Microstructure and Property Evolution of Weld Overlay on 42CrMo Continuous Casting Rolls

Literature Overview

This study, published in the Journal of Anhui University of Technology (Natural Science Edition) in 2020 by Liu Zhen, Gao Anyang, Zhao Shiguang, Pan Aisheng, Jiang Ji, and Xing Xueqiang from Anhui Magang Heavy Machinery Manufacturing Co., Ltd., addresses a critical engineering challenge in heavy-duty steel rolling equipment maintenance. The research was funded by the Anhui Provincial Key Research and Development Program (1804a09020067). The work focuses on understanding how weld overlay layers applied to 42CrMo continuous casting rolls evolve in terms of microstructure and mechanical properties during service and re-overlay cycles.

Continuous casting rolls are subjected to extreme thermal cycling, mechanical loading, and corrosive attack from molten steel and protective coatings. The base material, 42CrMo, is a medium-carbon alloy steel with excellent strength and toughness, but it lacks sufficient wear and corrosion resistance for prolonged contact with molten metal. Weld overlay provides a practical solution to extend roll life, but repeated overlay cycles introduce complex microstructural changes that must be understood to ensure reliable performance.

Core Technical Content

The research systematically examines the microstructural evolution of overlay layers on 42CrMo substrate through multiple overlay cycles. The key findings include:

• The overlay material typically employs a high-alloy austenitic or martensitic composition to provide resistance against thermal fatigue and abrasive wear.
• During each overlay cycle, the heat-affected zone (HAZ) experiences repeated thermal exposure, leading to grain coarsening, precipitate coarsening, and potential phase transformations.
• The bond interface between the overlay and substrate develops residual stress patterns that accumulate over successive cycles, potentially leading to delamination or cracking.
• Microhardness profiles across the overlay layer show a gradient from the surface toward the substrate, with the HAZ typically exhibiting peak hardness due to martensitic transformation during rapid cooling.

Microstructural Evolution Patterns

The study identifies three distinct zones within the overlay layer:

1. Surface zone: Characterized by fine-grained structure with retained austenite and carbide precipitates, providing wear resistance.
2. Intermediate zone: Mixed microstructure of martensite and bainite, transitioning from surface characteristics to substrate influence.
3. Heat-affected zone: Coarse grain boundary precipitation and potential tempering effects from previous overlay cycles.

Key Performance Parameters

Process Analysis and Engineering Implications

The welding process parameters play a decisive role in controlling the microstructural evolution. The study recommends the following process window:

• Welding current: 280-340 A for submerged arc overlay
• Arc voltage: 26-30 V
• Travel speed: 180-250 mm/min
• Preheat temperature: 200-250°C to reduce thermal stress
• Interpass temperature: 150-200°C to control HAZ cooling rate
• Post-weld treatment: Stress relief at 550-600°C for 2 hours per 25 mm thickness

A critical insight from this research is the concept of "overlay fatigue" — the progressive degradation of overlay properties with each re-application cycle. The study proposes a maximum of 3-4 overlay cycles before complete roll replacement is warranted. Beyond this threshold, the accumulated thermal damage to the base material compromises structural integrity regardless of overlay quality.

Defect Analysis

The most common defects identified include:

• Cracking: Primarily in the HAZ due to hydrogen-induced cracking and thermal stress, mitigated by proper preheating and post-weld heat treatment.
• Delamination: At the overlay-substrate interface caused by excessive dilution or poor surface preparation, addressed through strict cleaning protocols and controlled dilution rates.
• Porosity: From gas absorption during welding, controlled through flux selection and shielding gas management.
• Undercut: At roll edges where heat dissipation is rapid, corrected by adjusting travel speed and electrode angle.

Integration with Engineering Practice

In industrial practice at Anhui Magang, this research directly informed the development of a roll maintenance protocol that standardized overlay procedures and inspection criteria. The implementation of hardness mapping and bond strength testing after each overlay cycle enabled predictive maintenance scheduling, reducing unplanned downtime by approximately 30%.

The FMEA (Failure Mode and Effects Analysis) approach applied to overlay operations identified the top three risk factors: excessive interpass temperature, inadequate surface preparation, and improper welding parameter selection. Countermeasures included the installation of thermal monitoring systems, mandatory surface roughness verification before overlay, and parameter locking in welding power sources.

Study Insights and Reflections

This research provides valuable quantitative data on overlay degradation mechanisms that can be directly applied to maintenance planning for heavy-duty rolling equipment. The concept of overlay cycle limitation is particularly significant — it shifts the maintenance philosophy from reactive repair to proactive replacement scheduling. The correlation between dilution rate and overlay performance degradation offers a practical metric for determining when a roll has reached its serviceable limit.

The engineering implication is clear: overlay is not an indefinite solution for roll refurbishment. Each cycle introduces thermal damage that accumulates irreversibly. Manufacturers should establish overlay cycle tracking systems and integrate this data into their asset management strategies. The recommended maximum of 3-4 cycles provides a practical benchmark, though this may vary depending on roll diameter, service conditions, and overlay material selection.