Crack Failure Analysis of Cladding Layer on Remanufactured Large Hot Rolling Backup Roll

Literature Overview

This failure analysis report examines the cracking of the weld overlay cladding layer on a large hot rolling mill backup roll that was remanufactured through weld overlay repair. The backup roll, with a diameter of approximately 1200 mm, had been in service for a significant period before being remanufactured by removing the worn surface and depositing a new overlay layer. The cracks appeared during subsequent service, leading to non-productive downtime and potential safety hazards. This case study provides critical lessons for engineers involved in the remanufacture of large rolling mill components.

Core Technical Content

Component and Service Background

The backup roll was originally manufactured from a high-chromium cast iron (HCCI) with approximately 18% Cr, designed for hot rolling of carbon steel strips. After extensive service, the working surface exhibited wear of approximately 8 mm, necessitating remanufacture. The remanufacture process involved machining the worn surface to a depth of 12 mm, followed by depositing a 6 mm thick overlay layer using submerged arc welding (SAW) with a high-chromium alloy wire (similar to Stellite 6 composition).

Crack Characterization

The cracks were detected during routine inspection after 45 days of service. They appeared as multiple radial cracks extending from the roll surface inward, with lengths ranging from 2 to 15 mm. The cracks were predominantly located in the overlay layer near the fusion boundary with the base material, with a few cracks penetrating into the base material.

Metallographic examination revealed the following features:

• The cracks originated at the fusion boundary where microstructural discontinuities existed between the overlay and base material.
• The crack paths were intergranular in the overlay layer, indicating stress corrosion cracking or thermal fatigue cracking.
• The fusion boundary exhibited a region of coarse carbide precipitation, approximately 50–80 μm wide, which served as crack initiation sites.
• Inclusion analysis revealed the presence of MnS and TiN inclusions at the crack origins, acting as stress concentrators.

Root Cause Analysis

Using the 5-Why analysis methodology, the root causes of the cracking were identified:

Metallurgical Factors

The base HCCI material contained a complex microstructure of martensite, retained austenite, and chromium-rich carbides (M₇C₃ and M₂₃C₆). The overlay alloy, while chemically similar, had a different microstructure due to the welding thermal cycle. The mismatch in thermal expansion coefficients between the overlay (approximately 12 × 10⁻⁶/°C) and the base HCCI (approximately 14 × 10⁻⁶/°C) created significant thermal stresses during cooling from the welding temperature and during subsequent service at elevated temperatures.

The residual stress distribution was measured using X-ray diffraction and found to be approximately 350 MPa in tension at the fusion boundary, well above the yield strength of the overlay alloy at the service temperature. This tensile residual stress, combined with thermal cycling stresses during hot rolling service, exceeded the crack initiation threshold.

Key Technical Insights and Engineering Implications

PWHT Protocol for HCCI Backup Rolls

The most critical finding of this failure analysis is the inadequacy of the PWHT protocol. For HCCI backup rolls, the recommended PWHT temperature should be 750–800 °C for a minimum of 4 hours per 25 mm of wall thickness, followed by controlled cooling at a rate not exceeding 100 °C/h. The concern about softening the base material was misplaced, as HCCI retains its hardness above 700 °C due to the stability of chromium carbides.

Multi-Pass Overlay Strategy

For large backup rolls requiring overlay repair, a multi-pass strategy with interpass temperature control is essential. The recommended approach includes:

• Preheating the roll to 200–250 °C before welding to reduce thermal gradients.
• Limiting the interpass temperature to 250 °C to prevent excessive grain growth.
• Using a multi-pass sequence that alternates between the crown and flange regions to distribute thermal input evenly.
• Applying a minimum of 3 passes per 2 mm of overlay thickness to ensure adequate fusion and stress distribution.

Inspection and Quality Control

The failure analysis revealed that the pre-overlay inspection was inadequate. The following improvements are recommended:

• Perform ultrasonic testing (UT) of the base material before overlay to detect subsurface defects.
• Conduct magnetic particle inspection (MT) of the machined surface to identify surface cracks that could propagate during welding.
• Implement in-process monitoring of welding parameters to ensure consistency.
• Perform full-surface MT and PT inspection of the completed overlay before returning to service.

Study Insights and Reflections

This failure analysis serves as a powerful reminder that remanufacture of critical rolling mill components requires the same level of metallurgical rigor as original fabrication. The root cause was not a material defect or process error per se, but rather a failure to fully understand the thermal-mechanical interactions between the overlay and the base HCCI material. The tendency to compromise on PWHT parameters due to concerns about base material softening led to residual stresses that ultimately caused premature failure. For engineers responsible for rolling mill maintenance and remanufacture programs, this case underscores the importance of thorough root cause analysis, proper PWHT protocol development, and comprehensive inspection regimes. The economic cost of a 45-day failure versus the cost of proper PWHT is negligible, highlighting that quality investment in overlay repair is always justified.