Overlay Welding Cracks on Transfer Rolls – Formation Mechanism and Metallurgical Analysis

Literature Overview

This paper by Wang Qingbao, Li Zhuoxin, and Shi Yaowu (published in Transactions of the China Welding Institution, 2010, Vol. 31, No. 3, pp. 93–96) presents a detailed metallurgical investigation into overlay welding cracks on transfer rolls used in steel rolling mills. The authors from Beijing University of Technology and China Metalurgical Welding Technology Co., Ltd. conducted simulation overlay welding experiments, combined with optical microscopy and scanning electron microscopy (SEM) analysis, to identify the crack initiation sites and formation mechanisms.

Technical Context

Transfer rolls are critical components in hot rolling mills, responsible for transferring hot steel slabs between rollers. They operate under extreme conditions: high temperatures (up to 800–1000°C), high mechanical loads, and continuous thermal cycling. The roll surfaces are typically overlay welded with wear-resistant and heat-resistant alloys to extend service life. Cracking of these overlay layers is a major reliability concern, as it can lead to sudden roll failure and production shutdown.

Crack Classification and Morphological Analysis

The authors identified two distinct types of cracks in the overlay layers:

Crack Type	Morphology	Location	Primary Cause
Main cracks	Network-like, circumferentially distributed	Overlay surface	Low-melting-point eutectic formation
Micro cracks	Fine, surrounding main cracks	Near main cracks	Oxide segregation and accumulation

The main cracks form a network pattern distributed circumferentially around the roll, which is consistent with the thermal contraction pattern during cooling. The micro cracks are smaller in scale but numerous, surrounding the main cracks and potentially linking them to form a connected crack network.

Metallurgical Analysis of Main Cracks

SEM and Energy-Dispersive X-ray Spectroscopy (EDS) analysis of the main crack surfaces revealed elevated concentrations of carbon (C), sulfur (S), and oxygen (O). The mechanism is as follows:

During solidification of the overlay weld metal, carbon, sulfur, and oxygen segregate to the last-liquid regions at grain boundaries.
These elements interact synergistically to form low-melting-point eutectics:

Fe-Fe₃C eutectic (melting point approximately 1147°C)
Fe-FeS eutectic (melting point approximately 988°C)
FeO-Fe eutectic (melting point approximately 1370°C)
Ternary Fe-C-S and Fe-O-S eutectics with even lower melting points

As the weld metal cools below these eutectic melting points, the liquid films at grain boundaries are subjected to tensile stresses from thermal contraction.
The liquid films rupture, forming cracks that propagate along the grain boundaries.

The circumferential distribution of the main cracks is explained by the thermal contraction pattern: the roll contracts circumferentially as it cools, and the grain boundaries in the overlay layer are preferentially oriented in this direction due to the columnar grain structure typical of overlay welding.

Metallurgical Analysis of Micro Cracks

The micro cracks were found to contain high concentrations of oxide inclusions. The mechanism involves:

During welding, oxygen from the base material, the welding environment, and the flux dissolves into the molten pool.
As the weld metal solidifies, oxide inclusions (primarily Al₂O₃, SiO₂, and FeO·MnO spinel) form and segregate to grain boundaries and interdendritic regions.
The accumulation of oxide inclusions at grain boundaries creates weak interfaces with reduced cohesion.
During thermal cycling (both during welding and during subsequent service), the oxide-rich grain boundaries become preferential crack initiation sites.
The micro cracks form and link up, creating a network that surrounds and connects the main cracks.

Root Cause Analysis and Preventive Measures

The fundamental root cause identified is improper material selection or insufficient material purity. Even with proper welding process parameters and post-weld heat treatment, the presence of impurities (C, S, O) and oxide inclusions in the overlay alloy leads to cracking.

Root Cause	Preventive Measure
High carbon content in overlay alloy	Select low-carbon overlay alloys (C < 0.05%)
High sulfur content	Use low-sulfur alloys (S < 0.01%) or add desulfurizing elements (Ca, RE)
High oxygen content	Use high-purity shielding gas, thorough base material cleaning, and flux with low oxygen activity
Oxide inclusion formation	Add deoxidizers (Al, Si, Ti) to the alloy composition
Low-melting-point eutectics	Reduce C, S, and O content simultaneously; avoid FeS formation by controlling S and Mn ratios

Additional process measures include:

Using consumables with certified low impurity levels.
Ensuring thorough cleaning of the base material surface to remove oxide scales and contaminants.
Using appropriate flux or shielding gas to minimize oxygen and nitrogen pickup.
Controlling cooling rates to reduce thermal stress in the overlay.
Applying post-weld heat treatment to relieve residual stresses and promote healing of micro cracks.

Engineering Practice and Quality Control Implications

For transfer roll overlay welding in production, the following quality control measures are recommended:

Incoming material inspection – Verify the chemical composition of overlay welding consumables, with particular attention to C, S, and O content. Request mill test certificates that include oxygen content analysis.
Surface preparation – Implement rigorous surface cleaning procedures, including abrasive blasting to remove oxide scales and contaminants. The cleaned surface should be welded promptly to avoid re-oxidation.
Welding process monitoring – Monitor shielding gas purity, flux moisture content, and welding parameters to ensure consistent process conditions.
Post-weld inspection – Perform Magnetic Particle Testing (MT) or Liquid Penetrant Testing (PT) after each welding pass and after final heat treatment. Pay particular attention to the circumferential regions where main cracks are most likely to form.
Microstructural verification – Conduct metallographic examination of cross-sections to verify the absence of oxide-rich grain boundaries and low-melting-point eutectics.

Study Insights and Reflections

The most valuable contribution of this paper is the clear distinction between main cracks and micro cracks, and the identification of their different formation mechanisms. This distinction has important implications for preventive measures: main cracks are primarily addressed through alloy composition control (reducing C, S, and O), while micro cracks require attention to oxide inclusion control and deoxidation.

The finding that even proper welding process and heat treatment cannot prevent cracking if the material purity is insufficient is a critical insight for engineers. It shifts the focus from purely process-based quality control to material-based quality assurance. This means that the selection and qualification of overlay welding consumables must be as rigorous as the welding procedure qualification itself.

The synergistic interaction between C, S, and O in promoting low-melting-point eutectic formation is a reminder of the complexity of weld metal metallurgy. Individual impurity levels that might be acceptable in isolation can become problematic when present together. The alloy designer and welding engineer must consider the combined effect of all impurities, not just individual limits.

In conclusion, this paper provides a thorough metallurgical understanding of overlay welding cracks on transfer rolls, offering clear guidance for both material selection and process control. The practical recommendations are directly applicable to improving the reliability of overlay welded rolls in hot rolling mill applications.