Numerical Simulation of HSTT Welding with Misalignment on L360QS-N08825 Composite Pipe

Literature Overview

This paper by Han Yongkai, He Yazhang, Tang Deyu, Wang Tianqi, and Liu Jian, published in Hot Working Technology in 2021, investigates the welding behavior of L360QS-N08825 composite pipe using hot wire TIG (HSTT) welding under varying misalignment conditions. The study was supported by the National Natural Science Foundation of China and Tianjin Natural Science Foundation projects, indicating its relevance to offshore engineering applications. The authors developed a finite element model to simulate the temperature field distribution and penetration behavior under different misalignment magnitudes, and validated the simulation results through experimental welding trials.

Core Technical Findings

The study examined the effect of pipe joint misalignment on weld penetration in a composite pipe consisting of an L360QS carbon steel outer layer and an N08825 (Inconel 625) inner corrosion-resistant layer. The key findings are summarized as follows:

The study confirmed that the HSTT welding process is sensitive to joint misalignment, with increasing misalignment leading to asymmetric heat distribution and reduced penetration on the thin side of the misaligned joint. The finite element simulation results showed good correlation with experimental observations, validating the predictive capability of the numerical model.

Technical Interpretation and Process Analysis

HSTT Welding Process Characteristics

Hot wire TIG welding combines the advantages of conventional TIG welding with the enhanced deposition rate and penetration of a consumable filler wire that is preheated by a separate heating element. The hot wire provides additional heat input to the weld pool, improving the deposition efficiency and reducing the overall heat input required from the arc. This makes HSTT welding particularly suitable for welding thick-section composite pipes where deep penetration and high deposition rates are required.

The process parameters for HSTT welding typically include:

The interaction between the arc heat and the hot wire heat creates a complex thermal field that is difficult to characterize analytically. Finite element simulation provides a powerful tool for understanding and optimizing this thermal interaction.

Effect of Misalignment on Temperature Field and Penetration

Joint misalignment creates an asymmetric gap geometry, which fundamentally alters the heat flow pattern during welding. On the side with the larger gap (thin side), the heat input is spread over a larger area, resulting in lower peak temperatures and reduced penetration. Conversely, on the side with the smaller gap (thick side), the heat is concentrated, leading to higher temperatures and potential over-penetration or burn-through.

The numerical simulation captures this behavior by modeling the asymmetric boundary conditions imposed by the misaligned joint geometry. The temperature field distribution shows a clear shift toward the thick side, with the isotherms becoming compressed on the thin side. This compression reduces the effective heat input per unit area on the thin side, leading to incomplete melting and lack of fusion.

Application of Finite Element Method in Welding Analysis

The finite element method (FEM) used in this study employs a moving heat source model to simulate the welding thermal cycle. The heat source is typically modeled as a double-ellipsoidal or Gaussian distribution, with the heat flux concentrated at the weld pool surface. The material properties, including thermal conductivity, specific heat, and density, are temperature-dependent and must be accurately defined for both the carbon steel and the nickel alloy layers of the composite pipe.

The simulation must also account for the phase change behavior during melting and solidification, which is typically modeled using an enthalpy-porosity approach. The latent heat of fusion is distributed over a temperature range to avoid numerical instabilities. The boundary conditions include convective and radiative heat losses from the workpiece surface, which are particularly important for thin-section components.

Engineering Practice Integration

In offshore engineering, composite pipes such as L360QS-N08825 are used for subsea production lines and umbilicals where corrosion resistance is critical. The N08825 (Inconel 625) inner layer provides excellent resistance to chloride-induced stress corrosion cracking (SCC) and general corrosion in seawater environments, while the L360QS carbon steel outer layer provides structural strength at a lower cost.

The welding of composite pipes is challenging because the joint must maintain the integrity of both layers without compromising the corrosion protection provided by the inner layer. Any breakthrough of the carbon steel into the inner nickel alloy layer during welding can create a galvanic coupling that accelerates corrosion. Therefore, precise control of penetration depth is essential.

Joint misalignment in composite pipe welding is a common occurrence due to fabrication tolerances, thermal distortion during welding, and handling stresses. The study's findings highlight the need for tight control of joint preparation and assembly tolerances to minimize misalignment. In practice, the misalignment should be limited to less than 10% of the wall thickness, as specified in standards such as ASME B31.4 and DNV-ST-F101.

Key Questions and Reflections

One significant question is the effect of misalignment on the metallurgical quality of the weld metal. The asymmetric temperature field caused by misalignment may lead to uneven dilution of the base metals into the weld pool, resulting in variable weld metal composition across the weld cross-section. This compositional variation can affect the corrosion resistance and mechanical properties of the weld, particularly in the heat-affected zone near the nickel alloy layer.

Another reflection concerns the applicability of the simulation model to other welding processes. The HSTT welding process was studied, but the misalignment effects may differ for other processes such as submerged arc welding (SAW) or gas metal arc welding (GMAW), which have different heat source characteristics and heat input profiles. Engineers should be cautious about extrapolating the simulation results to other welding processes without independent validation.

Study Insights and Implications

The most significant contribution of this paper is the demonstration that finite element simulation can accurately predict the effect of joint misalignment on weld penetration in composite pipe welding. This capability enables engineers to optimize welding parameters and joint preparation procedures before physical welding trials, reducing development costs and improving first-pass quality. The study also highlights the importance of joint misalignment control in composite pipe welding, where even small misalignments can have significant effects on weld integrity.

For welding engineers, the study reinforces the value of numerical simulation as a complementary tool to experimental welding trials. The combination of simulation and experiment provides a more comprehensive understanding of welding behavior than either approach alone. This integrated methodology is particularly valuable for complex welding scenarios involving dissimilar metals, multi-layer welds, or unusual joint geometries.

Reference Value and Outlook

This paper holds high reference value for engineers involved in the welding of composite pipes for offshore and subsea applications. Its primary limitation is the exclusive focus on thermal simulation without mechanical or metallurgical analysis. Future research should extend the simulation to include residual stress prediction, microstructural evolution modeling, and corrosion performance assessment. The methodology presented, however, provides a robust framework for the analysis and optimization of composite pipe welding processes.