Cladding Repair Process for Continuous Casting Rolls

Literature Overview and Core Content

Continuous casting (CC) rolls are critical components in steel and aluminum production, subjected to extreme thermal cycling, mechanical loading, and abrasive wear from the passing metal strand. The typical service life of a CC roll is limited by surface degradation, including erosion, thermal cracking, and dimensional wear. This study presents a systematic cladding repair methodology for restoring worn CC rolls to serviceable condition, addressing the challenges of maintaining surface integrity, dimensional accuracy, and metallurgical compatibility.

The research focuses on the application of overlay welding techniques to repair the working surface of CC rolls, with particular attention to the selection of overlay alloy, welding process parameters, post-weld machining, and quality verification. The study provides practical guidance for maintenance engineers and fabrication shops responsible for roll refurbishment.

Service Conditions and Failure Modes

Operating Environment

CC rolls operate in a harsh environment characterized by:

Thermal cycling between ambient temperature and 800–1200°C (depending on the steel grade being cast)
Mechanical contact pressure of 10–50 MPa from the molten or semi-solid metal strand
Abrasive wear from oxide scale and refractory particles entrained in the metal
Thermal shock from water cooling systems applied to the roll surface
Corrosive attack from fluxes, mold powders, and molten metal vapors

Typical Failure Modes

Failure Mode	Description	Root Cause
Surface erosion	Progressive material removal from the roll surface	Thermal fatigue, abrasive wear from oxide scale
Thermal cracking	Network of fine cracks on the roll surface	Thermal shock, inadequate thermal conductivity
Dimensional wear	Loss of roll diameter and groove geometry	Mechanical abrasion, erosion
Surface roughness increase	Deterioration of surface finish	Erosion, spalling of the surface layer
Roll shell cracking	Macroscopic cracks in the roll shell	Thermal fatigue, mechanical overload

Cladding Repair Process

Surface Preparation

The first step in roll repair is thorough surface preparation. The worn surface must be ground to remove all damaged material, including eroded zones, cracked regions, and any previous overlay deposits. The grinding is performed to a depth of 2–5 mm below the original surface to ensure complete removal of degraded material. The resulting surface must be clean, free of oxide scale, oil, and contamination, and have a surface roughness of Ra 6.3–12.5 μm to promote good overlay adhesion.

For rolls with severe damage or deep cracks, additional preparation may be required, including crack removal by machining, TIG welding of through-thickness cracks, and stress relief of the roll shell before overlay application.

Overlay Alloy Selection

The selection of the overlay alloy is critical to the success of the repair. The alloy must provide:

High hardness and wear resistance at elevated temperatures (above 600°C)
Good thermal shock resistance to withstand the rapid temperature changes during casting
Compatibility with the roll shell material (typically low-alloy steel or cast iron)
Sufficient ductility to accommodate thermal expansion and contraction
Resistance to erosion and corrosion from molten metal and fluxes

Commonly used overlay alloys for CC roll repair include:

Overlay Alloy	Hardness (HV)	Key Characteristics	Typical Application
High-carbon martensitic (D2, H13)	500–600	High hardness, good wear resistance	Slab caster rolls, thick-section rolls
Austenitic stainless (309, 310)	200–300	Excellent thermal shock resistance, ductile	Bloom caster rolls, high-temperature applications
Nickel-based (Inconel 625, Stellite 6)	400–550	Superior erosion-corrosion resistance	Small-strand caster rolls, high-quality applications
High-speed steel (M2, M36)	600–700	Very high hardness, good red hardness	Precision rolls, thin-section rolls
Composite (martensite + austenite)	350–500	Balanced hardness and toughness	General-purpose roll repair

Welding Process Selection

The welding process must be selected based on the roll geometry, required deposit thickness, and available equipment. The most common processes for CC roll repair are:

Submerged arc welding (SAW): Suitable for thick deposits (3–10 mm) on large rolls. Provides high deposition rate and good penetration. Requires flux and wire consumables.
Plasma transferred arc welding (PTA): Provides excellent control over dilution rate and deposit quality. Suitable for medium-thickness deposits (2–5 mm). Preferred for high-quality applications.
Gas tungsten arc welding (GTAW/TIG): Suitable for thin deposits and repair of localized damage. Provides good visual control but lower deposition rate.
Laser cladding: Emerging technology offering very low dilution, high precision, and minimal heat input. Suitable for high-value rolls requiring precise dimensional control.

Welding Parameters

The following parameter ranges have been found effective for CC roll overlay repair:

Parameter	SAW	PTA	GTAW
Arc current	300–500 A	200–400 A	100–250 A
Arc voltage	28–35 V	25–35 V	15–25 V
Travel speed	200–500 mm/min	100–300 mm/min	50–150 mm/min
Wire/powder feed rate	8–15 m/min	0.5–2.0 kg/min	—
Shielding gas	Flux	Ar + 5% O₂	Ar
Preheat temperature	150–250°C	100–200°C	100–200°C
Interpass temperature	< 300°C	< 300°C	< 300°C

Post-Weld Machining

After overlay deposition, the roll surface must be machined to restore the original geometry and surface finish. The machining sequence typically includes:

Rough turning to remove 1–2 mm of the overlay deposit, exposing a clean, defect-free surface.
Semi-finishing to achieve the required dimensional accuracy (±0.05 mm).
Finishing to achieve the required surface roughness (Ra 0.4–1.6 μm for most applications).
Honing or grinding for precision rolls requiring Ra < 0.4 μm.

The machining allowance must be carefully planned to account for the overlay thickness, expected wear during service, and any dimensional distortion from the welding thermal cycle.

Quality Verification

Non-Destructive Testing

The following NDT methods are recommended for quality verification:

Magnetic particle testing (MT): For detection of surface and near-surface cracks in the overlay and HAZ.
Ultrasonic testing (UT): For detection of internal defects (porosity, lack of fusion) in the overlay and bond interface.
Visual inspection (VT): For assessment of surface quality, porosity, and cosmetic defects.
Hardness testing: For verification of the overlay hardness profile and dilution zone.

Dimensional and Geometric Verification

After machining, the roll must be verified for:

Diameter tolerance (typically ±0.02–0.05 mm)
Roundness (typically < 0.01 mm TIR)
Taper (typically < 0.01 mm/m)
Surface roughness (Ra 0.4–1.6 μm)
Groove geometry (for grooved rolls)

Study Insights and Reflections

The repair of CC rolls through overlay cladding is a well-established practice, but the study highlights several critical factors that are often overlooked in industrial practice. The most important is the selection of the appropriate overlay alloy for the specific service conditions. A one-size-fits-all approach is not acceptable; the alloy must be matched to the thermal, mechanical, and chemical environment of the specific caster application.

The study also emphasizes the importance of surface preparation and post-weld machining. Inadequate surface preparation can lead to poor overlay adhesion and premature failure, while insufficient machining allowance can result in residual defects that propagate during service. The quality of the repair is directly proportional to the care taken in these seemingly simple but critical steps.

From an economic perspective, the repair cost per roll is typically 30–50% of the cost of a new roll, with a service life of 80–100% of the original. This makes overlay repair a highly cost-effective maintenance strategy, provided that the repair quality is consistently maintained. For engineers responsible for roll maintenance, the establishment of standardized repair procedures, trained personnel, and rigorous quality control systems is essential to maximizing the return on investment in roll repair.