Numerical Simulation of Dynamic Thermal Process in Double-Sided Asymmetrical TIG Backing Welding of Thick Plates

Literature Overview

The study by Liu Dianbao, Zhao Huihui, Li Fuquan, Zhang Guangjun, and Wu Lin from the National Key Laboratory of Science and Technology on Precision Heat Processing of Metals, Harbin Institute of Technology, published in China Welding (2011, Vol. 20, No. 1, pp. 44-48), presents a finite element numerical simulation of the dynamic thermal process during double-sided asymmetrical TIG backing welding of large thick plates (1000 mm x 700 mm x 50 mm). This work addresses a critical challenge in heavy plate welding — achieving sound welds on both sides without backing materials, which is common in shipbuilding, nuclear power, and heavy equipment manufacturing.

Core Technical Content

The simulation was conducted using MSC.MARC finite element software, modeling the thermal process during double-sided asymmetrical TIG backing welding. The study specifically investigated the effect of arc distance on the thermal cycle in the weld zone.

The key findings are:

1. Double-peak thermal cycle: The workpiece experiences two distinct temperature peaks during double-sided welding — one from the fore arc and one from the rear arc.
2. Pre-heating effect: The fore arc preheats the material for the rear pass, with the preheating temperature dependent on arc distance, fore arc heat input, and initial workpiece temperature.
3. Post-heating effect: The rear arc provides post-heating to the fore pass, which can influence the final microstructure and residual stress state of the first pass.
4. Distance dependence: The mutual thermal effects between the two arcs decrease with increasing arc distance.

Technical Analysis and Thermal Process Interpretation

The double-sided asymmetrical TIG backing welding process involves welding one side of the joint, then flipping the workpiece (or moving the torch to the other side) and welding the second pass without mechanical backing. The asymmetry refers to the different thermal conditions experienced by each pass:

Fore pass (first side):

• Welds into cold or preheated base metal
• Experiences post-heating from the rear pass
• Final thermal cycle is influenced by the timing and heat input of the second pass

Rear pass (second side):

• Benefits from pre-heating by the fore arc
• Lower cooling rate due to elevated starting temperature
• May experience different solidification conditions

The double-peak thermal cycle has significant metallurgical implications:

Engineering Practice Implications

For thick-plate welding in heavy industry, the findings of this simulation have direct practical value:

1. Arc distance optimization: The optimal arc distance balances pre-heating benefits against excessive thermal input. For 50 mm plates, arc distances of 20-50 mm typically provide beneficial pre-heating without excessive grain growth.
2. Welding sequence planning: The mutual thermal effects mean that welding sequence and timing are critical process variables. Rapid transition between fore and rear passes maximizes pre-heating benefits, while long delays reduce the effect.
3. Heat input control: The fore arc heat input directly influences the pre-heating temperature for the rear pass. Higher fore arc heat input provides more pre-heating but may cause excessive HAZ softening in the fore pass.
4. Residual stress management: The post-heating effect of the rear arc on the fore pass can partially relieve residual stresses from the first pass, but may also introduce new stresses. The net effect depends on the thermal cycle parameters.

Application to Pipe and Fitting Welding

While this study focuses on plate welding, the principles directly apply to thick-walled pipe welding:

• Large-diameter pipe girth welds: Pipes with wall thickness exceeding 25 mm often require multi-pass welding with similar double-sided considerations
• Pipe fitting fabrication: Large elbows and tees fabricated from thick plate exhibit similar thermal challenges
• Nuclear reactor pressure vessel welds: These thick-section welds require careful thermal cycle control per ASME Section III requirements

For pipeline applications governed by API 1104 or ASME B31.3, the thermal cycle considerations become critical when welding thick-wall lines pipe (e.g., X80 grade with wall thickness exceeding 30 mm). The pre-heating and interpass temperature requirements in these codes are directly related to the thermal cycle effects quantified in this simulation.

Key Reflections and Study Insights

This numerical simulation work exemplifies the power of finite element analysis in welding engineering — it provides quantitative insight into thermal processes that are difficult to measure experimentally in thick-section welds. The double-peak thermal cycle concept is particularly important for understanding why double-sided welds sometimes exhibit different properties in each pass, even when identical welding parameters are used.

From a practical standpoint, engineers should consider the following when planning thick-plate or thick-wall welding operations:

• The fore pass and rear pass are not independent events — they thermally interact
• Arc distance is a critical parameter that should be optimized through simulation or trial welding
• The post-heating effect can be beneficial for stress relief but detrimental if it causes excessive grain growth
• Thermal simulation should be integrated into the welding procedure qualification process for critical thick-section applications

The methodology presented — using MSC.MARC with appropriate thermal boundary conditions and heat source models — represents a rigorous approach to welding process design. For engineers without access to commercial FEA software, simplified analytical models based on Rosenthal's moving heat source theory can provide approximate predictions of thermal cycles and cooling rates, which are the primary drivers of weld microstructure and properties.