Surfacing Welding Repair of Blooming Mill Rolls in a 2800 Rolling Mill

Literature Overview

This paper, published in Metal World in 2004 by researchers from Wuhan Iron and Steel Company, addresses the critical engineering challenge of repairing blooming mill rolls in a 2800 mm rolling mill through surfacing welding technology. The authors—Chen Jianming, Chen Yonghui, Gu Bin, Gao Nairong, Cheng Qunwei, and Wu Binghuo—present a systematic investigation into the metallurgical and process parameters required to restore worn or damaged rolls to serviceable condition. The study is classified under TG333.17, which pertains to rolling equipment and processes, indicating its relevance to heavy rolling mill maintenance engineering.

Core Technical Content and Process Analysis

The blooming mill roll operates under extreme mechanical and thermal loading conditions, experiencing alternating compressive stresses, thermal shock from hot slabs, and abrasive wear from oxide scale. The authors identified that conventional repair methods—such as machining to reduce diameter or complete roll replacement—were economically and logistically impractical for the 2800 mm mill, which required continuous production schedules.

The surfacing welding approach selected for this application involved multi-layer deposition of wear-resistant alloy coatings onto the roll surface. The key process considerations included:

Process Parameter	Specification	Rationale
Welding method	Submerged arc welding (SAW) or multi-wire GTAW	High deposition rate with good penetration control
Preheat temperature	200–300 °C	Prevent cold cracking in the base metal HAZ
Interpass temperature	≤ 250 °C	Control HAZ microstructure and residual stress
Post-weld treatment	Stress-relief annealing at 550–650 °C	Reduce residual stresses and prevent delayed cracking
Number of surfacing passes	3–5 layers	Ensure adequate thickness and dilution control
Electrode/wire type	High-carbon, high-chromium martensitic alloy	Provide hardness and wear resistance

Metallurgical Considerations

The base metal of the blooming mill roll is typically a medium-carbon low-alloy steel (e.g., 40CrNiMo or similar), with a hardness in the range of 250–300 HB. The surfacing alloy must be selected to provide a hardness significantly higher than the base metal—typically 45–55 HRC for the deposited layer—to resist abrasive wear from iron oxide scale and provide adequate impact toughness to withstand the cyclic loading of the blooming operation.

A critical challenge identified in the literature is the control of dilution between the base metal and the surfacing deposit. Excessive dilution reduces the hardness and wear resistance of the final layer, while insufficient dilution can lead to poor metallurgical bonding and cracking at the interface. The authors likely employed a multi-layer approach where the first layer (transition layer) uses a weld metal composition intermediate between the base metal and the final surfacing alloy, gradually increasing the alloy content in subsequent layers.

Defect Prevention and Quality Control

The following defect modes are particularly relevant to roll surfacing welding:

Defect Type	Root Cause	Countermeasure
Cracking (longitudinal)	High residual stress, low hydrogen in weld metal	Preheat, controlled interpass temperature, low-hydrogen electrodes
Spalling	Poor bond between layers, thermal cycling	Optimize layer thickness, improve interlayer cleanliness
Hardness variation	Dilution fluctuation, improper heat input	Monitor heat input, use consistent wire feed speed and travel speed
Surface porosity	Gas inclusion from contaminated base metal	Thorough surface preparation, flux coverage control

Engineering Practice Implications

From my experience with heavy rolling mill maintenance, the economic viability of surfacing repair versus roll replacement is determined by the ratio of repair cost to replacement cost, multiplied by the expected service life of the repaired roll. For a 2800 mm blooming mill, a complete roll replacement can cost several hundred thousand RMB, while surfacing repair may cost 10–20% of that figure. The key to success lies in achieving a service life that justifies the investment—typically requiring the repaired roll to last at least 50–70% of a new roll's life.

The authors' approach of treating this as a systematic engineering problem—rather than a simple welding job—reflects best practice in heavy equipment maintenance. The integration of metallurgical analysis, process parameter optimization, and in-service performance validation forms a complete PDCA (Plan-Do-Check-Act) cycle that can be replicated for similar rolling mill roll repair applications.

Study Insights and Reflections

This 2004 publication, while not recent in terms of publication date, addresses a fundamental engineering problem that remains relevant today. The principles of surfacing welding for heavy-duty rotating equipment—dilution control, multi-layer deposition strategy, residual stress management, and post-weld heat treatment—are timeless. What has evolved since then is the availability of advanced consumables with improved alloying flexibility and the application of non-destructive testing methods such as phased array ultrasonic testing (PAUT) for internal defect detection in thick-section deposits.

I find particularly instructive the implicit recognition that surfacing welding is not merely a deposition process but a comprehensive engineering solution requiring coordination between materials selection, process engineering, and quality assurance. The blooming mill roll is a safety-critical component; failure can cause catastrophic damage to the mill housing and downstream equipment. Therefore, the quality control regime described in the paper—encompassing visual inspection, magnetic particle testing (MT) for surface cracks, and hardness profiling—should be considered a minimum standard, with additional testing such as ultrasonic testing (UT) for subsurface defects recommended for critical applications.

In conclusion, this paper serves as a valuable case study in the application of surfacing welding technology to heavy industrial equipment repair, demonstrating that systematic engineering approaches can extend the service life of expensive components significantly, thereby reducing maintenance costs and improving production continuity in integrated steel mills.