This process involves three pours and forms three composite layers.
This design solves a key technical challenge: the low strength of the intermediate layer and core in ductile iron.
Advanced Material Design
Advanced chemical composition design and effective heat treatment bring significant improvements.
The high-speed steel layer forms a very hard martensitic matrix.
Within this matrix, fine, uniformly distributed high-hardness carbides (such as MC, M6C, and M2C) precipitate.
This microstructure enhances several critical roll properties:
- Wear resistance.
- High-temperature red hardness.
- Thermal fatigue resistance.
Critical Process Control: Timing and Temperature
However, two factors are critical during casting: the timing of the intermediate layer pour and the pouring temperature.
If the intermediate layer is poured too early:
- The previously poured high-speed steel layer will dissolve excessively.
- This results in a thinner high-speed steel layer.
- The final product fails to meet process requirements.
- The roll’s service life is reduced.
If the intermediate layer is poured too late:
The molten steel volume will be insufficient.
Its temperature will drop rapidly.
This often leads to poor bonding at the interface between the high-speed steel layer and the intermediate layer.
Defects such as porosity or inclusions are likely to occur.
The Optimal Pouring Time
The optimal pouring time is precise.
Pour the intermediate layer when the temperature of the high-speed steel layer has just reached the solidus line.
At this point:
The heat from the molten steel of the intermediate layer can melt a 10–20 mm thick section of the high-speed steel layer.
Meanwhile, the anti-oxidant added after the high-speed steel pour can diffuse to the inner surface of the intermediate layer.
This timing ensures two things:
The required thickness of the high-speed steel layer.
Good metallurgical bonding at the interface.
Evidence from Failed Rolls
Analysis of a failed roll confirms this.
Numerous loose pores were found at the interface between the high-speed steel layer and the intermediate layer.
This indicates that the intermediate layer was poured too late.
The result was poor bonding between the two layers.
The Role of the Core Ductile Iron Layer
The core ductile iron layer serves a filling and supporting function in the manufacturing process.
However, the quality of the molten iron matters greatly.
If the content of harmful gases (such as oxygen and nitrogen) in the molten iron is too high, inclusions will form.
These inclusions will rise to the surface.
They accumulate at the interface between the core and the intermediate layer.
This accumulation affects the metallurgical bonding between the intermediate layer and the core.
Evidence from the Failed Roll’s Core
Again, analysis of the failed roller provides evidence.
A 4.5 mm wide zone of inclusion enrichment was found on the side of the ductile iron.
This zone was near the interface with the intermediate layer.
It was formed by the rise of inclusions during the solidification of the ductile iron.
In summary, improper centrifugal casting processes and the low purity of the ductile iron core molten metal led to the presence of numerous loose pores and clusters of inclusions at the interface between the composite roll’s intermediate layer and the ductile iron core, as well as the high-speed steel layer, resulting in a significant reduction in the bonding strength at the interface; Under rolling stresses, cracks initiate preferentially at interface defects and propagate along the voids and inclusion zones, whilst the presence of large, brittle carbides near the interface accelerates crack growth, resulting in the formation of extensive delamination voids at the interface. When the void area reaches a critical size, extensive spalling failure occurs on the surface of the composite roll.
Conclusions and Recommendations
(1) Improper centrifugal casting processes and insufficient purity of the ductile iron core molten metal result in flake-like loose voids and aggregated bands of non-metallic inclusions at the interfaces between the high-speed steel composite roll’s intermediate layer, the high-speed steel layer, and the cast iron core, leading to poor interfacial bonding; Under the action of rolling forces, internal cracks in the roll initiate at these defects and propagate along the voids and inclusions; the presence of large, brittle carbides near the interfaces accelerates crack propagation, ultimately leading to extensive spalling of the roll surface.
(2) It is recommended that roll manufacturers improve their casting processes to reduce solidification porosity defects at the interfaces between the intermediate layer, the surface layer and the core; enhance the purity of the ductile iron core melt to reduce the content of inclusions; and optimise the heat treatment process for the roll body to refine the carbide size in the high-speed steel layer and the intermediate layer, thereby improving the balance of strength and toughness in the roll.