Residual Stress Measurement and Analysis at Welded Joints of Bimetal Composite Pipes

Overview of the Study

This literature review focuses on experimental investigation of residual stress distribution at welded joints of bimetal composite pipes, which is a critical topic in the design and integrity assessment of pressure-containing piping systems. Bimetal composite pipes, typically consisting of a corrosion-resistant inner layer (such as 316L stainless steel or nickel-based alloys) bonded to a structural carbon steel or low-alloy steel outer layer, are widely used in oil and gas, chemical, and nuclear industries where both mechanical strength and corrosion resistance are required. The welding of such composite pipes introduces complex residual stress fields due to thermal gradients, phase transformations, and the inherent mismatch in thermal expansion coefficients between the two layers. Understanding these residual stresses is essential for predicting stress-corrosion cracking susceptibility, fatigue life, and long-term structural integrity.

Core Technical Content

The study employs strain gauge methods, hole-drilling method, and X-ray diffraction techniques to map residual stress distributions at butt welds and fillet welds of bimetal composite pipes. The residual stress state is characterized by three principal components: longitudinal (along the pipe axis), hoop (circumferential), and radial (through-thickness) stresses. The literature reports that peak longitudinal residual tensile stresses at the weld toe and heat-affected zone can reach values approaching the yield strength of the base material, typically in the range of 200 to 320 MPa for carbon steel outer layers and 350 to 450 MPa for stainless steel inner layers.

The following table summarizes typical residual stress values reported in the literature for different weld configurations:

Process and Standards Analysis

The welding process parameters significantly influence residual stress magnitude and distribution. Gas metal arc welding (GMAW) with short-circuit transfer tends to produce lower peak stresses compared to pulsed GMAW or submerged arc welding (SAW) due to lower heat input per pass. The literature emphasizes the importance of welding sequence optimization to minimize restraint-induced stresses. For bimetal pipes, the welding strategy must also consider the bond line integrity; excessive heat input can degrade the metallurgical bond between the cladding layer and the base metal, leading to delamination or intermetallic compound formation.

According to NB/T 47014 and ASME IX, qualified welding procedures for bimetal composite pipes must ensure that the overlay material does not exceed the recommended heat input limits. For example, when welding 316L/SAE 1020 composite pipe, the heat input is typically limited to 1.5–2.5 kJ/mm to prevent chromium carbide precipitation at grain boundaries in the heat-affected zone, which would compromise both bond strength and corrosion resistance.

Engineering Practice Implications

From an engineering perspective, the residual stress data obtained from such studies directly informs post-weld heat treatment (PWHT) decisions. Stress relief annealing at 600–650°C for carbon steel outer layers can reduce peak residual stresses by 50–70%, but must be performed carefully to avoid sensitization of the stainless steel inner layer. Alternative methods such as vibration stress relief (VSR) and mechanical peening are also discussed as viable alternatives when PWHT is impractical or undesirable.

The study highlights that in high-consequence environments such as sour service (containing H2S), residual tensile stresses exceeding 150 MPa at the weld toe significantly increase the risk of hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC). This finding aligns with NACE MR0175/ISO 15156 requirements, which specify strict limits on residual stress for materials exposed to wet H2S environments.

Key Questions and Reflections

One critical question that emerges from this literature is whether current standards adequately account for the anisotropic residual stress state inherent to bimetal composite pipe welds. Most existing design codes treat residual stresses as uniform or symmetric, which may lead to non-conservative assessments in certain orientations. Another reflection is that the interaction between residual stress and applied operational loads (internal pressure, bending moments, thermal cycling) creates a complex multiaxial stress state that demands advanced evaluation methods such as finite element analysis combined with experimental validation.

Summary and Implications for Practice

The study on residual stress at bimetal composite pipe welded joints provides valuable quantitative data that bridges the gap between fundamental metallurgical understanding and practical engineering design. The key takeaway is that residual stress management—through process optimization, welding sequence planning, and appropriate post-weld treatments—remains one of the most effective strategies for enhancing the service life and integrity of bimetal composite pipe systems. Engineers involved in the design, fabrication, and inspection of such systems should treat residual stress assessment as a mandatory component of the fitness-for-service evaluation, particularly for critical applications in sour service, high-temperature service, or cyclic loading conditions.