Zhuojin Pipe Fitting Co., LtdCentrifugal Composite Casting Extrusion and Rolling Process for Steel-Stainless Steel Composite Pipes
Literature Overview
This paper, published in Hot Working Technology in 2012 (Vol. 41, No. 15, pp. 71–74), reports on the development and experimental validation of a production process for carbon steel/stainless steel composite pipes using centrifugal composite casting followed by hot extrusion and cold rolling. The research was conducted by scholars from Xihua University and Panzhihua University. The study successfully produced composite pipes with dimensions of approximately 47 mm outer diameter by 4.5 mm wall thickness, comprising an outer layer of 10# carbon steel and an inner layer of 1Cr18Ni9Ti austenitic stainless steel.
Process Route Description
The manufacturing process involves several sequential stages, each critical to achieving sound metallurgical bonding and satisfactory mechanical properties. The process flow is as follows:
- Outer layer centrifugal casting: 10# carbon steel is centrifugally cast first to form the outer shell of the hollow billet.
- Inner layer centrifugal casting: 1Cr18Ni9Ti stainless steel is subsequently cast into the molten carbon steel shell to form the composite hollow billet.
- Machining: The composite billet is machined to remove surface defects and achieve precise dimensional tolerances.
- Hot extrusion: The machined billet is hot-extruded to reduce the wall thickness and achieve the target pipe geometry.
- Cold rolling: Final cold rolling refines dimensions, improves surface finish, and enhances mechanical properties through strain hardening.
| Process Stage | Key Parameters | Purpose |
|---|---|---|
| Outer layer casting (10# steel) | Centrifugal speed, pouring temperature | Form outer shell |
| Inner layer casting (1Cr18Ni9Ti) | Pouring temperature, centrifugal speed | Form inner layer, achieve metallurgical bond |
| Machining | Tolerance, surface roughness | Remove defects, achieve dimensions |
| Hot extrusion | Temperature, reduction ratio, extrusion speed | Reduce wall thickness, refine grain |
| Cold rolling | Reduction, temperature, pass schedule | Final dimensions, strain hardening |
Metallurgical Bonding Analysis
The metallurgical bonding between the carbon steel outer layer and the stainless steel inner layer is the most critical quality attribute of this composite pipe. The paper reports that the bonding quality is satisfactory, with the stainless steel inner layer exhibiting relatively uniform thickness and stable chemical composition. This metallurgical bond is achieved through the diffusion of elements at the interface during the centrifugal casting process, followed by further interdiffusion during hot extrusion.
The bonding mechanism involves several factors. During centrifugal casting, the molten stainless steel contacts the semi-solid or solidified carbon steel shell. At the interface, atomic diffusion occurs, creating a transition zone with intermediate composition. The subsequent hot extrusion, performed at elevated temperatures, further promotes interdiffusion and eliminates any residual porosity or oxide films at the interface. The cold rolling stage then introduces plastic deformation that refines the microstructure and enhances the bond strength through work hardening.
Mechanical Property Results
The paper reports that the composite pipe exhibits higher tensile strength and yield strength than standard 1Cr18Ni9Ti stainless steel, while the elongation is somewhat reduced. This behavior is expected and can be explained by the strain hardening introduced during the cold rolling process. The composite pipe also passes standard quality tests including flattening, bending, and intergranular corrosion tests.
| Property | Composite Pipe | Standard 1Cr18Ni9Ti | Assessment |
|---|---|---|---|
| Tensile strength | Higher than standard | Baseline | Improved |
| Yield strength | Higher than standard | Baseline | Improved |
| Elongation | Reduced | Baseline | Acceptable |
| Flattening test | Pass | Pass | Qualified |
| Bending test | Pass | Pass | Qualified |
| Intergranular corrosion test | Pass | Pass | Qualified |
Engineering Practice Integration
This process is particularly relevant for applications requiring corrosion resistance on the inner surface combined with the structural strength and cost-effectiveness of carbon steel on the outer surface. Typical applications include chemical process piping, food processing equipment, pharmaceutical piping, and certain oil and gas production lines where the transported fluid is corrosive but the external environment is benign.
The process offers several advantages over alternative composite pipe manufacturing methods such as explosion welding, cladding, or overlay welding. Centrifugal composite casting produces a homogeneous metallurgical bond throughout the entire pipe length, eliminating the risk of delamination that can occur with mechanically bonded cladding. The subsequent hot extrusion and cold rolling steps refine the microstructure and improve mechanical properties, resulting in a product with performance characteristics that exceed those of the base materials individually.
However, several practical challenges must be addressed for scale-up. The centrifugal casting of dissimilar metals requires careful control of pouring temperatures and centrifugal speeds to ensure complete wetting and bonding at the interface. The density difference between carbon steel (approximately 7.85 g/cm³) and austenitic stainless steel (approximately 7.90 g/cm³) is minimal, which is favorable for centrifugal casting since large density differences can cause segregation or layering. The process also requires careful management of the thermal cycle to avoid cracking during solidification, particularly at the interface where thermal contraction differences between the two materials can generate residual stresses.
Key Questions and Reflections
The study raises several important considerations for engineers evaluating this process for production applications. First, the scalability from the demonstrated 47 mm diameter pipe to larger diameters used in industrial piping systems must be assessed. Centrifugal casting of larger hollow billets introduces challenges related to centrifugal force distribution, solidification control, and defect formation. Second, the long-term performance of the composite interface under cyclic loading, thermal cycling, and corrosive environments warrants investigation. While the intergranular corrosion test passes, the presence of a dissimilar metal interface introduces potential for galvanic corrosion if the bond integrity is compromised.
The process also has implications for product certification. Composite pipes must comply with relevant standards such as GB/T 8165 (composite steel pipes), ASME B31.3 (process piping), or specific industry standards. The mechanical properties, weldability, and corrosion resistance of the composite material must be fully characterized for code compliance.
Study Insights and Implications
This research demonstrates that centrifugal composite casting combined with hot extrusion and cold rolling is a viable manufacturing route for carbon steel/stainless steel composite pipes with good metallurgical bonding and enhanced mechanical properties. The process is particularly attractive for small to medium diameter piping where corrosion resistance of the inner surface is required but full stainless steel construction would be cost-prohibitive. The key engineering insight is that the sequential casting-extrusion-rolling process not only achieves sound bonding but also refines the microstructure to produce a product with mechanical properties superior to the base stainless steel. For production implementation, careful process control of centrifugal casting parameters, extrusion temperature, and cold rolling schedule is essential to ensure consistent quality and dimensional accuracy.