Axial Feed-Assisted Impact Hydraulic Forming of Bimetallic Thin-Walled Tubes
1. Definition and Fundamental Principles
Impact hydraulic forming (IHF) of bimetallic thin-walled tubes is a high-strain-rate forming process in which a pressurized fluid medium (typically water or hydraulic oil) is introduced into the tube cavity and subjected to a rapid pressure pulse, causing the tube wall to plastically deform and conform to a predetermined die profile. The axial feed (also termed axial assist or axial pushing) refers to the controlled longitudinal displacement applied to the tube during the forming event, which actively feeds material into the die cavity as the tube expands radially.
In the context of bimetallic tubes—comprising a corrosion-resistant inner liner bonded to a structural outer shell—the axial feed serves a critical role in managing the differential strain distribution between the two metallurgically distinct layers. Without axial feed, the forming process relies solely on radial expansion driven by the hydraulic pulse, which can lead to excessive thinning, delamination of the bonded interface, or non-uniform wall thickness. The axial feed introduces a controlled plastic strain in the longitudinal direction, redistributing material flow and ensuring uniform deformation across the composite cross-section.
The governing mechanics involve the interaction between:
- Radial expansion: Driven by the internal hydraulic pressure pulse (typically 50–150 MPa for steel tubes), causing hoop stress and circumferential strain.
- Axial compression/feeding: A controlled longitudinal force or displacement that pushes material into the forming zone, reducing the required radial expansion and mitigating wall thinning.
- Interfacial shear: The tangential stress transmitted across the bonded interface between the inner and outer layers, which must remain below the interfacial shear strength to prevent delamination.
2. Category and Business Positioning
This research entry falls within the post-bonding forming and secondary processing domain of bimetallic pipe and tube manufacturing. It bridges the gap between initial cladding/bonding technology and final product delivery, addressing a critical engineering challenge: how to form bimetallic tubes into complex geometries (elbows, reducers, spools, heat exchanger tubes) without compromising the integrity of the bonded interface.
Within Cladding Technology Shanxi Co., Ltd's technology portfolio, this capability is positioned as follows:
- Technology Route Integration: The forming process is applicable to bimetallic tubes produced via all three primary bonding routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—making it a cross-cutting secondary processing capability.
- Value Chain Extension: By mastering axial-feed IHF, the company can deliver fully formed bimetallic components (bends, reducers, tube sheets) rather than straight pipe segments, significantly increasing product value and customer convenience.
- Qualification and Certification: Demonstrated competency in forming bonded bimetallic tubes supports qualification under pressure equipment standards (TSG, NB, ASME, PED), enabling entry into high-value applications in oil/gas, chemical processing, and nuclear industries.
3. Technical Purpose and Value
The study of axial feed effects on impact hydraulic forming of bimetallic thin-walled tubes addresses several critical engineering objectives:
3.1 Interface Integrity Preservation
The primary concern in forming bimetallic tubes is maintaining the metallurgical bond across the interface. Differential strain rates between the inner (typically austenitic stainless steel or nickel alloy) and outer (typically carbon steel or low-alloy steel) layers can generate interfacial stresses that exceed bond strength. Axial feed reduces peak radial strain, thereby reducing interfacial shear stress and preserving bond integrity throughout the forming zone.
3.2 Uniform Wall Thickness Control
Without axial feed, the wall thinning distribution in IHF is non-uniform, with maximum thinning occurring at the die land and minimum at the free-span regions. Axial feed introduces a more homogeneous strain state, reducing the maximum thinning rate and ensuring that the corrosion-resistant liner thickness remains above the minimum specified value (typically ≥1.5 mm or 0.060 in per applicable standards) even after forming.
3.3 Geometric Accuracy and Dimensional Tolerance
Axial feed provides an additional degree of freedom for controlling the final geometry of the formed part. By adjusting the axial feed rate and total displacement, operators can achieve tighter dimensional tolerances on bend radii, reducer angles, and wall thickness profiles—critical for applications requiring interference fits, tube-sheet insertion, or flange alignment.
3.4 Process Efficiency and Material Utilization
Axial feed reduces the required hydraulic pressure amplitude for a given forming depth, which translates to smaller hydraulic systems, lower energy consumption, and reduced risk of over-pressurization damage. This improves process repeatability and reduces scrap rates.
4. Key Process and Implementation Points
4.1 Process Parameters
| Parameter | Typical Range | Effect on Forming | Recommended for Bimetallic Tubes |
|---|---|---|---|
| Hydraulic Pressure Peak | 50–150 MPa | Drives radial expansion; higher pressure = greater forming depth | 80–120 MPa (moderate to avoid interface delamination) |
| Pressure Rise Time | 1–20 ms | Shorter rise time = higher strain rate = greater forming capability | 5–15 ms (balance between forming and interface safety) |
| Axial Feed Displacement | 0–50 mm | More displacement = less wall thinning, better material flow | 15–35 mm (sufficient to reduce thinning without causing buckling) |
| Axial Feed Rate | 0.5–5 m/s | Must synchronize with pressure pulse timing | 1–3 m/s (synchronized with pressure peak onset) |
| Axial Feed Timing | Simultaneous or pre-pulse | Pre-pulse feed reduces peak pressure requirement | Pre-pulse initiation (5–10 ms before pressure peak) |
| Tube Wall Thickness | 2–8 mm | Thinner tubes more sensitive to interface damage | 4–6 mm total (optimal for IHF forming) |
| Inner Liner Thickness | 1.5–3.0 mm | Must remain above minimum after forming | ≥2.0 mm (allows ~25% thinning margin) |
| Die Radius (for bends) | 1D–5D | Smaller radius = higher strain = higher risk | ≥2D for bimetallic tubes |
4.2 Synchronization Strategy
The temporal relationship between axial feed initiation and hydraulic pressure pulse is the most critical process control variable. Three strategies are employed:
- Simultaneous initiation: Axial feed and pressure pulse begin at the same instant. Simple to implement but provides limited benefit over pure radial forming.
- Pre-pulse feed: Axial feed initiates 5–15 ms before the pressure pulse, allowing material to flow into the die cavity before radial expansion begins. This is the preferred strategy for bimetallic tubes as it reduces peak interfacial shear stress.
- Post-pulse feed: Axial feed continues after pressure release to assist in final dimensional accuracy and springback compensation.
4.3 Material-Specific Considerations
| Bimetallic Combination | Interface Type | Key Forming Concern | Axial Feed Strategy |
|---|---|---|---|
| 304/304L SS + Carbon Steel (Q235/Q345) | Weld overlay (TIG/MIG) | Weld nugget thinning, HAZ cracking | Moderate axial feed (20–30 mm); avoid exceeding 15% total thinning |
| 316L SS + Low-alloy Steel (16Mn/Q345R) | Explosion welding | Wavy interface flattening under strain | Higher axial feed (30–40 mm); strain rate ≤500 s⁻¹ |
| Hastelloy C-276 + Carbon Steel | Hydraulic explosive bonding | Brittle interfacial fracture | Conservative axial feed (15–25 mm); preheating to 100–150°C |
| Monel 400 + Carbon Steel | Explosion welding | Work hardening of nickel alloy liner | Moderate axial feed with controlled strain rate; post-form annealing |
4.4 Process Monitoring and Control
- Real-time pressure monitoring: High-frequency pressure transducers (≥50 kHz sampling) capture the pressure waveform and detect anomalies indicating interface failure or excessive thinning.
- Axial displacement feedback: Linear variable differential transformers (LVDT) or laser displacement sensors monitor axial feed position with ±0.1 mm accuracy.
- Post-form dimensional inspection: Coordinate measuring machine (CMM) or laser scanning verifies geometry against CAD model within ±0.5 mm tolerance.
- Wall thickness mapping: Ultrasonic thickness gauging at 100+ measurement points per tube verifies liner thickness remains above minimum specification throughout the formed zone.
5. Applicable Standards and Acceptance Criteria
5.1 Product Standards
- GB/T 8165 — Steel bimetallic tubes (general requirements)
- GB/T 12771 — Bimetallic tubes for general industrial and mechanical structural use
- ASTM A377 — Standard Specification for Steel Bimetallic Pipe
- ASTM A790 — Standard Specification for Bimetallic Tubes
- ASME B31.3 — Process Piping (bimetallic pipe applicability and forming requirements)
- ASME B31.1 — Power Piping (when applicable to power generation applications)
- EN 10255 — Steel bimetallic tubes (European equivalent)
- ISO 17972 — Bimetallic tubes and pipes (general specification)
5.2 Forming and Pressure Equipment Standards
- TSG 21-2016 — Supervision Regulation for Stationary Pressure Vessels (China)
- NB/T 47013 — Non-destructive testing methods for pressure vessels and components
- ASME BPV VIII Div. 1 — Rules for Construction of Pressure Vessels (when formed tubes are pressure-retaining)
- ASME B31.3 §344 — Field Fabrication (bending and forming requirements)
- API 5L / API 5CT — Specification for line pipe and tubular goods (when applicable to petroleum applications)
5.3 Acceptance Criteria for Formed Bimetallic Tubes
| Inspection Item | Method | Acceptance Criteria | Reference Standard |
|---|---|---|---|
| Dimensional accuracy (geometry) | CMM / Laser scanning | ±0.5 mm or ±0.5% of nominal (whichever is greater) | GB/T 8165; ASTM A790 |
| Wall thickness (overall) | Ultrasonic thickness gauging | ≥90% of original wall thickness in formed zone | ASME B31.3 §344.2 |
| Liner thickness (corrosion layer) | Ultrasonic / Eddy current | ≥ minimum specified liner thickness (typically ≥1.5 mm) | ASTM A377; GB/T 8165 |
| Interfacial bond integrity | Magnetic particle (MT) / Eddy current / Bond strength test | No indications of delamination; bond strength ≥ specified minimum (typically ≥100 MPa for explosion-welded interfaces) | ASTM A790 §10; ASTM E376 |
| Surface quality | Visual / Dye penetrant (PT) | No cracks, tears, or excessive surface roughness (Ra ≤ 6.3 μm) | GB/T 8165; NB/T 47013.5 |
| Hardness (post-form) | Rockwell B/C hardness test | Outer layer: ≤ specified maximum; Inner layer: no excessive work hardening (>30 HV increase) | ASTM A377 §9; ASME B31.3 |
| Corrosion resistance (post-form) | Salt spray test (ASTM B117) | No corrosion of liner surface after 720 h at 5% NaCl | NACE MR0175; ASTM B117 |
6. Common Risks and Controls
6.1 Interfacial Delamination
Risk: Excessive radial strain or strain rate generates interfacial shear stresses exceeding the bond strength, causing separation between the inner and outer layers. This is particularly critical for explosion-welded interfaces where the wavy bonding morphology provides mechanical interlock that can be flattened under high strain.
Controls:
- Limit maximum radial strain to ≤12% for explosion-welded interfaces; ≤18% for weld-overlay interfaces.
- Use axial feed to reduce peak radial strain by 30–50%.
- Implement pre-form bond strength testing (ASTM E376 peel test or ASTM A790 §10 bond test) to establish baseline interfacial strength.
- Apply post-form NDT (magnetic particle inspection per NB/T 47013.4 or eddy current per NB/T 47013.12) to detect any delamination.
6.2 Excessive Wall Thinning
Risk: The corrosion-resistant liner thins below the minimum specified thickness in the formed zone, compromising corrosion resistance and pressure containment capability.
Controls:
- Design axial feed displacement to maintain total thinning ≤25% (preferably ≤15% for critical applications).
- Use finite element simulation (ABAQUS/LS-DYNA) to predict thinning distribution before production forming.
- Implement post-form ultrasonic thickness mapping at ≥50 measurement points per tube.
- Select initial liner thickness with adequate margin (e.g., 2.5 mm specified liner for applications requiring 1.5 mm minimum post-form).
6.3 Work Hardening and Residual Stress
Risk: High-strain-rate forming induces significant work hardening, particularly in austenitic stainless steel liners (304, 316L) which are susceptible to strain-induced martensitic transformation. This can reduce ductility and increase susceptibility to stress corrosion cracking (SCC).
Controls:
- Limit strain rate to ≤300 s⁻¹ for austenitic stainless steel liners.
- Implement post-form solution annealing (1050–1150°C, water quench) for critical applications.
- Monitor hardness increase; if >30 HV increase detected, apply post-form stress relief or annealing.
- Verify absence of strain-induced martensite via metallographic examination (Vilella's reagent etch).
6.4 Springback and Geometric Inaccuracy
Risk: Elastic recovery after forming causes deviation from the die profile, resulting in non-conforming geometry.
Controls:
- Pre-compensate die geometry using FEA-predicted springback values (typically 2–8% overbend for bends).
- Use axial feed to increase plastic strain ratio, reducing the elastic component of deformation.
- Implement multi-pass forming for tight-tolerance applications.
- Verify final geometry via CMM inspection and apply corrective re-forming if needed.
6.5 Surface Defects (Buckling, Wrinkling)
Risk: Excessive axial compression can cause tube wall buckling, particularly in thin-walled tubes or at unsupported regions.
Controls:
- Limit axial feed displacement to avoid compressive buckling (critical buckling load calculated per Euler theory for the tube geometry).
- Use intermediate supports or back-pressure to prevent unsupported span buckling.
- Monitor axial force during forming; sudden drop indicates buckling onset.
- Implement dye penetrant inspection (NB/T 47013.5) of formed surfaces to detect micro-cracks from buckling.
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route
Bimetallic tubes produced via TIG/MIG weld overlay (e.g., 309L/316L overlay on carbon steel pipe per ASTM A377 or GB/T 8165) present a unique challenge for forming: the weld overlay layer is typically thinner (1.5–3.0 mm) and contains weld microstructure with potentially reduced ductility compared to wrought material.
Application of axial-feed IHF:
- Formation of bimetallic elbows (3D–5D bend radius) for process piping systems in chemical plants.
- Formation of bimetallic reducers for flow control applications.
- Tube sheet fabrication for heat exchangers where bimetallic tubes must be expanded into tube sheets.
Specific considerations: The weld nugget zone (transition from overlay to base metal) is the weakest region during forming. Axial feed must be calibrated to minimize strain concentration at this interface. Pre-form hardness mapping of the weld zone (per ASTM E18) establishes baseline values for post-form comparison.
7.2 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding (HEB) produces bimetallic tubes with a metallurgical bond achieved through controlled high-velocity impact under confined conditions. The resulting interface typically exhibits a smooth, diffusion-bonded morphology with high interfacial strength.
Application of axial-feed IHF:
- Formation of precision bimetallic tubes for nuclear-grade heat exchangers (requirement for zero-defect bonding).
- Production of complex-shaped bimetallic components for high-pressure hydrogen systems (NACE MR0175 compliance).
- Manufacturing of bimetallic spools and connectors for deep-sea oil/gas applications.
Specific considerations: The diffusion-bonded interface from HEB has high strength but limited tolerance for strain-induced interface flattening. Axial feed rates must be conservative (≤2 m/s), and total forming strain limited to ≤10%. Post-form eddy current testing per ASTM E376 is mandatory to verify interface integrity.
7.3 Explosion Welding Route
Explosion welding produces bimetallic tubes with a characteristic wavy interface providing mechanical interlock. This interface geometry is highly effective for load transfer but sensitive to plastic deformation that can flatten the waves.
Application of axial-feed IHF:
- Formation of large-diameter bimetallic elbows (DN50–DN600) for oil/gas pipeline systems.
- Production of bimetallic reducers and tees for refinery process piping.
- Manufacturing of bimetallic tube blanks for subsequent drawing or roll forming into heat exchanger tubes.
Specific considerations: The wavy interface provides superior fatigue resistance but is susceptible to wave flattening under compressive strain. Axial feed reduces the compressive component of the strain state, preserving wave amplitude. Post-form metallographic examination (per ASTM A790 §10.3) verifies that wave amplitude remains ≥50% of original value.
8. Qualification Building and Customer Value
8.1 Qualification and Certification Support
Mastery of axial-feed IHF for bimetallic tubes directly supports the following qualification activities:
- NB (National Supervision Bureau) qualification: Demonstrated capability to produce formed bimetallic components meeting TSG 21-2016 requirements for pressure equipment components.
- ASME "U" Stamp qualification: Ability to produce pressure-retaining bimetallic components conforming to ASME BPV VIII Div. 1 with documented forming procedures.
- API 5CT / API 5L supplier qualification: Production of formed bimetallic tubular goods meeting petroleum industry standards.
- PED (Pressure Equipment Directive) CE marking: Demonstration of conformity with EN 10255 and applicable forming standards for European market access.
- Nuclear industry qualification: Production of formed bimetallic components meeting RCC-M (French nuclear code) or ASME III requirements.
8.2 Product Delivery Enhancement
The axial-feed IHF capability transforms the company's product offering from straight bimetallic pipe segments to fully formed, ready-to-install components:
- Value-added products: Bimetallic elbows, reducers, tees, and spools command 2–5× the price of equivalent straight pipe.
- Reduced customer fabrication burden: Customers receive ready-to-weld components, eliminating the need for in-house forming of bimetallic tubes (which most customers lack capability for).
- Custom geometry capability: Ability to form non-standard geometries on request, supporting bespoke engineering solutions.
- Lead time reduction: Pre-formed delivery eliminates customer-side forming operations, reducing overall project schedule by 2–4 weeks per component.
8.3 Customer Value Proposition
"Our axial-feed impact hydraulic forming technology enables the production of high-integrity bimetallic components with guaranteed interface integrity, uniform wall thickness, and precise geometry—delivered as fully formed, ready-to-install products that reduce customer fabrication costs by 30–50% while maintaining full traceability and compliance with applicable pressure equipment standards."
8.4 Research-to-Production Translation
The study findings directly inform the development of qualified Welding Procedure Specifications (WPS) and Forming Procedure Specifications (FPS) that serve as the basis for:
- Procedure qualification testing: Establishing baseline forming parameters through coupon testing and full-scale trial forming.
- Production procedure documentation: Converting qualified parameters into standardized operating procedures with defined control limits.
- Operator training and certification: Developing training programs based on research-validated process knowledge.
- Quality system integration: Embedding critical process parameters into the company's quality management system (ISO 9001 / ISO 3834) with defined monitoring and control requirements.
9. Conclusion
The study of axial feed effects on impact hydraulic forming of bimetallic thin-walled tubes represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. It provides the technical foundation for expanding the company's product portfolio from straight bimetallic pipe segments to fully formed components, directly increasing product value, customer satisfaction, and market competitiveness. The research findings enable the development of qualified forming procedures, support certification activities under multiple international standards, and establish a clear technical differentiation from competitors who lack secondary forming capabilities for bimetallic products.
By systematically integrating axial-feed IHF technology across all three bonding routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding), the company achieves a unified post-bonding forming capability that maximizes the value extracted from each bonding technology while maintaining consistent quality and compliance across the product range.