Hydraulic Expansion Technology for Composite Stainless Steel Lined Pipes

Literature Overview

This paper by Wang Xuesheng, Li Peining, and Hui Hu, published in Chemical Equipment and Piping (2002, Vol. 39, No. 4, pp. 42-44), addresses a critical manufacturing challenge in the production of stainless steel lined composite pipes. The authors, affiliated with Shanghai Jiao Tong University and East China University of Science and Technology, developed a novel hydraulic expansion device specifically designed to overcome the limitations of existing hydraulic composite pipe manufacturing methods. The work is classified under TG394 (metal forming) and focuses on welded stainless steel pipe, manufacturing technology, and hydraulic expansion as the key forming method.

Core Technical Content

The fundamental problem addressed is the inadequate bonding quality and dimensional consistency achieved by conventional hydraulic expansion methods when producing composite pipes with stainless steel linings. Traditional approaches often suffer from uneven wall thickness distribution, incomplete interfacial bonding, and excessive plastic deformation of the outer carbon steel tube. The authors designed a new hydraulic expansion apparatus that employs a modified die geometry and a controlled pressure ramp to achieve uniform radial expansion of the inner stainless steel tube against the outer base tube.

The working principle relies on introducing high-pressure fluid into the cavity between the inner stainless steel liner tube and the outer carbon steel tube. As pressure increases, the inner tube undergoes controlled plastic deformation, expanding outward until its outer surface makes intimate contact with the inner surface of the outer tube. The key innovation lies in the die structure, which provides axial support and prevents buckling during the expansion process, while also allowing controlled release of pressure to maintain the bonded interface.

Process Parameters and Manufacturing Technology

The hydraulic expansion process involves several critical parameters that must be carefully controlled to ensure a reliable composite structure:

The manufacturing sequence typically follows these steps: first, the inner stainless steel tube is cut to length and the outer carbon steel tube is prepared with appropriate end treatments. The two tubes are assembled with the stainless steel tube inserted into the outer tube. The hydraulic expansion device is then mounted, and pressure is applied gradually through a calibrated pump system. The expansion continues until the target pressure is reached, at which point the pressure is held for a specified dwell time to allow uniform stress redistribution. Finally, the pressure is released, and the composite pipe is removed for inspection.

Quality Control and Defect Analysis

From a quality assurance perspective, several potential defects must be monitored during and after the expansion process:

1. Incomplete bonding: Occurs when the expansion pressure is insufficient or the expansion ratio is too low, resulting in gaps between the liner and base tube. This is detected through ultrasonic testing (UT) at the interface.
2. Over-expansion cracking: If the expansion ratio exceeds the plastic deformation capacity of the stainless steel liner, circumferential or longitudinal cracks may develop. This is mitigated by strict control of the expansion pressure and ratio.
3. Wall thinning: Excessive radial expansion causes significant thinning of the stainless steel liner wall, reducing its corrosion resistance margin. The minimum allowable thickness is typically specified at 70% of the original wall thickness.
4. Residual stress anomalies: Non-uniform expansion can lead to localized tensile residual stresses at the interface, which may promote corrosion cracking under service conditions.

The authors emphasize that the new device achieves more uniform expansion compared to conventional methods, resulting in better interface bonding and reduced risk of the above defects.

Engineering Practice Implications

This technology is particularly relevant for chemical processing industries where carbon steel pipes are exposed to corrosive media but must maintain structural integrity. The composite approach provides corrosion resistance from the stainless steel liner while utilizing the cost-effective carbon steel outer tube for pressure containment. Applications include reactor feed lines, distillation column internals, and heat exchanger tubes in petrochemical and chemical processing facilities.

In practice, the hydraulic expansion method must be complemented by appropriate welding procedures at the pipe ends. The dissimilar metal weld between the stainless steel liner and any connecting components requires careful selection of filler metals, typically ER309L or ER316L for austenitic stainless steel to carbon steel transitions. Preheating and post-weld heat treatment considerations must account for the different thermal expansion coefficients of the two materials.

Study Insights and Reflections

The paper represents a practical engineering solution to a real manufacturing problem. The development of a purpose-built hydraulic expansion device demonstrates the importance of process-specific equipment design in achieving consistent quality. The authors' approach of modifying the die structure and pressure control system addresses the root causes of quality variability in conventional methods. For engineers involved in composite pipe manufacturing, this work highlights the critical relationship between expansion parameters, material properties, and final product performance. The study also underscores the value of understanding the fundamental mechanics of plastic deformation in tube expansion, as this knowledge enables rational parameter selection rather than purely empirical trial-and-error approaches.