11 2003 pargeter s(crack welding)

7/21/2019 11 2003 PARGETER s(Crack Welding)

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Introduction

In the welding of ferritic steels, themost common form of fabrication crack-ing is that caused by hydrogen embrittle-ment. It is well known that there can besome delay between the completion of

welding and the formation of hydrogencracks in ferritic steels. Therefore, if in-spection is carried out too soon after weld-ing, these cracks may not be detected, withpotentially catastrophic consequences.On the other hand, excessive delays after

welding prior to inspection can have seri-

ous financial implications due to, for ex-ample, holdups in production.

Currently, there are recommendationsfor delays of between 16 and 48 hours in

various national and industry standards(Refs. 15), but there is no firm basis forthese times. Furthermore, there is gener-ally no discrimination between differentmaterials, joint geometries, or weldingconditions, with just one delay time rec-ommended for all circumstances.

It was against this background that aprogram of experimental work was initi-ated in which crack development wasmonitored using ultrasonic techniques ina variety of test welds.

Background

It is well established that hydrogencracking in ferritic steels only occurs whena critical combination of the four basic fac-tors involved is exceeded. These factorsare

1) Hydrogen2) Susceptible microstructure3) Tensile stress4) Temperature.The likelihood of cracking increases

with increasing hydrogen level , mi-crostructural susceptibility, tensile stress,and as the temperature approaches about

20C (68F). In the final stages of coolingfollowing welding, three of these parame-ters are slowly changing, as depicted

schematically in Fig. 1. Even when coolinghas stopped, however, and the developedcontraction stresses have stopped increas-ing, the hydrogen content will continue tochange. For the weld as a whole, the hy-drogen level will decrease as a result of dif-fusion out of the weld metal and heat-affected zone (HAZ) and the adjacentbase material. However, at local points

within the weld metal and HAZ, particu-larly those of high triaxial stress, the hy-drogen content will increase for a periodof time as a result of stress-assisted diffu-sion. Hydrogen will diffuse up stress gra-

dients from regions oflowertohighercon-centration. Thus, it could be some timeafter completion of welding and coolingthat, locally, the hydrogen concentrationfirst reaches a critical value and crackingcommences. This effect of hydrogen diffu-sion was noted as early as 1961 whenBeacham et al. (Ref. 6) demonstrated thatcracking in Lehigh restraint tests could bedelayed by storing test panels at low tem-peratures. In their tests, they found thatcracking occurred between one-quarterand one-half hour after welding undernormal conditions. For similar weldsquenched to 110F immediately after

welding, cracking was suppressed during

storage at 110F but did occur approxi-mately one-quarter hour after reachingroom temperature again. Hydrogen diffu-sion is therefore one feature that can con-tribute to the observed delayed nature ofhydrogen cracking.

A further aspect contributing to thedelay is due to the fact that hydrogencracking is frequently of a discontinuousnature. In part, this also arises from thestress-dependent diffusion characteristicsof hydrogen since, as the crack moves for-

ward, it enters a region of locally lower hy-drogen concentration and an incubation

time is then required for the hydrogenconcentration to increase locally, bystress-assisted diffusion, at the new pointof maximum stress near the new crack tip.

Andersson (Ref. 7) suggests that an addi-tional explanation for the time delay couldbe the formation of hydrogen traps, e.g.,

voids, dislocations, etc., at the crack tip byplastic straining, which temporarily lowerthe lattice hydrogen concentration. Athird aspect, which plays a part in the oftenobserved delayed nature of hydrogencracking, relates to the time required forhydrogen cracks, once formed, to grow to

a sufficient size to be detected by the NDEmethod being applied.

For a given weld, the delay time can beseen to be primarily a function of the com-bined magnitude of the three parameters(hydrogen level, microstructural suscepti-bility, and stress) in relation to the re-quired critical combination. When thecombined values of these are at very highlevels, i.e., the welding procedure has a

very high risk of cracking, cracking maycommence well before cooling to normalambient temperature is completed. Onthe other hand, the likelihood of a delayedcharacteristic is greatest when the combi-nation of factors involved in producing hy-

drogen cracking are only marginally abovethe critical combined value, and this isachieved only by the local enhancement ofhydrogen through stress-assisted diffu-sion. Hydrogen level is clearly an impor-tant variable influencing delay time sinceit is well established from constant load orstress rupture tests (Ref. 8) that, as the hy-drogen concentration decreases, this leadsto longer incubation times for rupture. Ul-timately, of course, continued lowering ofthe bulk hydrogen concentration leads toa situation where, no matter how long a

wait is imposed, local enhancement of hy-drogen concentration by stress-induceddiffusion cannot obtain the critical level

before overall loss of hydrogen results inthe local hydrogen level beginning todecline.

Other factors that are likely to increasethe delay time over and above those thatcontribute to a marginal procedure will bethose that prolong the time during whichlocal hydrogen concentrations can in-crease and stay close to critical concentra-

Evaluation of Necessary Delay beforeInspection for Hydrogen Cracks

Postweld crack development was monitored over time on a variety of test welds

BY R. PARGETER

R. PARGETER is Consultant, Ferritic Steels andSour Service, Metallurgy, Corrosion, Arcs & Sur-

facing Technology Group, TWI Ltd., Cambridge,U.K. Crown Copyright 2001 and TWI Ltd.

2001

KEY WORDS

EmbrittlementFabrication CrackingHydrogen CrackingStress-assisted DiffusionUltrasonic Inspection


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tions, or those that enhance the stress-induced diffusion. Thus both high jointthickness and high restraint levels could beexpected to be important factors. Thick-ness is important because bulk hydrogenconcentration will decay only very slowly atthe center of thick sections, while high re-

straint is important because higher stressescould increase the extent of locally en-hanced hydrogen through stress-induceddiffusion. Furthermore, the delay time canbe expected to be some function of the dif-fusion rate of hydrogen in steel, which, atroom temperature, can vary by at least twoorders of magnitude (Ref. 9) with materialconditions, compositions, and cleanliness;the slower the diffusion rate, the slower theoverall loss of hydrogen will be and thelonger the time for stress-induced diffu-sion to locally achieve critical hydrogenlevels. Finally, it is possible that the cracklocation (buried or surface breaking) willinfluence delay time because hydrogen

concentrations in near-surface regions willdecay more rapidly. In addition, a surface-breaking crack will itself provide a conduitfor hydrogen escape.

Experimental Approach

Multipass welds in butt joints weremade in a variety of steels, and the devel-opment of hydrogen cracking was moni-tored using mechanized ultrasonic exami-nation in which constant coupling wasensured. For the majority of the work,

welds were stop-ended bead in groove, in50-mm- (2-in.-) thick plate. Using thisbasic geometry, effects of base metal and

welding consumable, heat input, hydrogenlevel, and restraint were explored usingshielded metal arc welding. Some trials

were also carried out at higher heat inputusing larger submerged arc welded butt

joint panels.As explained in the previous section,

the greatest delays between completion ofwelding and cracking are anticipated at

near-threshold conditions. Thus, for eachof the above factors, a series of weldsaround threshold conditions was made. InC-Mn steels, this was achieved principallyby varying heat input, and in alloy steels by

varying preheat.

Experimental Details

Materials and Welding

Details of base materials are presentedin Table 1. Four C-Mn steels were used,two with 350-MPa (51-ksi) specified mini-mum yield stress (SMYS), and two with450-MPa (65-ksi) SMYS. The two 350-MPa steels were fairly closely matched inchemical composition, with the exceptionof S, Si, and O one being a clean steel(


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For the pulse/echo inspections, onlyindications having an amplitude greaterthan that from a 3-mm side-drilled holeplus 14 dB were displayed. Thus, the sen-sitivity was neither lower nor significantlygreater than that which is usual in prac-tice. It should be recognized, however,that the detectability of all defects usingfixed orientation probes is limited bycomparison with a manual inspection.Time-of-flight probes were used to com-pensate for this and to provide informa-tion on crack growth.

Interpretation of pulse/echo signalswas complicated by an observed increasein signal amplitude for all reflections, in-cluding stable geometrical features suchas the weld cap profile, as the weld cooled.Where features were observed on first in-spection (by either technique) and none ofthe observed features changed during sub-sequent monitoring, their nature was con-firmed by metallographic sectioning.

For submerged arc welds, immersion inthe oil bath was not possible. For these

welds, manually control led mechanizedscans were carried out. A probe pan andstepper motor were attached to the backface of each weld with magnetic feet assoon as it had cooled to about 30C, andkept in place for the duration of the test.Only time-of-flight probes were used forthese inspections.

All inspections were carried out at in-creasing intervals over a total period ofabout one week. The initial interval for theautomated, oil bath tests was one-halfhour, increasing logarithmically to 12hours at the end of the week. Test intervalson the submerged arc welds were 1 hourfor the first 3 hours, every 2 hours for thenext 6 hours, every 8 hours for the next 4days, and, finally, 24 hours until the end ofthe week.

Results

Examples of HAZ and weld metalcracks in groove welds are given in Fig. 5.The results of delay time measurementsare presented in Table 4. Four principaltimes were recorded, all relative to com-pletion of welding. These were the time offirst inspection, time of first detection of acrack, the last time when a new crack wasobserved, and the last time when a change

in crack size relative to the previous in-spection was observed. In addition, someinformation on cooling is included in thistable.

In the parametric studies, exploringthe effects of restraint, hydrogen level,and groove depth at approximately con-stant heat input, in the 350-MPa yield

steel, hydrogen cracking (when it oc-curred) was always present at the time offirst inspection. Growth of existing sub-surface HAZ cracks was observed for upto 6 hours after welding in the high-restraint welds at both high and lowhydrogen levels (e.g., W1-20 and W1-10).

Although such growth was not detected inlow-restraint welds, it was not possible toconclude that any of restraint, hydrogenlevel, or weld thickness over the rangesstudied had any effect on delay time.

The results of these parametric studieson the HY100 (690-MPa yield) steel, how-ever, showed many instances of measur-able delays in both initiation and growth of

weld metal hydrogen cracks (Table 4 andFigs. 69). When examining these data, itshould be recognized that for W1-15

Fig. 4 Oil bath and ultrasonic inspection equipment used for groove

welds.

A B

Fig. 5 Photomacrographs of: A HAZ crack in W1-14 (low-hydrogen, high-restraint 350-MPa C-

Mn steel); B weld metal crack in W1-2 (low-hydrogen, high-restraint HY100 steel).

Fig. 3 Dimensions of welded butt-joint test panels.


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(80C [176F] preheat, Fig. 6) the crackthat is recorded as appearing at 100 hours

was actually present, but just below the re-porting threshold, from about 20 hours. At90C (194F) and 120C (248F) preheat(W1-42 and W1-46), only one crack wasdetected. Overall, the expected greaterdelay time near threshold cracking condi-tions was confirmed. Results indicatedthat a low-hydrogen, high-restraint weldin HY100 will result in the greatest delaytime before the last initiation of discretenew cracks. The 40-mm groove welds ap-peared to result in the shortest delaytimes, and restraint had no clear effect ondelay times.

The anticipated decrease in hydrogendiffusivity in the higher sulfur C-Mn steeldid not have a measurable effect on delaytimes, but this was confused by the fact thatthis steel proved to be less hardenable(possibly because of its higher S content)despite having a marginally higher carbonequivalent, and cracking occurred in the

weld metal, as opposed to HAZ crackingin the cleaner steel. There was also no mea-surable delay time for the 450-MPa steel

welds at just below 1 kJ/mm (25 kJ/in.),which also cracked in the weld metal.

Although a particular series of groovewelds was carried out to explore the ef-fects of heat input, the effect is mostclearly demonstrated by examining all C-Mn steel data (350-MPa and 450-MPa

yield) together. These results are pre-sented in Fig. 10, and it can be seen thatthere is a trend of increasing delay time

with increasing heat input.Superimposition of the Q1N data on the

plot in Fig. 10 indicates that the results laywithin the scatter of the C-Mn steel results.It should be recognized, however, that thecracking in this weld was in weld metal,

which had a relatively lean compositionand, particularly in view of the longer delaytime observed in HY100 welds, it cannot beconcluded that HAZ cracking in Q1N maynot be subject to greater delays than inlower strength C-Mn steels.

The welds made in a wider groove didnot show a measurable difference in timefor crack initiation, with a maximum delayof 1 hour being recorded, by comparison

with a value of 4 hours for similar heatinput welds in a narrower groove. Crackgrowth was observed to continue for up to11 hours, by comparison with 5 hours withthe narrower groove.

Discussion

Material Effects

In the background section of thispaper, variables likely to influence the ten-dency for hydrogen cracking to be delayed

were identified based on the assumptionthat delayed cracking must be diffusioncontrolled. It was stated that delay times

would be expected to be influenced by thediffusivity of hydrogen, the tendency forhydrogen to migrate to or remain at sitesof high triaxial stress, and by the actual dis-tance hydrogen is required to diffuse be-fore it can concentrate sufficiently tocause cracking.

The most notable aspect of the resultsobtained was the very marked differencein behavior between the 350- and 450-N/mm2yield C-Mn steels and the higherstrength, more highly alloyed 690-N/mm2

yield HY100. Obviously there will be a sig-nificant difference between residual stresslevels within weldments in the two materi-als. However, another very significant dif-ference between these two steels and be-tween the two weld metals used is thelower hydrogen diffusivity expected in a

Fig. 6 Delay time measurements for HY100, low-hydrogen, high-restraint

groove welds.

Fig. 7 Delay time measurements for HY100, high-hydrogen, low-restraint

groove welds.

Fig. 8 Delay time measurements for HY100, high-hydrogen, high-restraint

groove welds.

Fig. 9 Delay time measurements for HY100, low-hydrogen, 40-mm-deep

groove welds.


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more highly alloyed steel. Both a higherstrength/residual stress level and a lowerhydrogen diffusivity would be expected toresult in longer delay times, but from theseresults, it is impossible to establish

whether either of these is dominant. Fromthe observation that the relatively lean

Q1N weld metal showed delay times char-acteristic of the C-Mn steels, however, itwould appear that diffusivity is more im-portant than strength per se.

It is also discernible from published lit-erature that delay time is greater for alloysteels. Interante and Stout (Ref. 10) foundincreasing delays on moving from A302 toHY80 to T-1 steels (Fig. 11), and Alcan-tara et al. (Ref. 11) similarly found in-creasing delays when welding C-Mn pipe

with increasing strength consumables be-tween 7018 and 13018.

It was expected that some delayedcracking might be observed in the weldsmade in the higher sulfur (and therefore

possibly lower hydrogen diffusivity) C-Mnsteel tested. As described in the resultssection, however, for this steel, crackingoccurred only in the weld metal, which wasof the same composition and diffusivity asused for the low-sulfur steel. The cleanli-ness of a C-Mn steel may, however, havean influence on delayed HAZ cracking,

which was not detected in this program. Itshould also be noted, however, that high-sulfur C-Mn steels do not necessarily havelower hydrogen diffusivities (Ref. 12).

Hydrogen Level and Restraint

Although the results revealed marked

difference in delay times observed be-tween the two main steel types (C-Mn andlow alloy), very little difference in delayedcracking behavior was seen as hydrogenlevel and restraint were varied in eachcase. For the C-Mn steel, the limitedamount of delayed growth that did occur

was only seen in high-restraint welds.However, the HAZ cracking in these

welds was replaced byweld metal cracking inthe low-restraint anddeep groove welds

which cracked withoutmeasurable delay,making it difficult to

discern whether thetype of cracking or therestraint level was re-sponsible for the lackof delayed crackgrowth. The behaviorof the HY100 welds,

which showed onlyvery small differencesin delayed cracking be-havior between highand low hydrogen levels and between highand low restraint, indicate that these fac-tors are indeed of little significance. The

very limited effect of hydrogen level uponcrack delay times is not, perhaps, surpris-

ing for alloy steels when the concepts ofrisk of hydrogen cracking and risk of de-layed cracking are separated. This is be-cause for cracking to occur in any given

weld, the hydrogen level at the time andplace where it occurs will be constant for athreshold condition since, in alloy steels,microstructural susceptibility varies verylittle. Thus, below a given hydrogen level,cracking will never occur, and above agiven level, excess hydrogen will haveevolved in order to achieve a thresholdcondition before the weld cools to a tem-perature where cracking can occur.

The fact that restraint seemed to haveno effect upon delay times may be ex-

plained by the fact that the low restraintwas not low enough to prevent near-yield-magnitude residual stresses developing inthe weld. This is not unrealistic for fabri-cation welds, and so although it may bepossible to increase delay times with arti-ficially low levels of restraint (and there-fore stress) in specially designed single-pass test welds, these are unlikely to be

relevant to real welds. For example, thelow-restraint Lehigh tests performed byInterrante and Stout (Ref. 10), in whichdelay times were seen to increase as re-straint decreased, rely upon a stress-con-

centrating root notch to induce longitudi-nal HAZ or weld metal cracking. This testis therefore largely insensitive to longitu-dinal residual stresses, and the transverserestraint (and, therefore, transverse resid-ual stress) had to be lowered to levels un-typical of a real fabrication before delayedcracking was observed.

Joint Thickness and Weld Volume

Joint thickness appeared to have no ef-fect with the C-Mn steel and the surpris-ing effect of reducing delay times atgreater thickness with the HY100. Agreater weld thickness would be expected

to increase delay times because bulk hy-drogen concentration will decay only veryslowly from the center of thick sections.The slightly reduced delay times seen inthis work between 20-mm- and 40-mm-(0.8- and 1.6-in.-) deep welds in HY100may be explained by the fact that thethicker welds took longer to complete and

were therefore at the preheat/ interpass

Fig. 10 Summary of all 350-MPa and 450-MPa yield steel results.

Fig. 11 Effect of steel type on delay time. After Interante and Stout (Ref.

10).


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temperature or above for a longer period,allowing hydrogen to accumulate at sites ofhigh triaxial stress during welding. It istherefore likely that had the weld beencompleted more quickly with higher arcenergies and thus greater bead sizes, thedelay times may have been increasedrather than reduced. It should also be rec-ognized that the increase in joint thickness

was achieved by machining a deepergroove, which made hydrogen-escapefrom the root of the weld (through the re-

verse face) easier. Nevertheless, the resultsof tests made in a wider (60-deg includedangle) groove also showed no measurableeffect on delay time. Overall, it is evidentthat extrapolation of results to different

weld geometries and procedures must bedone with caution.

Effect of Heat Input

One of the clearest effects to come outof this work is that of heat input. The re-sults plotted in Fig. 10 indicate that theupper bounds of all three parameters (firstobservation of a crack, last appearance ofa new crack, and end of crack growth) in-crease measurably over the range 0.85kJ/mm. Although there is no overlap be-tween the 3.55 kJ/mm welds made usingsubmerged arc and the 0.72.4 kJ/mm

welds made using shielded metal arc, theeffect is observable in both groups, andone bounding line can be drawn for bothsets, indicating that this is a heat input ef-fect and not a process effect.

Relation to Other Work

As well as enabling guidance to be givenon appropriate delay time before inspec-tion, this work has produced results thatare largely in agreement with previouspublished work on delayed cracking and,to a certain extent, explain a large amountof the anecdotal evidence of delayedcracking. A common feature of delayedcracking reported in the literature is rapidcrack initiation followed by significantlymore prolonged periods between bursts ofgrowth (Refs. 6, 8, and 13). This type of be-havior has also been seen in this work andhas significant implications for inspectionprocedures, as previously mentioned, as

well as explaining the very long delay timessometimes reported when only surface in-spection is used. In this work, crack initia-

tion was seen to occur as long as 65 hoursafter welding (in HY100), but in the ma-jority of cases, all detectable cracks werepresent within 24 hours, even when growth

was seen to continue for more than 100hours. Thus, it is highly probable that inmany cases where delay times in excess of24 hours have been reported before cracksare first detected, this has been due to the

Table 1 Chemical Compositions of Base Materials

Element wt-%

Low-Sulfur High-Sulfur 450 N/mm2 450 N/mm2 HY100 Q1N SteelC-Mn Steel C-Mn Steel QT Steel Yield Steel

C 0.19 0.18 0.09 0.06 0.17 0.13S


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Table 4 Results of Delay Time Measurements

Specimen Heat input, Time after end Temp. at Time taken Time after Temp. after Time up to Time up tokJ/mm / of welding start of to reach which which cracking which new which defectpreheat, when test test, C ambient, h cracking is is first defects start growth

C starts, h first noticed(a), h noticed, C appearing, h occurs, h

a) Low-hydrogen high-restraint welds, clean 350-MPa steelW1-14 0.56/20 0.66 35.3 11.3 0.66 35.3 0.66 0.66W1-10 0.64/20 1.00 24.0 8.9 1.00 24.0 1.00 5.14W1-18 0.72/20 1.00 27.5 9.8 1.00 27.5 1.00 1.00

W1-28 0.80/20 1.23 23.5 7.2 1.23 23.5 3.78 3.78W1-30 0.80/20 1.38 27.5 6.1

b) High-hydrogen high-restraint welds, clean 350-MPa steelW1-11 0.80/20 1.25 26.9 9.6 1.25 26.9 1.25 1.25W1-16 0.96/20 2.35 30.4 12.4 2.35 30.4 2.35 2.35W1-20 1.12/20 1.75 27.3 9.7 1.75 27.3 1.75 6.4

c) High-hydrogen low-restraint welds, clean 350-MPa steelW1-25 0.80/20 1.08 25.4 9.3 1.08 25.4 1.08 1.08W1-26 0.80/40 0.70 28.0 9.2 0.70 28.0 0.70 0.70W1-29 0.80/50 1.2 27.4 6.1 W1-27 0.80/60 2.08 27.2 10.4

d) Low-hydrogen 40-mm-deep groove high-restraint welds, clean 350-MPa steelW1-62 1.12/55 1.33 30.2 6.0 1.33 1.33 1.33 1.33W1-61 1.12/70 2.13 27.8 8.7

e) Low-hydrogen high-restraint welds, 690-MPa low-alloy steel (HY100)W1-2 1.12/40 1.66 30.0 9.8 1.66 30.0 21.3 >55.4(b)

W1-8 1.12/60 2.00 24.9 7.1 2.00 24.9 30.2 83.8W1-15 1.12/80 2.25 28.3 11.3 2.25 28.3 100.0 103.2W1-42 1.12/90 3.38 27.0 12.2 9.15 21.5 9.15 19.5W1-48 1.12/110 3.97 28.0 13.9 4.49 25.2 15.0 90.6W1-46 1.12/120 3.55 26.4 9.2 3.55 26.4 3.55 4.55W1-49 1.12/130 3.38 29.7 16.0 3.38 29.7 23.7 92.6W1-50 1.12/150 3.82 29.6 15.5 3.82 29.6 64.8 129.0

f) High-hydrogen high-restraint welds, low-alloy steel (HY100)W1-19 1.12/120 4.31 28.4 13.2 4.31 28.4 6.34 75.7W1-22 1.12/140 3.50 29.9 15.1 3.50 29.9 7.10 54.6W1-23 1.12/150 2.33 31.9 14.0 2.83 28.8 16.5 30.6W1-37 1.12/170 5.07 27.9 13.6 8.67 22.0 18.6 141.6W1-51 1.12/200 4.03 26.5 13.5 7.63 23.0 8.71 71.7

continued

Note: Welds are listed in lowest crack risk condition last.(a) If cracking is detected on the first inspection, it is the same as the time after welding when the tests start.(b) Inspection stopped due to an equipment problem.

time taken for either subsurface cracks tobreak surface and become detectable witha surface technique such as MPI or forburied cracks to grow to a detectable sizeby less sensitive NDE techniques such asradiography.

Although very long delay times similarto those previously reported have beenseen in this work, the phenomenon of de-

layed cracking has, nevertheless, beenshown to be restricted to particular weldingsituations. Conditions must be such thatsufficient hydrogen can be introduced that

will later just cause cracking when it con-centrates, without there being so much hy-drogen as to result in immediate crackingas the weld cools to ambient. This work in-dicates that such conditions are relatively

rare but are more likely to occur with high-strength, low-diffusivity steels such asHY100. This again is in agreement withprevious experience. The majority of re-ported delay in cracking has been in high-strength steels (Refs. 6, 10, 11, and 14).

An exception to this is the reporteddelay in cracking observed in the high-arc-energy submerged arc welded C-Mn steel

(Ref. 13) confirmed by the present work.In this case the particular circumstancesrequired for delayed cracking are thoughtto arise because of the greater diffusiondistances resulting in longer diffusiontimes rather than lower diffusivity. Thisobservation is very significant as it sug-gests that bead size/heat input is an im-portant variable affecting delay times.

Delay Times for Inspection in Fabrication

The purpose of this work was to pro-vide better supported, and thus both morecost effective and safer, guidelines for de-lays to be imposed between completion of

welding and inspection for fabrication hy-drogen cracks. Some considerationshould be given to the possibility that thedetectability of cracks may not always beas good in practice, and particularly dur-ing site work, as achieved during the lab-oratory work, even though reasonably re-alistic levels of detectability have beenadopted in the present work. Neverthe-less, the results for C-Mn steels do indi-cate that there is significant scope for re-duction in delay times before inspection.


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For a total of 29 cracked tests for heat in-puts up to and including 2.4 kJ/mm (61

kJ/in.) produced with SMAW, the greatestdelay time has not exceeded 4.7 hours.Proposed modifications to current

practice for delay times before inspectionfor C-Mn steels of yield strength up to andincluding 450 N/mm2 and of up to 50-mmthickness are given in Table 5 and incor-porate a considerable safety factor. Al-though it is believed that the increase in

delay times between shielded metal arcand submerged arc welds is an effect of

heat input, it is considered appropriate, inthe absence of other data, to restrict thegreatest reduction in delay time (to 12hours, compared with typical currentpractice, of 1648 hours) for heat inputs of 2.4 kJ/mm (61 kJ/in.) to SMAW.

Measured delay times for high-strength steels such as HY100 were muchlarger than for the C-Mn-type materials,

primarily due to the expected influence ofthe significant levels of alloying on hydro-

gen diffusivity. Thus, for these steels, atleast 72 hours is recommended, and itmust be recognized that crack growth maycontinue for some time beyond this.

A similar effect of heat input may alsoapply to the higher strength steel. How-ever, while in practice, steels of the HY100type are less likely to be welded at highheat inputs, further work would be re-

Table 4 continued Results of Delay Time Measurements

Specimen Heat input, Time after end Temp. at Time taken Time after Temp. after Time up to Time up tokJ/mm / of welding start of to reach which which cracking which new which defectpreheat, when test test, C ambient, h cracking is is first defects start growth

C starts, h first noticed, h noticed, C appearing, h occurs, h

g) High-hydrogen low-restraint welds, 690-MPa low-alloy steel (HY100)W1-32 1.12/150 3.16 28.3 13.1 3.16 28.3 14.8 18.65W1-33 1.12/170 5.38 26.1 16.3 W1-34 1.12/190 5.58 26.3 25.1 6.08 26.3 36.4 148.7

W1-35 1.12/210 5.76 27.0 27.9 8.30 23.0 25.4 113.64W1-36 1.12/230 7.67 24.0 26.0 21.2 20.5 21.2 89.5W1-38 1.12/250 4.91 29.7 27.9

h) Low-hydrogen 40-mm-deep groove high-restraint welds, 690-MPa low-alloy steel (HY100)W1-31 1.12/80 2.63 27.3 9.66 2.63 27.3 4.45 103.0W1-39 1.12/120 3.73 27.1 10.46 3.73 27.1 3.73 55.8W1-40 1.12/140 3.33 27.4 10.8 4.53 24.0 4.53 43.3

i) Low-hydrogen high-restraint welds, high-sulfur 350-MPa steelW1-56 0.72/20 0.80 22.2 7.6 0.80 22.2 0.8 0.8W1-52 0.80/20 0.75 0.9 7.2 0.75 23.9 7.5 7.5W1-55 0.72/40 1.67 21.8 6.0 W1-54 0.80/40 1.65 27.5 11.5

j) Low-hydrogen high-restraint welds, 450-MPa steelW1-57 0.80/20 1.16 22.3 7.5 1.16 22.3 1.16 1.16W1-59 0.80/40 1.67 27.5 9.8

W1-58 0.80/60 2.25 27.4 9.5 W1-53 0.80/20 0.58 24.9 9.9 W2-2 2.0/20 1.33 29 16.1 1.33 29 1.33 1.33W2-3 2.0/20 1.80 40 25.3 1.80 40 1.80 1.80W2-4 2.0/20 0.93 29 15.9 0.93 29 1.60 18.7W2-10 2.4/20 2.67 32 9.74 2.67 32 4.7 48.9W2-11 2.4/20 3.00 32 9.25 W2-12 2.4/20 3.00 27 15.3 3.00 27 3.00 10.3

k) Submerged arc welds, clean 350-MPa steelW1-101 5.0/170 9.2 33.1 W1-102 5.0/170 7.3 24.0 W1-100 5.0/170 5.5 26.7 5.5 26.7 16.5 33.5

l) Submerged arc welds, 450-MPa steelW2-5 3.5/75 2.15 48 18.7 4.18 27 8.48 8.48W2-8 3.5/95 2.95 41 22.5 2.95 41 12.32 110.0W2-9 3.5/120 3.1 44 25.3 10.9 20 10.9 47.6

m) Low-hydrogen high-restraint welds, 565-MPa HSLA steel (Q1N)W2-6 2.0/120 2.78 38 18.1 W2-7 2.0/90 1.97 40 18.1 W2-18 2.0/20 1.87 26 14.67 1.87 26 3.9 11.2

n) Low-hydrogen high-restraint wide groove welds, clean 350-MPa steelW2-13 0.8/20 0.5 32 8.57 0.50 32 0.50 0.50W2-14 0.8/40 0.58 36 18.24 0.58 36 1.10 11.4W2-15 0.8/60 1.00 34 17.33 W2-16 0.8/50 0.78 32 9.88 W2-17 0.8/40 0.63 32 10.57


9/9

WELDING RESEARCH


quired to investigate this factor for suchsteels.

The Q1N material is significantly morealloyed than the C-Mn steels studied inthe present project, and it could be ex-pected that it might produce delay timessignificantly longer than the C-Mn steelsand similar to those observed for HY100.However, typical consumables for weld-ing Q1N, such as the E9016G used, aresignificantly less highly alloyed than thoseused for HY100. This is considered to bethe reason why the delay times observed

were similar to those for C-Mn steel ssince the cracking produced in the Q1Ntests was in the weld metal and not theHAZ. Thus, the data generated can onlybe considered relevant to detection of

weld metal cracking when welding Q1Nwith the E9016G used or similarly alloyedconsumables. Delay times for HAZ crack-ing in Q1N, based on the above consider-ations, would be expected to be longer be-

cause of the slower diffusivity of hydrogenin the more highly alloyed HAZ material.To a first approximation, they would beexpected to be similar to those observedin HY100, based on the similar levels ofalloying. However, hydrogen loss fromthe HAZ would, in fact, largely take placethrough the weld metal, where diffusivity,based on relative alloy levels, might befaster. Until such considerations aredemonstrated, it is recommended that

where there is concern over HAZ crack-ing in Q1N, normal practice for delaytimes should be followed.

In view of the relevance of hydrogendiffusion, the indications from the present

results that thickness is not an importantvariable, within the range studied, shouldnot be taken to mean that greater thick-ness has no effect on delay time. There-fore, the present results and recommen-dations should be considered to onlyapply to material of 50-mm ( 2-in.)thickness. Until proven otherwise by ex-periment, it should be anticipated that forsignificantly greater material thickness,

longer delay time could be required. Theimportance of hydrogen diffusion indi-cates that a further variable, ambient tem-perature, should be considered. Althoughthe question of ambient temperature hasnot been addressed in this work, it is verylikely that lowering ambient temperature

will increase delay times (Ref. 6). Thisshould be borne in mind if welding is to beperformed at temperatures below 20C(68F), and the possibility of longer delaytimes should be allowed for.

Conclusions

1. For C-Mn-type steels of up to 450-MPa (65-ksi) yield strength, delay times

were found to increase with increasingheat input over the range studied of 0.7 to5 kJ/mm.

2. Delay times for weld metal crackingwhen welding Q1N steel at a heat input of2.0 kJ/mm with the E9016G used, or sim-

ilarly alloyed consumables, were similarto those for C-Mn steels.3. Significant delay times of up to 64

hours before the last initiation of a newcrack and up to 140 hours before all sub-sequent crack growth ceases have beenrecorded for welds in the 690-MPa (100-ksi) yield strength steel. The longest timebefore detectable cracks were first pro-duced was 21 hours.

4. Delay times before cracking in boththe 350-MPa (51-ksi) yield C-Mn steel1B789 and the 690-MPa (100-ksi) yieldHSLA steel 1B778 appear to be largely in-sensitive to the variables restraint, hydro-gen level, and weld thickness, within the

ranges studied.5. Increasing the weld volume (cross-sectional area) by changing the angle ofpreparation had no significant influenceon delay times in a 350-MPa (51-ksi) yieldstrength steel.

6. Guidelines for delay times before in-spection for hydrogen cracks have beenproduced. Seventy-two hours is recom-mended for 690-MPa (100-ksi) yield low-

alloy steel welded at

11 2003 pargeter s(crack welding)

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