Welding Research Sponsored by the Welding Research Counc of the Engineering Foundation

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1 Welding Research Spnsred by the Welding Research Cunc f the Engineering Fundatin SUPPLEMENT TO THE WELDING JOURNAL, APRIL 972 The Establishment f Safe Welding Prcedures fr Steels A study f the hydrgen cracking behavir f CMn and lw ally steels has led t the develpment f statistically based welding prcedures fr a range f CMn steel cmpsitins BY N. BAILEY Intrductin Organizatins like The Welding Institute are frequently asked fr guidance n hw t weld steels which are new t a particular fabricatr. It is therefre necessary t have a ratinal way f devising welding prcedures which are safe, but which are nt s cnservative that they are unecnmic. Abut 7 years ag Baker et al. laid dwn the steps needed t determine welding prcedures fr steels in which hydrgen induced delayed cld cracking was expected t be the majr prblem. The steps can be briefly summarized: N. BAILEY is with the Metallurgical Labratry f The Welding Institute, Cambridge, England. 4. Establish the transfrmatin characteristics f the steel s that heat affected zne (HAZ) hardness and micrstructure can be predicted Determine the susceptibility f the steel t hydrgen cracking when welded Predict safe welding prcedures Establish cnfidence in the predicted prcedures by sme frm f welding test. The present paper describes prgress in this field since that time t test the validity f the abve cncept. 2 This has led t a technique fr the predictin f statistically based safe welding prcedures fr carbnmanganese steels, t a better understanding f hydrgen cracking and t an understanding f hw mre ecnmical prcedures can be develped. Experimental Apprach Fur carbnmanganese and lw ally steels were selected and an assessment made f their weldability as utlined in steps () and (2) abve. Welding prcedures designed t avid hydrgen cracking were then devised and checked using the same casts f steel in a specially devised fillet welded fabricatin. Hydrgen cracking is clsely assciated with hardening f the HAZ as it cls rapidly after welding. A high speed dilatmeter determined the cntinuus cling transfrmatin (CCT) behavir f the test steels at cling rates typical f manual shielded metalarc welding. 3 Hllw cylindrical specimens, 0.95 m m(0.038 in.) thick were inductin heated t 325C (247 F), cled in argn at different rates and the micrstructure examined and hardness measured. This enabled HAZ WELDING RESEARCH S U P P L E M E N T [ 9s

2 test weld. Thermcuple * ^^S 7mm 7 mm ^ JViWrVr 25 mm 4 lmm thick shim 02 mm Fig. CTS test assembly shwing details f shim, psitin f thermcuple and methd f cling after welding. C = cmbined thickness f test weld; L = nminal leg length f test weld hardnesses t be predicted frm HAZ cling rates which culd be determined frm welding parameters.* Althugh a variety f tests are available t assess the susceptibility f a steel t hydrgen cracking, the cntrlled thermal severity (CTS) test was chsen s that dilatmetric predictins culd be cmpared with actual HAZ cling rates, micrstructures and hardnesses and als the bundary between cracking and n cracking btained fr welding prcesses f different hydrgen ptentials. The CTS test has lng been used t examine hydrgen cracking behavir but it is imprtant t pint ut tw mdificatins that have been fund necessary t simulate the behavir f real weldments mre accurately. One is the presence f a rt gap f. mm (4 in.) t represent pr fit which is present in mst structures. This has been shwn 5 t increase the risk f hydrgen cracking when using lw hydrgen cnsumables in a fillet weld if the rt gap exceeds 0.4 mm ( in.). The ther mdificatin is that the test piece is cled after welding as shwn in Fig. t prevent the temperature f the test piece equalizing at a temperature abve ambient, an effect which des nt happen in large fabricatins, particularly if the preheat is applied lcally t the weld area. Temperature equalizatin wuld reduce the risk f cracking in the CTS test because mre hydrgen wuld diffuse ut during the slwer cl frm the equalizatin temperature s that the HAZ wuld cntain less hydrgen at the temperature at which cracking is mst likely, abut 20C (70F). In additin t the abve mdificatins, limited thermal recrds were made using a thermcuple in the HAZ near the te f the test weld (a lcatin easy t mnitr in practice) t cmpare cling rates with thse taken near the rt which had been dne in previus wrk. 4 Welding prcedures devised frm these experiments were tested n a jint simulatin test piece (Fig. 2) designed t incrprate mst f the jints, mstly fillet welds, fund in prtable military bridging. Since the test piece, standing abut m (39 in.) high, was smaller than a real bridge, care had t be taken t spread the welding ver a lng enugh time t avid temperature buildups which wuld have reduced the likelihd f hydrgen cracking. Results HAZ Transfrmatin Behavir The steels examined, tw CMn structural steels, a 0.5% MB steel and a lean multially steel (0.5C.5 Mn 0.5Cr 0.2M 0.05V) were all f lw hardenability s that HAZ hardness culd be cntrlled by cntrlling weld bead size (arc energy) and t a lesser extent preheat. Cmparisn f dilatmetric and HAZ micrstructures revealed that in sme cases the micrstructure and hardnesses (Fig. 3a) were very similar, bearing in mind that a maximum HAZ hardness was being cmpared with a mean value frm the dilatmetry. In ther instances (Fig. 3b) crrelatin was nt s gd; tw effects were thught t be respnsible. One was that in dilatmetry the upper part f the thermal cycle was nt quite ht enugh r lng enugh t give as much austenite grain grwth as in a weld HAZ s that the latter were mre hardenable because f the carser austenite grain size; cnditins used fr dilatmetry have since been mdified t btain a clser apprach t an HAZ grain size. The ther effect was mre cmplex and was prbably due t the cling f the HAZ being described by a single parameter, the 300C (572F) cling rate. 4 This parameter wrks well fr steels which transfrm near 300C but the structural steels used in the present wrk started transfrming between 350 and 30C (0 t 70F) and it was realized that the use f a preheat t reduce a 300C cling rate wuld nt reduce the cling rate at a higher temperature by the same prprtin. The HAZ wuld therefre cl t fast thrugh the intended startftransfrmatin temperature and wuld finish up harder than intended. A crrectin due t Graville 2 was made t the 300C cling rate, which takes accunt f the startftransfrmatin (T s ) and the 70s APRIL 97 2

3 preheat (P) temperatures. The crrected 300C cling rate, R is expressed by: R (300P)(T s 20)\ 2 \ 280(T S P) ) This crrectin, shwn by the arrwed pints in Fig. 3c, imprved the crrespndence between CCT and HAZ data. Susceptibility t Hydrgen Cracking The susceptibility t cracking was assessed in terms f the critical maximum HAZ hardness fr cracking. This was determined by making and examining a series f CTS test welds using different energy inputs and finding the sftest HAZ in which cracking ccurred. Full details f these tests are given in (ref. 2) but the results can be briefly summarized as fllws:. With prcesses f relatively high hydrgen ptential (which gave all weld metal hydrgen cntents f abut 30 and 7 ml00 g determined by a methd 4 similar t the IIW methd) critical HAZ hardnesses f 350 HV were fund fr the CMn and the lean multially steels and 375 HV fr 0.5 MB 2. With a lwer hydrgen prcess (C0 2 welding giving 8 ml H 2 00g) the critical hardness fr all steels was 400 HV 3. When preheat was used, higher HAZ hardnesses culd be tlerated withut cracking, presumably because mre hydrgen was able t escape by diffusin during the lnger cl dwn t ambient temperature at which cracking is pssible. In fact the main benefit f preheating appeared t be in reducing HAZ hydrgen cntents, rather than reducing hardness. 4. It was fund satisfactry t predict preheats t avid cracking by using an earlier diagram giving preheat in terms f expected maximum HAZ hardness. This diagram is reprduced in Fig. 4 and it is ntewrthy that with the steels examined hardnesses were belw 450 HV s that pstheating wuld nt have been necessary accrding t the diagram. Predictin f Welding Prcedures Fr the different steels examined, safe welding prcedures were devised fr cnsumables f different hydrgen ptential; these are illustrated fr the lean multially steel in Fig. 5. One pint which had t be reslved was the range f cmpsitins fr which the prcedures are valid. Since the risk f cracking increases when bth the cntent f carbn and allying elements is increased, a cmpsitin as near t the maximum regularly prduced shuld be the ne n which safe welding prcedures are based. The cmpsitin can be expressed in terms f a carbn equivalent (CE) frmula but the calculatin f this frm the maximum allwable specificatin values can give unduly cautius prcedures which can prve ruinusly expensive fr the fabricatr. Fr the lean multially steel illustrated, Table shws specified cmpsitins and the highest cmpsitin examined in the present wrk. Since this was btained frm the steelmaker as a "high" cmpsitin and the carbn plate analysis was, in fact, abve the ladle maximum, the CE f 0.2 was taken as a tp end cmpsitin fr the establishment f safe welding prcedures rather than the maximum value f 0.70 pssible if every element was at the tp f its range. The safe welding prcedures shwn in Fig. 5a and b are apprpriate t cnsumables f high and medium hydrgen ptential, respectively. Thse used in the experiments described gave weld metal hydrgen cntents f 7 and 8 ml00 g when sampled t BS 79:93 (Part l) 4 (similar t the IIW methd) and the crrespnding critical HAZ hardnesses withut preheat, were 350 and 400 HV. The higher critical hardnesses used with preheat in cnstructing Fig. 5 were taken frm the "higher restraint" curve f Fig. 4 fr bth levels f hydrgen as insufficient results were available t see hw the use f a lw hydrgen prcess mdified this diagram. It shuld be nted that the value f 8 ml00 gm is at the tp end f the range expected fr slid wire C0 2 welding and mre wrk wuld be necessary t see whether a lwer critical hardness culd be used if the weld metal hydrgen cntent culd be kept lwer. A final pint abut the diagrams is that when the minimum preheat lines terminate in a hrizntal prtin, the inflectin represents the limiting plate thickness; a further increase in thick Fig. 2 Jint simulatin test assembly after welding and befre sectining and examinatin fr cracks. This item std ver meter high, was made frm plate 50 mm thick and cntained jints typical f thse fund in prtable military bridging WELDING RESEARCH SUPPLEMENT! 7s

4 buu i i i i i i i i 500 i i i i I i i I II i i i CCT results Maximum HAZ hardness (HV2Hmm D Maximum HAZ hardness (HV ZH 3mm >L i < DO C CTi esult s M QXi mum HAZ nr dness It V2 \l) 200 n i l i i i i 200 b) 500 II l<j Mil e* ' V * \ < *"7 < \ CC T ri suit) Maximum HAZ hardness (HV 2^ c Cling rate f HAZ withut preheat crrectin ZOO.i,ui. i i 4 «. 3 2 i k Q0*C COOLING RATE,deg Csec Fig. 3 Cmparisn f average CCT and maximum HAZ hardness values: (a) gd crrelatin, steel 0.8C,.47Mn, 0.5Cr, 0.2 M, 0.05V; (b) pr crrelatin, steel 0.0C, 0.3Mn, 0.2Si, 0.44M, 0.003B; (c) pr crrelatin imprved by preheat crrectin, steel 0.22C,.2Mn, 0.27Si, 0.03Nb This result indicates that pstweld cling f the CTS test made it behave in a manner similar t a large structure, althugh the crrespndence was just shrt f the ideal. 3. Taken as a whle, the effects f varying the fitup, measured n the sectins examined fr cracking, cnfirmed earlier wrk using CTS tests 5 that a gap size f 0.4 mm was critical, larger gaps shwing a greater tendency t cracking. Sme cnsideratin was given t the assessment f the effective cmbined thickness f cmplex jints by cmparing their HAZ cling curves with similar data frm CTS tests f different cmbined thicknesses. This was fund t be a useful technique. It als shwed that the cling behavir at lwer temperatures culd be influenced by metal t remte t influence the 300C cling rate (i.e. further away than the 75 mm (3 in.) frm the jint line, ver which distance cmbined thicknesses are cnventinally calculated). ness des nt increase the 300C cling rate. Jint Simulatin Tests Tw test pieces f the type shwn in Fig. 2 were made frm each steel in the prgram, ne representing an average and the ther a high cmpsitin. The same cnsumables were used fr the different tests as in the earlier experiments and duplicate jints in each test were welded with prcedures (devised in the same way as thse f Fig. 5) aimed at being either side f the brderline cnditins fr cracking. Where pssible, fitup was als varied. Full details f the tests and results are given elsewhere 2 but the results can be summarized fairly briefly.. The use f "safe" welding prcedures in all cases resulted in crackfree welds; this was cnfirmed by cmprehensive sectining and micrscpic examinatin. 2. Critical HAZ hardnesses were marginally lwer (n mre than 0 HV) fr the jint simulatin test than fr the CTS tests. This is best shwn in Fig., which is a plt f the results fr preheated jints, in terms f preheat used and maximum HAZ hardness fund fr cracked and uncracked welds in cmparisn with the "high restraint" curve f Fig. 4. Discussin Fllwing the cmpletin f the wrk described abve, which was centered arund particular steels used in fillet welded bridging structures, it was appreciated that sufficient data existed n CMn steels, with and withut silicn, t enable welding prcedures t be devised by assessing statistically the chances f a HAZ being harder than the critical value. Frm the data available 2 * 3 * 7 a linear relatinship was fund between CE and the reciprcal f the square rt f the critical 300C cling rate. The existence f a linear relatinship, in cntrast t the curves shwn 72s I APRIL 972

5 COMBINED THICKNESS.in COMBINED THICKNESS,! n. j i Prehet CI 20 i t* _ 50* 2 30 fl 7 w w r 0 7 «y 7 7 *. f w Z 5 ^ (0! 20 J50l= '.n = 5. 75* i P'H. K\ y y [30 h "I 3 «4 05 (Ve 0 IS 20 RUN OUT RATIO COMBINED THICKNESS,mm 0 * RUN OUT RATIO n COMBINED THICKNESS, mm n Fig. 5 Safe welding prcedures fr welding lean multially steel t BS5027 having a carbn equivalent nt exceeding 0.2 using a prcess invlving (left) relatively high hydrgen ptential (typical weld metal hydrgen cntent 7 ml 00 g) and (right) a medium hydrgen ptential (typically 8 mlioo g) ' i EXPECTED HAZ HARDNESS. HV Special methds advisable Fig. 4 Temperature which will prevent HAZ cld cracking if applied as preheat, interpass and pst heat temperature by ther wrkers 3 * 8 wh did nt use the VR relatinship, allws results t be treated statistically in a simple manner as shwn in Fig. 7. Silicn has an influence n the hardenability f CMn steels and the use f a factr f Si in the CE frmula was fund t minimize scatter n the VR CE plt. This factr' has been cnfirmed by subsequent wrk 3. Fr a critical hardness f 350 HV, the best, fit straight line fr the experimental dilatmetric results was fund t be: V R S50 2. CE 0.3 fr 400 HV "V R400 fr 450 HV VR450 =.9 CE CE O.t The scatter was such that in the case f 350 HV the ttal width f the band f ± 2.58 standard errrs, which shuld cntain 99% f the results, was equivalent t a CE f 0.. The band width increased fr the higher hardnesses, mainly because hardnesses f 400 and 450 HV were nt pssible in the sfter steels even with very fast cling rates s that fewer results were available. It was reasned that a in 200 chance f an HAZ being abve the critical hardness was an acceptable risk fr, even thugh an actual HAZ tended t be slightly harder than dilatmeter specimens cled at the same rate, the risk f cracking in the CTS tests appeared t increase gradually, rather than suddenly, as the hardness was increased abve the critical. The ther factr liable t vary is the cmpsitin f the steel being welded. Frtunately, sme data are available 9 fr a typical CMn steel BS98 (nw BS430 grade SOC) shwing the expected variatin in CE WELDING RESEARCH SUPPLEMENT! 73s

6 Cracked test Minimum preheat, interpass and pstheat curves frm Fig. 4 ZbU ZOO n 3 09 ' % T" " " " Uncracked test Sftest cracked unpreheated HAZ T High restraint MAXIMUM HAZ HARDNESS, HV 2V 2 Fig. Preheat and maximum hardness f jint simulatin tests. Pints representing preheat temperatures and maximum HAZ hardnesses f cracked and uncracked jints have been pltted n the "high restraint" curve f Fig. 4. Of many unpreheated jints, nly the sftest cracked ne has been pltted. All jints made with high ptential hydrgen cnsumables value fr several prducts. The standard deviatin,, varied between 0.08 and 0.029, averaging CE. Structural steels in the U.K and elsewhere are available with maximum CE values specified. If it can be assumed that the "aim pint" CE is 2(0.05) belw the specified maximum, then it can be shwn by multiplying individual prbabilities that by using prcedures based n the upper scatter band f Fig. 7, the verall risk f btaining a hardened HAZ (assessed dilatmetrically) is abut in If thrugh a difference between ladle and prduct analyses, segregatin r ther causes, a high cmpsitin is being welded the risk f a hardened HAZ will nly apprach in 2 if the maximum CE is exceeded by Having the danger level 0.05 CE abve the specificatin maximum seems t be reasnable since it was similar t the maximum difference between CEs calculated frm ladle and prduct analyses (0.055) fund in the earlier part f the research wrk described abve. Weldability Diagram A diagram f safe welding prcedures based n the principles described abve is reprduced in Fig. 8. It cnsists f tw main graphs cnnected by an (unlabelled) vertical scale f values f R,, the scale being different in size fr each preheat level. The left hand half f the diagram is a graph relating critical cling rate t energy input (E, klni.) fr different cmbined thicknesses (C, in.) using the expressin 4 ~ VR 54T(+0.00T) (4C+0.5) E+4000C where T is 300C minus the preheat temperature. On this diagram have been superimpsed lines representing the maximum effective cmbined thickness fr fillet welds at different preheat temperatures and als vertical lines shwing maximum cntrlable bead sizes in nrmal psitinal welding. It will be appreciated that these bead sizes cannt be attained in all circumstances and a value f 35 kjin. might be a better value fr a hrizntalvertical butt weld and as little as 20 klin. fr a capping pass in this psitin. The energy input scale is cnnected t different parameters which can be cntrlled in manual welding, namely fillet weld leg length and electrde runut length and rati suitable fr bth fillet and butt welds. The right hand graph is a series f lines relating CE value t l\ R c with the safety margin f 2 standard errrs included. The vertical, l\ R c scale has been adjusted fr each preheat temperature s that the tw sides f the graph crrespnd. The cling rates relate t critical hardnesses f 350 HV fr preheats up t 00C (20F) increasing t 450 HV fr a preheat f 200C (390F). This des nt mean when welding a steel t Fig. 8 with a preheat f 200C that the HAZ hardness will be as high as 450 HV; even if the steel has the actual maximum CE value used there is nly a in 200 chance that it will reach that value. It des, hwever, mean that there will be a risk that the hardness will exceed 350 HV in asdepsited, untempered HAZs and that this shuld be brne in mind if service cnditins include a risk f stress crrsin under very crrsive cnditins, r if there is cncern abut the tughness f such relatively hard heat affected znes. Belw the nrmal CE scale, apprpriate t a critical hardness withut preheat f 350 HV, additinal scales apprpriate t hardness f 400 and 450 HV are shwn. The 400 HV scale f these is well established 2 fr prcesses giving weld metal hydrgen cntents 4 f abut 8 ml00 g and less. The maximum limit has nt yet been determined, althugh it is knwn t be less than 7 ml00 g. The 450 HV scale shws the ptentialities f really lw hydrgen prcesses such as gd quality slid wire GMAW and GTAW and pssibly even manual shielded metal arc welding with electrdes dried at high temperatures (e.g. 450C, 850F). Wrk is cntinuing t establish critical hardness values ap 74s I APRIL 972

7 prpriate t prcesses f very lw hydrgen ptential. Reactin in the U.K. t this nmgram has been smewhat mixed. That f the pressure vessel industry has tended t be favrable, prbably because its inspectin standards are stringent and because the cnsequences f failure n pressure test can be catastrphic and thse f leakage embarrassing. Structural welding engineers have tended t cnsider the diagram verly cnservative and it is wrth cnsidering the prbable reasns fr this. The prime factr is, f curse, ecnmic because greater cst f increased preheat temperatures r larger weld bead sizes immediately makes a steel structure less attractive than, say, a cncrete ne. In supprt, as it were, f this attitude are the lwer inspectin standards attainable n site, particularly in fillet welded cnstructins, and the relatively high tlerance fr cracks in this type f structure resulting frm the gd tughness f the steels nrmally used and their use in relatively thin sectins. Furthermre thin plate is likely t have CE values well belw the specificatin maxima. Indeed a fabricatr wh keeps a clse watch n his mill sheets shuld sn be able t devise his wn ecnmical welding prcedures frm Fig. 8. If he sails very clse t the wind he shuld keep a clse lk ut fr high cmpsitin plate and if any is fund he shuld mdify his welding prcedures accrdingly. In using the nmgram it is imprtant t bear in mind the cnditins upn which it was based and t take apprpriate actin when they are nt being met. Mre severe cnditins might be fund in the rt runs f very highly restrained butt welds in thick plate, cnditins similar t thse in the Lehigh and Ygrve r Tekken tests 0 where cracking has been assciated with HAZ hardnesses well belw 350 HV. On the ther hand, when cnditins are less cnducive t cracking relaxed prcedures can be used always prvided either that the techniques used can be backed up by adequate recrds f previus satisfactry experience n the same type f jb, welding methds and steel, r that adequate jint simulatin tests have been carried ut. Mre relaxed prcedures are pssible either where the later stages f cling f the weldment permit mre hydrgen t diffuse ut r where the whle weld is heated up by the next run and sftened by tempering r where the stress level is lw. Fr example in a shrt butt weld the rt pass may be depsited with deep penetratin cellulsic electrdes and tem Carbn equivalent,, Fig. 7 Relatinship between critical cling rate giving 350 HV and carbn equivalent fr CMn steels cntaining 0.9.7Mn and 00.Si Table Cmpsitin f Ducl W30A Plate Range f specificatin BS507 Maximum examined CE C Si Mn Cr M Ni Cu CE* Mn Cr + M + V Ni + Cu pered by subsequent passes applied befre the rt pass has had time t crack. Passes fllwing in rapid successin effectively build up a preheat fr the later runs while the stresses n the upper runs are lessened because the rt gap has been filled and ther strain cncentratrs minimized. The capping passes which can leave untempered HAZs are depsited with interpass temperatures at a maximum and the stress cncentratin at a minimum. Final cling f HAZs will be delayed if the heat sink in ne directin is limited (e.g. a narrw flange f a beam), if preheat is applied ver a wide area arund the weld r if the jint is warmed by anther weld made in clse prximity Jint simulatin tests (frmerly referred t as prcedural tests 23u, are intended t simulate the real fabricatin with all the cntrls n welding set at a slightly lwer level than they will be in the fabricatin 2 s that if cracking is encuntered, it will be in the test and nt the jb. Prbably the main difficulty in rganizing a jint simulatin test is t btain steel fr the test which is at the tp end f the cmpsitin range likely t be used fr the subsequent fabricatin, particularly if the mill sheets fr the steel refer t ladle and nt prduct analysis. Hwever a nmgram such as Fig. 8 can be used t adjust the prcedures used fr a jint simulatin test t prcedures which will be safe fr a slightly higher r lwer cmpsitin. WELDING RESEARCH S U P P L E M E N T ' 75s

8 Future Develpments A reassessment f the nmgram f Fig. 8 will becme necessary t take accunt bth f current wrk and f experience f fabricatrs wh have made use f it. The increasing amunt f CCT infrmatin being btained, particularly since discrepancies between dilatmeter and HAZ specimens were fund, may reduce the scatter in Fig. 7 and thereby make the diagram less cnservative and mre ecnmical. Wrk is als being carried ut t clarify the effect f Si in CMn steels. This has s far cnfirmed that Si increases the hardenability f such + it \^ * * D J< C n Cj <U cj,r D «> a: ^ ''S aj ' t JV OI P t~' ''Ol CO c "5 c. c V'>~ ps. Cu IO i ''fa ^_ 2: 7 a; ^'. * ' 7*. <' "> Si ~ k ~ ~ 7 O 3 i _ w fe IO ** c^ O > } f O ^ ', ' V). cb * IO sf c ^ P CD c. JO "O i. i f^ " <P Uj t ID " _. O c.c ai c O) OJ " isga. ic t..c Ci> O <N "4 g Si 2 apjjaiaj wat *" "g WOJI Q)Buai w liny g»> c, 3> *$ O " *"H + U<J «Q, fc ^ CJ ~ t: i J CO Q. *~.P csj O <*> t ^ <N O jj C\ <\^ «^ «^ * * O) sapjjaa pajaa S OIIDJ )n uny j; ^ N? i: <? =t a> OJO c cu I" <3s *,. *. : " > c i c la s. "O C : a <u i >*c 2 i c: P. 7s APR I L 972

9 steels 3 but suggests that it may als decrease the susceptibility t cracking. If this is the case, and tests are being made t check the pint, then the easiest slutin might be t exclude Si frm the CE frmula n the grunds that the increased hardenability balances the decreased susceptibility t a sufficient degree fr practical purpses. Further wrk is aimed at establishing r cnfirming the critical hardness values, nt nly fr different welding prcesses and hydrgen levels, but als fr different jint cnfiguratins and strength levels. Hwever, the greatest debate will result frm the statistical treatment f cracking. Are custmers prepared t accept that different jints will be welded with prcedures in which there is a risk f cracking, hwever small? It may seem a strange philsphy t be prpunding but, in essence, it is nt very different frm the results f researches int fracture mechanics which can give the purchaser estimates f the size f crack which can be tlerated with safety in a structure. Nr is it very different frm the lng established custm that structures upn which life and limb depend shuld be the nes mst thrughly inspected and tested. Summary A study f the hydrgen cracking behavir f CMn and lw ally steels has led t the develpment f statistically based welding prcedures fr a range f CMn steel cmpsitins. The nmgram shuld nt be regarded as a rigid set f instructins n hw t weld any CMn steel but as a set f guidelines against which t cmpare current welding prcedures s that new nes can be devised fr new steels, prcesses and prducts with the minimum f risk and the maximum speed and ecnmy. Acknwledgements This paper was based largely n wrk made pssible thrugh the supprt f the Military Vehicles and Engineering Establishment, Christchurch, England. The authr is als grateful t clleagues at The Welding Institute fr help and advice. References. Baker, R. G., Watkinsn, F., and Newman, R. P., "The Metallurgical Implicatins f Welding Practice as Related t Lw Ally Steels," Prceedings 2nd Cmmnwealth, Welding Cnference 9, 253, Institute f Welding, Lndn. 2. Bailey, N., "Welding Prcedures fr Lw Ally Steels," The Welding Institute Reprt Series, July Watkinsn, F., and Baker, R. G., "Welding f Steel t BS 98: 92, a Dilatmetric Investigatin," British Welding Jurnal 97, Vl. 4, N., pp Cttrell, C. L. M., and Bradstreet, B. J., "A Methd fr Calculating the Effect f Preheat n Weldability," British Welding Jurnal 955, Vl. 2. N. 7, pp Graville, B. A., "Effect f Fitup n Heat Affected Zne Cld Cracking," British Welding Jurnal 98, Vl. 5, N. 4, pp Ce, F. R.. and Watkinsn, F., "Weld Metal Hydrgen Levels." The Welding Institute Res. Bulletin 99, Vl. 0, N. 2, pp Watkinsn, F., "Atlas f High Speed CCT Data," The Welding Institute internal dcument, Bldgett. O. W., "Design f Welded Structures," "Weldability and Welding Prcedures," sectin 7.2, 9. Cleveland, Ohi. 9. Gledhill. P. K. and Gult. E. L., "Prductin f Universal Beams and Clumns in High Yield Stress Steel." "Strng Tugh Structural Steels" ISI publicatin 04, 97. pp and discussin p Kihara. H. Suzuki. H. and Nakamura. H.. "Weld Cracking Tests f High Strength Steels," Welding Research 92, Vl. 27, N., pp. 3s48s.. Bailey. N.. "Welding Carbn Manganese Steels." Metal Cnstructin 970, Vl. 2, N. 0, pp Bailey, N., "Jint Simulatin Testing against Hydrgen Cracking" The Welding Institute Research Bulletin 97, Vl. 2. N. 0, pp Watkinsn, F., et al., t be published. 4. British Standard BS 79 Part : "Classificatin, Cding and Marking f Cvered Electrdes fr Metal Arc Welding," 93, Appendix B. WRC Bulletin N. 70 February 972 "Mig Welding with Pulsed Pwer" By A. Lesnewich This reprt, prepared fr the Interpretive Reprts Cmmittee f the Welding Research Cuncil, presents a cncise summary f the present stateftheart f pulsedarc MIG welding. The effect f pulsed current n MIG welding is s significant that sme authrities accrd the technique the status f a new welding prcess, calling it pulsed spray t differentiate it frm ther frms f pulsed pwer. Actual changes assciated with this methd invlve a new cncept in pwer supply design and, t a minr degree, sme adaptatins in the way the welding equipment is used. Included in the reprt is an explanatin f why pulsed pwer was develped fr welding and hw the pulsed spray technique wrks. The effect f variables such as pulse frequency, pulse amplitude and pulse width are described in detail. Als cvered are the fundamentals f the pwer supply and the adaptability f the prcess fr welding steel, aluminum, magnesium and cpper. The price f Bulletin 70 is $2.00 per cpy. Orders fr single cpies shuld be sent t the American Welding Sciety, 250 N.W. 7th St., Miami, Fla Orders fr bulk lts, 0 r mre cpies, shuld be sent t the Welding Research Cuncil, 345 East 47th Street, New Yrk, N. Y WELDING RESEARCH SUPPLEMENT! 77s