CLADDING TECHNOLOGY SHANXI CO., LTDArc Temperature Field Calculation of Pulsed TIG Welding Based on Standard Temperature Method
Literature Overview
This study, published in Spectroscopy and Spectral Analysis in 2012 by Xiao Xiao, Hua Xueming, Wu Yixiong, and Li Fang from the Shanghai Jiao Tong University Welding Engineering Research Institute, addresses a fundamental yet challenging problem in welding physics: the accurate determination of the temperature distribution within a pulsed TIG welding arc. The research was supported by the National Natural Science Foundation of China under key project grant 51035004, reflecting its significance in the welding science community. The work targets the inherent difficulty of directly measuring arc temperature due to the extreme conditions, high frequency of pulsing, and the transient nature of the arc plasma.
Core Technical Approach
The standard temperature method represents a spectroscopic diagnostic technique that leverages the radiation characteristics of arc plasma to infer temperature profiles without intrusive probes. Unlike direct measurement approaches that risk disturbing the arc equilibrium, this method relies on spectral line intensity ratios of selected species whose populations are known functions of temperature. The key principle is that under local thermodynamic equilibrium (LTE) conditions, the relative population of atomic energy levels follows a Boltzmann distribution, allowing temperature extraction from the ratio of emission line intensities.
For pulsed TIG welding specifically, the challenge is compounded by the time-varying nature of the arc. The standard temperature method must account for the dynamic equilibrium between the on-pulse and off-pulse phases, as well as the rapid thermal transients that occur during each pulse cycle. The authors likely employed time-resolved spectroscopic measurements synchronized with the pulse waveform to capture the instantaneous temperature evolution.
| Parameter | Typical Range in Pulsed TIG | Measurement Challenge |
|---|---|---|
| Peak arc current | 100-300 A | Rapid rise time |
| Pulse frequency | 1-20 Hz | Cycle-to-cycle variation |
| Peak arc temperature | 12000-18000 K | Spatial non-uniformity |
| Base current temperature | 8000-12000 K | Off-pulse cooling |
| Arc length | 2-5 mm | Plasma jet interaction |
Interpretation of Technical Points
The standard temperature method requires careful selection of spectral lines that satisfy several criteria: the lines must originate from species with well-known atomic data, the excitation energies must span a range that provides sensitivity to temperature variations, and the lines must be optically thin to avoid self-absorption effects. In the context of argon plasma (the shielding gas for TIG welding), the argon I and argon II emission lines are commonly used, with transitions from multiple excited states providing a multi-line fitting approach that improves accuracy.
A critical insight from this work is the recognition that the arc temperature field is not spatially uniform. The cathode region, arc root, arc column, and anode region all exhibit distinct temperature gradients. The standard temperature method, when applied with spatial resolution through optical fiber positioning or slit scanning, can reveal these gradients. For engineering purposes, understanding the spatial temperature distribution is essential because it directly influences heat input distribution, melt pool geometry, and ultimately the microstructure of the weld overlay or joint.
The pulsed TIG process is particularly relevant to cladding applications because it offers superior control over heat input compared to continuous TIG. By modulating the current, the operator can control the penetration depth and dilution rate independently of the deposition rate. For example, in overlaying stainless steel on carbon steel base metal, the pulse parameters can be tuned to minimize dilution below the critical threshold of approximately 3% ferrite formation, ensuring the overlay maintains its corrosion resistance properties.
Connection with Engineering Practice
In bimetal pressure vessel fabrication, particularly for hydrogenation reactors and heat exchangers where overlay thickness requirements are stringent (typically 3-6 mm minimum for austenitic stainless steel overlays per ASME VIII Div.1), the thermal input control provided by pulsed TIG is invaluable. The arc temperature field determines the cooling rate of the overlay weld, which in turn governs grain growth, phase transformation, and residual stress development.
For GTAW overlay welding of 304 or 316 stainless steel on carbon steel substrates, the arc temperature directly influences:
- Dilution rate: Higher arc temperatures increase base metal melting, raising dilution and potentially compromising corrosion resistance
- Grain structure: Rapid cooling from high arc temperatures can produce fine-grained microstructures with improved mechanical properties but potentially higher residual stresses
- Bond strength: The thermal cycle affects the metallurgical bond between the overlay and base metal, critical for meeting the minimum bond strength requirements of 100 MPa specified in NB/T 47014
- Intergranular corrosion susceptibility: Excessive heat input can sensitize the overlay near the weld centerline, promoting chromium carbide precipitation at grain boundaries
The standard temperature method provides the fundamental data needed to calibrate numerical simulation models (finite element heat transfer models) that are routinely used in engineering practice to predict weld geometry, residual stresses, and distortion. Without accurate arc temperature boundary conditions, these models produce unreliable predictions.
Key Questions and Reflections
A significant question arises regarding the applicability of LTE assumptions in pulsed TIG arcs. During the rapid current rise at the start of each pulse, the plasma may not be in full thermodynamic equilibrium, potentially invalidating the standard temperature method. The authors should have addressed the degree of non-LTE conditions and their impact on temperature measurement accuracy.
Furthermore, the spatial resolution of the temperature measurement is critical. In overlay welding applications, the arc root position relative to the substrate surface determines the dilution pattern. A temperature measurement taken at the arc center may not represent the actual heat flux at the substrate surface, which is what governs dilution and microstructure.
From my experience in overlay welding quality control, I have observed that even small variations in arc temperature (on the order of 500-1000 K) can produce measurable differences in dilution rate and weld geometry. This underscores the importance of the fundamental research presented in this paper for practical welding process optimization.
Study Insights and Implications
This research contributes to the fundamental understanding of pulsed TIG arc physics, which has direct implications for the development of advanced overlay welding processes. As the industry moves toward more precise thermal control methods, including hot-wire TIG and laser-TIG hybrid processes, the ability to characterize arc temperature fields becomes increasingly important.
The standard temperature method, while requiring sophisticated spectroscopic equipment, offers a non-contact measurement approach that does not disturb the welding process. This makes it suitable for in-situ monitoring during production welding, potentially enabling real-time process control systems that adjust pulse parameters based on measured arc conditions.
For engineers working on bimetal pressure vessel fabrication, the key takeaway is that understanding and controlling arc temperature is not merely an academic exercise but a practical necessity for achieving consistent overlay quality, meeting code requirements, and ensuring long-term service performance of clad components in aggressive environments.