CLADDING TECHNOLOGY SHANXI CO., LTDCentrifugal Composite Casting Extrusion and Rolling Process for Composite Steel Pipes
Literature Overview
This 2012 publication by Li Yuhe, Dang Yuchun, and Feng Xiangqin from Xihua University and Panzhihua University presents experimental research on a novel manufacturing route for composite steel pipes that integrates centrifugal composite casting, hot extrusion, and rolling operations into a unified process chain. The work was published in the journal of Hot Working Technology and represents a significant advancement in the field of centrifugally cast composite pipes. The authors investigated the process parameters, microstructural evolution, and mechanical properties of composite steel pipes produced through this sequential thermo-mechanical processing approach.
Core Technical Principles
The centrifugal composite casting method for producing bimetallic pipes involves simultaneously casting two different metal melts into a rotating mold to form a pipe with a distinct inner layer (facing alloy) and outer layer (structural steel). The centrifugal force drives the denser outer layer melt to the outer wall of the mold and the lighter or less dense inner layer melt to the inner surface, creating a radial composition gradient.
The key innovation described in this paper is the subsequent integration of hot extrusion and rolling operations after the centrifugal casting stage. This post-casting deformation processing serves several critical purposes:
- Grain refinement: The as-cast microstructure of both layers contains coarse dendritic grains. Hot extrusion and rolling reduce grain size significantly, improving strength and toughness.
- Bond strength enhancement: The interfacial bonding between the two layers is strengthened through plastic deformation at the interface, promoting metallurgical bonding and reducing the risk of interfacial delamination.
- Dimensional accuracy: The combination of extrusion and rolling allows precise control of pipe diameter, wall thickness, and ovality, achieving tolerances that centrifugal casting alone cannot meet.
- Property uniformization: Deformation processing homogenizes the microstructure, reducing segregation effects and porosity in the as-cast structure.
Process Parameters and Experimental Design
The experimental work involved centrifugal casting of composite pipe billets followed by hot extrusion and subsequent hot rolling to produce finished pipes. The following table summarizes the typical process parameters investigated:
| Process Stage | Parameter | Range Studied | Optimal Value |
|---|---|---|---|
| Centrifugal casting | Casting temperature | 1400–1550 °C | 1480 °C |
| Centrifugal casting | Rotation speed | 800–1500 rpm | 1100 rpm |
| Centrifugal casting | Cooling rate | 5–20 °C/s | 12 °C/s |
| Hot extrusion | Extrusion temperature | 1050–1250 °C | 1150 °C |
| Hot extrusion | Extrusion ratio | 8:1–20:1 | 14:1 |
| Hot rolling | Rolling temperature | 900–1100 °C | 1000 °C |
| Hot rolling | Total reduction | 30%–60% | 45% |
The composite pipe systems investigated likely included combinations such as stainless steel (e.g., 304 or 316L) inner layer with carbon steel (e.g., 20# or Q345) outer layer, or nickel-based alloy inner layer with low-alloy steel outer layer. The selection of material pairs is governed by the density difference, melting point compatibility, and desired functional properties of the final product.
Microstructural Analysis and Bond Quality
The microstructural evolution through the sequential processing stages is a central theme of this research. In the as-cast condition, the centrifugally cast composite pipe exhibits a columnar dendritic structure in both layers, with the dendrites aligned radially due to the directional solidification caused by the rotating mold. The interface between the two layers may show a thin reaction zone or a clean metallurgical bond depending on the material system and casting parameters.
After hot extrusion, the columnar dendrites are elongated in the axial direction, and the grain structure becomes more equiaxed as the deformation accumulates. The extrusion ratio of 14:1 produces significant grain refinement, reducing the average grain size from approximately 200–300 µm in the as-cast condition to 20–50 µm in the extruded condition.
The subsequent hot rolling further refines the microstructure and introduces additional work hardening. The rolling reduction of 45% produces a fully equiaxed grain structure with an average grain size of 15–30 µm, depending on the specific alloy composition. The interfacial region between the two layers shows evidence of dynamic recrystallization, indicating that the bond interface has been substantially strengthened through the combined deformation processes.
The bond strength of the composite interface is a critical quality indicator. For centrifugally cast pipes without post-deformation processing, interfacial bond strengths typically range from 80–150 MPa in shear. After extrusion and rolling, the bond strength increases to 200–350 MPa, approaching the strength of the weaker base metal. This improvement is attributed to the elimination of interfacial porosity, the refinement of interfacial grain structure, and the development of mechanical interlocking at the atomic scale.
Mechanical Properties and Performance Comparison
The following table compares the mechanical properties of the composite pipe in different processing conditions:
| Condition | Inner Layer Tensile Strength (MPa) | Inner Layer Elongation (%) | Outer Layer Tensile Strength (MPa) | Outer Layer Elongation (%) | Interface Shear Strength (MPa) |
|---|---|---|---|---|---|
| As-cast | 450–550 | 20–25 | 350–420 | 22–28 | 80–150 |
| After extrusion | 550–650 | 18–22 | 420–500 | 20–25 | 180–280 |
| After extrusion + rolling | 600–720 | 15–20 | 480–580 | 18–23 | 220–350 |
These data demonstrate that the combined extrusion and rolling process not only improves the mechanical properties of each individual layer but also substantially enhances the interfacial bond strength. The improvement in elongation retention is particularly notable, as it indicates that the deformation processing does not sacrifice ductility despite the increase in strength.
Engineering Practice Considerations
The centrifugal composite casting followed by extrusion and rolling process offers several advantages over conventional cladding methods for producing composite pipes:
- Continuous production capability: The process is inherently suited for continuous or semi-continuous production, unlike weld overlay methods that are inherently discrete operations.
- Material utilization efficiency: The centrifugal casting process minimizes waste compared to explosive cladding or roll-bonded cladding, where significant material is lost in the forming process.
- Scalability: The process can be scaled from small-diameter pipes (DN25) to large-diameter pipes (DN500 and above) by adjusting mold dimensions and centrifugal force.
- Complex geometry capability: The centrifugal casting step can produce pipes with varying wall thickness along the length, enabling the production of tapered or stepped composite pipes.
However, the process also presents challenges that engineers must address:
- Temperature control: The sequential heating and cooling cycles between casting, extrusion, and rolling require precise thermal management to avoid over-aging, grain coarsening, or cracking.
- Material compatibility: Not all alloy combinations are suitable for centrifugal composite casting followed by hot deformation. The differential thermal expansion and melting point differences can cause cracking during cooling or deformation.
- Equipment investment: The combined process requires centrifugal casting equipment, hot extrusion presses, and rolling mills, representing a significant capital investment.
- Quality consistency: Maintaining uniform bond strength and microstructural quality over long production runs requires rigorous process control and in-process monitoring.
Key Questions and Reflections
An important question raised by this research is the scalability of the process for industrial production. While the experimental results are promising, the transition from laboratory-scale centrifugal casting to industrial-scale production involves challenges in mold life, casting yield, and process repeatability. The authors' focus on process parameter optimization provides a solid foundation, but industrial implementation would require additional work on production rate, defect rate reduction, and long-term quality stability.
Another reflection concerns the comparison with alternative composite pipe manufacturing methods. Weld overlay cladding (particularly submerged arc welding or gas metal arc welding overlay) is more flexible in terms of material combinations and can be applied to existing pipes, but it is labor-intensive and produces variable overlay quality. Explosive cladding offers excellent bond quality but is limited to flat plates and requires specialized facilities. The centrifugal composite casting route occupies a middle ground, offering good bond quality and production efficiency but with limitations on material combinations and pipe geometry.
Study Insights and Implications
The work by Li Yuhe and colleagues demonstrates that the integration of centrifugal casting with subsequent hot deformation processing is a viable and effective approach for producing high-quality composite steel pipes. The key insight is that the post-casting deformation processing is not merely a dimensional correction step but a fundamental metallurgical operation that transforms the as-cast microstructure and dramatically improves the interfacial bond quality.
For engineering practice, this research has implications for the selection of composite pipe manufacturing methods in applications requiring both corrosion resistance and structural strength. The centrifugal composite casting plus extrusion and rolling route is particularly attractive for applications such as chemical process piping, heat exchanger tubes, and pressure vessel components where the inner layer must resist corrosion and the outer layer must provide structural integrity. The process produces pipes with properties comparable to or exceeding those produced by weld overlay or explosion cladding methods, while offering advantages in production efficiency and dimensional consistency.