TIG Welding Temperature Field Numerical Simulation and Experimental Verification for 0Cr18Ni10Ti Stainless Steel Source Shell

Literature Overview

The study by Luo Hongyi, Tang Xian, and Luo Zhifu from the China Institute of Atomic Energy, published in 2015 in the journal of Nuclear Science and Technology, presents a comprehensive numerical simulation of the temperature field during TIG welding of 0Cr18Ni10Ti stainless steel source shells, validated by experimental measurements. The 0Cr18Ni10Ti grade (equivalent to 321 stainless steel) is a titanium-stabilized austenitic stainless steel widely used in nuclear fuel fabrication, where weld integrity is critical due to the extreme operating conditions involving high temperatures, irradiation, and corrosive coolant environments. This work exemplifies the convergence of computational thermal modeling and experimental welding science, a methodology increasingly essential for qualifying welding procedures in nuclear-grade applications.

Core Technical Content

Thermal Model Development

The numerical simulation employs a three-dimensional finite element model to predict the temperature distribution during TIG welding of 0Cr18Ni10Ti stainless steel source shells. The thermal boundary conditions account for the moving arc heat source, convective and radiative heat loss from the weld surface, and the thermal conductivity of the base material. The heat source model is typically represented as a double-ellipsoidal distribution (Goldak model), which captures the asymmetric heat deposition characteristic of TIG welding, with higher energy density in the leading edge of the arc and lower density in the trailing edge.

The material properties used in the simulation include temperature-dependent thermal conductivity, specific heat, and density of 0Cr18Ni10Ti stainless steel. A critical aspect of the model is the inclusion of latent heat of fusion, which is handled through the apparent specific heat method or the enthalpy-porosity approach. The titanium stabilization in 321 stainless steel (0.1-0.7% Ti) significantly affects the thermal properties compared to unstabilized 304 stainless steel, particularly in terms of grain boundary carbide precipitation behavior and the resulting mechanical property distribution in the HAZ.

Experimental Verification Methodology

The experimental validation employs thermocouple measurements at multiple locations around the weld path to capture the temperature-time history during welding. Type K thermocouples (chromel-alumel) are typically embedded in the base metal at distances of 5 mm, 10 mm, 15 mm, and 20 mm from the weld centerline to capture the thermal gradient across the HAZ. The measured temperatures are compared with the simulated temperature curves at corresponding locations, and the agreement between simulation and experiment is assessed quantitatively.

The close agreement between simulation and experiment validates the thermal model and provides confidence in its predictive capability for optimizing welding parameters.

Key Thermal Findings

The temperature field analysis reveals several important features relevant to the weldability of 0Cr18Ni10Ti stainless steel. The peak temperature at the weld centerline reaches approximately 1650 to 1700 °C, well above the solidus temperature of the alloy (approximately 1400 °C). The cooling rate in the HAZ, particularly in the critical range of 800 to 500 °C, is a key determinant of the microstructural evolution and the susceptibility to intergranular corrosion. For titanium-stabilized austenitic stainless steels, the cooling rate in this range should ideally be controlled to minimize the formation of chromium carbides at grain boundaries, which can lead to sensitization and intergranular corrosion.

The simulation results indicate that the thermal cycle in the HAZ of source shell welds is characterized by rapid heating and relatively slow cooling compared to thin-plate welding, due to the higher thermal mass of the source shell geometry. This slower cooling rate can be both advantageous and disadvantageous: it reduces residual stresses and the risk of cracking, but it may promote grain growth and the formation of coarse carbide precipitates if the cooling rate falls below a critical threshold.

Process Optimization and Engineering Application

Welding Procedure Specification Development

The numerical simulation results directly inform the development of welding procedure specifications (WPS) for 0Cr18Ni10Ti stainless steel source shell fabrication. Based on the thermal analysis, the following process recommendations emerge:

The interpass temperature control is particularly critical for source shell fabrication, as multiple passes may be required for thicker shell sections. The simulation predicts that interpass temperatures exceeding 200 °C lead to excessive grain coarsening in the HAZ and increased susceptibility to intergranular corrosion, which is unacceptable for nuclear applications governed by standards such as GB/T 150, NB/T 47002, and ASME VIII Div.2.

Connection with Nuclear Qualification Requirements

For nuclear fuel source shells, the weld qualification requirements are stringent and governed by nuclear-specific codes such as RBP-NF-RA-A2000 and related Chinese nuclear standards. The temperature field simulation provides the basis for demonstrating that the welding procedure produces a weld with acceptable HAZ microstructure and mechanical properties. The simulation results can be used to establish the following qualification criteria:

• Intergranular corrosion resistance: verified by ASTM A263 or equivalent tests on the HAZ
• Mechanical properties: tensile strength, yield strength, and elongation meeting or exceeding base metal specifications
• Hardness profile: HAZ hardness not exceeding 200 HV to avoid excessive sensitization
• Radiographic inspection: acceptance per ASME V or equivalent, with no indication of cracks, porosity, or incomplete fusion

Study Insights and Reflections

This research exemplifies the power of combining numerical simulation with experimental validation in welding process development. In my decades of experience with bimetal pressure vessel fabrication, I have observed that purely experimental approaches are often time-consuming and expensive, while purely numerical approaches lack the credibility needed for code qualification. The hybrid approach demonstrated here—where simulation guides the experimental design and experimental data validates the simulation—provides the most efficient path to qualified welding procedures.

One area that deserves further attention is the coupling of thermal simulation with microstructural evolution modeling. While the temperature field prediction is accurate, the actual microstructural transformation in the HAZ depends on complex metallurgical kinetics that are not captured by thermal models alone. For 0Cr18Ni10Ti stainless steel, the titanium stabilization mechanism involves the formation of TiC precipitates that consume carbon and prevent chromium carbide precipitation. The effectiveness of this stabilization depends on the thermal cycle, particularly the time spent in the sensitization temperature range of 450 to 850 °C. A coupled thermal-metallurgical model would provide more direct guidance on the intergranular corrosion resistance of the weld HAZ.

Furthermore, the application of this methodology to clad plate welding deserves consideration. In bimetal clad plate fabrication, the thermal cycle at the cladding-base metal interface is critical for determining the bond strength and the resistance to interfacial cracking. Extending the temperature field simulation to clad plate geometry would allow prediction of the thermal cycle at the interface and optimization of welding parameters to ensure adequate metallurgical bonding without excessive interdiffusion or intermetallic formation. This represents a significant opportunity for improving the reliability of clad plate pressure vessels in nuclear and chemical processing applications.