Welding of Stainless Steel for LNG Applications

Stainless steel welding is an essential process in the construction and maintenance of LNG facilities. It involves joining stainless steel components to create robust, resilient structures that can withstand the extreme cold temperatures and cryogenic conditions required for the storage and transport of LNG.

The welding process must be carefully executed to prevent damage to the stainless steel, as any imperfections can compromise the integrity of the entire system. For this reason, highly skilled welders are employed to ensure the joints are strong and seamless, using specialized techniques like TIG (Tungsten Inert Gas) welding to produce high-quality welds.

Material Characteristics

General Considerations

Various materials are selected to withstand the onerous service conditions imposed by LNG, including aluminum, 9 % nickel steel and austenitic stainless steels. The materials and welding consumables suitable for use at various nominal temperatures are shown in Table below.

This Article is concerned with the specialist area of arc welding consumables for joining 304L and 316L austenitic stainless steels that will be subject to service or design temperatures down to -196 °C.

These steels are among the most widely used corrosion resistant alloys and have the benefit of being naturally tough and resistant to catastrophic brittle failure at the lowest temperatures, unlike lower alloy ferritic steels which display a sharp and temperature-dependant ductile-brittle transition.

Toughness Requirements

Design temperatures encountered for austenitic stainless steels used in LNG facilities may vary but for simplicity and ease of testing, Charpy toughness tests are normally carried out at -196 °C because this is an easily achieved, and convenient, test temperature obtained by cooling in liquid nitrogen. The standard Charpy impact specimen is 55 mm long and 10 × 10 mm in section with a V-notch to initiate a fracture path when the specimen is struck with a pendulum striker. Toughness is proportional to the impact energy absorbed by fracture and lateral expansion is a measure of the specimen deformation or fracture ductility.

The most commonly specified toughness requirement is based on Charpy lateral expansion. The requirement for 0,38 mm lateral expansion at -196 °C, which can be found in the ASME Code (e. g. ASME B31,3 for Piping System of pressure vessels on gas tankersprocess piping), is frequently quoted even for projects that are not being fabricated to ASME Code requirements.

Although 0,38 mm lateral expansion is probably the most widely specified criterion, some European projects do have a Charpy energy requirement. For example, projects carried out under the scope of TUV sometimes specify a minimum Charpy energy of 40 J/cm², corresponding to 32 J on a standard Charpy impact specimen.

Stainless Steel Welding

The Welding Processes – Where They are Used

There are five main Aluminum Welding Techniques: Advanced Methodsarc-welding processes:

• Gas tungsten arc welding (GTAW) or tungsten inert gas (TIG) welding.
• Gas metal arc welding (GMAW) or metal inert gas (MIG) welding.
• Shielded metal arc welding (SMAW) or manual metal arc (MMA) welding.
• Flux cored arc welding (FCAW).
• Submerged arc welding (SAW).

Each of the processes has areas and applications where it excels and the following briefly describes the strengths of each process.

GTAW

The GTAW process is used for root welding pipes and tubes, but is also used for completing joints in smaller diameter thinner wall pipe. The GTAW process is very controllable and produces high integrity weld metal making it an excellent choice for applications requiring careful control or where weld integrity is more important than productivity.

GMAW

The GMAW process using solid wire has not found widespread use for critical fabrication work and although it is capable of achieving good cryogenic toughness. Many of the applications where GMAW could be used are now being welded with flux-cored wires.

SMAW

The SMAW process is still widely used for many applications because of its simplicity and adaptability. The process requires relatively simple equipment and does not require a shielding gas, making it an attractive process for site welding. The success of the process is dependent, not only on the characteristics of the electrode, but also the skill of the welder; so electrodes with good operability and welder appeal are of great benefit.

FCAW

The gas-shielded FCAW process has found significant use in areas where SMAW was traditionally used but where a continuous wire process can provide a valuable productivity advantage. The gas-shielded FCAW process is similar to GMAW but uses a tubular cored wire in place of the solid wire. The cored wire can provide a number of advantages including positional capability, scope to vary chemical analysis of the weld deposit, and excellent weld appearance. The flux cored wires can offer productivity advantages over SMAW electrodes in applications involving pipes of diameter above ~200 mm and (depending on actual diameter) wall thickness greater than ~9,5-12,5 mm.

SAW

The SAW process is not normally used for general pipe work or for site welding but is a highly productive process for joining thick sections that can be manipulated so that welding can be carried out in the flat position. This could apply to longitudinal seams in tanks or vessels, or to circumferential joints that can be rotated. SAW would not normally be used for material less than ~15 mm thick or less than ~200 mm in diameter.

Toughness

The gas-shielded arc welding processes (GTAW and GMAW) produce welds with a low level of microscopic non-metallic inclusions, leading to inherently good toughness at all temperatures. Excellent cryogenic impact properties can be achieved consistently without special control measures, using standard commercially available ER308L/ER308LSi and ER316L/ER316LSi wires.

However, the non-metallic inclusion level is unavoidably higher with the flux-shielded processes (SMAW, FCAW and SAW) and consequently these 308L/316L consumables require additional metallurgical controls to ensure that welds will achieve the required cryogenic toughness. Three areas in particular are important:

• ferrite content;
• alloy control;
• specific to SMAW and SAW;
• the type of flux.

A FERRITE

While the typical microstructure of 304L/316L parent material is fully austenitic, it is considered essential for weld metal compositions to be balanced to solidify with some ferrite (expressed as ferrite number, FN) to ensure resistance to hot cracking. However, it has long been recognized that the accepted ferrite levels normally applied to flux-shielded welding processes may not be compatible with satisfactory cryogenic toughness.

Various standards specify ferrite limits for austenitic stainless steel weld metals. For example, the article to American Welding Society (AWS) specification A5,4 for stainless steel SMAW electrodes describes the relevant issues and recommends 3FN maximum or fully austenitic welds and a basic flux system in preference to rutile. API 582 Guidelines require 3FN minimum, although it is noted that for cryogenic service lower FN may be required.

It has been found that it is possible to achieve the 0,38 mm lateral expansion requirement by controlling the weld metal ferrite of SMAW electrodes and flux cored wires in the range 2-5FN.

A particular concern with austenitic weld metals having low ferrite levels is the risk of hot cracking.

B ALLOY CONTROL

By controlling the weld metal ferrite in the range 2-5FN and simultaneously balancing the Cr:Ni equivalent ratio to eliminate any potential risk of hot cracking, the deposit composition of SMAW electrodes and flux cored wires becomes restricted to the “lean” area of their respective weld metal specification ranges. One consequence of this is that with 316L types molybdenum is preferably controlled in the range 2,0 – 2,5 %.

C FLUX TYPE

With CMn and low alloy ferrritic steels, it is traditionally accepted that the best impact properties using SMAW electrodes are achieved with fully basic flux systems. With austenitic stainless steels the effect is less pronounced, although it has long been recognized and reported that SMAW electrodes with basic flux coverings (AWS E3XX-15 types) give somewhat better results than rutile (AWS E3XX-16/17) coatings.

Solid wire is usually specified for the SAW process, so there is much less scope for the kinds of alloy control which allow SMAW and FCAW to be optimized for cryogenic toughness; this makes it even more important to select the correct flux.

Weld Procedure

Having selected the correct consumables, the weld procedure then needs to be optimized. Contrary to the usual trend with low alloy steel welds, the best toughness is obtained in austenitic welds when the number of weld runs deposited in the joint is minimized. This can be demonstrated by a series of submerged arc welds produced in 20 mm plate with heat inputs ranging from 1,0 kJ/mm (27 runs) up to 2,7 kJ/mm (only 10 runs).

These welds showed an increase in toughness, from 0,30 mm lateral expansion (28 J) up to 0,48 mm (46 J), with increasing heat input. The reason for the improvement in toughness is not clear, but it is suspected that the reduction in the number of runs deposited reduces the strain ageing effect and hence improves the impact properties.

Many authorities do not allow this effect to be taken advantage of when welding stainless steels for aqueous corrosion applications because of concerns about the potentially adverse influence of high heat input on corrosion performance.

However, published work has shown no detrimental effect on joint performance in 304L/316L stainless steel with interpass temperatures up to 300 °C and heat inputs as high as 2,9 kJ/mm. For LNG service the corrosion requirement is minimal and there should be no detrimental effects on performance with heat inputs up to -2,5 kJ/mm.

The practical application of this is that in order to achieve good cryogenic toughness it is not necessary to use low heat inputs and controlled stringer bead welding techniques; in fact it is beneficial to use higher heat inputs and larger weld beads.

Read also: Weld Failures and Weld Testing Techniques in Oil and Gas Industry

This means that when welding with SMAW electrodes or flux cored wires it is possible to use a full width weave without having a detrimental effect on toughness.

The use of a wide weave can be particularly helpful when welding positionally because it helps with weld pool control and is far quicker for joint filling. The selection of welding parameters should be based on joint configuration, material thickness and component size; with any proposed parameters being proven by a weld procedure qualification test.

Author

Olga Nesvetailova

Freelancer

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