Overlay Layer Cracking Failure Analysis in Remanufactured Large Hot Rolling Backup Rolls

Literature Overview

This 2022 paper by Nie Binying, published in "Materials Science and Engineering," presents a detailed failure analysis of early-service cracking in the overlay welded layer of remanufactured large hot rolling backup rolls. The study employs hardness testing, microstructural examination, and fractographic analysis to identify the failure mechanism and propose corrective measures. The work is particularly significant in the context of industrial remanufacturing, where the restoration of worn rolling mill components through overlay welding is a cost-effective alternative to complete replacement, but where the metallurgical quality of the overlay layer directly determines the service life and reliability of the restored component.

Failure Mechanism Identification

Low-Cycle Contact Fatigue as the Root Cause

Through systematic examination of the crack morphology, hardness distribution, and microstructural characteristics of the overlay layer, the authors concluded that the cracking failure mechanism is low-cycle contact fatigue damage. This determination is critical because it distinguishes the failure from other common overlay welding defects such as hot cracking, hydrogen-induced cracking, or thermal fatigue cracking, each of which would require different preventive measures.

Low-cycle contact fatigue in this context refers to the progressive damage accumulation at the surface of the overlay layer under repeated contact loading during the rolling process. The backup roll, during hot rolling operations, experiences extremely high contact stresses at the roll gap where it supports the work rolls. These cyclic contact stresses, combined with the thermal cycling of the hot rolling environment, create a severe combined loading condition that can initiate and propagate subsurface or surface cracks in the overlay layer.

The Role of Retained Austenite

The central metallurgical finding of this study is that the excessive content of retained austenite within the overlay layer is the primary metallurgical cause of the premature cracking. The overlay layer, designed to provide high hardness and wear resistance through a martensitic microstructure, instead contained an insufficient proportion of martensite due to the excessive retention of austenite. This resulted in:

The retained austenite in martensitic overlay welds is a well-known metallurgical phenomenon, occurring when the cooling rate from the austenitizing temperature is insufficient to fully transform the austenite phase, or when the chemical composition of the deposit promotes austenite stability (for example, through elevated carbon, manganese, or nickel content). In the context of overlay welding on backup rolls, the high thermal mass of the large roll body acts as a heat sink, potentially promoting slower cooling rates that favor retained austenite formation.

Corrective Measures and Heat Treatment Strategy

The authors propose a high-temperature tempering heat treatment to promote the transformation of retained austenite to martensite, thereby adjusting the metastable phase balance within the overlay layer. This approach is metallurgically sound because:

1. High-temperature tempering (typically in the range of 500-650°C for martensitic steels) can destabilize retained austenite through the precipitation of carbides within the austenite phase, reducing its thermodynamic stability and promoting its transformation to martensite during subsequent cooling.
2. The resulting increase in martensite content directly improves the hardness and strength of the overlay layer, enhancing its resistance to contact fatigue damage.
3. The transformation of retained austenite to martensite during cooling also generates additional volume expansion, which can create beneficial compressive residual stresses at the overlay surface, further improving fatigue resistance.

The study emphasizes that this heat treatment approach not only addresses the immediate failure issue but also improves the quality stability and service life of remanufactured backup rolls, representing a practical and economically viable solution for industrial remanufacturing operations.

Engineering Practice Integration

For engineers involved in the remanufacturing of large rolling mill components, this study offers several actionable insights:

• Pre-welding dilution analysis should be performed to predict the expected chemical composition of the overlay deposit, as excessive base metal dilution can alter the austenite stability and promote retained austenite formation.
• Post-weld heat treatment specifications should be validated through microstructural examination to confirm that the retained austenite content has been reduced to acceptable levels (typically below 10-15% for martensitic overlay applications).
• Hardness mapping across the overlay layer thickness should be performed to ensure uniform hardness distribution, as localized soft spots corresponding to retained austenite-rich regions represent potential crack initiation sites.
• The selection of overlay welding consumables should prioritize compositions that minimize retained austenite formation, considering factors such as carbon content, manganese content, and the overall hardenability of the deposit.

Critical Reflections

The study's identification of low-cycle contact fatigue as the failure mechanism, combined with the root cause attribution to excessive retained austenite, represents a rigorous and logical analytical approach. However, the study would benefit from additional quantitative data, such as specific retained austenite percentages measured by X-ray diffraction, hardness profiles across the overlay thickness, and quantitative fractographic analysis to further characterize the crack propagation mechanism. The proposed high-temperature tempering solution, while metallurgically sound, requires careful parameter optimization to avoid over-tempering that could reduce hardness below minimum service requirements.

Summary and Outlook

This failure analysis provides a clear and actionable framework for understanding and preventing overlay layer cracking in remanufactured hot rolling backup rolls. The identification of excessive retained austenite as the root cause, and the proposal of high-temperature tempering as a corrective measure, represent practical solutions that can be immediately implemented in industrial settings. The study underscores the importance of microstructural control in overlay welding applications, particularly for heavy-duty applications where contact fatigue resistance is paramount. Future work should focus on developing predictive models for retained austenite content as a function of welding parameters, base metal thermal mass, and consumable composition, enabling proactive rather than reactive quality control in overlay welding operations.