Weld Overlay Repair Process for Continuous Casting Rolls

Literature Overview and Technical Background

Continuous casting rolls represent one of the most demanding applications for weld overlay repair in the steel industry. These rolls operate under extreme thermal cycling (surface temperatures reaching 800-1200°C during casting), mechanical loading (contact pressure up to 400 MPa), and thermal shock conditions that can induce temperature gradients exceeding 50°C per minute. The study examines a comprehensive repair methodology for segmental water-cooled copper rolls and cast iron rolls used in slab and bloom casting machines, addressing both surface restoration and dimensional recovery.

The repair challenge is compounded by the requirement to maintain precise roll diameter tolerance (±0.05 mm for the working surface), ensure metallurgical compatibility between the overlay deposit and the base roll material, and achieve service life comparable to new rolls. The study documents a multi-technique approach combining GTAW打底 (root pass), SAW (Submerged Arc Welding) filling, and finishing grinding operations, with specific attention to residual stress management and thermal distortion control.

Base Material Characterization and Overlay Design

The study addresses two primary roll types: copper alloy rolls (Cu-Cr-Zr, typically CuCrZr with 0.25-0.4% Cr and 0.05-0.15% Zr) used for slab casting, and high-chromium cast iron rolls (Cr 12-18%, C 2.5-3.5%) used for bloom and beam casting. Each requires a distinctly different overlay strategy.

For copper alloy rolls, the overlay material selection prioritizes thermal conductivity matching and thermal fatigue resistance. A Cu-Cr alloy with 0.3% Cr content was selected to provide enhanced thermal fatigue resistance while maintaining thermal conductivity above 300 W/m·K. The dilution control is critical because excessive iron dilution from the substrate reduces thermal conductivity and accelerates thermal fatigue cracking.

For high-chromium cast iron rolls, the overlay must address the inherent brittleness of the base material. The selected overlay alloy (Cr 22%, C 2.8%, Mo 0.5%) provides a more ductile microstructure with 15-20% retained austenite, which accommodates thermal cycling without catastrophic cracking. The bond line is designed to achieve a gradual hardness transition from 650 HV (base) through the dilution zone to 580-620 HV in the overlay proper.

Welding Procedure and Process Control

The repair procedure follows a strict sequence designed to minimize thermal distortion and residual stress accumulation. The process begins with mechanical preparation of the damaged area, followed by a systematic multi-pass welding sequence.

Preparation and Preheat

Surface preparation involves grinding back all damaged material to a sound metal surface, verified by magnetic particle inspection (MT) or dye penetrant testing (PT) per JB/T 4730. The repair area is ground to a uniform profile with a maximum undercut of 0.5 mm to prevent stress concentration. Preheating is applied using induction heating to achieve a uniform temperature of 150-200°C for copper rolls and 200-300°C for cast iron rolls. Temperature uniformity across the repair zone must be within ±20°C to prevent differential expansion.

Multi-Pass Welding Sequence

The GTAW root pass is critical for achieving full bond strength. The arc is struck on the base material with a slight overlap (3-5 mm) into the sound metal to ensure metallurgical bonding. The first SAW fill pass uses a reduced current to minimize dilution from the root, gradually increasing current for subsequent passes as the thermal mass of the deposit increases.

Thermal Management During Welding

A key innovation documented in the study is the use of active cooling plates positioned behind the repair zone. These plates, maintained at 20-30°C using forced water circulation, extract heat from the roll body and reduce the effective thermal gradient. This technique reduces peak temperatures at the bond line by 150-200°C compared to uncontrolled cooling, significantly reducing the risk of thermal fatigue cracking in subsequent service.

Post-Weld Treatment and Quality Assurance

After welding, the deposit undergoes a controlled stress relief operation. For copper alloy rolls, stress relief is performed at 540°C for 2 hours with furnace cooling to below 100°C. This temperature is carefully selected to be below the solidus temperature of any eutectic phases at the bond line while being sufficient to relieve 80-90% of welding residual stresses. For cast iron rolls, stress relief at 580°C for 3 hours is applied, followed by controlled cooling at a rate not exceeding 50°C per hour.

The final grinding operation restores the roll to dimensional specification using a CNC grinding machine with diamond grinding wheels. The grinding allowance is typically 1.0-1.5 mm, with the final pass using a fine-grit wheel (60# diamond) to achieve surface roughness Ra ≤ 1.6 μm. Post-grinding surface inspection using white light interferometry verifies the surface profile accuracy.

Defect Prevention and FMEA Analysis

A Failure Mode and Effects Analysis (FMEA) was conducted for the repair process, identifying the following critical failure modes:

The highest RPN (Risk Priority Number) of 240 for bond line cracking justifies the emphasis on preheat temperature control and interpass temperature monitoring. The study recommends the use of infrared thermography for real-time temperature monitoring during welding, with automated arc interruption if interpass temperature exceeds 300°C.

Service Performance and Life Assessment

Field validation was conducted on a slab caster operating at 120 t/h capacity. Repaired rolls demonstrated an average service life of 180 casting heats, compared to 200 heats for new rolls, representing a 90% life recovery ratio. The primary failure mechanism for repaired rolls was thermal fatigue cracking initiating at the grinding surface, which is consistent with the expected behavior for surface-restored components. No bond line failures were observed, confirming the effectiveness of the dilution control strategy.

Study Insights and Engineering Implications

The systematic approach to continuous casting roll repair documented in this study provides a comprehensive framework that integrates metallurgical understanding, process engineering, and quality assurance. The most significant insight is the role of active thermal management during welding, which transforms what was traditionally considered an intractable repair problem into a controlled, predictable process. The residual stress reduction achieved through combined stress relief and active cooling extends service life by 30-40% compared to conventional repair methods that rely solely on post-weld heat treatment. This methodology is directly transferable to other thermally cycled rolling components including finishing mill rolls and cold rolling work rolls, where similar thermal fatigue challenges exist.