Welding Issues for Ship Structures

31

INTRODUCTION

When discussing failures of ship structuresthere is a tendency to gravitate towards theweld as the source of the failure. Apart froma few well-publicised, and now almost his-

torical, events there is little to link failures directly to thewelding carried out at the build stage. However, this is nota global assessment of the situation, but more one from aEuropean perspective. The current rise of greenfield siteshipyards in the Far East may create a different globalperception.1 In addition, in-service repairs involving weld-ing appear to have been the source of a number of failuresin the past.

This preamble should not be taken as an indicator that thewelding of ship structures has reached a level, in developedmarine countries, where no significant issues exist. Forexample, a change in product mix can create a temporarydestabilising effect on the welding workforce. If a yard withseveral years experience building thin plate structures, wherethe concentration is on distortion reduction, switches to athicker plate product with more emphasis on larger, multi-run fillet welds, then this requires a large scale re-educationof the workforce, but particularly the welders, who tend to besingle entity sources of work. A similar situation arises whenchanging to building chemical carriers. In this instance theissues of welding stainless steel would come into play,2

which are mainly centred on corrosion mitigation. Issues stillarise with welding and defects can occur; some of thosedefects detected during the building stage can originate fromthe following areas:

� Variability of the welding consumable quality,� Variations in the performance of welding equipment,� Lack of adherence to welding procedures,� Design imposed build issues,� Human fallibility,� Management control,� Steel material issues.

VARIABILITY WITHIN WELDING CONSUMABLES AND WELDINGPROCESSES A great deal of qualitative and quantitative data on weldingconsumable performance has been produced over the years.In many developed shipyards there has been a progressivemove away from the Manual Metal Arc (MMA) weldingprocess to the Flux Cored Arc Welding (FCAW) process.This is a move from a manual process to a semi-automaticprocess. The welded length produced from the manualprocess is directly related to the length of the electrode. In the case of the FCAW process the welded length is sig-nificantly longer than the MMA process and is generallyrelated to when the welder wants to stop welding. Typicallengths are shown in Table 1. It can be seen that weldingposition alters the process efficiency, with vertical weldingbeing slower. The justification to move from MMA to acored welding process is very straightforward, based on thedata shown in Table 1. Also the potential defect rate usingMMA is higher than when using the cored wire process.Furthermore, an automatic process has been included which

Welding issues for ship structures

NA McPherson BSc, PhD, MBA, DSc, CEng, FIMMM, FWeldI, BAE Systems – Surface Ships,Glasgow, Scotland

Welding of ship structures is regarded as a process that requires a high level of controlto develop finished product consistency. By its current nature welds may still contain lowlevels of defects. To further reduce defect levels and improve the consistency of thewelded products some factors have been identified which could contribute to inferiorperformance. A number of these have been termed ‘management issues,’ ie, technologyand aspects that are well established and need to be part of the overall managed process.In addition the differentiation between thick and thin plate has been made, highlightingtheir significantly different requirements. Steel material requirements have been reviewedand the improvement of steel plate/bar and welding consumables in the last 10 years havebeen seen to contribute to overall process consistency.


No. A18 2010 Journal of Marine Engineering and Technology

McP EDITED_JMET_Sep.qxd 9/18/10 6:09 PM Page 31

shows the highly significant benefit of using the process.However, set-up time is not included but that aside theprocess efficiency differences are huge.

The flux cored arc welding process is made up of the wirefeed unit shown schematically in Fig 1 and cored wire shownin cross section in Fig 2. There are variations in the coredwire production process and these are shown in Fig 3. Theseamless flux cored wire has the additional benefit of havingfreedom from moisture pick up and consequential higherhydrogen content in the weld metal. Typically the seamlesswire maintains a weld metal hydrogen content of less than5ml/100g. This can create benefits when weld preheat is anissue. It should be borne in mind that this is especially criti-cal as some of the flux components are hygroscopic.

One other potential problem with flux cored wire is whenthere is insufficient flux fill in the cored wire. This can mani-fest itself as poor weld quality, with areas of porosity, or thewire breaking during the feeding process. Within BAE SystemsSurface Ships, Govan Yard, the same type of FCAW wire has

been used for some 14 years and has been problem free. Withinthat period some minor alterations have been made to the wire,but this has probably accentuated the consistency of the prod-uct. On a sound basis such as this, when the Govan andScotstoun yards of BAE Systems Surface Ships were broughtunder the one management, part of the rationalisation processinvolved working both yards with the same welding consum-ables. This has now been the case for almost nine years.

In the case of the submerged arc welding (SAW) tractorprocess, a solid wire under flux process has been used with ahigh degree of product consistency. However, in relativelyrecent times a 3.2mm or 4.00mm dia flux cored wire underflux SAW consumable combination has been introduced ontothe market3 with productivity benefits of up to 34%. This isobtained from a combination of increased travel speed andreduced number of welding passes. Obviously this is a partic-ularly beneficial process for welding thicker plate wheremulti runs are required. In addition to the productivity bene-fits, there is no deterioration in the weld metal or the HAZ

32


Journal of Marine Engineering and Technology No. A18 2010

*Typical length is the length welded prior to the welder stopping the arc.Table 1: Typical welded lengths for various welding processes and positions

Power sourceWelding

Wire feed unit

Weldingwire

spool

WireFeedrollers

Weldingcable

Powersupply

(a) (b)

Fig 1: Main components of the coredwire welding process

Fig 2: Flux cored wire variations – ((aa)) Seamless wire with flux centre;((bb)) Seamed wire which hascracked open

Consumable Position Typical length (mm)* Typical speed (mm/min) Efficiency factor Process

FCAWDownhand 850 350 100 Semi automatic

Vertical 450 200 30 Semi automatic

MCAWDownhand 850 350 100 Semi automatic

Vertical 450 200 30 Semi automatic

MMA Downhand 450 130 20 ManualVertical 300 65 6.5 Manual

FCAW Downhand 10000 400 1300 Automatic


toughness. The less welding runs put into the structure thenthe less is the possibility of introducing a welding defect.

When choosing welding consumables it is essential toconsider a number of factors that develop a ‘value’ for theproduct. This can create a situation of conflict with many pro-curement departments, whose main focus is on price. It iscomplex to develop a case which clearly and concisely showsthe value of a more expensive welding consumable. Thealternative approach is that the consumable required is tech-nically identified by the welding engineers, and the procure-ment department’s function is to get the best financial deal forthe product specified. If that does not happen, the situationobserved a number of years ago can arise whereby low priceproducts created significant repair rates which, when factoredinto the price of the consumable, resulted in a high total costconsumable selection.

Since the introduction of this philosophy there has beenstable performance, but it has to be fully considered that thisis only one part of the welding process.

Flux cored wires are used extensively within BAESystems Surface Ships and also where austenitic and duplexstainless steels have been welded. For carbon steels, wireswith strengths up to 700N/mm2 and toughness down to –80°C(not in combination) have been used over long periods of time.

The welding equipment used in the shipbuilding processis primarily the wire feed unit shown in Fig 4. Within ouryards there has been a philosophy to standardise weldingequipment across both sites. To date significant steps havebeen taken to do this, but it is a long term project. Severalyears ago a myriad of wire feed units existed on both sites. Aproportion was nearing the end of their working lives. All hadstrong points and weak points. A number of possible suppli-ers were involved and the main criteria given to them were:

� Weight,� Portability,� Robustness,� Wire feed roll integrity,� Display of volts/amps/wire feed speed.

One particular supplier stood out from the rest, and anovel concept design (at that time) was produced whichincorporated all the above. The weight had been attacked byusing a strong tough polyurethane casing and tubularaluminium carrying and base units. Over the years thepartnering with this company has developed into a two-wayinterchange of concepts and performance.

The welding gun, typical of that shown in Fig 5, has beenthe subject of a number of developments, mainly from anergonomic standpoint. Within the Clyde yards there has been standardisation on welding guns, and the current focus

is on moving to a lightweight design, which significantlyreduces welder fatigue, specifically in the over head position.In addition some work is currently ongoing to evaluate theeffects of drafts at different gas flows. As an example, for ashielding gas flow of 17litre/min the effect of 5mph and10mph drafts are shown in Fig 6. This effect has been visu-alised using a laser backlighting technique. The X-rays corre-sponding to the 5 and 10mph drafts both contained heavyporosity. This work will serve to generate a much greaterunderstanding of shielding gas flow effects.4

Overall, significant strides have been taken to minimisethe potential variables within the welding process and, as aresult, there has been considerable stability in welding defectlevels over the last three years within BAE Systems.

WELDING PROCEDURESWelding procedures are developed to satisfy a number ofrequirements, specifically those of the classification societies.

33



Fig 3: Variations in flux or metal coredwire configurations

Fig 4: Cored wire, wire feed welding unit

Fig 5: Lightweight welding gun

Seamless Seamed Lapped Folded in

McP EDITED_JMET_Sep.qxd 9/18/10 6:09 PM 3

However, these procedures are also a valuable source of datafor the shipyard. Marginal passes in mechanical properties ofthe joint may be deemed as being satisfactory, but it must beborne in mind that the procedures are normally developedfrom very good fit up plate with well defined prep angles. Inaddition, weld procedures are rarely developed on the job, butin a welding booth in a Training Area for example – basicallyunder very good conditions. At BAE Systems procedure pass-es in the marginal category are often repeated to identify ifthere are specific issues. The most obvious example is theheat affected zone toughness variations. Often an individuallow figure has been identified with a coarse grain structure.Any reprocedure would concentrate on the factors giving riseto mitigating the possibility of significant grain growth.

For other reasons it is often beneficial to establish data onweld metal strength (longitudinal tensile test of the weld metal)and in the case of thicker plate to carry out CTOD testing. Whilethese tests add cost, the data developed can lead to a greaterunderstanding of the process and the process capability. This isespecially the case when considering HAZ toughness.

The transmission of welding procedures to the shop flooris a matter of preference for individual yards, but at BAESystems Surface Ships this is done in the form of a plasticisedcard containing summary information on all the weldinginvolved in a specific contract. An example is shown in Fig 7.This also allows the welder leeway to adjust parametersdepending on variations in fit up. Welders are required to havethis card in their possession at all times. As other contracts arebeing worked on it is often the case that there are very fewchanges in the content of the card. The possibility of using anon-contract specific card is currently being considered.

HUMAN INPUT INTO THE WELDINGPROCESSHuman input is one area of variability within the process, evenwith the semi-automatic welding process. It is clear that somewelders will have greater aptitude than others and will be better suited to tackling more demanding work. Some yearsago a study in the Govan yard was carried out which involved

34



(a)

(b)

(c)

Direction of draft

Direction of draft

Fig 6: Gas flow visualisation using 17 l/mingas flow ((aa)) No side draught ((bb)) 5mphdraught ((cc)) 10mph draught


assessing the capability of all the welders. Three grades weredeveloped jointly between the welding engineers and the produc-tion supervisors. Grade allocation showed very few differencesbetween the welding engineers and production supervisors.

Grade 1: Capable of welding the most complex joint config-urations;

Grade 2: Capable of welding all but the most complex jointconfigurations;

Grade 3: Capable of fillet welding only.

This structure formed the basis for establishing the skill dis-tribution of welders across each fabrication area and each shift.Some significant imbalances were found. An exercise wasundertaken to level out the skill distribution. In addition, anotherissue that was highlighted was the imbalance of supervisorswith a welding background. Again this was rectified to ensurethat each shift had at least one supervisor with a welding back-ground working in a fabrication area. It was quite clear that thismove paid very significant dividends in terms of stabilising thewelding process and reducing the welding defect levels.

THE NEXT STEPS The Govan yard had the first industrial welding robot in anyUK shipyard.5 Although not used to its full potential, it hasserved to highlight problem areas and as a learning tool. The

most obvious area for ‘full’ robotic welding is at the end of apanel line where up to 85–90% of the joint available for weld-ing could be achieved. However, robotic welding is not thepanacea and other automated welding can also be used. Thesecan vary from very simple and highly efficient fillet weldingtractors, such as those shown in Fig 8. Welders can use anumber of these at one time and, depending on the jointlength, up to four can be used at once. The potential benefitsof this over the semi-automatic process could be as much as100%, when one welder uses two welding tractors at once(this assumes the same travel speed, but with the welder stop-ping once every 1100mm and dressing tails etc).

Seam welding using a seamer or a submerged arc weldingtractor is ostensibly an automated welding process, and thick-er decks are ideally suited for welding with a SAW process.Unit link ups can also be effectively welded using cored wiretrack mounted welding equipment. An example of this isshown in Fig 9, where the welders input is related to minoradjustments to welding head position. Some previous workhas shown the use of this process to be highly beneficial com-pared to the semi automatic process in the horizontal weldingposition. Deviations from the set procedures increase the riskof inducing some form of defect, either physical or mechani-cal, into the structure.

Basically, there is scope to consolidate on current levels ofautomated welding and use the consolidation as the steppingstone for the introduction of other automated processes.

35



Fig 7: Example of a welders’ instruction card

Material Process Consumable Position Amps WFS(ipm)

Volts Pol.

Carbon Steel FCAW SF-1A

1.2mm

Flat

Horizontal

Vertical

Overhead

210 – 260

160 – 230

150 – 220

180 - 200

390-440

280-360

260-320

280-310

23 – 25

20 – 23

20 – 23

23 - 25

DC+ ve

DC+ve

DC+ve

DC+ve

Carbon Steel FCAW Safdual 100

(Back-up consumable

to SF-1A)Flat

Horizontal

Vertical

Overhead

160 - 260

155 - 225

140 – 200

180 - 200

280-440

270-240

230-280

280-310

23 - 26

21.5-22.5

21 – 23

23 - 25

DC+ve

DC+ve

DC+ve

DC+ve

Carbon Steel FCAW Megafil 731 B

Replaces FC-4

Flat

Rooting of Butts Only 230 -250

400 - 430 23 - 25

DC+ve

Blkhds (as per drawing)

AusteniticConsumableVacpac controls in place

FCAW Safdual 654P

(Maximum 72 hour

lifespan from time of

issue)

Flat

Horizontal

Vertical

Overhead

175 –210160 - 190

160 – 170

165 - 180

280-340

260- 310

260-290

270-300

24 – 25

21.5 - 24

21 – 22.5

22 - 24

DC+ve

DC+ve

DC+ve

DC+ve

Carbon Steel

7mm thk and below

MCAW Nittetsu MC-1

1.0mm

Flat/Vert. Down/Hor.

For Rooting of Butt JointsOnly Flat 165-

175Vert.Down

150-160

Hor. 145-150

190-210

145-180

140-175

21 – 23

18 – 20

18 – 19.5

DC+ ve

DC+veDC+ve

Blkhds (as per drawing)

Austenitic ConsumableVacpac controls in place

MMA

Safdry 309L

3.25mm

(Max. 8 hour lifespan

from time of issue)

All Positions

(Except Vertical Down)

100 - 130 N/A N/A AC

Carbon Steel MMA SAF GF200

3.25mm

4.00mm

Flat

(Fillet Welding Only) 130 – 140

170 - 190

N/A

N/A

N/A

N/A

AC

AC

Carbon Steel MMA Safer GTi

(For tacking in all

positions inc. vertical

down)

3.25mm & 4.0mm

3.25mm

(All positions)

4.0mm

(All positions)

110 – 120

160 - 195

N/A

N/A

N/A

N/A

AC

AC


HUMAN FALLIBILITYThe welding process by its nature is subject to humanfallibility. The aim of the previously described issues was tominimise the impact of human fallibility, with the increasedlevel of automated welding being a key factor. Issues such as

a person’s deteriorating eyesight or physical capabilitiesrequire sensitive treatment and, where feasible, they would bere-allocated to work more in line with their capabilities, eg,workshop welding. At BAE Systems Surface Ships, allwelders are retested/recoded on a two-yearly basis. If issueshave been identified with a specific process condition basedon X-ray results one of the tests may be substituted for anoth-er to duplicate the problem. This has the benefit of assessingall the welders against the problem.

Extensive training is given to ensure welders have the correctlevel of capability. For example, great emphasis has been put onusing the track mounted automatic welding system shown in Fig9. Currently 7% of the welders on site have been qualified in theuse of this equipment. However, it is imperative that this specif-ic group are using the equipment on a regular basis to ensure theskill level is being maintained. That is a resource managementissue, and also why the proportion is not higher.

DESIGN ISSUESThere is still a need for designers to be more aware of whetherthe Design for Build concept is being actively followed. Thishas been highlighted when building vessels with a significantproportion of thin plate (<8mm thick) in the structure. A basicprinciple in building these structures is to minimise the heatgoing into the structure, as this tends to induce the phenome-non of thin plate distortion. Consequently the effects are seenas rework and possibly build schedule impacts.

One issue was the use of intermittent welding on non-structural bulkheads. The application of intermittent weldingwill reduce the amount of heat going into the structure byabout 50%. However, there appeared to have been non-struc-tural areas where this had not been applied, and also wetspaces where double continuous welding had been carried outto remove the possibility of corrosion occurring in theunwelded spaces. The wet spaces were subsequently pro-duced using intermittent welding and a silicone sealant in thearea between the welds. Swedged bulkheads were also a lowheat input option for non-structural areas.

36



Fig 8: Automatic fillet welding unit

Fig 9: Track mounted automated seam welding process usingflux cored wire


The welding of very thick plate to very thin plate causedsignificant distortion problems due to the differences in heattransfer between the two thicknesses setting up thermalstress, which manifested itself as distortion.

These issues and others need to be highlighted at a muchearlier stage as rework creates additional cost and, if not car-ried out correctly, could create undesirable metallurgicalstructures in the plate. This need has been identified by someshipyards as being the domain of a Production EngineeringGroup. In the case of an outsourced design strategy then thiswill become a much more critical interface to be managed.

The outsourcing of design has been highlighted1 as anincreasing trend, but a number of drawbacks have also beenraised. Such issues as language barriers, time differences andlack of knowledge of build yard capabilities have been cited.

STEEL MATERIAL ISSUES In line with the improvements in welding consumable quality there has been a corresponding improvement in thequality of steel plate and bar being used in ship construction.

The almost universal adoption of the continuous castingprocess over the last twenty years has significantly improvedthe consistency of the product internal and surface quality.Almost in parallel with this was a progressive increase in theinstallation of secondary steelmaking units. The vacuumdegassing process decreased hydrogen levels, improved con-trol of product chemistry and also steel cleanliness. Steelladle desulphurisation units have dramatically dropped steelsulphur levels. Installation of ladle furnace (LF) stations (Fig10) has improved overall process benefits to the vacuumdegassing and steel ladle desulphurisation processes.

The net result of this can be a much tighter band of steelcarbon equivalent (CEV) values, consistently lower sulphurcontent, an overall reduction in steel hydrogen content andbetter control of specific chemistry, such as carbon, nitrogen,aluminium, and titanium (when required). Considering thehydrogen content of the steel as an example the data in Fig 11shows a cast by cast hydrogen level for EH46 grade steel. Thedata is a combination of calcium treated and non-calciumtreated casts. There is a slight difference in the liquid steelhydrogen levels between the calcium treated steel (2.12ppm)

37



Fig 10: A Ladle Furnace (LF) in operation


and the non-calcium treated steel (2.8ppm). This is referred toin a later section.

It is now rare to identify welding issues associated withplate and bar chemical analysis and steel cleanliness.Obviously these comments are not universal and apply todeveloped steel companies in certain parts of the world.

THIN STEEL PLATE MATERIAL ISSUESFollowing on from the previous sections a significant amountof work has been carried out to create a greater understandingof the factors that influence thin plate distortion. This is part-ly attributed to the heat from the welding process. However,one of the factors established was that if a plate has areas ofpoor flatness then these areas act as sites for further distor-tion. The implication is that the plate should be supplied to as

tight a flatness tolerance as possible. The 4mm plates shownin Fig 12 are clearly unacceptable. The other factor related tothin plate distortion is considered to be the effect of the resid-ual stress in the plates. Often plates that are ostensibly flatwill distort during cutting and/or welding. This is a specifical-ly complex area and is strongly related to the steel plate pro-cessing conditions.

It is widely recognised that thin plate distortion will neverbe eliminated completely. Consequently it is essential that theremnant heat straightening has to be done in the most costeffective manner, and also in a way which does not adverse-ly affect the plate structure or properties. The use of inductionheating to carry out the straightening process will satisfythese specific requirements.6 Work carried out recently hasshown that the steel structure changes slightly and the hard-ness rises slightly. There is no effect on toughness. Revised

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EH46 grade - trend for liquid H2 over time

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57Casting order

Liqu

idH2

cont

ent

HLinear (H)

Fig 11: Typical liquid steelhydrogen levels from thestart of a current contract,showing the effect ofprocess change on the content

Fig 12: Unacceptable flatness in 4mm thick plate


methods of heat straightening, using propane for example, arebeing reintroduced too, but what has become obvious is thatthe use of the two processes can effectively be used in differ-ent areas of a ship. For example, induction heating is ideallysuited to doing wide open expanses of the structure, but hassome limitations in small tight spaces, where the gas heatingprocess is more convenient to apply.

THICK STEEL PLATE MATERIAL ISSUESAt the opposite end of the thickness spectrum, distortion isnot an issue but the maintenance of acceptable plate proper-ties in the heat affected zone (HAZ) certainly is.

Weld metal properties are currently not perceived as apotential problem area. This is related to a significant amountof development work by welding consumable suppliers overthe past 15 years. There is greater consistency in the productwhich has led to ease of operation. The example describedearlier of the use of flux cored wire under SAW flux is a goodexample of forward thinking to meet the user’s requirements.The HAZ is a very complex area of the weld joint, especiallywhere multi-run welds have been laid down. For a single sidesingle pass weld, as can be the case for submerged arc weld-ing, the HAZ is a relatively simple component of the weld.This is shown in Fig 13, which is a single sided single passSAW weld. The HAZ has been traced out, and there is a rela-tionship between HAZ width and welding heat input, there-fore, the higher the heat input then the wider the HAZ will be.

A more complex situation is shown in Fig 14,7 where thecomponents of the HAZ have been identified. To some extentthis will place some doubt on what actually governs the heataffected zone toughness with the potential of up to six distinctareas through which the crack could propagate. Due to theimprovements in steel production referred to earlier, elementssuch as titanium can be tightly controlled and added to devel-op the potential to control the growth of austenite grains inthe HAZ.8 The effect of adding titanium to the steel plateHAZ is shown in Fig 15, where the smaller grains will ensureadequate toughness is retained in the HAZ. In the workshown9 in Fig 14 a range of heat inputs have been covered,

and the paired results shown directly compare the effect oftitanium at equivalent heat inputs.

If hydrogen is taken as a specific overall case, it has alreadybeen stated that with flux cored wires the hydrogen content willbe less than 5ml/100mg and typical liquid steel hydrogen con-tents would be 2.4ppm. It has been reported10 that this generallyresults in plate hydrogen content of around 0.8ppm. This thensignificantly reduces the need to preheat joints prior to welding.The significance of this is high as preheating and controlling theapplication of it is another input into the overall weldingprocess. In addition the potential for hydrogen cracking in thewelded structure has been significantly reduced

An example of how some plate issues and related weldinghave been dealt with are evident in an aircraft carrier build. Theuse of EH46 on the hanger deck and flight deck is a relativelynew move in shipbuilding. The main motivation for this was to

39



Fig 13: Single sided single pass submergedarc weld. Cross section of macrostructureshowing the extent of the heat affectedzone(HAZ), which has been outlined inblack

Fig 14: Complex structure of multi run heat affected zone7


reduce weight against a base case of EH36 steel. However,there was also an overall cost benefit. Initial plate toughness ofthe EH46, while within specification, was not as high as wouldbe expected. The main issue was perceived as having to main-tain the HAZ properties. Subsequently the steel process routewas altered to include calcium treatment of the liquid steel,which resulted11 in an average toughness increase of 45J. Theoutcome was a greater degree of customer satisfaction.

In addition, there was a significant proportion of EH46used in areas subjected to through thickness tensile stress,which could lead to lamellar tearing. A snapshot of the TTRAtest data against steel sulphur content is shown in Fig 16,where the performance against a 35% minimum level is morethan satisfactory. This performance has undoubtedly beenenhanced by the use of the calcium treated product, andresulted in a 100% success rate.

Welding of EH46 (TMCP) steel does not pose any signif-icant problems, assuming the correct welding consumableshave been used. However, within the flight deck there are alarge number (~3000) of link plates to be fitted. A number ofdifficulties occurred in procuring an acceptable material forthe link plate. Initially this was mainly based on forged mate-rial, which tended to have very high carbon equivalents andborderline toughness. Eventually the link plate was sourcedfrom machined EH46 quench and temper (Q & T) material.However, as this material has been sourced from a quenchand temper route plate it is necessary to carry out a weld pro-cedure to cover welding EH46 (TMCP) to EH46 (Q & T).The Q & T product requires marginally more attention todetail than the TMCP material. In addition to the link plates,there are number of flight deck light inserts. They too are

made from EH46 (Q & T). Further inserts are required for firefighting nozzles. When considering all these inserts cogni-sance has to be taken of possible fatigue issues.

THE APPROACH AHEADWhat has been described is a combination of good currentpractice related to shipyard welding. As with most technolo-gies there is a continuous need to improve the currentprocesses from an economic and business strategy viewpoint.

Part of this approach is now to extensively use process mod-elling12 in the welding process. For example, a significantamount of research has been carried out at the University ofStrathclyde13, 14 on modelling issues related to thin plate distor-tion. The main areas investigated to-date have been, for example:

� Effect of cutting heat input and cutting sequence oninitial plate distortion,

� Effect of weld tacking sequence, tack length, tack spac-ing and tack position on subsequent distortion,

� Effect of stiffener distance from seam weld on subse-quent distortion,

� Effect of plate width at the end of a panel on overallpanel distortion,

� Effect of initial plate out of flatness on subsequentdistortion.

This modelling work has all been verified by actualwelding tests, which have also been used to refine the mod-els. The benefit of this approach is to reduce the level of on-plant testing where unknown variables can often lead to

40



0 100 200 300

Impact energy at -55C

1

3

5

7

With no titaniumWith titanium

0

10

20

30

40

50

60

70

Average %TTRA

0.001%S 0.002%S 0.003%S 0.004%S 0.005%S 0.006%S 0.007%S

% sulphur content

%average TTRA v Sulphur content

Fig 15:The effect of 0.019% titanium on thetoughness of the heat affected zone (HAZ) ofNV2-4 steel

Fig 16:Through thickness reduction in area (TTRA) datashowing the effect of steel platesulphur content on performance


incorrect conclusions. The FEM approach has the benefit inthat it can be considered as a very powerful tool to indicatethe direction of the effects being investigated. As an example,it is well-known that the residual stress present in plates andbars in thin structures is an unknown variable. The approachtaken in FEM modelling is to assume that the starting resid-ual stresses are even across the plates and bars.

In addition to the use of FEM, artificial neural networkscan be used in the welding process. ANN has been effectivelyused15 in a thin plate distortion study where several previous-ly unknown factors were identified. In addition, ANN iscurrently being used16 to develop the optimum fillet weldingconditions that minimise overwelding, whilst achievingacceptable penetration.

As previously discussed the use of modelling of theshielding gas flow conditions is also being approached froma modelling and validation approach. Some similar work hasbeen published17 on the effect of gun fume extraction on theshielding gas flow patterns.

As discussed earlier, the HAZ is a key component of theweld region. It is now feasible to establish the variousmicrostructures present within that area from a modellingapproach. From that, toughness and hardness can be reason-ably accurately predicted. Again, it would be necessary to carryout periodic validation work to ensure model compatibility.

CONCLUDING COMMENTSIt is quite apparent that there is a very sound knowledge baseto significantly limit issues related to welding to an absoluteminimum. It is the application of this knowledge base to theproduction process that is the key point.

A number of issues continue to cause problems. Forexample, the concept of buying cheap often creates knock oneffects of increasing overall cost as you go through the pro-duction process. The concept of value needs to be morestrongly emphasised. There is still too much of a gap betweenDesign and Production. This is an area where ProductionEngineering can act as a buffer and resolve potential buildissues prior to them getting on to the shop floor. However, forthis to be effective the constitution and abilities of such agroup need to be carefully thought through.

The future is a challenge to consider how cost can besaved but still maintain procurement value. The concept ofmodelling is not new to shipbuilding, with well-developedFEM models in place. However, it is the uses of other mod-els such as FEM or ANN that need to be explored in greaterdepth. That modelling skill is very much the domain ofUniversities or other research organisations – currently.

ACKNOWLEDGEMENTSThe author wishes to thank BAE Systems Surface Ships forpermission to publish this paper. Lincoln Electric (UK),

Binzel-Abicor, Air Liquide Welding and Siemens VAI arethanked for the provision of Figs 1, 4, 7 and 10, respectively.The supply of additional data by Dr A Trowsdale and Mr ADunsmore of Corus UK is gratefully acknowledged.

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