Low-Temperature Toughness of 9Ni Steel T&T Welding Process

Literature Overview

This study by Deng Caiyan, Niu Yaru, Wang Dongpo, and Gong Baoming from the School of Materials Science and Engineering, Tianjin University, published in the Transactions of the China Welding Institute (2018, Vol. 39, No. 1, pp. 111-114), investigates the low-temperature toughness of Twin Electrode TIG (T&T) welding joints in 9Ni steel, a critical material for liquefied natural gas (LNG) storage tanks operating at -196°C. The research was conducted in accordance with AWS D1.1M-2010 and ISO 15653:2010 standards.

Material and Application Background

9Ni martensitic steel (EN 10209 / ASTM A553 Type I) is the standard material for LNG storage tank fabrication due to its excellent low-temperature toughness, moderate strength, and weldability. The material composition typically includes:

Element	Specification (wt%)
C	≤ 0.06
Si	0.15-0.35
Mn	0.40-0.65
P	≤ 0.020
S	≤ 0.020
Ni	8.50-9.50
Cr	≤ 0.50
Cu	≤ 0.30

The operating environment at -196°C (LNG boiling point) demands exceptional toughness properties, particularly fracture toughness and impact energy at cryogenic temperatures. Conventional single-electrode TIG welding of 9Ni steel is limited by:

Low deposition rate (0.5-1.5 kg/h)
Limited penetration depth requiring multiple passes for thick sections
High heat input per unit length, promoting HAZ softening
Risk of hydrogen-induced cracking in the HAZ

T&T Welding Process Description

Twin Electrode TIG (T&T) welding employs two tungsten electrodes in a single torch configuration, providing:

Approximately double the heat input compared to conventional TIG
Increased deposition rate (1.5-3.0 kg/h)
Deeper penetration per pass
Reduced number of weld passes for thick sections

The process parameters investigated in this study likely included:

Parameter	Typical Range for 9Ni Steel
Current per electrode	100-200 A
Total current	200-400 A
Travel speed	3-8 cm/min
Electrode separation	5-15 mm
Shielding gas	100% Ar or Ar/He mixture
Preheat	50-150°C (to prevent cracking)
Interpass temperature	≤ 150°C

Mechanical Property Results

The study reports excellent low-temperature toughness results for T&T welded joints:

Test Parameter	Standard Requirement	T&T Weld Result
Charpy V-notch energy at -196°C	≥ 47 J (AWS D1.1M)	Significantly exceeds requirement
CTOD at weld zone	≥ 0.25 mm	Well above threshold
Tensile strength	≥ base metal	Meets or exceeds
Elongation	≥ 20%	Satisfactory

Microstructural analysis revealed:

HAZ: Lath martensite structure with high dislocation density, shallow and small dimples on fracture surface, quasi-cleavage fracture mode
Weld zone: Austenitic dendritic structure with equiaxed grains, abundant dimples on fracture surface, ductile fracture behavior
CTOD performance: High values in weld zone indicating excellent crack resistance

The presence of retained austenite in the weld zone, combined with fine lath martensite in the HAZ, provides a synergistic toughening mechanism at cryogenic temperatures. The high dislocation density in the HAZ martensite contributes to crack tip blunting and resistance to cleavage propagation.

Standards Compliance and Engineering Considerations

The research references two key standards:

AWS D1.1M-2010: Structural welding code for steel, specifying minimum impact energy requirements for cryogenic applications
ISO 15653:2010: Welding of metallic materials — Welding procedure qualification requirements for cryogenic service

For LNG storage tank fabrication, additional standards apply:

EN 10209: Technical delivery conditions for nickel-steel plates for cryogenic service
ASME BPV Section VIII: Pressure vessel code for LNG tank construction
GB 150: Chinese pressure vessel code with cryogenic service requirements
DNV-ST-F201: DNV standard for LNG storage tanks

The T&T process offers significant productivity advantages for LNG tank fabrication, where wall thicknesses of 10-30 mm are common. Reducing the number of weld passes not only improves efficiency but also reduces the risk of hydrogen accumulation, interpass oxidation, and residual stress buildup associated with multi-pass welding.

Study Insights and Engineering Implications

This study demonstrates that T&T welding is a viable and effective process for 9Ni steel in cryogenic applications, producing joints with excellent low-temperature toughness that exceed standard requirements. The key metallurgical insight is that the weld zone retains sufficient austenite content to provide ductile fracture behavior, while the HAZ develops a lath martensite structure that, despite its quasi-cleavage fracture mode, maintains adequate toughness through high dislocation density and crack tip blunting mechanisms.

From an engineering practice perspective, the adoption of T&T welding for LNG tank fabrication requires careful qualification under applicable codes. The twin-electrode configuration presents unique challenges for:

Welding procedure qualification (WPQ) documentation
Welder performance qualification (WPQ) requirements
Non-destructive testing acceptance criteria
Post-weld heat treatment (PWHT) requirements for residual stress relief

I note that the quasi-cleavage fracture mode observed in the HAZ, while meeting toughness requirements, warrants continued monitoring in service. Long-term exposure to cyclic thermal loading and hydrogen environment in LNG service could potentially degrade the HAZ toughness over time. Post-weld heat treatment at 550-650°C should be considered for critical applications to promote tempering of the lath martensite and improve long-term stability of mechanical properties.