Issues and Discussions on the Development of Composite and Bonded Steel Pipes

Literature Overview

This two-part article by He Defu and Wang Jingying, published in Steel Pipe (2017), provides a comprehensive examination of metallurgical composite steel pipes and mechanically bonded steel pipes. The authors, affiliated with Shanghai Jiuli Trade and Development Co., Ltd. and Zhejiang Dechuan Pipe Industry Co., Ltd., address critical technical challenges in the development and application of these advanced pipe products. The article covers the essential differences between metallurgical bonding and mechanical bonding, problems in internal-surface overlay welding of corrosion-resistant alloy (CRA) layers, optimization of weld quality, heat treatment standard comparisons, bond quality inspection methods, and material selection criteria.

Core Technical Content

The fundamental distinction between metallurgical composite pipes and mechanically bonded pipes is central to this discussion. Metallurgical composite pipes achieve a true atomic-level bond between the base steel and the CRA overlay through processes such as explosion welding, roll bonding, or centrifugal casting. Mechanically bonded pipes rely on plastic deformation and surface roughness interlocking, typically achieved through roll bonding of two separate layers under high pressure and temperature.

Comparison of Composite Pipe Bonding Methods

Interpretation of Technical Points

The authors identify several persistent problems in the development of internally overlay-welded CRA composite pipes. The primary challenges include:

1. Weld dilution control: When overlay welding a CRA layer (such as 310SS, 6Mo, or Ni-base alloys) onto a carbon steel base, dilution from the base metal into the weld deposit can significantly reduce the corrosion resistance of the overlay. Multiple passes with proper welding sequence design are essential to minimize dilution.
2. Longitudinal seam weld quality: The longitudinal seam weld in a composite pipe presents unique challenges because the weld must penetrate through both the base steel and the CRA layer. The heat input must be carefully controlled to avoid cracking in the high-alloy CRA while ensuring full penetration of the base material. Weld groove design, preheat temperature, interpass temperature, and post-weld heat treatment (PWHT) requirements all differ from those of a conventional single-material pipe weld.
3. Filler metal selection: The choice of filler metal for the composite pipe weld is critical. A filler metal matched to the CRA layer may be too dilution-sensitive, while a filler metal matched to the base steel may not provide adequate corrosion resistance at the interface. The authors advocate for a carefully qualified filler metal system that accounts for the expected dilution ratio.

The discussion on heat treatment standards is particularly valuable. The authors compare Chinese standards with international counterparts (ASTM, EN, and ISO standards) and identify significant differences in temperature ranges, soaking times, and cooling rate requirements. These differences can lead to non-conformity issues when products manufactured to one standard are intended for use in jurisdictions governed by another standard. The bond quality inspection methods discussed include peel testing, shear testing, and macroscopic examination of the bond line.

Process and Standards Analysis

The welding procedure for composite pipes requires careful consideration of the following parameters:

The post-weld heat treatment for composite pipes is particularly challenging because the base steel and CRA layer have different transformation temperatures and sensitization thresholds. A PWHT temperature suitable for stress relief in carbon steel (around 600°C) may cause sensitization in austenitic stainless steel overlays, leading to intergranular corrosion susceptibility. Conversely, a PWHT temperature required to homogenize the CRA weld metal (around 900°C) may cause grain coarsening and reduced toughness in the carbon steel base.

Integration with Engineering Practice

In engineering practice, composite pipes are increasingly specified for applications where corrosion resistance is critical but the full cost of an all-alloy pipe is prohibitive. Typical applications include:

• Oil and gas production tubing in sour service (H2S-containing environments)
• Chemical processing equipment piping
• Desalination plant piping
• Pulp and paper industry equipment

The material selection guidance provided by the authors is practical and should be followed closely. The base steel should be selected based on the mechanical and pressure requirements, while the CRA layer should be selected based on the specific corrosive environment. Common CRA selections include:

• 316L or 321SS for moderate chloride environments
• 6Mo (SAF-2205) for higher chloride and pH sensitivity
• Alloy 625 or C-276 for aggressive reducing environments
• Ni-base alloys (Hastelloy, Inconel) for extreme corrosion conditions

A critical observation from the authors' discussion is that the longitudinal seam weld represents the weakest link in the composite pipe system. The weld must simultaneously satisfy the mechanical requirements of the base steel, the corrosion resistance requirements of the CRA layer, and the bond integrity requirements at the interface. This triple requirement makes the weld procedure qualification process significantly more complex than for conventional pipes.

Key Questions and Reflections

One question that deserves further exploration is the long-term performance of mechanically bonded composite pipes under cyclic loading and thermal cycling conditions. While the initial bond strength may be adequate, the differential thermal expansion between the base steel and CRA layer can induce interfacial stresses over time. In applications involving frequent thermal cycling—such as steam systems or cryogenic service—this could potentially lead to bond degradation.

Another area of concern is the inspection and qualification of the bond quality. Peel testing is destructive and cannot be applied to finished pipes in routine quality control. Non-destructive alternatives, such as ultrasonic testing of the bond line, are being developed but are not yet fully standardized. Until reliable NDT methods are available, the bond quality verification relies heavily on process control and periodic destructive testing of coupon samples.

Study Insights and Implications

This literature provides valuable guidance for engineers and manufacturers involved in the production and application of composite steel pipes. The systematic approach to identifying problems, proposing solutions, and comparing standards is commendable. The emphasis on weld quality optimization and bond integrity verification reflects a mature understanding of the technical challenges involved.

For welding engineers, the key takeaway is that composite pipe welding requires a fundamentally different approach from conventional pipe welding. The welding procedure must be developed and qualified with specific attention to the multi-material nature of the joint. Welding procedure qualification tests should include not only mechanical testing but also corrosion testing of the weld metal and heat-affected zone to ensure that the CRA overlay's protective function is maintained throughout the weld.

The standardization gap identified in the heat treatment section is particularly concerning for international trade. Manufacturers should strive to meet the most stringent applicable standard to ensure global marketability, and end-users should clearly specify the governing standard in their purchase specifications to avoid ambiguity. The future development of composite pipes will depend on the harmonization of these standards and the development of reliable, non-destructive bond quality assessment methods.