Cladding Repair Process for Continuous Casting Rolls

Literature Overview

The study by Miao Hailiang (published in Welding Technology, 1995) addresses the cladding repair process for continuous casting rolls, conducted at the Tianjin No. 2 Metallurgical Machinery Factory Repair Center. Continuous casting rolls are critical components in steelmaking that experience extreme thermal cycling, mechanical loading, and contact with molten steel. Their repair and restoration through cladding is essential for economic steel production, as roll replacement is prohibitively expensive.

Core Technical Points

Continuous Casting Roll Service Conditions

Continuous casting rolls operate under demanding conditions:

Parameter	Typical Value	Impact on Roll
Molten steel temperature	1500–1600 °C	Thermal shock, oxidation
Roll surface temperature	800–1000 °C	Phase transformation, softening
Thermal cycling rate	50–200 cycles/hour	Thermal fatigue cracking
Contact pressure	5–20 MPa	Contact fatigue, indentation
Sliding speed	0.5–2.0 m/min	Friction, wear
Cooling water temperature	20–40 °C	Thermal gradient stress

The combination of thermal shock, mechanical wear, and chemical attack (from slag and scale) leads to progressive roll surface degradation. Cladding repair restores the roll surface with a wear-resistant, thermal-fatigue-resistant alloy layer.

Cladding Materials for Casting Rolls

The selection of cladding material depends on the roll position and service conditions:

Roll Position	Primary Damage Mechanism	Recommended Cladding Material
Mold roll	Thermal fatigue, thermal shock	Ni-Cr-Si-B (Ni-based), Co-based
Secondary cooling roll	Contact fatigue, wear	Cr-Mo alloy steel, high-carbon steel
Support roll	Mechanical wear, indentation	Hardened alloy steel
Descale roll	Abrasive wear, impact	High-carbon chrome steel

Common cladding alloys include:

Ni-based alloys (e.g., Stellite 6, Ni-Cr-Si-B): Excellent thermal fatigue resistance, oxidation resistance
Co-based alloys (e.g., Stellite 21): Superior wear resistance, good thermal shock resistance
High-carbon chrome steels (e.g., AISI 52100 equivalent): High hardness, good contact fatigue resistance
Cr-Mo-V alloy steels: Good balance of toughness and wear resistance

Process Parameters for Roll Cladding

The cladding of cylindrical rolls requires specialized process considerations:

Parameter	Typical Range	Considerations
Welding process	SAW, GTAW, FCAW	Depends on roll diameter and overlay thickness
Current	200–400 A	Adjusted for roll curvature
Travel speed	150–350 mm/min	Controls heat input per unit length
Wire feed rate	4–8 m/min	Maintains consistent deposition rate
Shielding gas	Ar, CO2, Ar/CO2 mix	Prevents oxidation of molten pool
Preheating	150–350 °C	Reduces thermal stress, prevents cracking
Interpass temperature	150–300 °C	Controls cooling rate and residual stress
Overlay thickness	2–8 mm	Depends on roll size and service life requirement
Number of passes	2–6	Multi-pass for uniformity and thickness

Key Process Challenges

1. Roll curvature effects:

Heat input distribution varies with roll position
Dilution is non-uniform around the roll circumference
Distortion is constrained by the cylindrical geometry
Weld access and visibility are limited

2. Thermal stress management:

The large thermal mass of the roll creates complex temperature gradients
Preheating is essential but must be uniform to avoid differential expansion
Post-weld cooling must be controlled to prevent cracking
Residual stresses can be significant and must be managed through stress relief

3. Multi-pass welding strategy:

First pass: Establishes bond with controlled dilution
Intermediate passes: Build thickness with consistent composition
Final pass: Optimized for surface quality and hardness uniformity
Each pass requires parameter adjustment for the increased roll diameter

Quality Control and Inspection

Inspection Requirements

Inspection	Method	Purpose	Frequency
Visual examination	VT	Surface defects, profile	Every weld
Magnetic particle	MT	Surface cracks	Every repair
Ultrasonic	UT	Bond quality, internal defects	Sampling or every repair
Hardness	HB/HRC	Overlay hardness uniformity	Every repair
Penetrant testing	PT	Surface cracks (non-ferrous areas)	As required
Dimensional	CMM, profile gauge	Geometry, runout	Every repair
Bond strength	Peel test	Interface integrity	Qualification

Common Defects and Solutions

Defect	Root Cause	Solution
Cracking	Thermal stress, hydrogen, composition	Preheating, controlled cooling, proper filler
Spalling	Poor bond, residual stress	Surface prep, stress relief, proper parameters
Uneven thickness	Parameter variation, operator skill	Automation, parameter monitoring
Distortion	Thermal expansion, constraint	Preheating, symmetric welding, fixtures
Porosity	Contamination, gas shielding	Cleaning, proper gas flow, flux drying
Excessive dilution	High heat input, few passes	Multi-pass, lower heat input

Study Insights and Reflections

This 1995 study reflects the industrial reality of continuous casting roll maintenance during a period of rapid steel industry expansion in China. The economic importance of roll repair cannot be overstated: a single continuous casting roll can cost hundreds of thousands of dollars, and replacement downtime can cost millions. Effective cladding repair extends roll life by 2–5 times compared to replacement.

The study's process parameters and quality control procedures remain largely applicable today, though modern implementations incorporate:

Automated welding systems: Robotic cladding with real-time parameter monitoring
Advanced filler metals: New Ni-based and Co-based alloys with improved thermal fatigue resistance
In-situ monitoring: Thermocouples, strain gauges, and acoustic emission for real-time quality assessment
Computational modeling: Finite element analysis for residual stress prediction and process optimization
Surface engineering: Post-weld surface treatments (shot peening, laser texturing) to enhance thermal fatigue resistance

One particularly important insight from this research is the recognition that roll cladding is not a simple surface application but a complex engineering process that must account for the roll's entire service life. The cladding must survive thousands of thermal cycles, contact with molten steel, and mechanical loading without cracking, spalling, or excessive wear. This requires careful consideration of:

Thermal fatigue crack initiation and propagation resistance
Oxidation and scale formation at high temperatures
Contact fatigue resistance under cyclic loading
Wear resistance during sliding contact with steel
Dimensional stability during thermal cycling

The study also highlights the importance of operator skill and procedural discipline in roll cladding. Even with optimal process parameters, inconsistent execution can lead to quality problems. Modern practice addresses this through automation, standardized procedures, and comprehensive training programs.

From a materials engineering perspective, the evolution of cladding materials for casting rolls has been significant since 1995. The development of Ni-Cr-Si-B alloys with improved thermal fatigue resistance, the introduction of functionally graded overlays, and the exploration of ceramic-reinforced composites have all expanded the possibilities for roll surface engineering. Engineers today should consider these advanced materials while maintaining the fundamental process principles established in earlier research.