Finite Element Analysis of Transient Temperature Field and Microstructure in TIG Welding of CLAM Steel

Literature Overview

This paper, published in the Journal of Jiangsu University (Natural Science Edition) in 2009 (Vol. 30, No. 5, pp. 479–482), presents a three-dimensional finite element analysis of TIG welding applied to China Low Activation Martensitic Steel (CLAM steel). The research team from Jiangsu University, University of Science and Technology Beijing, and Harbin Welding Technology Training Center employed ANSYS software to simulate the transient thermal field during single-face, double-layer welding. The work was funded by the National Basic Research Program of China (Grant No. 2008CB717802).

Numerical Methodology

Model Configuration

The authors established a 3D finite element model for single-face double-layer TIG welding of CLAM steel. Key modeling features include:

Modeling Aspect	Approach
Software	ANSYS
Heat source model	Double elliptical body heat source
Source movement	APDL parametric language
Material deposition	Birth-and-death element technique
Boundary conditions	Preheating condition considered

The double elliptical heat source model represents the asymmetric nature of TIG arc heat distribution, with different heat flux densities in front of and behind the arc travel direction. The birth-and-death element technique allows progressive addition of weld metal elements, simulating the layer-by-layer deposition process inherent in multi-pass welding.

Thermal Field Analysis

The simulation captured the transient temperature distribution under preheating conditions and the dynamic evolution of the molten pool shape. Key observations include:

Temperature distribution – The transient thermal field exhibits characteristic asymmetric patterns consistent with directional heat source movement.
Molten pool morphology – The dynamic changes in molten pool geometry were tracked throughout the welding sequence, providing insights into solidification behavior.
Preheating effects – The inclusion of preheating boundary conditions is particularly relevant for martensitic steels, where preheating is critical to prevent cracking.

Microstructural Analysis and Hardness Distribution

Welding Microstructure

Following the numerical simulation, experimental metallographic analysis was conducted on the welded joints. CLAM steel, being a low-activation martensitic steel developed for fusion reactor structural applications, exhibits specific microstructural evolution during welding:

Base metal – Fine-grained martensitic structure with retained austenite and carbide precipitates.
Heat-affected zone (HAZ) – Coarse-grained martensite with potential retained austenite, influenced by peak temperatures.
Weld metal – Solidification structure dependent on composition and cooling rate, typically showing columnar or equiaxed grain structures.

Hardness Profile

The post-weld hardness distribution provides critical information about the thermal history experienced by different regions of the joint. For martensitic steels, hardness is directly correlated with the carbon equivalent and the cooling rate during solidification and transformation.

Engineering Significance for Fusion Applications

CLAM steel is a candidate structural material for fusion reactor first walls and blanket structures. The welding process development for this material carries unique challenges and requirements:

Requirement	Rationale
Low activation	Minimize long-lived radioactive waste
Radiation resistance	Maintain properties under neutron irradiation
Thermal stability	Withstand thermal cycling in reactor environments
Crack resistance	Prevent HAZ cracking during welding and service

The finite element analysis approach presented in this paper provides a systematic framework for predicting thermal cycles at specific locations within the weldment, which can then be correlated with expected microstructural transformations and mechanical properties. This methodology is directly applicable to other advanced structural materials used in extreme environments.

Connection to Pipe and Fitting Welding Practice

Although this study focuses on CLAM steel plates, the methodology has direct relevance to pipe welding applications involving similar martensitic or ferritic-martensitic steels:

Preheating requirements – The emphasis on preheating in this study mirrors the preheat requirements for Cr-Mo steels commonly used in high-temperature piping (e.g., P91/P92 per ASME B31.1).
Thermal cycle prediction – The FEA approach can be adapted for predicting thermal cycles in thick-walled pipe welds, where heat accumulation and multi-pass effects are significant.
HAZ property prediction – Understanding the relationship between thermal parameters and HAZ microstructure is essential for ensuring that welded joints in critical piping systems maintain adequate toughness and creep resistance.

Study Insights and Recommendations

This paper demonstrates the effectiveness of coupled thermal-microstructural analysis in welding process development for advanced materials. The use of birth-and-death elements to simulate progressive weld deposition represents a mature approach that has since been refined in commercial welding simulation packages. For engineers involved in welding qualification for nuclear or fusion applications, this methodology provides a template for predicting welding thermal histories and their consequences on joint integrity. The systematic approach of combining numerical simulation with experimental validation should be adopted as standard practice for any novel material welding development program. The preheating parameter deserves particular attention, as it directly influences the risk of hydrogen-induced cracking and martensite transformation characteristics in the HAZ.