TIG Welding Process Test of Thin-Plate Dissimilar Steel Spherical Body

Literature Overview

The paper by Jiang Youqing from Huazhong University of Science and Technology, published in Welding Technology in 1989, addresses the TIG welding process for thin-plate dissimilar steel spherical bodies. This early research is significant in the context of pressure vessel fabrication, where spherical shells are commonly used for storage tanks, reactors, and heat exchangers. The study of dissimilar steel welding in spherical geometries is particularly relevant to bimetal pressure vessel applications.

Core Technical Content

Spherical Shell Geometry and Welding Challenges

Spherical pressure vessels present unique welding challenges compared to cylindrical vessels:

Challenge	Description	Impact on Welding
Curvature	High curvature radius	Distortion, fit-up difficulty
Dissimilar metals	Different thermal/mechanical properties	Residual stress, cracking
Thin plate	Low thickness (typically 3–10 mm)	Burn-through, distortion
Multi-pass	Multiple welds on sphere	Accumulated distortion
Position	Variable welding positions	Process control difficulty

Material Combinations Studied

The study examines welding of dissimilar steel combinations, likely including:

Base Metal 1	Base Metal 2	Application
Carbon steel (Q235)	Low-alloy steel (16Mn)	General purpose spheres
Low-alloy steel (16Mn)	Stainless steel (304)	Corrosion-resistant interiors
Carbon steel	Copper-nickel alloy	Marine applications
Low-alloy steel	Nickel-based alloy	High-temperature service

TIG Welding Parameters for Thin-Plate Dissimilar Steel

Parameter	Value	Rationale
Current	60–150 A	Low current for thin plate
Current type	DCEN	Tungsten erosion, penetration
Travel speed	100–300 mm/min	Control heat input
Shielding gas	100% Ar	Inert shielding
Gas flow	8–10 L/min	Adequate protection
Back purge	100% Ar, 5–8 L/min	Internal oxidation prevention
Filler wire	Matching or transition alloy	Dilution control
Preheat	50–150°C (dissimilar)	Reduce cracking risk
Interpass temperature	≤ 150°C	Control HAZ properties

Microstructural Considerations for Dissimilar Steel Welds

The weld metal in dissimilar steel joints exhibits a composition intermediate between the two base metals. Key considerations include:

Dilution rate: Typically 30–60% for single-pass welds, depending on current and travel speed
Phase composition: The weld metal may contain a mixture of phases from both base metals
Hardness mismatch: Significant hardness differences between weld zones can lead to stress concentration
Corrosion resistance: The weld metal composition must meet the requirements of the more demanding environment

Process-Standard Analysis

Comparison of Welding Methods for Spherical Shells

Method	Productivity	Quality	Cost	Applicability
Manual TIG	Low	High	High	Thin plate, dissimilar
Automatic TIG	Moderate	High	Moderate	Consistent geometry
SAW	High	Moderate	Low	Thick plate, flat
FCAW	High	Moderate	Low	Medium plate
Laser + TIG	Very high	High	High	Thin to medium
Orbital	High	Very high	Very high	Pipe joints

Standards for Pressure Vessel Welding

Spherical pressure vessels must comply with:

ASME VIII Div.1: For spherical vessels, the design stress is higher than cylindrical (1.25 × allowable stress)
ASME VIII Div.2: For higher design stress and more rigorous requirements
GB/T 150: Chinese pressure vessel code with specific requirements for spherical shells
NB/T 47002: Chinese standard for pressure vessel steel plates
ASME IX: Welder and welding procedure qualification

Non-Destructive Testing Requirements

For spherical pressure vessels, NDE requirements include:

NDE Method	Coverage	Acceptance Criteria
RT (radiographic)	100% of welds	ASME V Article 2, T-2741
UT (ultrasonic)	100% of welds	ASME V Article 4, T-420
MT (magnetic particle)	100% of welds	ASME V Article 7, T-710
PT (penetrant)	100% of welds	ASME V Article 6, T-610
Hydrostatic test	100%	1.3 × MAWP or per code

Key Questions and Reflections

The welding of thin-plate dissimilar steel spherical bodies presents several engineering challenges:

Distortion control: Spherical shells are particularly susceptible to distortion due to their geometry. Welding sequence optimization is critical, with alternating welds on opposite sides of the sphere to balance thermal input.
Residual stress: Dissimilar steel welds develop high residual stresses due to differential thermal expansion. Post-weld stress relief (PWSR) at 550–650°C is often required, but this must be compatible with both materials.
Cracking susceptibility: Dissimilar steel welds are susceptible to both hot cracking (in the weld metal) and cold cracking (in the HAZ). Preheat and post-weld heat treatment are essential for high-carbon or high-hardness base metals.
Corrosion compatibility: The weld metal composition must be compatible with the service environment. For dissimilar joints involving stainless steel, the weld metal must maintain adequate chromium content to ensure corrosion resistance.

Study Insights and Implications

This early research provides foundational knowledge for the welding of dissimilar steel spherical pressure vessels. The principles outlined remain relevant to modern bimetal pressure vessel fabrication, where dissimilar metal joints are common in applications requiring corrosion resistance on the interior and structural strength on the exterior.

For modern applications, the TIG welding process described can be enhanced with:

Pulsed TIG: For better control of heat input and dilution
Active TIG: For improved wetting and reduced dilution
Laser-TIG hybrid: For increased productivity while maintaining quality
Robotic TIG: For consistent weld quality in repetitive production

The study highlights the importance of process development for specific geometries and material combinations. Each dissimilar steel combination requires individual procedure qualification, with careful attention to dilution control, residual stress management, and corrosion compatibility.

For pressure vessel applications, the welding of dissimilar steel spherical shells must satisfy the rigorous requirements of applicable codes. The weld procedure qualification must demonstrate adequate mechanical properties, corrosion resistance, and non-destructive examination acceptance criteria. The residual stress state after welding and any post-weld heat treatment must be characterized to ensure the vessel meets design life requirements.