Novel Bimetal CRA Composite Pipe Hot Hydroforming New Process

Overview and Background

Hydroforming is a bulk forming process that uses internal hydraulic pressure to shape metal tubes into complex geometries, offering advantages in material efficiency, dimensional accuracy, and part consolidation over traditional machining. When applied to bimetal composite pipes, hot hydroforming introduces unique challenges related to differential thermal expansion, strain compatibility between the two materials, and the preservation of the metallurgical bond integrity under large plastic deformation. The study examines a novel hot hydroforming process for bimetal pipes incorporating a corrosion-resistant alloy (CRA) inner layer, evaluating process parameters, deformation behaviour, and the resulting product quality.

Process Principles and Parameter Optimisation

The hot hydroforming process for bimetal CRA composite pipes involves heating the pipe assembly to a temperature within the recrystallisation range of both materials, followed by application of internal hydraulic pressure and axial feeding to form the desired shape. The study identifies three critical process parameters: forming temperature, hydraulic pressure, and axial feed rate.

The forming temperature must be high enough to ensure adequate formability of the CRA layer (which typically has a higher flow stress than carbon steel at elevated temperatures) but low enough to avoid excessive grain growth or intermetallic formation at the interface. For a typical 316L/16Mn bimetal pipe, the optimal forming temperature is 900 to 950 degrees Celsius. Below 850 degrees Celsius, the 316L layer exhibits insufficient ductility and is prone to cracking at high strain levels; above 1000 degrees Celsius, the interface becomes susceptible to intermetallic compound formation and the carbon steel outer layer may experience excessive oxidation.

The study demonstrates that the strain compatibility between the CRA layer and the structural steel layer is a governing factor in the forming limit. Because the two materials have different flow stress-strain curves at elevated temperatures, the thinner CRA layer tends to thin more rapidly than the outer steel layer under internal pressure. The study recommends that the CRA layer thickness should not exceed 25 percent of the total pipe wall thickness to maintain adequate strain compatibility. For applications requiring thicker CRA layers, a pre-forming step or a multi-stage forming sequence is necessary.

Deformation Analysis and Defect Prevention

The study employs finite element simulation to model the deformation behaviour of bimetal CRA composite pipes during hot hydroforming. The analysis reveals that the maximum strain localises at the inner surface of the CRA layer, where the hydrostatic tensile stress is highest. For a forming reduction of 40 percent in the circumferential direction, the maximum strain in the CRA layer reaches approximately 0.35, which is well within the ductility limit of 316L at 925 degrees Celsius (approximately 0.80). However, for more severe forming reductions exceeding 55 percent, the strain in the CRA layer approaches 0.55, which approaches the upper limit and introduces a risk of cracking.

The study also identifies a previously under-recognised defect mechanism: interface delamination caused by differential plastic flow. When the axial feed rate is too high relative to the hydraulic pressure, the outer steel layer experiences greater axial strain than the CRA layer, creating a shear stress at the interface that can exceed the bond strength. The study recommends a feed-to-pressure ratio (FPR) of 0.008 to 0.015 mm/s per MPa to maintain uniform deformation across both layers.

Engineering Practice and Application Cases

The study presents a successful application of the hot hydroforming process in the manufacture of bimetal CRA composite pipe elbows for a high-pressure hydrogenation unit. The pipe assembly consisted of a 316L inner layer (3 mm thick) bonded to a 16Mn outer layer (12 mm thick), with a total wall thickness of 15 mm. The elbows were formed from straight bimetal pipe sections using the hot hydroforming process at 925 degrees Celsius, 280 MPa hydraulic pressure, and an axial feed rate of 5 mm/s. The resulting elbows exhibited a uniform wall thickness variation of less than 5 percent, a metallurgical bond strength of 320 MPa (measured by ring tensile test), and no evidence of interface delamination or CRA layer cracking after full dimensional and non-destructive testing.

The economic benefit of the hot hydroforming process is significant: compared to traditional machining from forged blanks, the hydroformed elbows achieved a material utilisation rate of 85 percent versus 40 percent, resulting in a material cost reduction of approximately 55 percent. Additionally, the elimination of machining operations reduced the production lead time from 6 weeks to 2 weeks per batch.

Key Reflections and Study Insights

The hot hydroforming of bimetal CRA composite pipes is a process that demands careful balance between the forming requirements of the two constituent materials. The key to success lies in understanding the deformation compatibility between the CRA and structural layers and in selecting process parameters that ensure uniform strain distribution without exceeding the forming limits of either material. Engineers involved in the design and specification of bimetal hydroformed components must engage the process engineers early to define the forming reduction limits, the acceptable wall thickness variation, and the required post-forming inspection protocol. The process offers substantial economic and performance advantages over traditional manufacturing methods, but its successful application requires a thorough understanding of the material deformation behaviour under combined thermal and mechanical loading.