Postweld Heat Treatment for Hydrogen Removal

Transcription

1 Pstweld Heat Treatment fr Hydrgen Remval Heat treatment recmmendatins are prvided t reduce hydrgen cncentratins t safe levels whereby hydrgen cracking can be avided BY W. S. KYTE AND B. CHEW Intrductin Disslved hydrgen is always intrduced int a weld as a result f the welding prcess and, if intrduced in sufficient quantities, will cause cracking f the jint. The cncentratin at which cracking ccurs depends n a cmbinatin f factrs which include the inherent susceptibility f the micrstructure and the magnitude f the residual welding stresses. The separate and cmbined rle f these extensive factrs has been studied ly _i.-_.-i In principle, hydrgen cracking culd be avided if as-welded stresses were eliminated. At present, hwever, there are n practical methds fr achieving this. Stresses can be reduced by pstweld heat treatment, but cracks can still frm during the perid between cmpletin f welding and applicatin f stress relief heat treatment. Similarly, preventin f hydrgen cracking slely thrugh cntrl f the micrstructure is a limited practical ptin. The micrstructure f the weld metal and heat-affected zne is largely dependent n the cmpsitin. This cmpsitin must clearly be chsen s as t fulfil a brad range f perfrmance requirements based n strength, tughness, creep perfrmance, etc., and nt simply fr preventing hydrgen cracking. Welding prcess parameters such as heat input, travel speed, etc., will have a limited effect in s far as they alter the cling rate thrugh the transfrmatin range and, in general, increased heat input leads t sfter micrstructures. Hwever, alteratin f prcess parameters can have a whle range f effects, nt least f which, fr example, wuld be an increase in grain size in bth weld metal and heat-affected zne resulting frm an increased heat input. Thus, while the latter might wrk in the directin f a sfter structure, the resulting grain carseness might be detrimental in terms f jint perfrmance. The applicatin f preheat t the jint is beneficial in the same sense that it t alters the nature f the transfrmatin prducts and prmtes sfter micrstructures. Again, hwever, this ccurs nly within the cnfines f the basic cmpsitin being welded. If the methds referred t abve prve inadequate, then a further line f attack is t exercise cntrl ver the cncentratin f hydrgen in the weld. In principle, if this cncentratin can be kept belw the critical level, then cracking will nt ccur. This leads t tw further ptins. The first f these is t cntrl the entry f hydrgen int the weld in the first instance by applying suitable baking and strage treatments t the cnsumables used. This is the subject f a separate study sme f which has been reprted elsewhere.' The secnd is t reduce by specifying a suitable pstweld heat treatment the cncentratin f hydrgen present t a safe level; this is the aim f the present wrk. W. S. KYTE is with the Central Electricity Research Labratries, CEGB, Leatherhead, Surrey, England, and B. CHEW is with the Marchwd Engineering Labratries, CEGB. Marchwd, Suthamptn, Hampshire, England. Diffusin Mdel T specify a heat treatment fr hydrgen remval r t quantify the effect f an existing ne, e.g., a stress relief cycle, it is necessary t calculate the cncentratin prfile in the weld as a functin f time. Sme heat treatments invlve lengthy heating and cling perids. Because f this, much f the diffusinal transprt can take place under nnisthermal cnditins. The diffusin cefficient, D, which is an expnential functin f temperature will thus vary with time, t, and direct integratin f Fick's law: A = D(t) _! Sx 2-1 5yU 0) where c is cncentratin and x.y are spatial crdinates, but is nt pssible. Fr this reasn a parameter, T, has been used"' and is defined as: T = jddt (2) which n insertin int equatin (1) gives: _!! 6T ~ Sx 7 " 1 " Sy 7 (3) Equatin (3) can be slved analytically"' r numerically" t give the cncentratin, c, as a functin f psitin (x,y) and the parameter, T. The slutin fund must satisfy the bundary and initial cnditins impsed by the particular weld being treated. These usually take the frm f zer cncentratin at the surface f the structure at all times and a given distributin f hydrgen at zer time. T may 54-s I FEBRUARY 1979

2 WELDING RESEARCH SUPPLEMENT j 55-s 200 U D 2 8. E > 0 20 Fig. 2-Finite-element nzzle weld mesh fr duble- Time, s i CN Key c/c0 c Fig. 3 Hydrgen cncentratin cnturs at Til 1 = fr duble- nzzle weld Fig. I Develpment l a T value be evaluated in a given situatin in the fllwing manner. Any heat treatment schedule can be cnverted frm a temperature/time cycle t a D vs. time curve (Fig. 1) if D is knwn as a functin f temperature. The area under this D/time curve gives a value fr T, i.e., T = D dt If the simpler cnstant temperature case is cnsidered then, T--D.M where D e is the diffusin cefficient at temperature 0. Thus, pint cncentra- t Time, s tins and average cncentratins can be btained at any time in the heat treatment cycle, r after any perid f time at cnstant temperature, frm the slutin t equatin (3) btained analytically r numerically. In a previus paper" the numerical methd f slving equatin (3) was intrduced based n the finite element technique using a heat cnductin cmputer prgram. 7 T demnstrate the methd it was applied t a fillet weld fr which an analytical slutin was nt available. The purpse f this paper is t reprt the applicatin f the technique t several ther imprtant weld gemetries with the aim f specifying in general terms heat treatments that will reduce hydrgen cncentratins in a weld t any desired value. As in the fillet weld case, strict analytical slutins d nt exist fr these welds. Selected Welds and Results In treating the fillet weld," the weld shape was btained by tracing the utline f a sectin frm an actual weld. Similar utlines have been prduced fr fur further typical welds: a single- butt weld, a duble- butt weld, a single- nzzle weld, and a duble- nzzle weld. Meshes have been prduced, based n these drawings. An example is shwn in Fig. 2. Figure 3 shws typical cnstant cncentratin cnturs btained at selected values f the parameter T/L' 2, where L, a linear dimensin f the weld, is defined in the illustra-

3 - S2 2L - 2L T T/L Fig. 4-Fractinal change in weld metal average cncentratin as a functin f T/L 1 butt weids Fig. 5 Fractinal change in weld metal average cncentratin as a functin f T/L 1 nzzle welds tin. The bundary cnditins were the same as in the previus wrk: at t = 0, a unifrm cncentratin f hydrgen within the weld metal and zer cncentratin everywhere else, and at t > 0, zer cncentratin everywhere utside the structure. The cnturs can be prduced fr any value f T/L- and illustrate the way in which hydrgen diffuses away frm the weld area. Using an averaging prcedure described in the earlier paper, the fractinal average cncentratin c/c f the weld metal regin can be cmputed, again as a functin f T/L 2, These are pltted in Figs. 4 and 5, and the apprpriate curve fr the fillet weld is reprduced in Fig. 6. It has been shwn in the previus sectin hw, knwing the temperature histry f the weld, a T value can be btained. This, tgether with a knwledge f the linear dimensins f the weld, enables c/c values t be read frm graphs such as thse shwn in Figs It shuld be nted that these curves relate specifically t the actual welds fr which the calculatins were dne. Strictly, any ther example f ne f the given weld types which is nt gemetrically similar will give a slightly different result. Hwever, in practical terms the differences are nt likely t be significant; mst welds may be apprximated by ne f thse shwn in Figs Several practical examples f hydrgen cncentratin reductins may nw be given. 1 Q s y-- i* Fig. 6 Fractinal change in weld metal average cncentratin as a functin f T/L' fillet welds Stress Relief Heat Treatment Fr many welds a pstweld tempering and stress relief heat treatment will be specified, and it is instructive t calculate the reductin in hydrgen cntent due t sme f the current specificatins. These demand cntrlled heating and cling f the weld and assciated base material. Mrever, it shuld be nted that a cnsiderable amunt f hydrgen dispersal can ccur during these heating up and cling dwn perids. 5 Fr welds in 2%Cr-1M and icr-ttm-iw pipewrk, the fllwing heat treatment is specified: 8 heating rate < 0 C/h (180 F/h); hlding temperature C (1,274-1,328 F); hlding time-2h/25mm (2 h/1 in.); cling rate dwn t 400 C (752 F) < 50 C/h (90 F/h). Fr welds in pressure vessels, BS 1515 (1965) sets ut the fllwing: 1. Heating Rate: 200 C/h (r 360 F/h) up t 25 mm (1 in.) in thickness; 5080 C (r 9,176 F)/d/h where d, the thickness, is < 0 mm (3'i.i in.); 50 C/h (90 F/h) where d > 0 mm (3 15 /, 6 in.). 2. Hlding Temperature: C (1,076-1,148 F). 3. Hlding Time: 1h/25 mm (1 h/1 in.). 4. Cling Rate t 400 C (752 F): 250 C (450 F)/h up t 25 mm (apprx 1 in.); 5080 C (r 9,176 F)/d/h u here d < 0 mm (3"/i«in.); 50 C/h (90 F/h) where d > 0 mm (3' 5 /, 0 in.). In Table 1 the 2y4Cr-1M/ 1 /2Cr-'/2M- 4 cycle has been applied t the butt and fillet welds, and the pressure vessel cycle has been applied t the nzzle welds. The hlding times at the tp temperature are thse required by the specificatins, and the relative average cncentratin f hydrgen remaining as a result f this plus the heating and cling prtins f the cycle are given. Diffusin cefficients have been taken frm the review by Ce 9 and are the middle f the range f quted measured values. The results illustrate the effect f: 56-s I FEBRUARY 1979

4 Table 1 Relative Average Hydrgen Cncentratin Remaining as a Result f Stress Relief Heat Treatment Butt welds"" Fillet welds"" Nzzle welds" 2L = 12.7 mm 2L = 76 mm 2L = 6 mm 2L = 12.7 mm 2L = 25.4 mm '"L values are defined in rigs ~5% 1% 2L = 127 mm -24% -% Table 2-Time in Hurs at ~250 C (482 F) fr Reductin f Relative Average Hydrgen Cncentratins t the Stated Levels Weld metal hydrgen cne, % "»2L = 12.7 mm Butt welds 2L = 76 mm 2L = 6 mm Fillet welds 2f = 12.7 mr 2L = 25.4 mm Nzzle welds 2L = 127 mm (130)"" 1.7 (750) (62) 1.3 (570) 3.2 (5 X 3 ) 60 (3 X -) (2 X : ') 45 (2 X -) 115 (2 X : >) (1 X ') (7 X ') (5 X ').14 (66).66 (300) 1.1 (5).58 (260) 2.6 (1 X ') 4.4 (2 X ').54 (250) 4.4 (2 x s ) (90) 2.0 (920) 9.5 (5 X ") (4 X s ) (6 X s ) (2 x s ) (5 x ') (2 X ') (1 X ') (1 X s ) ln 'L values are defined in Figs ""Bracketed figures are times al 20 C (68 F), D, I X mill's '; D = 4.5 X 'mm s 1. Sectin thickness vs. single- weld preparatin, thin sectins and duble- preparatins being the mst favrable. The effect f sectin thickness is perhaps bvius. The mre rapid utgassing f the duble- gemetries examined here arises because less weld metal is used and the "average" linear dimensin acrss the weld width is smaller. Transprt by diffusin is a prcess which depends upn the square f these dimensins, and it is clearly advantageus frm this pint f view t reduce the amunt f weld metal used in making the weld. As stated previusly, the reductin in hydrgen cncentratin necessary t ensure freedm frm cracking will depend n the inherent susceptibility f the micrstructure and the level f residual stress. The latter will be determined by several factrs amng which are the degree f restraint applied t the weld by the surrunding material, the temperature interval between end f transfrmatin, and the preheat temperature.'" High stresses and susceptible micrstructures require a lwer hydrgen cntent. At the present time there is n way f fully quantifying these effects in a real welding situatin, and the maximum tlerable hydrgen cntent must be estimated frm experience. There is sme evidence" that the weld metal cncentratin immediately after welding is nt likely t be > 50% f thse values determined fr the cnsumables using the standard test." Since these shuld lie mainly in the range -15 ml H,NTP/0g weld metal if welding prcesses f relatively lw hydrgen ptential are used, the weld itself may cntain 5-7 ml H.NTP/IOOg. Reductin f this cntent t < 1 ml H,NTP/0g shuld give freedm frm cracking in mst cases" s that a tenfld decrease in weld hydrgen cntent shuld suffice. Table 1 shws that the standard heat treatments achieve reductins t < % f the starting cncentratin in almst every case and shuld, therefre, subsequently give freedm frm hydrgen-induced cracking. Effect f Pstheat There are three situatins at least in which cracking may still ccur: 1. A stress relief heat treatment after welding may nt be required by the specificatin. 2. Where ne is specified, there may be a lengthy perid at rm temperature between the cmpletin f welding and its applicatin. 3. There may be a lapse in preheat during actual welding. The last f these culd allw the weld t cl sufficiently t enter a "crack-sensitive" regin while still cntaining a relatively high cncentratin f hydrgen. Preheat temperatures generally lie in the range C ( F) and cracking has rarely, if ever, been bserved in this regin, prbably because f increased slubility f hydrgen in the metal matrix. It fllws, therefre, that as lng as the preheat is maintained between cmpletin f welding and heat treatment, and is nt allwed t lapse during welding, tw f the three pssible dangers referred t abve may be avided. Returning t the first case where n heat treatment is called fr, preheat must be maintained fr sufficient time after welding t allw a safe cncentratin f hydrgen t be reached. Examples f the time needed at 250 C (482 F) t allw the cncentratin t reach 50, and 1% f that present initially fr the different weld types are given in Table 2. Fr cmparisn, the bracketed figures shw the time required at 20 C (68 F) which wuld represent the extreme situatin where n pstheating was used. The diffusin cefficients, D 20 and D.,. used in the calculatin are again taken frm the middle f the range f measured values. It can be seen frm Table 2 that, in rder t reduce the hydrgen cncentratin t a tenth f its riginal value, a preheat f 250 C (482 F) must be maintained after welding fr a perid varying frm a few hurs t at least 2 days, depending mainly n the sectin thickness 2L. In the case f the 127 mm (5 in.) single- nzzle weld, a 2 day pstheating at 250 C (482 F) wuld lead nly t a 75% reductin. Hwever, a weld in this sectin thickness wuld almst certainly require stress relief. Cnsequently, the safe curse f actin wuld be t maintain the preheat if pssible until stress relief heat treatment was applied. If, fr example, it were intended t inspect the weld at ambient temperatures fr defects be- WELDING RESEARCH SUPPLEMENT I 57-s

5 Discussin fre heat treatment, then it must be recgnized that sme risk f hydrgen cracking is incurred due t the difficulty f utgassing a weld f this thickness. The heat treatment recmmendatins utlined abve shuld prvide freedm frm cracking under mst circumstances. Hwever, it is imprtant t re-examine the assumptins n which these are based. The chice f reductin t an average cncentratin level % f the initial value is made t cver as wide a range f circumstances as pssible. It may, fr several reasns, be t cnservative. In the case f a stress relieved jint, the heat treatment cycle is mandatry and it turns ut that the % criterin is met anyway. In ther cases, cmparing the pstheating times required by the analysis (Table 2) with industrial practice suggests that the requirement t meet the % level wuld be t stringent in sme instances and the times given fr the 50% level may be mre in line with sme f the practical experience. There are several pssible reasns fr this. First, if manual metal arc electrdes are baked and stred apprpriately/ they will generate (under test cnditins) nearer 5-7 ml H 2 /0 g weld metal than -15 ml as assumed. Thus, the starting cncentratin in the actual weld will be lwer, and lwer percentage reductins due t pstheating will be required. Cnversely, f curse, pr industrial practice in terms f electrde care might lead t levels > 15 mi and give rise t cracking where it therwise might nt have ccurred. Secnd, aiming fr < 1 ml H, as a final cncentratin des nt take accunt f variatins in the susceptibility f different micrstructures. Steels vary in their hardenability and transfrmatin prducts. It is well knwn' 2 that these factrs affect the cracking tendency. Sme f the sfter micrstructures will be able t tlerate > 1 ml H, withut cracking under a given system f residual stresses., The thickness parameter must be cnsidered. Thickness has a square law effect n hydrgen diffusin, and this has been taken accunt f in the analysis. Hwever, welds in thick sectins take lnger t make and this means greater pprtunity fr hydrgen t escape frm the system during welding, resulting in a lwer starting cncentratin. Against this must be set the fact that because thicker welds are mre heavily restrained, the residual stresses built up are likely t be greater and, in this respect, will demand a lwer final hydrgen cncentratin t ensure freedm frm cracking. The extent t which the first effect helps in achieving a safe level is unknwn. Summarizing, there are several factrs t take int accunt when selecting a pstheating schedule fr a particular applicatin. In terms f. an all-embracing recmmendatin, aiming fr the % level shuld certainly give freedm frm cracking in almst every case, but may be t stringent in sme circumstances. If the heat treatment required t achieve this level is t lengthy and/r cstly, then the ther ptin is t intrduce less hydrgen int the weld in the first instance. This may be achieved thrugh the prper baking and strage f electrdes. 4 It may then be sufficient t aim fr the 50% level in Table 2. The results in Tables 1 and 2 have been btained using literature average values fr the diffusin cefficient D L. If a mre accurate value, D M, is available in a particular case, then a time t, read frm Table 2 may be crrected t a mre accurate value t M using the relatinship: D M t M = D_t L This arises frm the fact that all cncentratins are functins f the dimensinless parameter T/L J which at cnstant temperature is equal t Dt/ L 2. Table 1 results are fr nn-isthermal cnditins and althugh the functin is still T/L 2, T is given by D(t) dt. Thus, t imprve accuracy in this case D must nw be knwn ver the whle temperature range f the heat treatment cycle. These mdificatins are unlikely t prduce significant differences in the general cnclusins. Cnclusins 1. Finite element techniques can be applied t real weld shapes t derive hydrgen cncentratin curves. These then frm the basis fr an evaluatin f pstweld heat treatment in terms f hydrgen dispersal frm welds. 2. Any chsen level can be reached after welding by maintaining pstheat fr a perid which may be calculated frm the finite element results. 3. Hydrgen utgassing in the regin f 20 C (68 F) is a lengthy prcess s that welds requiring stress relief heat treatment will incur sme risk f cracking if allwed t cl t ambient between cmpletin f welding and applicatin f the heat treatment. 4. In general, the mandatry stress relief heat treatments are sufficient t lwer hydrgen cncentratins t less than % f that present initially. Evidence available in the literature suggests that this level will ensure freedm frm cracking in mst cases. Acknwledgment This wrk was carried ut at the Central Electricity Research Labratries and the Marchwd Engineering Labratries and is published by permissin f the Central Electricity Generating Bard. References 1. Beachum, E. P., Jhnsn, H. H., and Stut, R. D., "Hydrgen and Delayed Cracking in Steel Weldments," Welding jurnal, 30(4), April 1961, Research Suppl., pp. 155-s t 159-s. 2. Interrante, C. C, and Stut, R. D., "Delayed Cracking in Steel Weldments," Welding lurnal, 43(4), April 1964, Research Suppl., pp. 145-s t 160-s. 3. Interrante, C. C, Nelsn, C. A., and Hudgins, C. M., "Interpretive Reprt n Effect f Hydrgen in Pressure-essel Steels," WRC Bulletin 145, Octber Chew, B.. "Misture Lss and Regain by Sme Basic Flux Cvered Electrdes," Welding lurnal, 55 (5), May 1976, Research Suppl., pp. 127-s t 134-s. 5. Chew, B., "Calculated Hydrgen Cncentratin Changes Due t a Stress-Relief Cycle," Welding Research Abrad, l. XX (2), Feb 1974, pp Chew, B., and Kyte, W. S., "Diffusin f Hydrgen in Fillet Welds," Metals Technlgy, l 2, Feb 1975, pp Fullard, K., CEGB Reprt RD/B/N1849, (1974). 8. CEGB Standard 23584, (1974). 9. Ce, F. R., "Welding Steels withut Hydrgen Cracking," The Welding Institute, Cambridge (1973).. lnes, W. K. C, and Alberry, P. )., Unpublished Wrk. 11. British Standards, BS 639, (1976). 58-s FEBRUARY 1979