CLADDING TECHNOLOGY SHANXI CO., LTDHydraulic Expansion Device and Technical Points for Stainless Steel Lined Composite Pipe
Literature Overview
This paper presents the design, operation, and quality control of a hydraulic expansion device used for manufacturing stainless steel lined composite pipes through the hydraulic expansion (hydroforming) method. The technique is widely applied in the production of stainless steel-lined carbon steel pipes for chemical processing, oil refining, and pressure vessel fabrication. The study covers the device configuration, expansion process parameters, bond strength verification, and common defects, providing a comprehensive technical reference for engineers involved in bimetal pipe manufacturing.
Device Configuration and Operating Principles
The hydraulic expansion device consists of a hydraulic power unit, a mandrel assembly with segmented expansion plugs, a pipe fixture, and a pressure monitoring system. The operating principle involves inserting a segmented steel mandrel into the carbon steel outer pipe, then applying hydraulic pressure to radially expand the mandrel, which in turn forces the stainless steel inner pipe outward until intimate contact is achieved between the two layers. The bond is maintained by the elastic recovery of the outer pipe, which clamps the inner pipe under residual compressive stress.
Key Process Parameters
| Parameter | Typical Range | Critical Value |
|---|---|---|
| Hydraulic pressure (MPa) | 20-40 | 25-30 |
| Expansion ratio (%) | 1.5-3.5 | 2.0-2.5 |
| Expansion rate (mm/min) | 0.5-2.0 | 1.0 |
| Surface roughness of outer pipe (Ra, μm) | 3.2-6.3 | ≤6.3 |
| Surface roughness of inner pipe (Ra, μm) | 1.6-3.2 | ≤3.2 |
| Bond strength requirement (MPa) | ≥20 | ≥25 |
The expansion ratio is the most critical parameter. Below 1.5%, insufficient plastic deformation prevents reliable bonding. Above 3.5%, the risk of surface cracking in the stainless steel inner pipe increases significantly. The optimal window of 2.0-2.5% provides a balance between bond integrity and surface quality.
Quality Control and Defect Prevention
The primary quality control method for hydraulic expansion bonding is the bond strength test, typically performed by the peel test or the ring tensile test in accordance with ASTM A263 or API 934. The paper reports an average bond strength of 28-35 MPa for properly expanded joints, with the minimum acceptable value set at 20 MPa per API 934 Section 6.
Common defects include:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Insufficient bond | Low expansion ratio | Increase pressure to 30 MPa |
| Inner pipe cracking | Excessive expansion ratio | Reduce to 2.0% |
| Surface scratches | Poor surface preparation | Polish to Ra ≤ 3.2 μm |
| Local delamination | Contamination at interface | Clean with solvent before expansion |
| Uneven expansion | Mandrel misalignment | Verify concentricity < 0.1 mm |
The study emphasizes the importance of surface preparation, noting that any contamination (oil, rust, scale) at the interface can reduce bond strength by 40-60%. A mandatory cleaning step using acetone or a dedicated degreaser is recommended before expansion, and the surfaces should be inspected under 10x magnification for any visible defects.
Engineering Practice Cases
In a recent project for a 316L-lined carbon steel pipe used in a chlor-alkali plant, the hydraulic expansion device was used to manufacture 200 meters of DN150 pipe. The process parameters were optimized to a pressure of 28 MPa and an expansion ratio of 2.2%, achieving an average bond strength of 32 MPa. Non-destructive testing using magnetic flux leakage detected no delamination in 98% of the inspected length, with the remaining 2% showing minor surface imperfections that did not affect bond integrity. The pipes passed hydrostatic testing at 1.5 times the design pressure without any leakage or bond failure.
Study Insights
The hydraulic expansion method offers significant advantages over explosive cladding and roll-bonding for pipe products, particularly in terms of scalability, cost, and surface quality. However, the success of the method is highly dependent on precise control of the expansion parameters and rigorous surface preparation. The paper's systematic approach to process optimization, supported by bond strength testing and non-destructive evaluation, provides a replicable quality framework that can be adapted to different pipe diameters and material combinations. For engineers specifying bimetal pipes in pressure vessel designs, understanding the expansion process parameters is essential for ensuring that the specified bond strength requirements are achievable and verifiable.