Repair Weld Overlay of Cast Frames for New Rolling Mills

Literature Overview

This 2000 study by Zhang Liangfeng from Xiangtan Institute of Mechanical and Electrical Engineering addresses the repair welding of cast frames used in new-generation rolling mill equipment. The topic sits at the intersection of heavy machinery maintenance, cast iron welding repair, and surface hardening technology. Rolling mill frames are massive cast structures—often exceeding 50 tons—that endure cyclic loading, thermal gradients, and abrasive wear during steel rolling operations. When localized wear or cracking occurs, complete replacement is economically prohibitive, making weld overlay repair the preferred engineering solution.

Core Technical Content

The fundamental challenge in repairing cast mill frames lies in the metallurgical incompatibility between the base cast iron (typically grey cast iron or ductile iron) and the deposited overlay material. The high carbon equivalent of the base material promotes brittle martensite formation in the heat-affected zone during welding, while the rapid solidification rate in thin overlay layers can produce microcracking. The study emphasizes several critical process parameters:

Parameter	Typical Range	Rationale
Preheating temperature	250–350 °C	Reduces cooling rate below the martensite start temperature
Interpass temperature	200–300 °C	Controls thermal cycling and residual stress
Post-weld heat treatment	500–550 °C for 2–4 h	Stress relief and tempering of hard phases
Weld current density	20–40 A/mm²	Balances dilution control with adequate penetration
Overlay hardness target	45–55 HRC	Ensures wear resistance without excessive brittleness

Process Analysis and Engineering Practice

The repair methodology typically involves a multi-pass approach:

Surface preparation — Removal of loose scale, oil, and severely worn material by grinding or flame cutting, followed by inspection for hidden cracks using magnetic particle testing (MT) or ultrasonic testing (UT).
Transition layer deposition — A low-dilution nickel-based or austenitic manganese alloy (such as Ni-Fe or Ni-Cr-Mo type) is applied as the first pass to buffer the cast iron substrate and prevent cold cracking.
Overlay layer application — One or more passes of high-carbon chromium alloy (e.g., Cr12MoV-type or Cr-C-Mo-V) are deposited to achieve the target hardness and wear resistance.
Post-weld treatment — Stress relief annealing is mandatory to prevent delayed cracking, particularly in thick-section castings where residual stresses can reach 200–300 MPa.

Defect Analysis and Countermeasures

Defect Type	Root Cause	Countermeasure
Hot cracking in overlay	High sulfur/phosphorus in base metal	Add sulfur and phosphorus scavengers to filler
Cold cracking in HAZ	Rapid cooling forming untempered martensite	Increase preheat; apply thicker transition layer
Poor bond strength	Excessive dilution or incomplete cleaning	Reduce current; improve surface preparation
Undercut at boundary	Uneven thermal distribution	Use backing plate or filler strip at joint edge

Study Insights and Engineering Implications

The key insight from this work is that successful repair of cast iron structures demands not merely a hard overlay but a carefully designed metallurgical transition. The concept of a "buffer zone" using austenitic or nickel-based fillers has since become standard practice in heavy equipment repair worldwide. For rolling mill applications, the cyclic stress environment requires that the repair joint fatigue strength approach that of the virgin casting—a requirement that is often overlooked in practice. The study's emphasis on post-weld heat treatment as a non-negotiable step remains valid, as residual stress relief can improve the service life of the repaired area by a factor of 2 to 3. In modern practice, this approach has been supplemented by low-temperature plasma arc welding (LT-PTA) and laser remelting, which further reduce heat input and dilution, but the fundamental metallurgical principles remain unchanged.