Finite Element Simulation of Temperature Field in Strip Electrode Submerged Arc Welding Overlay on Thick Plates

Literature Overview and Research Context

This study by Wang Zhifeng, Chen Peiyin, Wu Wei, Chen Yan, Chen Jianmin, and Bao Hong, published in the Welding Journal in 2009, addresses a critical yet often under-appreciated aspect of weld overlay engineering: the three-dimensional thermal behavior during strip electrode submerged arc welding (SAW) overlay on thick plates. The research originates from the Harbin Institute of Welding at the Mechanical Science Institute and China First Heavy Machinery Group, two institutions deeply embedded in heavy industrial fabrication. The work targets the practical challenge faced when overlaying corrosion-resistant or wear-resistant layers onto thick base plates, where plate thickness significantly influences heat distribution, dilution, and the resulting overlay layer quality.

In heavy industry applications such as pressure vessel fabrication, hydrogenation reactor construction, and large-diameter pipe overlay, base plate thicknesses frequently exceed 30 mm and may reach beyond 100 mm. The thermal mass of such thick plates creates complex heat sink effects that alter the temperature field profile in ways that differ substantially from thin-plate scenarios. Understanding this thermal behavior is essential for controlling dilution rates, managing residual stresses, and ensuring the metallurgical integrity of the overlay layer.

Core Technical Content and Methodology

The research employs finite element analysis (FEA) to model the transient temperature field during strip electrode SAW overlay. The methodology involves establishing a three-dimensional finite element model of the thick plate geometry and applying a moving heat source representing the strip electrode. The key modeling parameters include:

Parameter	Typical Range in Study	Engineering Significance
Base plate thickness	40–80 mm	Determines heat sink capacity and thermal gradient
Strip electrode width	40–60 mm	Controls weld bead width and deposition rate
Welding current	500–1000 A	Primary driver of heat input
Welding voltage	25–35 V	Influences arc stability and penetration
Welding speed	200–500 mm/min	Governs heat input per unit length
Heat input	30–80 kJ/mm	Critical for dilution and HAZ properties
Initial temperature	20°C (ambient)	Baseline condition
Flux coverage	Continuous	Provides thermal insulation

The thermal model incorporates the following governing equations and assumptions:

Heat conduction equation: The transient three-dimensional heat conduction equation governs the temperature distribution, expressed as ρCp(∂T/∂t) = ∇·(k∇T) + Q, where ρ is density, Cp is specific heat, k is thermal conductivity, and Q represents the volumetric heat source.
Moving heat source: The strip electrode is modeled as a rectangular or Gaussian heat source that traverses the plate surface at the welding speed. The heat source geometry and power distribution directly affect the predicted temperature profiles.
Boundary conditions: Convective and radiative heat losses at the plate surfaces are modeled, along with the thermal insulation effect of the flux layer. The back surface of the plate is typically assumed to be adiabatic or subject to convective cooling.
Material properties: Temperature-dependent thermal conductivity, specific heat, and density are incorporated, reflecting the nonlinear behavior of carbon steel and low-alloy steel at elevated temperatures.

Key Findings and Technical Interpretation

The simulation results reveal several important thermal characteristics that have direct implications for overlay welding practice:

Thermal Gradient Distribution

The temperature gradient within the thick plate exhibits a pronounced non-uniformity. Near the weld centerline, peak temperatures exceed 1500°C, while the thermal influence zone extends significantly deeper into the plate compared to thin-plate scenarios. At distances of 50–80 mm from the weld centerline, temperatures still remain above 200°C, indicating a large heat-affected zone (HAZ). This extensive HAZ is a primary concern in overlay welding because it can lead to:

Dilution of the overlay layer with base material, reducing corrosion resistance
Formation of brittle microstructural phases in the HAZ
Development of significant residual stresses due to thermal gradients

Effect of Plate Thickness on Temperature Field

The study demonstrates that increasing plate thickness from 40 mm to 80 mm results in:

A reduction in peak temperature at the weld surface by approximately 15–25%, due to increased heat dissipation into the bulk material
A deeper penetration of the thermal influence zone, with the 200°C isotherm extending further from the weld centerline
An increase in the cooling time from 800°C to 500°C (t8/5), which is critical for predicting HAZ microstructure and hardness

The cooling time t8/5 is a particularly important parameter because it directly correlates with the hardness of the HAZ in carbon and low-alloy steels. For thick plates, t8/5 values can exceed 20 seconds, which may lead to coarse microstructures and reduced toughness in the HAZ.

Multi-Pass Thermal Accumulation

When multiple overlay passes are deposited, the thermal history becomes significantly more complex. The second and subsequent passes encounter a pre-heated base material, which reduces the thermal gradient and modifies the solidification behavior of the overlay metal. The simulation captures this thermal accumulation effect, showing that:

Peak temperatures increase by 100–200°C for subsequent passes compared to the first pass
The cooling rate decreases substantially, potentially leading to coarser grain structures in the overlay layer
The thermal cycle experienced by previously deposited layers involves reheating to temperatures that may fall within the intercritical range, affecting the final microstructure

Engineering Practice Implications

Process Parameter Optimization

Based on the thermal simulation results, the following process recommendations emerge for thick-plate overlay welding:

Heat input management: For plates exceeding 50 mm in thickness, heat input should be carefully controlled to balance deposition efficiency against dilution and residual stress concerns. A heat input range of 40–60 kJ/mm is generally recommended for single-pass strip electrode SAW overlay.
Preheating strategy: Preheating the base plate to 150–250°C can reduce the thermal gradient and minimize the risk of cold cracking in the HAZ, particularly for materials with higher carbon equivalent values.
Interpass temperature control: Maintaining interpass temperatures between 150–250°C helps manage the thermal cycle and prevents excessive softening of previously deposited layers.
Post-weld heat treatment (PWHT): The simulation-predicted residual stress distribution suggests that stress relief annealing at 550–650°C for a duration of 2 hours per 25 mm of thickness is advisable for thick-plate overlay weldments.

Connection to Standards and Codes

The thermal analysis findings have direct relevance to the qualification requirements specified in standards such as:

Standard	Requirement	Connection to Thermal Analysis
NB/T 47014	WPS qualification for weld overlay	Thermal parameters must be within qualified range
GB/T 150	Pressure vessel design and fabrication	Residual stress and HAZ properties must be controlled
ASME VIII Div.1	Welding procedure qualification	Heat input limits and PWHT requirements
API 934	Weld overlay requirements	Dilution limits and overlay layer properties

The thermal simulation provides a quantitative basis for understanding why certain process parameters are specified in these standards and for rationalizing procedure qualification parameters.

Study Insights and Reflections

This research exemplifies the value of computational modeling in overlay welding engineering. While the finite element model necessarily involves simplifications and assumptions, it provides insights that are difficult or impossible to obtain through experimental measurement alone. The three-dimensional thermal field, particularly in the depth direction of thick plates, cannot be easily measured using thermocouples without disrupting the welding process.

One critical insight from this work is the recognition that thick-plate overlay welding is fundamentally different from thin-plate overlay welding in terms of thermal behavior. Engineers who simply scale up process parameters from thin-plate experience without accounting for the thermal mass of thick plates may encounter unexpected problems such as excessive dilution, HAZ cracking, or inadequate bond strength.

The study also highlights the importance of multi-pass thermal accumulation, which is often overlooked in practice. In heavy industrial applications where multiple overlay passes are required to achieve the specified overlay thickness, the thermal history of each subsequent pass differs significantly from the first pass. This has implications for both the metallurgical quality and the mechanical properties of the final overlay layer.

From a practical standpoint, the thermal simulation results should be used in conjunction with experimental validation. The predicted temperature fields can guide the placement of thermocouples for experimental verification and provide a framework for interpreting measured data. The combination of computational modeling and experimental measurement offers the most reliable approach to understanding and controlling the thermal behavior of overlay welding processes.

The work by Wang et al. represents a solid contribution to the understanding of overlay welding thermal phenomena, and its findings remain relevant to contemporary practice in thick-plate overlay welding for pressure vessels, heat exchangers, and heavy machinery components.