Current Status of Continuous Casting Roll Overlay Technology

Literature Overview

This study note examines a technical review published in 2006 by Yang Zhengyong from Baosteel Equipment Maintenance Company, providing an overview of the current status of overlay welding technology applied to continuous casting rolls. Continuous casting is a critical process in steelmaking, where molten steel is cast into semi-solid billets, slabs, or blooms through a water-cooled mold and subsequently solidified as it travels through a secondary cooling zone. The casting rolls, which support and shape the solidifying steel shell, are subjected to extreme thermal and mechanical loading, including temperatures up to 1500°C at the steel contact surface, cyclic thermal fatigue, and contact stress from the moving steel shell. Overlay welding is the primary technology used to restore and enhance the surface properties of casting rolls, extending their service life and reducing production costs.

Core Technical Content

Continuous casting rolls are typically made of high-chromium cast iron (14-20% Cr) or high-speed steel, with a surface that must withstand repeated thermal cycling, thermal shock from water cooling, and contact with hot steel at temperatures exceeding 1200°C. The overlay technology applied to casting rolls includes several approaches: (1) surfacing welds using hardfacing alloys to restore worn surfaces; (2) overlay of corrosion-resistant alloys for specific service conditions; (3) laser cladding for precision surface modification; and (4) hot isostatic pressing (HIP) to improve substrate properties. The overlay must provide excellent thermal fatigue resistance, high hardness, and good bond strength to the base material, while also accommodating the thermal expansion mismatch between the overlay and roll substrate.

Overlay Alloy Selection and Properties

Alloy Type	Composition (wt%)	Hardness (HRC)	Thermal Fatigue Resistance	Application
High-Cr martensitic	C 2.5-3.0, Cr 20-25	55-62	Good	Mold rolls, secondary cooling rolls
Nickel-hardened	Ni 12-15, Mo 2-3, Cr 8-10	48-55	Excellent	High-temperature zone rolls
Austenitic	C 1.0-1.5, Mn 12-14, Cr 10-12	35-45	Good	Slab caster rolls
High-speed steel type	W 5-7, Mo 5-8, V 3-5, Cr 4-6	60-65	Moderate	High-wear zones
Nickel-based (Inconel 625)	Ni balance, Cr 20-22, Mo 8-10	30-35	Excellent	Special applications

Welding Process Comparison

Process	Heat Input	Dilution Control	Surface Quality	Throughput	Cost
Submerged arc welding (SAW)	High (30-50 kJ/mm)	Poor	Rough	High	Low
Shielded metal arc welding (SMAW)	Medium (15-25 kJ/mm)	Moderate	Moderate	Moderate	Moderate
Gas tungsten arc welding (GTAW)	Low (5-15 kJ/mm)	Good	Excellent	Low	Moderate
Plasma transferred arc (PTA)	Medium (10-20 kJ/mm)	Good	Excellent	High	High
Laser cladding	Low (3-10 kJ/mm)	Excellent	Excellent	Moderate	High
Oxy-fuel welding	Low (5-10 kJ/mm)	Poor	Rough	Low	Low

Process Challenges and Solutions

The overlay of continuous casting rolls presents several unique challenges that distinguish it from general industrial cladding applications. The primary challenge is thermal fatigue resistance: the roll surface experiences repeated heating and cooling cycles as the steel shell solidifies and is water-cooled, with temperature gradients of up to 1000°C across the roll wall thickness. This cyclic thermal loading can cause cracking of the overlay layer, particularly if the thermal expansion coefficient mismatch between the overlay and base material is significant. The solution involves selecting overlay alloys with thermal expansion coefficients closely matched to the base material and employing multi-pass overlay strategies with controlled interpass temperatures to minimize residual stresses.

Common Defects and Countermeasures

Defect	Mechanism	Detection	Countermeasure
Thermal fatigue cracking	Cyclic thermal stress	MT, PT, UT	Match thermal expansion, reduce residual stress
Overlay spalling	Poor bond strength, thermal shock	Visual, UT	Improve surface preparation, transition layer
Excessive dilution	High heat input, poor process control	Metallographic, XRF	Reduce heat input, use low-dilution process
Hardness non-uniformity	Inconsistent cooling rate	Hardness traverse	Multi-pass strategy, parameter control
Cracking at interface	Residual stress, thermal mismatch	UT, MT	Preheat, PWHT, reduce heat input

Engineering Practice and Industry Trends

The continuous casting roll overlay industry has evolved significantly since the early 2000s, with several key trends shaping current practice. First, there is a shift from conventional arc welding processes toward advanced thermal spray and laser cladding technologies, which offer superior control over dilution, microstructure, and surface quality. Second, the use of nickel-based overlay alloys has increased for high-temperature applications, despite their higher cost, because of their superior thermal fatigue resistance and corrosion resistance in cooling water environments. Third, the integration of computational modeling and finite element analysis into overlay design has improved the prediction of residual stress distributions and crack initiation sites, enabling more rational process optimization. Fourth, the adoption of robotic welding systems has improved process consistency and reduced operator variability, leading to more reliable overlay quality.

Study Insights and Conclusions

The continuous casting roll overlay technology represents a critical enabler of efficient steelmaking operations, with the overlay quality directly impacting caster productivity, steel quality, and overall production cost. The selection of overlay alloy must be driven by a thorough understanding of the specific service conditions, including temperature, cooling water chemistry, steel grade, and casting speed, rather than by generic recommendations. The process selection should balance throughput requirements with quality demands: for high-volume production with moderate quality requirements, SAW or PTA may be appropriate; for critical applications requiring excellent surface quality and low dilution, laser cladding or GTAW is preferred. Engineers should invest in comprehensive quality control, including non-destructive testing, metallographic examination, and hardness verification, to ensure that overlay layers meet the required performance specifications. The continuous evolution of overlay technology, driven by advances in materials science, process engineering, and quality control, will continue to improve the reliability and efficiency of continuous casting operations in the global steel industry.