INTRODUCTION

With rising demand for LNG, theconstruction of LNG facilities is onthe increase worldwide. Variousmaterials are selected to withstandthe onerous service conditions, in-cluding aluminium, 9% nickel steeland austenitic stainless steels. Theconstruction and fabrication ofLNG facilities will inevitably invol-ve welding pipework which usuallyincludes 304L or 316L austeniticstainless steel that will be subject toservice below -160°C or design tem-peratures down to -196 °C. 304Land 316L are among the most wide-ly used corrosion resistant alloysand have the benefit of being natu-rally tough and resistant to cata-strophic brittle failure at the lowesttemperatures, unlike lower alloy fer-

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Is Welding Stainless Steel For LNG Applications Easy?Authors: Graham Holloway, Adam Marshall, Metrode Products Ltd, UK

Keywords:LNG, austenitic stainless steel,arc welding, toughness, lateralexpansion, cryogenic, 304L,316L

AbstractGrades 304/304L and 316/316L stainless steel base materials have a fullyaustenitic microstructure and therefore have very good toughness at cryogenictemperatures. Although these stainless steels are easy to weld, it is not neces-sarily easy to achieve good weld metal toughness at the temperatures requiredby LNG plant.There are a number of options to ensure that good weld metal toughness isachieved:• Solution annealing improves toughness but is not a practical option for most

fabrications• Use of fully austenitic weld metals. This is feasible but requires weld metal

that is over-alloyed compared to the base material and the use of consum-ables which may not meet any national standards. It also presents problemsbecause many specifiers insist on delta ferrite in weld metal to guarantee free-dom from hot cracking

• Use of gas-shielded processes (gas tungsten arc welding, TIG/GTAW and gasmetal arc welding, MIG/GMAW). TIG is a solution for root welding and alsofor thin wall tube, but is very slow for larger joints. MIG has only found limit-ed use for general fabrication work and would not normally be considered forpositional welding or site welding

• Use of specially designed 308L and 316L MMA/SMAW consumables capableof meeting 0.38mm lateral expansion at -196°C

For LNG applications, which have generally involved the joining of pipework,the use of specially designed 308L and 316L consumables is proving to be thefavoured solution. The consistently good toughness that can be achieved bycareful consumable design is supported by an extensive database of all-weldmetal data. The successful application of the LNG dedicated consumables isdemonstrated by reference to a number of projects that have made use ofthem.

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ritic steels which display a sharpand temperature-dependant ductile-brittle transition.

It has long been recognised that thesatisfactory cryogenic toughness ofweld metals for austenitic stainlesssteel grades 304 and 316 cannot betaken for granted. Many of the inf-luential factors are quite wellknown. For example, low carbonweld metals (below 0.03-0.04%C)are always used and deliberate nitr-ogen additions are avoided. Thereare also unavoidable process-depen-dent influences such as the level ofnon-metallic inclusions which mayreduce the baseline toughness, sothat compensating beneficial fac-tors need to be considered such asthe choice of flux system for fluxcovered electrodes or submerged arcwelding and the control of weldmetal ferrite. The authors have re-cently reviewed many of these issu-es [1] and the present paper ex-

Figure 1: Correlation between impact energy and lateral expansion for E316L-16 MMA elec-

trodes impact tested at -196°C. Data plotted is from individual Charpy specimens.

pands on the previous overviewwith the results of new tests.

TOUGHNESS REQUIREMENTS

Design temperatures encounteredfor austenitic stainless steels used inLNG facilities may vary but forsimplicity and ease of testing, Char-py impact tests are normally carriedout at -196°C because this test tem-perature is conveniently obtainedby cooling in liquid nitrogen.Toughness is proportional to theimpact energy absorbed by fractureand lateral expansion is a measureof the Charpy test specimen defor-mation or fracture ductility.

In principle, the Charpy specimen

might be examined to assess thepercent ductile shear fracture, andsome specifiers require this to be re-ported. However, in austeniticwelds, the mixture of cleavagefacets and areas of shear on thefracture surface means that Charpyfracture appearance cannot be as-sessed by the convenient visual cri-teria of ASTM E23, so test houseswill usually decline to report it.

The most commonly specifiedtoughness requirement is based onCharpy lateral expansion. The re-quirement for 0.38mm lateral ex-pansion at -196°C, which can befound in the ASME Code (eg ASMEB31.3 for process piping), is fre-quently quoted even for projectsthat are not being fabricated to AS-ME Code requirements. Although0.38mm lateral expansion is proba-bly the most widely specified crite-rion, some European projects mayhave a Charpy energy requirement.For example, projects carried outunder the scope of TÜV sometimesspecify a minimum Charpy energyof 40J/cm2, corresponding to 32J ona standard Charpy impact speci-men. For a given set of data and testtemperature there is normally a li-near relationship between lateralexpansion and impact energy, Figure 1. Weld metal data are pre-sented to show the relationship be-tween Charpy energy and lateral ex-pansion, however, this paper focu-

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Table 1: Solution annealed (1050°C/1 hour + WQ) versus as-welded all weld metal properties.

Figure 2: Charpy transition curve for a range of fully austenitic weld metals.

They all show >0.5mm lateral expansion at -196°C.

Figure 3: Correlation between impact energy and lateral expansion for ER316L TIG and

ER316LSi MIG, impact tested at -196°C. Data plotted is for individual Charpy specimens.

ses on the -196°C design require-ment of 0.38mm lateral expansion.

SOLUTION ANNEALING

Solution annealing, heat treatmentat 1050°C followed by water quen-

ching, is one method of producinghigher toughness weld metal. Ex-perience has shown, for example,that an "off-the-shelf" rutile typeE316L MMA electrode giving as-welded impact energy of only 10-25J and 0.10-0.28mm lateral expan-sion at -196°C would be restored to32-40J and 0.50-0.70mm lateral ex-pansion after solution annealing,Table 1. In addition to improvingtoughness, there will also be a re-duction in yield (proof stress) andferrite content of the weld metal.As-welded 0.2% proof stress valuesalways overmatch base metal butheat treatment brings the proof-to-tensile ratio close to the annealedbase material value, while normallystill meeting the base materialstrength requirement.

Solution annealing has a number ofpractical disadvantages: it is diffi-cult to carry out on large fabricatedcomponents; on thin material thereis a danger of distortion; complexfabricated components may alsodistort, and it is not really practicalor economic to carry out a solutionannealing heat treatment on a pipe-line. However, in the foundry suchheat treatments are more appropria-te and often specified for welds ap-plied to castings.

Although solution annealing willprovide the necessary toughness re-quired for LNG applications it isnot an option that provides a prac-tical solution for pipework.

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Figure 4: Correlation between ferrite (WRC ferrite calculated from the wire analysis) and lat-

eral expansion for ER316L TIG and ER316LSi MIG tested at -196°C. Data plotted is based on

the average of a set of Charpy specimens.

Table 2: Representative all-weld metal mechanical properties for the Metrode 308L 'CF' consumable range.

Figure 5: Effect of weld metal ferrite on lateral expansion (tested at -196°C) for a range of

308L consumables. Data from a variety of sources.

FULLY AUSTENITIC STAINLESS

STEEL WELD METALS

There are a number of fully austeni-tic low carbon weld metals, all ofwhich provide very good impactproperties at cryogenic temperatu-res; in fact some of the fully auste-nitic stainless steel weld metals willmaintain useful toughness down to-269°C, Figure 2.

The fully austenitic stainless steelweld metals will be more highly al-loyed than the standard 308L or316L compositions, and this willadd to the cost. It also means usinga weld metal which although suita-

ble for the application does not ha-ve the same analysis as the base ma-terial and in some circumstances itmay not meet an AWS or EN speci-fication. Some compositions maynot be readily available for all wel-ding processes. Experience has alsoindicated that many specifiers donot allow fully austenitic weld me-tal compositions but require weldmetal containing delta ferrite to gu-arantee freedom from hot cracking.

Although fully austenitic weld me-tals will comfortably meet thetoughness requirements of LNG ap-plications there are a number of dis-advantages in their use which me-ans they are not the ideal solution.

GAS SHIELDED WELDING

PROCESSES

The gas-shielded arc welding pro-cesses - TIG and MIG - producewelds with a low level of microsco-pic non-metallic inclusions, leadingto inherently good toughness at alltemperatures. Excellent cryogenicimpact properties can be achievedconsistently without special controlmeasures, using standard commer-cially available ER308L/ER308LSiand ER316L/ER316LSi wires. Figure 3 shows the relationship be-tween impact energy and lateral ex-pansion for 316L; with the TIG pro-cess comfortably meeting 0.38mmlateral expansion and in the testscarried out to date the MIG process

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Figure 6: Correlation between impact energy and lateral expansion for a series of basic coated

308L (E308L-15) electrodes and a series of basic coated 316L (E316L-15) electrodes impact

tested at -196°C. Data plotted are from individual Charpy specimens.

Figure 7: Correlation between ferrite and lateral expansion from -196°C impact tests. Data is

from the same series of 308L and 316L basic coated electrodes plotted in Figure 6. Data plot-

ted are from individual Charpy specimens.

Figure 8: Correlation between ferrite and lateral expansion for E308L-15 electrodes impact

tested at -196°C. The low, medium and high alloy levels refer to total alloying content of the

weld deposit but all compositions fall within the AWS A5.4 E308L-15 analysis range.

also meets a minimum 0.38mm la-teral expansion. With the gas-shiel-

ded processes there does not appearto be the same dependence of

toughness on ferrite content thathas been seen for the flux shieldedprocesses, Figure 4. This graph isbased on the predicted ferrite con-tent of the wire calculated from thewire analysis so it is possible thatthe weld ferrite content wouldshow more of a correlation withtoughness.

The gas-shielded processes, TIG andMIG, can meet the impact require-ments and have an important roleto play in fabrication. But if thewelding process options are restric-ted to TIG and MIG then there willbe unnecessary restrictions imposedon the fabricator. The ability to useflux shielded processes (MMA,FCAW and SAW) would provide thefabricator with the scope to usewhichever process was preferred fora particular application. However,the non-metallic inclusion level isunavoidably higher with the flux-shielded processes - MMA, FCAWand SAW - and consequently these308L/316L consumables require ad-ditional metallurgical controls toensure that welds will achieve therequired cryogenic toughness.

SPECIALLY DESIGNED 308L AND

316L CONSUMABLES

Previous work has been publishedexamining the effect of ferrite con-tent on the toughness of 308L and316L weld metals and the generaltrend is demonstrated by Figure 5which shows that, up to a certainpoint, as ferrite increases the tough-ness is reduced. The region beyond

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Figure 9: Correlation between impact energy and lateral expansion for 316L MMA elec-

trodes. The advantage of the controlled ferrite consumable being clearly demonstrated.

Data plotted is from individual Charpy specimens.

Figure 10: Correlation between impact energy and lateral expansion for 308L flux cored wires

impact tested at -196°C. The controlled ferrite version (SC308LCF) consistently meets

0.38mm lateral expansion. Data plotted is from individual Charpy specimens.

Table 3: Representative all-weld metal mechanical properties for the Metrode 316L 'CF' consumable range.

this point, where some recovery oftoughness occurs, is not the subjectof this paper and the authors arenot aware that specifiers are yet pre-pared to acknowledge this effect.

To investigate the trend up to~10FN further, two series of MMAwelding consumables were produ-ced: one E308L and one E316L.Both used the same design of basicflux coating (AWS A5.4 'EXXXL-15',EN 1600 'E XXX L B'). Basic fluxsystems are commonly consideredto offer better toughness than rutiletypes. The two series of consuma-bles were designed to cover a rangeof ferrite contents from ~0.5FN to~10FN by varying the %Cr/%Ni ra-tio of the weld metals, but aimingto stay within the AWS and EN ana-lysis ranges. All of the data fromthese tests are presented in Figure 6,showing that it is possible to get awide range of impact properties fr-om weld metals conforming to re-cognised national standards. Thedata covered a range of ~0.3-0.9mmlateral expansion and ~25-55J, withthe 308L tending to produce slight-ly higher values than 316L.When the lateral expansion is plot-ted against ferrite content, Figure 7,it can be seen that for these data,that 0.38mm lateral expansion can-

not be guaranteed with a ferritecontent above ~4.5FN. However, ifthe data are further divided it canbe shown that it is not only the fer-rite content of the weld deposit thataffects toughness but also the alloylevel, Figure 8. In Figure 8 it can beseen that for weld metals havingthe same ferrite content the loweralloyed deposit will tend to producethe best toughness.

The trends that are demonstratedby these data have been used to ma-nufacture commercial, controlledferrite, MMA and flux cored wireconsumables, Tables 2 and 3. Theconsistency of the impact proper-ties that can be achieved by usingcontrolled ferrite consumables is de-monstrated by the all-weld metaldata plotted in Figures 9 and 10,which show the advantage in con-

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Figure11: Gas shielded flux cored arc welding, controlled ferrite 308L, being used for the first

time during construction of the Grain-LNG importation facility on the Isle of Grain, UK.

Photograph courtesy of P M Associates UK Ltd.

Figure 12: Isle of Grain importation terminal showing the pipeline connecting the jetty to the

storage tanks.

trolling ferrite and alloy level whencompared to standard commercialconsumables, closing the gap be-tween rutile and basic electrodes.

APPLICATIONS OF CONTROLLED

FERRITE CONSUMABLES

Many tonnes of Ultramet308LCF/316LCF electrodes havebeen used on projects all round theworld and numerous successfulweld procedures have been carriedout. Most of the projects these elec-trodes were used on were pipelinesand process pipework, all of whichhad stringent low temperaturetoughness requirements.

The Ultramet 316LCF MMA electro-de was originally designed in theearly 1990's to satisfy the cryogenictoughness requirements of Mobil/Ralph M Parsons for the ScottishArea Gas Evacuation (SAGE) projectterminal at St Fergus, Scotland.More recently, tonnage quantitiesof the controlled ferrite 'CF' MMAconsumables have been used in Ka-zakhstan on the Karachaganak Pro-ject. There have also been signifi-

cant quantities of cryogenic pipew-ork welded with the 'CF' consuma-bles on the natural gas to liquidsplant of the Mesaieed Q-Chem pet-rochemical complex in Qatar.

The first commercial use of the Su-percore 308LCF flux cored wire wasfor the Isle of Grain LNG terminalin the UK. The flux cored wire wasused in the fabrication of a threemile pipeline in 915mm (36inch)diameter, 10mm (0.4inch) wallthickness, 304L stainless steel, seeFigures 11 and 12. Productivity wasoptimised by root welding with thepulsed-MIG process and joint fillingwith Supercore 308LCF flux coredwire, a combination which also metthe other project criteria with re-spect to toughness. Table 4 showsexamples of selected weld procedu-re tests for the Grain project.

CONCLUSIONS

Is welding stainless steel for LNGapplications easy? Just over 25 yearsago a classic review of and presenta-tion of test data for the cryogenictoughness of austenitic stainless

steel MMA weld metals concludedthat "Dual weld metal requirementsfor 3FN minimum and 0.38mm mi-nimum lateral expansion at -196°Care at odds and sharply limit thechoice of welding filler metal com-positions conforming to the requi-rements of AWS A5.4…" [2]. Thosesame requirements are frequentlycalled for today, but with improve-ments in consumable design andmanufacture based on careful inves-tigative testing of otherwise vulne-rable weld metals, it is possible tosay "Yes, welding stainless steel forLNG applications is easy" - assu-ming the correct consumables havebeen selected.

REFERENCES

[1] Holloway, G B; Zhang, Z & Marshall, A W.

"Stainless steel arc welding consumables

for cryogenic applications" Paper P0449,

Stainless Steel World 2004 Conference,

Houston, USA.

[2] Szumachowski, E R & Reid, H F "Cryo-

genic toughness of SMA austenitic stain-

less steel weld metal: Part II - Role of

nitrogen" Weld.J, 58 (2), Feb 1979,

p34s-44s.

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Table 4: Representative mechanical properties for the Metrode 'CF' consumables from weld procedure tests.