Cracking Failure Analysis of Overlay Welded Layer on Remanufactured Large Hot Rolling Backup Rolls

Literature Overview

The 2022 study by Nie Binying, published in Materials Science and Technology, addresses a critical failure mode observed in remanufactured large hot rolling backup rolls where the overlay welded layer cracks during early service life. The investigation combines hardness testing, microstructural examination, and fractographic analysis to identify the root cause of premature cracking and proposes a corrective heat treatment approach. This work carries direct relevance to rolling mill maintenance engineers and remanufacturing specialists who rely on overlay welding to restore worn roll surfaces to functional dimensions.

Failure Mechanism and Root Cause Analysis

The study definitively identifies low-cycle contact fatigue as the cracking failure mechanism. This determination is significant because it shifts the focus from simple tensile or shear failure to a cyclic loading regime where material degradation accumulates over repeated stress cycles. The fractographic evidence, combined with hardness and microstructural data, reveals that the overlay layer contains an excessive proportion of retained austenite with insufficient martensite content.

The excessive retained austenite content results from the specific composition and cooling rate of the overlay welding process. Retained austenite is a metastable phase that provides ductility but lacks the hardness and strength necessary for contact fatigue resistance. In the high-pressure contact environment of hot rolling, where backup rolls experience Hertzian contact stresses on the order of several GPa, the soft retained austenite regions deform preferentially, initiating microcracks that propagate under cyclic loading.

Corrective Heat Treatment Approach

The proposed remedy involves high-temperature tempering heat treatment designed to transform retained austenite into martensite. This transformation occurs because the elevated temperature allows carbon diffusion and phase rearrangement, followed by controlled cooling that promotes martensitic transformation upon quenching. The resulting increase in martensite content directly enhances the overlay layer hardness and strength, thereby improving contact fatigue resistance.

The effectiveness of this approach depends on careful control of the tempering temperature and cooling rate. Temperatures that are too low will not fully transform retained austenite, while temperatures that are too high may cause over-tempering of existing martensite, reducing hardness below acceptable levels. The optimal temperature window must be determined through dilatometry or thermomechanical analysis specific to the overlay alloy composition.

Engineering Practice Integration

For rolling mill remanufacturing operations, this study establishes several critical quality control checkpoints. First, post-weld hardness testing of overlay layers must be conducted systematically, with acceptance criteria set to ensure sufficient martensite content. Second, retained austenite quantification through X-ray diffraction or metallographic methods should be incorporated into routine quality assurance protocols. Third, post-weld heat treatment procedures must be validated to confirm that the transformation of retained austenite is achieved without introducing new defects such as temper embrittlement or excessive grain growth.

In the context of backup roll remanufacturing governed by industry standards such as ISO 2556 or mill-specific specifications, the overlay welding procedure qualification must include evaluation of the as-welded and heat-treated microstructure. The study underscores that visual inspection and dimensional verification alone are insufficient; metallurgical characterization is essential to ensure long-term service reliability.

Key Questions and Reflections

A critical question arises regarding the composition design of overlay welding consumables for backup roll applications. If retained austenite is detrimental to contact fatigue performance, should the filler metal composition be modified to minimize retained austenite formation in the first place? Alloying additions such as molybdenum, vanadium, or titanium could potentially stabilize the ferrite phase or promote carbide precipitation that reduces retained austenite stability.

Additionally, the study highlights the importance of understanding the specific loading conditions in hot rolling service. Backup rolls experience not only contact stresses but also thermal cycling, oxidation, and scale spalling. The interaction between these multiple degradation mechanisms and the overlay microstructure warrants further investigation. Engineers responsible for roll remanufacturing should consider the full spectrum of service conditions when specifying overlay welding procedures and post-weld treatments.

This failure analysis demonstrates the value of systematic metallurgical investigation in diagnosing premature service failures and establishing corrective measures that extend component life and improve manufacturing quality consistency.