Centrifugal Composite Casting-Extrusion-Rolling Process for Carbon Steel-Stainless Steel Composite Tubes

Literature Overview

This paper by Li Yuhe, Dang Yuchun, and Feng Xiangqin (2012), published in Hot Working Technology (Vol. 41, No. 15, pp. 71-74), presents a process development study for producing bimetallic composite steel tubes through a combined centrifugal composite casting, hot extrusion, and cold rolling sequence. The research was conducted at Xihua University and Panzhihua University, with the objective of establishing a viable manufacturing route for carbon steel/stainless steel composite tubes with a nominal size of φ47mm × 4.5mm. The outer layer consists of 10# carbon steel (a low-carbon structural steel equivalent to ASTM A106 Grade A), while the inner layer is 1Cr18Ni9Ti stainless steel (equivalent to AISI 321), selected for its excellent corrosion resistance in aggressive chemical environments.

Process Description and Technical Parameters

The manufacturing process follows a sequential approach where two distinct molten metals are combined during centrifugal casting to form a hollow composite billet, which is subsequently refined through hot extrusion and cold rolling to achieve the final tube dimensions and mechanical properties.

The sequence of pouring—carbon steel first, followed by stainless steel—is critical to the metallurgical bonding mechanism. When the 10# steel melt is poured into the rotating mold, it solidifies against the mold wall under centrifugal force, forming the outer layer. The subsequently poured 1Cr18Ni9Ti melt contacts the partially solidified carbon steel surface, creating a diffusion bond at the interface. The temperature gradient at the interface during solidification governs the formation of intermetallic compounds and the quality of the metallurgical bond.

The choice of 1Cr18Ni9Ti (321 stainless steel) for the inner layer is technically justified. The titanium stabilization prevents chromium carbide precipitation at grain boundaries during hot working, thereby mitigating intergranular corrosion susceptibility. This is particularly important given that the subsequent hot extrusion and cold rolling processes expose the stainless steel to temperatures in the sensitization range (450-850°C).

Metallurgical Bonding and Microstructural Analysis

The quality of the metallurgical bond at the carbon steel-stainless steel interface is the single most critical quality attribute of the composite tube. The authors report that the bonding is metallurgically sound, with no observable delamination or unbonded regions. This finding is consistent with the thermodynamic expectations for Fe-C/Fe-Cr-Ni systems, where mutual solubility in the austenite-ferrite system facilitates diffusion bonding during solidification.

The grain refinement achieved through hot extrusion and cold rolling is a significant metallurgical benefit. Hot extrusion introduces severe plastic deformation and dynamic recrystallization, while cold rolling further refines the grain structure through work hardening and stored energy accumulation. The combined effect results in a composite tube with mechanical properties that exceed those of the base 1Cr18Ni9Ti material, as evidenced by the reported increases in tensile strength and yield strength. The slight reduction in elongation is an expected consequence of the increased dislocation density and grain refinement, reflecting a typical strength-ductility trade-off.

Quality Verification and Testing Results

The authors conducted a comprehensive suite of quality verification tests to validate the composite tube against relevant standards. The test results provide confidence in the process feasibility:

The flattening and bending tests are particularly important for composite tubes, as they assess the integrity of the metallurgical bond under plastic deformation. Failure in these tests would indicate insufficient bonding strength, which could lead to layer separation during service loading. The crystal corrosion test (intergranular corrosion resistance) validates that the hot working temperatures did not sensitize the 1Cr18Ni9Ti layer to intergranular corrosion, confirming the effectiveness of titanium stabilization.

Engineering Practice Considerations

From a practical manufacturing standpoint, the centrifugal composite casting-extrusion-rolling process offers several advantages over alternative composite tube production methods such as explosion welding, cladding, or mechanical bonding. The process is inherently continuous, scalable to production volumes, and does not require specialized equipment beyond standard centrifugal casting machines and rolling mills. The metallurgical bond eliminates the need for subsequent welding or mechanical fastening of the layers, reducing manufacturing complexity and cost.

However, several engineering challenges must be addressed for industrial scale-up. The dimensional accuracy of the composite tube, particularly the uniformity of the stainless steel inner layer thickness, requires careful control of the centrifugal casting parameters including rotational speed, pouring temperature, and cooling rate. Variations in these parameters can lead to thickness variations in the inner layer, which directly affect corrosion protection performance. Additionally, the process must be carefully controlled to prevent mixing or interdiffusion that could compromise the corrosion resistance of the stainless steel layer.

The process is particularly suited for applications requiring corrosion-resistant inner surfaces with structural outer layers, such as chemical process piping, desalination plant tubing, and heat exchanger tubes. The ability to combine the high strength of carbon steel with the corrosion resistance of stainless steel in a single, seamlessly bonded tube represents a significant engineering advantage over welded bimetallic alternatives.

Study Insights and Conclusions

This paper demonstrates a technically viable and economically promising manufacturing route for bimetallic composite steel tubes. The combination of centrifugal composite casting with subsequent hot extrusion and cold rolling achieves a metallurgical bond of sufficient quality for structural and corrosion-resistant applications. The mechanical property improvements resulting from grain refinement during the deformation processes are a valuable byproduct of the manufacturing sequence. For engineers considering composite tube solutions for aggressive service environments, this process route warrants serious evaluation, provided that dimensional control and interfacial quality are rigorously monitored throughout production.