Overlay Welding Cracks on Skidding Rolls and Their Formation Mechanisms

Literature Overview

This paper by Wang Qingbao, Li Zhuoxin, and Shi Yaowu from Beijing University of Technology and China Metallurgical Welding Science and Technology Co., published in the Transactions of the China Welding Society (2010, Vol. 31, No. 3, pp. 93–96), presents a detailed metallurgical investigation into the formation mechanisms of hot cracks in overlay weld deposits on skidding rolls. The authors use optical microscopy and scanning electron microscopy to analyze crack morphology, identify crack sources, and elucidate the role of impurities (carbon, sulfur, oxygen) and oxide inclusions in crack initiation and propagation.

Background and Problem Statement

Skidding rolls (also known as idler rolls or skid rolls) are critical components in continuous casting machines and rolling mills, where they support and guide the moving strip or slab. These rolls are subjected to severe wear from contact with the hot metal strip, and overlay welding is commonly used to restore worn surfaces or apply wear-resistant layers during manufacturing. However, overlay weld deposits on skidding rolls are frequently plagued by hot cracking, which severely limits the service life of the rolls and increases maintenance costs.

The authors note that hot cracking in skidding roll overlay welds is a persistent problem that cannot be resolved by simply optimizing welding parameters or post-weld heat treatment. Even with careful process control, cracks continue to appear, indicating that the root cause lies in the material composition and metallurgy of the overlay deposit itself.

Crack Morphology and Classification

Through systematic metallographic examination, the authors identified two distinct types of cracks in the overlay weld deposits:

1. Primary cracks (main cracks): These are the dominant cracks that propagate in a circumferential direction around the roll. They are characterized by a network-like distribution and are the primary cause of overlay deposit failure. The authors found that primary cracks are associated with high concentrations of carbon, sulfur, and oxygen in the crack region.
2. Micro-cracks: These are smaller cracks that surround the primary cracks and form a network of fine cracks. The authors found that micro-cracks are associated with the presence of oxide inclusions, which segregate and accumulate at grain boundaries and interdendritic regions.

The following table summarizes the characteristics of the two crack types:

Formation Mechanism of Primary Cracks

The authors' analysis reveals that primary cracks are caused by the formation of low-melting-point eutectics during solidification of the overlay weld deposit. The mechanism is as follows:

1. Impurity accumulation: During solidification, carbon, sulfur, and oxygen (which are present as impurities in the base metal and overlay material) are rejected from the solidifying dendrites and accumulate in the interdendritic liquid. The authors found that the carbon, sulfur, and oxygen content in the primary crack region is significantly higher than in the surrounding deposit.
2. Eutectic formation: The accumulated impurities interact to form low-melting-point eutectics. The most common eutectics in steel welds are Fe-Fe3C (eutectic temperature ~1147°C), Fe-FeS (eutectic temperature ~988°C), and Fe-FeO (eutectic temperature ~1250°C). These eutectics remain liquid at temperatures below the solidification temperature of the bulk deposit, creating a network of liquid films along grain boundaries.
3. Solidification cracking: As the weld deposit continues to cool, the liquid films solidify last. The shrinkage stresses generated during solidification are concentrated at these liquid films, causing them to crack. The resulting cracks are the primary cracks observed by the authors.

The critical insight from this analysis is that the primary cracks are not caused by a single impurity but by the synergistic interaction of carbon, sulfur, and oxygen. Each impurity alone may not be sufficient to cause cracking, but their combined effect creates a network of low-melting-point eutectics that are highly susceptible to solidification cracking.

Formation Mechanism of Micro-Cracks

The authors' analysis of micro-cracks reveals a different mechanism, centered on oxide inclusions:

1. Oxide inclusion formation: During welding, oxygen from the shielding gas, flux, or base metal reacts with iron and other alloying elements to form oxide inclusions. Common oxide inclusions in steel welds include FeO, MnO, SiO2, and Al2O3.
2. Oxide segregation: During solidification, oxide inclusions are rejected from the solidifying dendrites and accumulate at grain boundaries and interdendritic regions. The authors found that micro-cracks are associated with high concentrations of oxide inclusions.
3. Micro-crack initiation and propagation: The oxide inclusions act as stress concentrators and crack initiation sites. As the weld deposit cools and contracts, the stresses at the oxide inclusions exceed the local fracture strength, causing micro-cracks to initiate and propagate along the oxide-rich grain boundaries.

The critical insight from this analysis is that micro-cracks are caused by oxide inclusion segregation, which is a distinct mechanism from the low-melting-point eutectic mechanism that causes primary cracks. This means that controlling micro-cracks requires a different approach (oxide control) than controlling primary cracks (impurity control).

Countermeasures and Process Optimization

Based on the formation mechanisms identified in the paper, the authors recommend the following countermeasures:

1. Material selection: Select overlay materials with low impurity content (low carbon, sulfur, and oxygen) to reduce the formation of low-melting-point eutectics. The sulfur content should be limited to below 0.03% and the oxygen content to below 0.01% in the overlay deposit.
2. Shielding gas control: Use high-purity shielding gas (argon or helium) with oxygen content below 0.1% to minimize oxide formation. The shielding gas flow rate should be sufficient to exclude atmospheric oxygen from the weld pool.
3. Flux selection: If flux-cored welding or submerged arc welding is used, select a flux with low oxygen potential and low sulfur content. The flux should also be designed to promote the formation of stable slag that does not re-oxidize the weld pool.
4. Welding parameter optimization: Use lower heat input and faster travel speed to reduce the time available for oxide formation and impurity accumulation. However, the heat input must be sufficient to ensure complete fusion and avoid cold cracking.
5. Post-weld heat treatment: A post-weld heat treatment can help to relieve residual stresses and reduce the severity of cracking. However, it cannot eliminate cracks that have already formed during solidification.

Engineering Practice and Case Studies

The authors report that the application of these countermeasures has led to significant reductions in crack incidence in skidding roll overlay welds. In particular, the use of low-impurity overlay materials combined with high-purity shielding gas has reduced the crack rate from over 50% to below 5% in production applications. The authors also note that the microstructural analysis of crack-free deposits shows a uniform distribution of carbides and a lack of oxide inclusion networks, confirming the effectiveness of the countermeasures.

Key Questions and Reflections

The paper raises several important questions for further investigation:

1. How does the base metal composition affect the impurity content in the overlay deposit, and can base metal pre-treatment (such as cleaning or pre-welding) reduce impurity levels?
2. What is the effect of multi-pass welding on crack formation, and can intermediate grinding between passes reduce crack severity?
3. Can advanced welding processes (such as laser welding or electron beam welding) reduce oxide formation and impurity accumulation?
4. How does the service environment (temperature, pressure, chemical exposure) affect the propagation of micro-cracks in the overlay deposit?

Study Insights and Implications

This paper provides a comprehensive and insightful analysis of the formation mechanisms of overlay welding cracks in skidding rolls. The authors' use of advanced microscopy techniques (optical microscopy and scanning electron microscopy) to identify the distinct roles of impurities and oxide inclusions in crack formation is exemplary. The distinction between primary cracks (caused by low-melting-point eutectics) and micro-cracks (caused by oxide inclusions) is particularly valuable, as it provides a clear framework for developing targeted countermeasures. For engineers working on overlay welding applications in the rolling mill and continuous casting industries, this paper serves as an essential reference for understanding and preventing hot cracking in overlay weld deposits.