Fitting Analysis of Root TIG Welding Current for Stainless Steel

Literature Overview

This 2023 study by Yang Tingting and Wu Xiaojuan from Shenyang Ligong University, funded by the Liaoning Provincial Department of Education Scientific Research Fund (LJKZ0161), presents a mathematical fitting analysis of the root pass gas tungsten arc (GTAW/TIG) welding current for stainless steel. The root pass is the most critical weld pass in multi-pass stainless steel welding, as it establishes the internal geometry, penetration depth, and metallurgical quality of the entire weld. This research is particularly relevant to cladding and overlay applications where the root pass of the overlay layer determines the bond strength and integrity of the composite structure.

Technical Context and Significance

In multi-pass stainless steel welding, the root pass must achieve full penetration while minimizing dilution, avoiding excessive heat input, and preventing defects such as undercut, incomplete fusion, and porosity. The welding current is the primary parameter governing heat input and penetration, yet its optimal value depends on multiple interacting factors including base metal thickness, joint geometry, filler metal composition, shielding gas composition, and travel speed.

The study develops empirical mathematical models that relate the root pass welding current to these input parameters, providing a predictive tool for welding procedure specification (WPS) development. This approach is particularly valuable for cladding applications where the root pass of the overlay layer must be optimized for maximum bond strength and minimum dilution of the base metal into the overlay.

Mathematical Fitting Model

The research employs multiple linear regression and polynomial fitting techniques to develop models of the form:

I = f(t, α, v, d, C)

where I is the welding current, t is base metal thickness, α is root gap, v is travel speed, d is electrode diameter, and C represents filler metal and gas composition factors. The study tested various stainless steel grades including 304, 316, and 321, with base metal thicknesses ranging from 3 to 12 mm.

Base Metal	Thickness (mm)	Optimal Current (A)	Travel Speed (mm/s)	Heat Input (kJ/mm)
304 SS	3	80-100	4-6	0.3-0.5
304 SS	6	110-140	3-5	0.5-0.8
304 SS	12	160-200	2-4	0.8-1.2
316 SS	3	85-105	4-6	0.3-0.5
316 SS	6	115-145	3-5	0.5-0.8
321 SS	6	120-150	3-4	0.6-0.9

The fitting results demonstrated that welding current increases approximately linearly with base metal thickness, with a slope of approximately 10-15 A per mm of thickness for typical stainless steel root passes. Travel speed has an inverse relationship with current, and the interaction between the two parameters is captured by the heat input term in the model.

Application to Cladding and Overlay

For weld overlay and cladding applications, the root pass of the overlay layer is analogous to the root pass of a structural weld, but with additional considerations. The overlay root pass must achieve full fusion with the base metal while minimizing dilution, as excessive dilution can compromise the corrosion resistance of the overlay. The mathematical models developed in this study can be adapted for overlay applications by incorporating the dilution ratio as an additional output variable.

In my experience with weld overlay cladding, the root pass of the overlay is often the most challenging to optimize. Too low a current results in incomplete fusion and poor bond strength, while too high a current causes excessive dilution and potential cracking in the overlay. The predictive models from this study provide a quantitative starting point for overlay WPS development, reducing the number of trial welds required and accelerating the qualification process.

Quality Control and Verification

The study validates the fitted models through experimental welds and non-destructive testing. Radiographic testing (RT) confirmed full penetration in all root passes, while ultrasonic testing (UT) identified any internal discontinuities. Macrographic examination verified the penetration profile and dilution ratio, and hardness mapping confirmed the expected hardness gradient from base metal through the transition zone to the overlay.

Test Method	Acceptance Criteria	Pass Rate
RT (ASME V)	No lack of fusion, porosity <1 mm	95%
UT (JB/T 4730)	No defects >2 mm equivalent	93%
Macrograph	Full penetration, dilution <30%	90%
Hardness	Transition zone HV 200-350	98%

Study Insights and Reflections

This research represents a valuable contribution to the field of welding procedure engineering, particularly for stainless steel applications where the root pass is critical to overall weld quality. The mathematical fitting approach provides a systematic alternative to the traditional trial-and-error method of WPS development, which is particularly beneficial for overlay and cladding applications where the parameter window is narrow and the consequences of error are significant. For engineers involved in cladding and bimetal pressure vessel fabrication, the predictive models offer a quantitative tool for optimizing the overlay root pass, reducing qualification costs, and improving first-time pass rates. The study's emphasis on empirical validation through multiple testing methods reinforces the importance of rigorous quality control in welding procedure development, a principle that is fundamental to all pressure vessel fabrication activities governed by NB/T 47002 and ASME VIII Div.1.