Failure Mechanism of Overlay Layer Cracking in Remanufactured Large Hot Rolling Backup Rolls

Literature Overview

This paper by Nie Binying (2022), published in Materials Science and Process, addresses a critical industrial problem: early-stage cracking of the overlay (surfacing) layer on remanufactured large hot rolling backup rolls. The study employs hardness testing, microstructural analysis, and fractography to identify the root cause of failure and proposes a post-weld heat treatment solution to restore serviceability. The relevance to piping and heavy equipment engineering is significant, as backup rolls are analogous to thick-walled cylindrical components where overlay welding is used for surface restoration.

Core Findings and Failure Mechanism

The investigation concluded that the cracking mechanism is low-cycle contact fatigue damage rather than a simple stress-corrosion or hydrogen-induced failure. The key metallurgical root cause was identified as excessive residual austenite content in the overlay layer, which resulted in insufficient martensite volume fraction. This directly led to:

• Reduced hardness of the overlay deposit
• Lower tensile strength of the surfacing metal
• Degraded contact fatigue resistance of the roll surface

The engineering implication is clear: when overlay welding deposits retain too much metastable austenite, the surface cannot sustain the cyclic contact stresses imposed during hot rolling operations.

Technical Analysis of the Metallurgical Problem

The residual austenite issue is a well-known challenge in overlay welding of high-alloy steels. During the rapid solidification of the weld pool, austenite stabilizers (Cr, Ni, Mn) can promote a high fraction of retained austenite at room temperature. If the cooling rate is insufficient to fully transform austenite to martensite, or if the alloy composition favors austenite stability, the resulting microstructure is softer and more susceptible to plastic deformation under contact loading.

Proposed Solution: High-Temperature Tempering

The author recommends a high-temperature tempering heat treatment to promote the transformation of residual austenite to martensite. This approach works through the following metallurgical mechanism:

1. Heating to a temperature range where retained austenite becomes unstable (typically 550–650°C for Cr-based overlay alloys)
2. Retention at temperature allows austenite to transform to martensite upon subsequent cooling
3. The resulting microstructure achieves a balanced austenite-martensite ratio optimized for contact fatigue resistance

This is a practical and economical solution that avoids the need for complete re-welding of the roll surface.

Engineering Practice Implications

For engineers involved in remanufacturing of heavy cylindrical components (backup rolls, mill rolls, large-diameter pipe surfaces), the following lessons are critical:

• Post-weld heat treatment is not optional when overlay deposits contain significant austenite stabilizers.
• The volume fraction of residual austenite must be quantified before service, using XRD or magnetic methods.
• The target microstructure for contact fatigue applications should prioritize martensite with controlled tempering, not high-carbon austenite.
• Low-cycle contact fatigue is a distinct failure mode from rolling contact fatigue (Hertzian fatigue) and requires different design criteria.

Key Questions and Reflections

A critical question arises: why was the residual austenite content not controlled during the initial overlay welding process? The answer likely lies in the selection of filler metal composition and the welding parameters used. For large backup rolls, the heat input during multi-pass overlay welding tends to be high, which promotes austenite retention. Process modifications such as:

• Reducing interpass temperature
• Using filler metals with lower Ni and Mn content
• Applying a dilution control strategy between base metal and overlay

would be effective preventive measures. Additionally, the study does not address the interface bonding quality between the overlay layer and the substrate, which is another potential failure site in remanufactured components.

Study Insights

This paper provides a clear example of how metallurgical understanding directly translates to engineering solutions. The failure was not due to a welding defect per se, but rather a metallurgical design issue that went undetected until service. This reinforces the importance of post-weld microstructural verification for overlay welding applications in heavy industry. The tempering solution is elegant in its simplicity—it requires no additional welding, only a controlled thermal cycle. For piping engineers who oversee overlay repair of heat exchanger tubes or pressure vessel surfaces, this case study underscores that residual austenite content must always be considered when evaluating the long-term durability of overlay deposits.