xiangtan Institute of Mechanical and Electrical Engineering addresses the weld overlay repair of casting frames used in new rolling mill equipment. Casting frames for rolling mills are massive structural components that bear enormous rolling forces, and localized wear or damage at contact surfaces can lead to production shutdowns of significant economic consequence. The document represents the intersection of heavy equipment manufacturing and repair welding technology in the Chinese steel industry.

Core Technical Content

Rolling mill casting frames typically consist of high-strength cast iron or cast steel components designed to withstand repeated impact loading from rolling forces. Wear occurs primarily at:

• Bearing seat surfaces where journal contact produces sliding wear
• Guide surfaces for roll positioning
• Connection interfaces with other structural elements

The cladding repair approach described involves removing damaged material through grinding or machining, preparing the substrate surface, and applying a wear-resistant overlay to restore dimensional accuracy and surface hardness.

Key Technical Parameters

Process Analysis and Engineering Insights

The repair of rolling mill casting frames demands attention to several critical factors that distinguish this application from simpler hardfacing jobs:

1. Thermal management of massive castings: The high thermal mass of rolling mill frames means that heat accumulates slowly but also dissipates slowly. This creates a paradox — the local weld zone can cool rapidly (promoting brittle microstructures) while the overall casting remains hot (risking distortion of adjacent areas). Controlled interpass temperatures and symmetric welding sequences are essential.
2. Gray iron graphite morphology at the bond line: When welding onto gray cast iron, the graphite flakes in the base metal create discontinuities at the fusion line. If the heat input is too low, the base metal does not fully melt, and the overlay bonds to unmelted graphite, resulting in poor bond strength. If the heat input is too high, excessive austenitization occurs, leading to high hardness and cracking in the HAZ.
3. Dimensional restoration: After cladding, the surface must be machined back to precise dimensions. The rolling mill frame alignment tolerance is typically ±0.05 mm/m, which means the overlay must be applied with sufficient excess and then ground to specification.

Defect Analysis and Prevention

Engineering Practice Integration

In practical rolling mill maintenance operations, the decision to repair versus replace a casting frame is driven by economic analysis. A typical rolling mill casting frame weighs 20–80 tonnes and costs 500,000–2,000,000 CNY to manufacture. A cladding repair costs 50,000–150,000 CNY including downtime, making it economically justified for localized damage.

The welding procedure specification (WPS) for such repairs must be qualified per NB/T 47014 or equivalent standards. Qualification testing should include:

• Shear bond strength test (≥ 150 MPa for cast iron repairs)
• Hardness traverse across the weld zone
• Visual and magnetic particle inspection of the overlay surface
• Dimensional check after machining

Process Window Optimization

The optimal welding parameters for rolling mill frame repairs depend on the specific casting material:

• For HT300 gray cast iron: Use Ni-Fe transition layer (1 pass, 2 mm) followed by Cr-C hardfacing (2 passes, 2 mm each); preheat 350 °C; interpass ≤ 400 °C
• For ZG270-500 cast steel: Direct application of Cr-Mo hardfacing is feasible; preheat 250 °C; interpass ≤ 350 °C
• For heavily damaged areas (> 5 mm removal): Consider building up with low-hydrogen steel first, then applying hardfacing as a separate operation

Key Questions and Reflections

The 2000 publication reflects a period when Chinese rolling mill equipment was being extensively renewed and upgraded. Many of these frames were originally designed without consideration for future repair welding, meaning that metallurgical compatibility between the original casting and the repair overlay was not optimized during initial design.

A critical insight from this work is that the repair welding of massive castings requires a fundamentally different approach from welding of plate or forgings. The thermal mass effect means that conventional WPS parameters developed for plate welding may be inadequate. Engineers must develop casting-specific procedures that account for the slow thermal response and the unique microstructure of cast materials.

Another important consideration is the service environment. Rolling mill frames operate in environments with iron oxide particles, scale, and sometimes coolant sprays. The overlay material must resist not only mechanical wear but also mild corrosion from acidic condensate that forms on iron oxide deposits. Cr-Mo type hardfacing offers better corrosion resistance than Cr-C type and is generally preferred for this application.

Study Value and Outlook

This document contributes to the body of knowledge on heavy equipment repair welding in the Chinese steel industry. The principles described — careful preheating, transition layer application, controlled cooling, and post-weld stress relief — remain the foundation of successful casting repair welding today. Modern additions would include TOFD or PAUT inspection of the bond line for critical applications, and laser cladding for highly localized repairs where thermal distortion must be minimized. The work reminds engineers that even in the age of advanced manufacturing, the ability to repair massive cast structures remains an essential skill that determines plant availability and operational economics.