A New Weld Overlay Process for Continuous Casting Rolls

Literature Overview

This paper presents a novel weld overlay process developed specifically for the repair and refurbishment of continuous casting (CC) copper alloy rolls. Traditional repair methods—such as hot-dip plating, hot isostatic pressing (HIP), and conventional arc welding—have limitations related to thermal distortion, base metal dilution, and residual stress accumulation. The new process combines preheating control, multi-layer deposition with tailored consumables, and post-weld treatment to achieve a durable overlay layer that maintains dimensional accuracy and metallurgical integrity of the roll body.

Problem Statement and Industry Context

Continuous casting rolls operate under extreme conditions: direct contact with molten steel at 1400-1600 degrees Celsius, cyclic thermal loading, mechanical contact stress from the solidifying strand, and corrosion from flux and scale. The working surface of CC rolls typically consists of a copper alloy base (Cu-Cr or Cu-Ni-Sn) with a hard chrome or tungsten carbide overlay. When the overlay wears down, the roll must be refurbished, and the quality of the repair directly affects surface quality of the cast product and roll service life.

Conventional repair approaches face several challenges:

Copper alloy base metals have high thermal conductivity and low melting point, leading to excessive heat-affected zone (HAZ) softening during welding
High residual stresses can cause distortion of the precision-ground roll surface
Dilution of the overlay alloy by the copper base metal reduces hardness and wear resistance
Intermetallic compound formation at the Cu/Fe interface can lead to brittle fracture

The Proposed Process

Process Architecture

The new process consists of the following sequential steps:

Surface preparation: Mechanical grinding to remove the worn overlay and oxide layer, followed by ultrasonic cleaning to eliminate residual oil and contaminants
Controlled preheating: Induction heating to 150-200 degrees Celsius, carefully managed to prevent excessive softening of the copper alloy substrate
Transition layer deposition: A single pass of a Cu-Ni intermediate alloy to establish a metallurgical bond between the copper base and the hardfacing overlay
Multi-pass hardfacing: 3-5 passes of a high-carbon chromium-cobalt alloy (e.g., Stellite 6 or equivalent) with controlled interpass temperature below 150 degrees Celsius
Post-weld stress relief: Low-temperature annealing at 250-300 degrees Celsius for 2-4 hours to relieve residual stresses without softening the overlay
Precision grinding and finishing: Restore the roll to original dimensional specifications

Consumable Selection and Metallurgical Design

The consumable system is the core innovation of this process. The authors designed a three-tier consumable scheme:

Layer	Material	Function	Key Composition
Transition	Cu-20Ni-2Fe	Wetting and bonding	High Cu, moderate Ni
Build-up	Ni-20Cr-5Fe-3Mo	Dilution buffer	Nickel-based solid solution
Working	Co-Cr-W-C (Stellite-type)	Wear resistance	High Co, Cr, W, C

The transition layer is critical because it addresses the fundamental metallurgical incompatibility between copper and iron-based hardfacing alloys. The Cu-Ni alloy has a melting range compatible with the copper base and provides adequate wetting, while the Ni-Cr buffer layer gradually transitions the composition from copper-rich to iron-rich, minimizing intermetallic formation.

Process Parameters

The following parameters were optimized through systematic experiments:

Welding process: GTAW (gas tungsten arc welding) for transition and buffer layers; SAW (submerged arc welding) for build-up passes
Arc voltage: 18-22 V (GTAW); 28-32 V (SAW)
Travel speed: 80-120 mm/min (GTAW); 350-500 mm/min (SAW)
Shielding gas: Argon with 2% hydrogen (GTAW); granulated flux (SAW)
Interpass temperature: Strictly maintained below 150 degrees Celsius
Number of passes: 5-7 total (1 transition + 1 buffer + 3-5 working)

Performance Evaluation

Microstructural Analysis

Metallographic examination reveals a sound metallurgical bond between the copper substrate and the overlay system. The transition layer shows a dendritic microstructure with Cu-Ni solid solution matrix and minor Fe-rich precipitates. The buffer layer exhibits a single-phase austenitic structure with dissolved Cr and Mo, providing excellent ductility and resistance to thermal fatigue. The working layer consists of a matrix of M7C3 carbides in a Co-Cr-W solid solution, with hardness ranging from 450 to 550 HV.

Importantly, no intermetallic compounds such as CuFe or Cu3Ni were observed at the Cu/overlay interface, confirming the effectiveness of the transition layer design. This is a significant improvement over conventional approaches where brittle intermetallics often form at the interface, leading to spalling failure.

Mechanical Properties

Property	Conventional Repair	New Process	Improvement
Overlay hardness (HV30)	380-420	480-550	25-35%
Bond strength (MPa)	85-120	150-210	50-70%
Residual stress (MPa)	250-350 (tensile)	80-150 (tensile)	60-70% reduction
Thermal fatigue cycles to failure	800-1200	2000-2800	2-2.5x

The significant reduction in residual stress is attributed to the controlled interpass temperature and post-weld annealing. The improved bond strength results from the elimination of intermetallic phases and the gradual composition transition.

Service Performance

Field trials on 200 mm and 280 mm diameter CC rolls showed that the new process extends roll service life by 60-80% compared to conventional repair methods. The overlay layer maintained consistent hardness throughout the service period, with no evidence of spalling, cracking, or excessive wear. The surface quality of the cast slab was also improved, with fewer surface defects and better dimensional tolerance.

FMEA Analysis of the Process

Applying Failure Mode and Effects Analysis (FMEA) to the new process reveals the following critical risk areas:

Failure Mode	Cause	Effect	Severity	Occurrence	Detection	RPN	Countermeasure
Overlay spalling	Insufficient bond strength	Roll failure	9	3	4	108	Bond strength testing before reinstallation
Cracking at interface	Excessive residual stress	Surface defects on slab	8	3	3	72	Stress relief annealing; UT inspection
Excessive dilution	High heat input	Reduced hardness	7	4	2	56	Controlled interpass temperature; dilution monitoring
Surface porosity	Inadequate shielding	Reduced fatigue life	6	3	4	72	Enhanced shielding; PT inspection
Dimensional deviation	Thermal distortion	Poor slab quality	7	2	3	42	Precision grinding; dimensional inspection

The highest risk items—overlay spalling and interface cracking—require systematic monitoring through bond strength testing and non-destructive inspection. The process design effectively mitigates these risks through the transition layer approach and controlled thermal input.

Engineering Practice and Implementation Considerations

Equipment Requirements

The new process requires standard welding equipment with some modifications:

GTAW machine with pulsed current capability for transition layer
SAW machine with automatic wire feed for build-up passes
Induction heating system for controlled preheating
Precision post-weld annealing furnace or induction heating for stress relief
Surface inspection equipment: magnetic particle inspection (MT), penetrant testing (PT), ultrasonic testing (UT)

Quality Control Protocol

A comprehensive quality control plan should include:

Pre-weld inspection: surface cleanliness verification, dimensional measurement of the worn area
In-process monitoring: interpass temperature logging, visual inspection of each pass, dilution spot checks
Post-weld inspection: hardness mapping, bond strength testing (destructive sampling), NDT for cracks and porosity
Final verification: dimensional inspection after grinding, surface roughness measurement

Cost-Benefit Analysis

Although the new process requires additional consumables (transition and buffer layers) and post-weld treatment, the extended service life and reduced unplanned downtime result in significant cost savings. A typical CC roll repair using the new process costs approximately 30-40% more than conventional repair but extends service life by 60-80%, yielding a net cost reduction of 25-35% per roll cycle.

Study Insights and Conclusion

This paper presents a well-engineered solution to a persistent industrial problem. The key innovation lies not in any single technological breakthrough but in the systematic integration of metallurgical design (transition layer), process control (interpass temperature management), and post-weld treatment (stress relief) into a coherent process package.

The most valuable lesson for engineers is the importance of addressing metallurgical incompatibility through intermediate layers rather than attempting to force a direct bond between dissimilar materials. This principle applies broadly to bimetallic component fabrication and repair, from copper-steel joints in heat exchangers to titanium-steel clad plates in chemical processing equipment.

The process also highlights the critical role of residual stress management in precision component repair. In applications where dimensional accuracy is paramount—such as continuous casting rolls, turbine blades, and precision shafts—residual stress control is not merely a quality enhancement but a fundamental requirement for functional reliability.

Future work should focus on extending the process to larger diameter rolls (above 400 mm), developing automated welding sequences for roll refurbishment shops, and investigating the applicability of the transition layer concept to other dissimilar metal welding scenarios in the metallurgical industry.