TIG Welding of High-Temperature Alloy GH3030 for PTFE Cracking Equipment

Literature Overview

This paper by Wei Xin-Hua from Juhua Group Engineering Company, published in Welding (Issue 5, 1999), documents the TIG welding of GH3030 high-temperature alloy for a PTFE (polytetrafluoroethylene) production cracking device (R1101). The equipment operates at temperatures of 700–800°C with thermally corrosive gas media, creating extremely demanding service conditions. GH3030 was selected as the primary material due to its excellent thermal stability, thermal strength, and corrosion resistance. The device has a sleeve-type structure with diameters up to φ225 mm and wall thicknesses of 4 mm, and TIG welding was selected as the most suitable welding method with single-sided double-sided weld formation for both longitudinal and circumferential seams.

Material Properties and Weldability Assessment

GH3030 is a nickel-chromium-iron alloy with a nominal composition of approximately 20% Cr, 10% Ni, and balance Fe, with minor additions of Ti, Al, and other elements for precipitation hardening. This alloy is known for its excellent resistance to thermal cycling, oxidation, and sulfidation at elevated temperatures. The following table presents the key properties and weldability characteristics of GH3030:

Property	Value	Relevance to Welding
Melting point	~1400°C	Moderate melting range, low cracking susceptibility
Thermal conductivity	20–25 W/(m·K) at 20°C	Low, requires high heat input for penetration
Thermal expansion	13–14 μm/(m·K)	Moderate, manageable residual stress
Weldability	Good	Austenitic structure, low carbon
Hot cracking susceptibility	Low	Due to low carbon and stable austenite
Sensitization risk	Low	Low carbon content

The weldability of GH3030 is generally good due to its austenitic microstructure and low carbon content. However, the high operating temperature (700–800°C) imposes strict requirements on the weld quality, particularly regarding resistance to thermal fatigue cracking and corrosion. The weld metal must have mechanical properties comparable to the base metal at elevated temperatures, and the heat-affected zone must not be susceptible to intergranular corrosion or stress corrosion cracking.

Welding Process and Parameters

For the sleeve-type structure with 4 mm wall thickness, TIG welding (GTAW) is the optimal choice due to its precise heat input control, clean weld formation, and ability to produce high-quality welds without filler metal contamination. The welding is performed with single-sided double-sided formation, meaning the weld is deposited from one side but achieves sound formation on both sides.

The following table presents the typical welding parameters for GH3030 at 4 mm thickness:

Parameter	Value	Notes
Welding current	80–120 A	DCEN polarity
Arc voltage	13–16 V
Travel speed	60–100 mm/min
Shielding gas	Pure argon	Flow rate 10–15 L/min
Tungsten electrode	WC-20, φ2.4–3.2 mm
Fill wire	ERNiCrFe-7 (or equivalent)	φ2.4–3.2 mm
Preheat temperature	100–150°C	To reduce thermal gradient
Interpass temperature	≤ 250°C	To prevent sensitization
Post-weld heat treatment	Solution anneal at 1150°C	To homogenize microstructure

The use of a matching or slightly strengthened filler metal is critical. ERNiCrFe-7 (equivalent to Inconel 625) is commonly used for GH3030 welding as it provides good weld metal strength and corrosion resistance. The filler metal composition should be carefully controlled to avoid excessive segregation of high-melting-point elements that could lead to hot cracking.

Quality Requirements and NDT

For equipment operating at 700–800°C with corrosive media, the weld quality requirements are extremely stringent. The following non-destructive testing (NDT) methods are typically required:

NDT Method	Purpose	Acceptance Criteria
Radiographic testing (RT)	Detect internal defects (porosity, lack of fusion, cracks)	ASME V Section 9, T-2741, Level II
Dye penetrant testing (PT)	Detect surface cracks and lack of fusion	ASME V Section 7, Level II
Hardness testing	Verify HAZ and weld metal hardness	≤ 350 HV, no local hardening
Tensile testing	Verify weld joint strength	≥ 90% of base metal tensile strength

The post-weld solution annealing treatment at 1150°C for 1–2 hours is essential to dissolve any precipitated phases in the heat-affected zone and restore the full thermal stability of the alloy. This treatment also relieves welding residual stresses and homogenizes the microstructure.

Study Insights and Engineering Implications

The TIG welding of GH3030 for high-temperature PTFE cracking equipment demonstrates the critical role of welding technology in enabling the use of advanced materials in demanding industrial applications. The selection of TIG welding for this application was driven by the need for precise heat input control, clean weld formation, and the ability to achieve full penetration in thin-wall structures. The single-sided double-sided weld formation technique is particularly important for the sleeve-type geometry, where access to both sides of the joint may be limited.

For future engineering applications involving similar high-temperature alloys, the key lessons are: (1) TIG welding remains the preferred process for thin-wall high-temperature alloy fabrication; (2) careful control of interpass temperature and post-weld heat treatment are essential for long-term service reliability; (3) comprehensive NDT including both volumetric and surface methods is necessary to ensure weld integrity; and (4) filler metal selection must be carefully matched to the base metal to ensure weld metal properties at elevated temperatures. This case study provides valuable guidance for engineers designing and fabricating equipment for extreme thermal and chemical environments.