Development and Application of 45-Degree Full-Position Automatic TIG Welding Equipment for Pressure Vessels

Overview and Background

This study, conducted by Wang Yankun, Liu Wancun, Xiao Peng, and Gao Yongguang from the Technical Center of Dalian Nuclear Power and Petrochemical Co., Ltd. (a subsidiary of China First Heavy Industries Group) in 2021, addresses a significant engineering challenge in the fabrication of pressure vessels: achieving consistent, high-quality welds in inclined 45-degree positions using fully automatic TIG welding. The 45-degree full-position welding configuration is particularly relevant in the construction of large-diameter pressure vessels, nuclear-grade equipment, and petrochemical reactors where manual welding faces inherent limitations in repeatability and fatigue resistance. The research was motivated by the need to reduce reliance on skilled welders while maintaining compliance with stringent standards such as ASME VIII Div.1 and GB/T 150.

Core Technical Approach

The fundamental challenge of 45-degree position welding lies in the gravitational influence on the molten pool, which tends to sag and create uneven bead profiles. The developed automatic TIG system integrates several critical subsystems to address this issue:

Subsystem	Function	Key Parameters
Torch tracking system	Maintains constant torch-to-workpiece distance and angle	Positioning accuracy ±0.5 mm
Wire feed mechanism	Consistent filler metal delivery	Feed rate adjustable 0.5–5.0 m/min
Arc length control	Stabilizes arc voltage and heat input	Arc length maintained at 2–4 mm
Rotation drive	Synchronized workpiece rotation	Speed range 0.5–10 rpm
Gas shielding system	Adequate inert atmosphere coverage	Flow rate 8–15 L/min Ar

The equipment employs a rotary fixture that positions the workpiece at a 45-degree inclination relative to the horizontal plane, with the torch remaining in a fixed vertical orientation. This configuration requires precise coordination between the rotation speed and the welding parameters to ensure that the molten pool does not flow excessively toward the lower side of the weld seam.

Welding Parameter Optimization

Through systematic experimental trials on typical pressure vessel materials including 16MnR carbon steel and 304 stainless steel, the following parameter windows were identified for 45-degree position welding:

Parameter	Carbon Steel (16MnR)	Stainless Steel (304)
Welding current (A)	120–180	100–150
Arc voltage (V)	18–22	16–20
Travel speed (mm/min)	150–300	200–400
Filler wire diameter (mm)	2.4	2.4
Shielding gas flow (L/min)	12–15	15–20
Nozzle diameter (mm)	14–16	14–16

The key finding was that reducing the travel speed by approximately 20–30% compared to flat-position welding, while slightly increasing the arc voltage, produced acceptable penetration and bead geometry without excessive sagging. The filler wire feed rate was synchronized with the travel speed to maintain a consistent reinforcement height of 1.5–2.5 mm.

Quality Control and Defect Analysis

Non-destructive testing (NDT) of completed welds was performed according to JB/T 4730, utilizing radiographic testing (RT) for volumetric defect detection and magnetic particle testing (MT) for surface discontinuities. The defect analysis revealed the following:

Defect Type	Frequency	Primary Cause	Countermeasure
Undercut	8–12%	Excessive current or speed	Reduce current by 10–15%
Excess reinforcement	5–8%	Slow travel speed	Increase speed by 10%
Porosity	2–4%	Inadequate shielding	Increase gas flow; check nozzle
Lack of fusion	1–3%	Insufficient penetration	Increase current or reduce speed
Sagging bead	3–5%	Gravity effect at 45°	Reduce heat input; optimize speed

The overall weld acceptance rate achieved through this automated process exceeded 92% on the first pass, compared to approximately 85–88% for manual welding under similar conditions. This improvement translates directly into reduced rework costs and shorter fabrication schedules for pressure vessel projects.

Engineering Practice and Standard Compliance

The equipment was validated against the requirements of ASME Section IX for weld procedure qualification, with specific attention to the Qualified Welding Procedure Specification (WPS) parameters for position qualification. The study confirmed that once the equipment parameters are locked within the qualified range, the consistency of weld quality significantly outperforms manual methods, particularly for long production runs. For nuclear-grade applications, additional requirements from RCC-M and NB/T 47014 were considered, including the need for welder qualification and procedure requalification intervals.

The practical application demonstrated that the 45-degree automatic TIG system is most effective for circumferential welds on pressure vessels with diameters ranging from 800 mm to 3000 mm and wall thicknesses of 12–40 mm. For thicker sections, multi-pass welding with root pass performed by the automatic TIG system and fill/cap passes performed by automatic TIG or SAW was recommended.

Study Insights and Reflections

This research exemplifies the critical role that automation plays in advancing fabrication capabilities for critical pressure equipment. The 45-degree position, which has traditionally been considered a challenging configuration for automated welding, becomes tractable when the equipment is designed with sufficient degrees of freedom in parameter adjustment and precise mechanical control. The study underscores an important principle in pressure vessel fabrication: the investment in specialized welding equipment often yields substantial returns through improved quality, reduced labor costs, and enhanced schedule predictability. For engineers involved in pressure vessel design and fabrication, the lessons from this work reinforce the importance of process qualification and the systematic approach to parameter optimization when introducing new welding methods into production environments.