Weld Thermal Efficiency of the GTAW Process

Transcription

1 Weld Thermal Effcency of the GTW Process study of the arc energy dstrbuton, materal response, and changes n weldng parameters usng the GTW process show mproved meltng and process thermal effcency as travel speed s ncreased. BY R. W. NILES ND C. E. JCKSON BSTRCT. The lmtatons of arc energy nput to certan ranges may not provde an adequate means of predctng or controllng mechancal propertes when weldng hardenable materals. Ths has been shown n the followng nvestgaton to be due, prmarly, to the varaton n energy dstrbuton whch occurs at the weldng arc wth changes n weldng parameters. Ths energy dstrbuton has been expermentally determned as a functon of weldng parameters usng the GTW process and has been expressed n terms of thermal energy effcences. The total energy enterng the base plate and the energy used to melt the weld nugget have been compared wth the total arc energy avalable and expressed respectvely as process and meltng effcences. Results of ths nvestgaton show that the energy dstrbuton at the weldng arc can be sgnfcantly changed by varatons n current level, travel speed, sheldng atmosphere, and process selected. n ncrease n current level and a decrease n the travel speed produces a decrease n the process effcency when weldng wth the GTW process. Ths ndcates that a lower percentage of the avalable heat enters the base plate materal. n ncrease n travel speed R. W. NILES, formerly a graduate student at the Oho State Unversty, s now Weldng Engneer, Eastman Kodak Company, Rochester, New York. C. E. JCKSON s Professor of Weldng Engneerng, The Oho State Unversty, Columbus, Oho. or current level, on the other hand, produces an ncrease n meltng effcency wth the GTW process ndcatng that a greater percentage of the avalable arc energy s used to form the weld bead. Introducton Of major concern to the weldng engneer has been the effect whch the heat of weldng has on the mechancal propertes and structure of materals to be joned. Many methods have been tred n an attempt to predct the effects of the weld thermal cycle on mechancal and structural propertes for the varous materals and thcknesses encountered. By lmtng the energy nput to certan ranges, some control of the metallurgcally mportant peak temperatures and coolng rate has been obtaned. However, the nonlnear effect of travel speed, current and voltage on the energy nput for any process s not generally recognzed. The total energy nput developed per unt length n the weldng zone may be calculated by the equaton: E t - = V/S (joules/lnear n.), or = V/S (joules/lnear mm) (1) where: E, = Total arc energy nput (joules/lnear n. or mm) V = rc voltage (volts) = rc current (amperes) S = rc travel speed (n./mn or mm/s) Ths approach s lmted n ts control of metallurgcal response because t does not take nto consderaton changes n the dstrbuton of the arc energy produced by varatons n weldng parameters. s shown n Fg. 1, only part of the total arc energy avalable s transferred to the weldment. lso, only part of the energy whch enters the weldment s used to melt the weld bead; the remander heats up the base metal and forms the heat-affected zone. n accurate predcton of the quantty of heat or energy (E ) enterng the base metal s needed to predct the metallurgcal response. For a gven arc energy nput, the rato between the total arc energy and the heat enterng the base metal s sgnfcant. The partcular weldng parameters and process selected for use also affect the metallurgcal response. In changng from one weldng process to another, large varatons n metallurgcal response can result despte the fact that the total nput energy of the two processes s dentcal. Shultz, Jackson, and others (Refs. 1, 2) have recognzed that total energy nput may predct only n a general manner the metallurgcal response; hence, other methods of control have been suggested. Shultz and Jackson (Ref. 1) show a correlaton between weld metal yeld strength and bead sze or weld nugget area. Several nvestgators have studed the relatonshp of weld metal yeld strength to weld metal coolng rates. The coolng rates for weld metal have WELDING RESERCH SUPPLEMENT! 2-s

2 Fg. 1 Energy dstrbuton at the weldng arc. E, = total energy nput/unt length; E, = total energy enterng plate; E m = energy to melt weld bead; E t = E-, x losses been determned expermentally by measurng the weld metal thermal curves wth thermocouples nserted nto the molten weld pool. nother approach used by many nvestgators has been to calculate temperature dstrbuton and coolng rates usng varous mathematcal heat transfer expressons (Refs. 3-6). In order to use these and other approaches to predct and control mechancal propertes, an understandng of the energy dstrbuton durng weldng s needed. Thermal Effcency Generally, effcency s expressed as the rato of output to nput energes. In ths nvestgaton, two thermal effcences are used to descrbe the energy dstrbuton n the weldng arc. These are referred to as the process effcency (Z t ) and the meltng effcency (Z m ). Process Effcency The process effcency (Z,) s defned as the rato of the total energy whch enters the plate or secton beng welded per unt dstance traveled, to the total energy nput of the arc over that same dstance. Ths relatonshp s stated as follows: where: Z = E, = E, = (E./E.) X 0 (2) Process effcency (%) Energy per unt dstance enterng the work Total arc energy avalable per unt length Ths relatonshp s mportant snce most heat transfer expressons de- rved for predctng weldng temperature dstrbuton and coolng rates depend upon the nserton of an effcency term for ther soluton. If the transfer effcency of the total energy nput s not consdered, the energy nput term does not represent the actual energy enterng the plate. Meltng Effcency Equally mportant s the relatonshp between meltng effcency and weldng parameters. The meltng effcency, Z m, can be defned as beng the rato of the energy requred to melt the weld bead to the total energy nput over the same dstance. Ths relatonshp can be expressed as: Z m = (E m /E.) X 0 (3) where: Z E, E, = Meltng effcency (%) Energy per unt length requred to melt the weld bead Total arc energy avalable per unt length The Equaton of Heat Dstrbuton Extensve studes have been carred out by Rosenthal (Refs. 4, 7), Rykaln (Ref. ), Chrstensen (Ref. 8) and others. Rosenthal mathematcally descrbed the weld thermal cycle for several of the most common weldng cases. Hs work s based on the heat conducton equaton as derved by Fourer. Rosenthal assumed In the soluton of ths equaton that: 1. Materal physcal coeffcents were constant. 2. The weldng heat source was a pont heat source. 3. Heat losses to the surroundng atmosphere were neglgble. Joule heatng effects n the plate materal were neglected. Ronsenthal has shown that for the case of weldng on the surface of a sem-nfnte body, the temperature dstrbuton can be expressed by the relatonshp: T-To = (P a where: T To = = Peff r = /27rkr). e exp. (-v/2a) (x + r) (4) Temperature of the pont n queston Intal temperature of the pont n queston Effectve power nput (x 2 + y 2 + z 2 )" 2 dstance from the pont n queston to the heat source where x, y, and z are the coordnates of poston (see Fg. 2). k = Thermal conductvty (Ref. 9) of the base materal whch s assumed to be a constant equal to 0.11 cal/(s) (cm 2 ) (deg C/m). a = Thermal dffusvty (Ref. 9) of the base materal whch s also assumed to be a constant equal to 0.12 cm 2 /s. v = Rate of travel of the heat source. By arbtrarly makng the ntal temperature level the zero level of temperature, the T 0 term may be elmnated and T becomes a measure of the temperature ncrease at the pont n queston. It s mportant to recognze the lmtatons of ths expresson. For example, as "r" approaches zero, the temperature "T" ncreases to nfnty, whch s an mpossble stuaton. Ths results from assumng an nfntely small pont heat source when dervng the equaton. Snce the weld bead whch acts as the heat source n the sem-nfnte body, s of fnte sze, the equaton s only vald at a dstance removed from the fuson area. Calculaton of Process Effcency, Z ; In the equaton for process effcency Z, (eq. 2) E, can be determned by usng eq. 1. E can be determned by usng Rosenthal's equaton for the heat dstrbuton n a sem-nfnte body n conjuncton wth a measured weld thermal cycle. Equaton 4 can be expressed n the form: T - To e exp.(-v/2a) 7E7 x +r) () where all terms have been defned, T-To for a partcular pont n the measured weld thermal cycle can be determned emprcally, and snce the rato of temperature to effectve 26-8 I JNURY 1 97

3 power nput s known from applcaton of Rosenthal's equaton, the effectve power nput needed to produce the measured thermocycle can be determned. The effectve power nput can be converted to joules per unt length for calculaton of the process effcency. Hence, E,(J/ In.) = P eff (J/s) (mn/n.) (6) 0r E (J/ mm) = P eff (J/ s) (s/ mm) (7) and Z can be determned by usng eqs. 1 and 6. Calculaton of Meltng Effcency, Z The weld metal volume represents the total amount of metal whch has reached the molten state. Ths volume has a heat content per unt length proportonal to ts cross-sectonal area whch remans constant along the length of the weld bead when constant weldng condtons are mantaned (Refs., 11). Based on measurements of the nugget area, the calorfc heat content of the molten weld bead per unt length can be calculated, compared wth the total arc energy avalable, and used to determne the thermal effcency of meltng. Ths s done by calculatng frst the heat content of a gram of ron heated to the average effectve maxmum temperature of the weld metal. The maxmum temperature of weld metal has been the subject of many nvestgatons. The effectve temperature must be above the meltng temperature of the base metal. The temperature at the weld metal-base metal nterface s at the meltng temperature of the base metal. The maxmum temperature s dependent upon the process (Refs. 8, 12, ) and has been generally reported at temperatures below 00 C wth some nvestgators suggestng temperatures up to C. The effectve temperature wll depend upon the arc envronment assocated wth the partcular process. For ths nvestgaton the effectve temperature to whch the molten metal has been heated s taken as 0 C. The heat content of a unt weght of molten weld metal then can be calculated as follows: The temperature of the weld metal - 0 C (23 K) The heat necessary to melt 1 gram of steel = q =. calores/ mole 00 weght (Ref. ) C p 23 So: q.61 + /o 23 dt q = =.9 mol. wt. kcal/ For Fe, mole weght =.8 g and 1 calore = joules Thus, q = (.94 kcal/mol. wt.) (4.186 J/cal) (- 3 cal/kcal).8 g/ mol. wt. (8) and fnally, q = 9.3 J/ gram Ths s the heat necessary to rase one gram of steel to a temperature of 0 C. Knowng the heat content of one gram of ron, the calculaton of the meltng effcency of the ron weld bead can be made. E t would be determned from the prevously stated relatonshp of eq. 1. E m would be determned from the weght of the weld bead per lnear mllmeter multpled by the above derved heat content per gram of ron and converted to unts comparable to those for E t. Thus, the weght of the weld metal/ lnear mm = nugget area (mm 2 ) (1 mm) (densty of ron) and the weght of weld metal/ lnear mm = na (mm 2 ) (1 mm) ( g/ mm 3 ). The heat content per gram of ron s 9.3 Joules, therefore: E m (J/ mm) = na(mm 2 ) (1 mm) ( g/ mm 3 ) (9.3 J/ g) (9) Now Zm can be calculated usng equatons 1 and 9. Objectve The objectve of ths nvestgaton was to examne and to show the effect of changes of weldng parameters on the energy dstrbuton durng weldng. The process effcency of the gas tungsten-arc weldng (GTW) process was studed. The program <L m(mm{{mtt was amed also at measurng the meltng effcency wth the GTW process. Expermental Procedure utomatc gas tungsten-arc weldng systems were used n ths nvestgaton to control the weldng varables of current, voltage and travel speed. standard chromel-alumel thermocouple, made from 28 gage wre, was capactor dscharge welded to the top surface of a one-nch thck steel base plate to measure the weld thermal cycle as the weldng arc passed ts poston. Measurements of thermocouple to weld centerlne dstance, thermocouple to weld edge dstance, and weld wdth were recorded. The weldng parameters used wth the gas tungsten-arc weldng process n ths nvestgaton are reported n Table I. /32 n. (4 mm) dameter 2% thorated tungsten electrode wth concal tpped vertex angles of to deg was used durng ths nvestgaton. The effects of varyng vertex electrode tp angle, sheldng gas composton, and current level on the process and meltng effcences were examned. ll weld samples were cross-sectoned and polshed. Photomacrographs were taken of the weld and heat-affected zone from whch nugget areas and heat-affected zone areas were determned. Usng the nugget areas, and applyng eq. 9, the calorfc heat content per unt length of molten weld metal was calculated for each sample. Ths heat energy was then compared wth the total heat energy developed per unt length by the weldng arc to obtan the meltng effcency. To determne process effcency, the peak temperature and poston were used n calculatng the effectve power nput. Snce Rosenthal's equaton was derved for a pont source travelng n a straght lne, and the molten weld bead nterface actually acts as the heat source, the edge of the weld bead was used to represent the Y coordnate dstance from the thermocouple to the lne heat source Heat Source q = H p - H Known quanttes are: / ' C p dt Thermocouple Poston H 18 oo =.61 klocalores/ weght (Ref. ) mole Fg. 2 Thermocouple poston as related to the heat source at the tme ot the peak temperature WELDING RESERCH SUPPLEMENT! 27-s

4 Zb l l l l I l 1 amp 1 Volts ^ pm (2.lmm/S) /l Vertex / 4I C / - - rgon Sheldng Gas L EC END O mp 12/ mm/s) D IO mp (42mm/s) 1 mp 16 3 mm/s! 0 mp (81 mm/s) n Fg. 3 r0 I I I I 2C 40 ^0 sc 70 Tme - Seconds Thermal cycle from weld sample number. 0 Electrode Tp Vertex ngle (degrees ) Fg. 6 Nugget area versus vertex angle for gas tungsten-arc surface welds made on HY-80 plate wth an electrode to work dstance of 0. n. (2.4 mm) and O mp (2 I mm/s) Q mp (4 2mm/s ) 1 mp (63 mm/s) 0 mp (84 mm/s) Vertex ngle (degrees) Fg. 4 Effect of sheldng gas and vertex angle on arc voltage for the gas tungstenarc process wth an electrode to work dstance ot 0. n. (2.4 mm) and on HY-80 plate as shown n Fg. 2. The dstance (r) between the thermocouple and the heat source was used n the calculaton of effectve power nput. Fgure 3 shows a typcal thermal curve as drawn by a X-Y recorder. The process effcency was determned by comparng the effectve power nput wth the total arc energy avalable per unt length. The bead sze or nugget area, heataffected zone area, process and meltng effcency were determned for each welded sample. These results are reported n Table 1. Varatons In rc Characterstcs Usng the GTW Process Varatons n sheldng gas composton and electrode tp vertex angle were nvestgated for the GTW welds made on HY-80 plate. n Increase n current causes the arc plasma to clmb the surface of the 28-s I JNURY 7 Helum Sheldng Gos pm (21 mm/s) pm ( 42 mm/s) 1 pm (6.3 mm/s) 0 SO pm (84 mm/s) 0 Electrode Tp Vertex ngle (degrees) Fg. Effect of vertex angle on meltng effcency for gas tungsten arc surface welds made on HY-80 plate wth an electrode to work dstance of 0. n. (2.4 mm) and concal tp of the electrode. The amount of clmb was found to be approxmately proportonal to the ncrease n current thus the current densty of the emttng surface remaned essentally constant. For a small electrode vertex angle, the amount of clmb necessary to obtan a certan emttng surface area on the electrode tp was greater than the amount of clmb necessary to mantan a gven emttng surface area wth a larger vertex angle. Snce the arc voltage drop s proportonal to the length of the arc column, the longer arc column produced wth the greater electrode clmb for the small vertex angles, produces a greater total arc voltage drop. Ths was found to be the case n ths nvestgaton as Illustrated by the results of Fg. 4. Results of smlar tests reported by Savage et al (Ref. 1) confrm these observatons. The effect of usng argon and helum sheldng gases on the arc voltage s also llustrated n Fg. 4. It should lkewse be noted that snce the arc voltage ncreases wth a decreasng vertex angle whle at a constant current of that the arc energy avalable also ncreases wth a decreasng vertex angle. s a result, the meltng effcency should ncrease wth ncreasng vertex angle. Ths trend was determned for gas tungsten-arc welds made wth ether helum or argon sheldng as shown n Fg.. Both of these cases ndcate that the heat dstrbuton at the plate surface vares wth changes n the vertex angle. s the vertex angle ncreases, a greater percentage of the heat avalable n the arc s used to melt the weld nugget. It s also nterestng to note that for the two cases where arc nstablty was observed, due to cathode spot wanderng on the deg electrode surface, that the effcency of meltng decreased as shown by the dashed lnes of Fg.. s shown n Fg. 6, the weld nugget areas remaned essentally constant wth changes n vertex angle. The process effcency results shown n Fg. 7 ndcate that the total heat enterng the plate materal remans essentally constant for changes In vertex angle. Snce the meltng effcency, as shown n Fg., ncreases wth ncreasng vertex angles, ths suggests that wth ncreasng vertex angle, an ncreasng percentage of the total heat enterng the plate materal s used to form the weld nugget. The rato of nugget area to heat-affected zone area should therefore ncrease wth Increasng vertex

5 Table 1 -- Weldng Condtons Used for Gas Tungsten-rc Weld Beads Made on One Inch Thck Base Plate,a) Weld no. Current" 3 ' Voltage, V Travel speed pm Vertex angle, deg Nugget area, n. 2 HZ area, n. 2 Total heat transfer effcency, Z, (%) Meltng effcency, Z m (%) (a) Welds 1 to 2 on HY80; 3 to 64 mld steel; and 6 to 67 HY 0. (b) Electrode negatve argon sheldng used n all cases except welds 2 to 42, nclusve were shelded wth helum. WELDING RESERCH SUPPLEMENT! 29-s

6 ^ o c ^- t/1 o Helum = O S pm 0 Opm / pm U Sheldnc U L EGEND 1 Gas (21 mm/s) ( 4.2 mm /s) (63 mm/s) rgon Sh eldng Gas H -8 a y o _fh - - pm(2.l mm/s) Travel Ipm (6.3mm/s) Travel Helum Sheldng Gas O rgon Sheldng Gas Helum Sheldng Gas O pm (21 mm/s) pm (42 mm/s) 1 pm (63 mm/s) rgon Sheldng Gas Electrode Tp Vertex ngle (degrees) Fg. 7 Process effcency versus vertex angle for gas tungsten arc surface welds made on HY-80 plate wth electrode to work dstance of 0. n. (2.4 mm) and 0 I Electrode Tp Vertex ngle (degrees) Fg. 9 Effect ot sheldng gas and vertex angle on meltng effcency wth electrode to work dstance of 0. n. (2.4 mm) and Weldng Current ( amps ) Fg. Process effcency versus weldng current for gas tungsten-arc surface welds made on HY-80 plate wth electrode to work dstance of 0. n. (2.4 mm) 40 Helum Sheldng Gas O pm (2 I mm/s) D IOpm (42mm/s) 1 pm (6 3 mm/s) O pm D pm 1 pm pm (HYI) HY80 Sheldng Gas Helum rgon rgon Sheldng Gas E N o c a> 0 -HYI 0 Electrode Tp Vertex ngle (degrees) Fg. 8 Rato of nugget to HZ area for gas tungsten arc surface welds made on HY-80 plate usng electrode to work dstance of 0. n. (2.4 mm) and angle. Ths result s shown n Fg. 8 and can be consdered sgnfcant snce the heat-affected zone usually represents the area of greatest change and varaton n propertes, and any method whch could offer an effectve means of reducng the sze of the heat-affected zone would be of practcal mportance. The effect of sheldng gas composton on the effcency of meltng for dfferent vertex angle at and 1 pm (2.12 and 6.3 mm/s) travel speeds s shown n Fg. 9. The greater LU cn c IO _ Mld V O Steel 3 pm 'pm pm rgon Weldng Current (amps) Fg. 11 Meltng effcency versus weldng current tor gas tungsten-arc surface welds made on HY-80 plate compared wth surface welds made on HY-0 plate and mld steel. Electrode to work dstance 0. n. (2.4 mm) -s I JNURY 7

7 effcency of meltng exhbted by helum at both travel speeds can agan be related to the arc characterstcs developed. In the arc column, a balance between power Input and radal heat losses exsts. Wth the use of helum, a hgher heat transfer effcency gas, the arc column contracts, reducng ts surface area n order to balance the heat losses wth energy nput. Ths contracton has the effect of ncreasng the current densty of the arc, and snce the voltage gradent has also ncreased, a hgh power densty resultng n hgher arc temperature s concentrated on a smaller surface area. The end result of ths heat concentraton s that a greater percentage of the heat avalable s used for meltng. Ths results n a hgher meltng effcency for helum sheldng. Effect of Weldng Parameter Varatons on Process Effcency Usng the GTW Process plot of process effcency versus weldng current shown n Fg. ndcates that a decreasng percentage of the total heat enters the plate wth ncreasng current levels when usng ether argon or helum sheldng gases. Ths ndcates that heat losses from the arc column ncrease more rapdly wth current level than does the percentage of the total heat enterng the base metal. The changes n the process effcency varyng from about 3% to 6% agree favorably wth results obtaned by Rykaln usng calormetrc methods (Ref. ). Effect of Weldng Parameter Varatons on Meltng Effcency Usng the GTW Process Varyng the current between 12 and 22 and mantanng the vertex angle at deg, the effect of current level on heat dstrbuton was examned. plot of meltng effcency versus weldng current for gas tungstenarc surface welds made on HY-80 plate, usng both helum and argen sheldng gas, s shown n Fg. 11. s expected from the prevous analyss on the effect of sheldng gas compostons, the helum-shelded weld deposts show a hgher meltng effcency than those made wth argon sheldng. lso of sgnfcance s the ncrease n meltng effcency wth ncreasng current and travel speed. The relatve ncrease n the dameter of the arc plasma at the plate surface s small n comparson wth the ncrease n current. Thus, the hgh heat concentraton resultng from the ncreased current densty results n an ncreased meltng effcency wth hgher current. The varatons n weld nugget area wth current level for the gas tung o 0.02 cn CT I! I Helum Sheldng O pm (2.1 mm/s) pm (42 mm/s) > 1 pm ( 6.3mm/s) / rgon Sheldng Weldng Current (amps) ^y^o Fg. 12 Weld nugget area versus weldng current for gas tungsten-arc surface welds made on HY-80 steel plate usng argon sheldng gas. Electrode to work dstance of 0. n. (2.4 mm) sten-arc process usng helum and argon sheldng gases are shown n Fg. 12. s would be expected, the weld nugget area ncreases wth the current level. It s to be noted that the weld nugget areas of the surface beads made wth helum sheldng are at least twce as large as those made wth argon sheldng at the same level of current and travel speed. Conclusons descrpton of energy dstrbuton durng gas tungsten arc weldng has been determned and presented n terms of two weld thermal effcences, the process effcency and the meltng effcency. Through an examnaton of these effcences, better understandng has been ganed as to how the weldng energy dstrbuton changes as a functon of weldng parameters and arc varatons. lso obtaned s an apprecaton of how these changes n weldng energy dstrbuton may affect the mechancal propertes of hardenable materals. In partcular, t has been shown that: 1. The process effcency remans constant wth changes n tungsten I 2 electrode tp vertex angle when usng the GTW process, but ncreased from approxmately 40% to % wth an ncrease n travel speed from pm (2.1 mm/s) to 1 pm (6.2 mm/s) wth ether helum or argon sheldng. The process effcency decreases by approxmately one-half from 6% to 3% wth ncreasng current level from 12 to 22 usng the GTW process and helum sheldng. The process effcency also decreases less drastcally wth ncreasng current level at a gven travel speed when argon sheldng s used wth the GTW process. 2. It s sgnfcant n these tests that the rato of the nugget area to the heat-affected zone area ncreases as the vertex angle of the tungsten electrode ncreases. 3. The meltng effcency ncreases wth an ncrease n the tungsten electrode tp vertex angle when usng the GTW process and ncreases wth a change n sheldng from argon to helum gas. The meltng effcency ncreases wth ncreasng current level and travel speed from approxmately % to 22% wth argon sheldng and from approxmately 22% to % wth helum sheldng when usng the N E E WELDING RESERCH SUPPLEMENT! 31-s

8 GTW process. The basc relatonshp and the sgnfcance of parameter changes n the GTW process should be useful as a gude n programmng studes for other arc processes. Other processes wll undoubtedly show a varaton n meltng and process effcences dependent upon weldng arc parameters and materal constants. cknowledgments The authors wsh to express ther apprecaton for the support provded by the Unon Carbde Corporaton, Lnde Dvson. Ther fnancal support provded the Fellowshp tund and donatons of equpment made the program possble. Techncal and materal assstance from others s also acknowledged. References 1. Shultz, B. L. and Jackson, C. E., "Influence of Weld Bead rea on Weld Metal Mechancal Propertes," Weldng Journal, 2 (1), Jan. 73, Res. Suppl., pp 26-s to 37-s. 2. Jackson, C. E. and Shrubsall,. E., "Energy Dstrbuton n Electrc Weldng," Weldng Journal, 49 (), May 1, Res. Suppl,, pp 231-s to 241-s. 3. dams, C. M., Jr., "Coolng Rates and Peak Temperatures n Fuson Weldng." Weldng Journal, 37 (), May 8, Res. Suppl., pp 2-s to 21-s. 4. Rosenthal, D., "Mathematcal Theory of Heat Dstrbuton Durng Weldng and Cuttng," Weldng Journal, (), May 41, Res. Suppl., pp 2-s to 234-s.. Rykaln, N. N., "Calculaton of Heat Flow n Weldng," translated by Zv Paley and C. M. dams, Jr., Contract Number UC C38 1, Moscow. 6. Hess, W. F., Merrll, L. L, Nppes, E. F. and Bunk,. P., "The Measurement of Coolng Rates ssocated wth rc Weldng and Ther pplcaton to the Selecton of Optmum Weldng Condtons," Weldng Journal, 22 (9) Sept. 43, Res. Suppl., pp 377-s to 422-s. 7. Rosenthal, D. and Schmerber, R., "Thermal Study of rc Weldng," Weldng Journal, (4), prl 38, Res. Suppl., pp 2 to Chrstensen, N., Daves, L. and Gjermundsen, K., "Dstrbuton of Temperatures n rc Weldng," Brtsh Weldng Journal, Feb. 6, pp 4 to Carslow, H. S. and Jaeger, J. C, Conducton of Heat n Solds, 2nd ed., Oxford Unversty Press, 9, p 26.. Jackson, C. E., "The Scence of rc Weldng," Weldng Journal, Part I, 39 (4), Res. Suppl., pp 129-s to 0-s; Part II, 39 (), Res. Suppl., pp -s to 1-s; Part III, 39 (6), Res. Suppl., pp 22-s to 2-s,. 11. Jackson, C. E. and Goodwn, W. J., "Effect of Varatons n Weldng Technque on the Transton Behavor of Welded Specmens, Part II," Weldng Journal, 27 (), May 48, Res. Suppl., pp 23-s to 266-s. 12. ndo, K. and Nshguch, K., "verage Temperature of the Molten Pool on TIG and MIG rc Weldng of Steel and lumnum," IIW Document , 69.. Jackson, C. E., "Fluxes and Slags n Weldng," Weldng Research Councl Bulletn 1, Dec Sms, C. E., Electrc Furnace Steelmakng, Vol. II, IME, 67, p Savage, W. F., Strunck, S. S. and Ishkawa, Y., "The Effect of Electrode Geometry on Gas Tungsten-rc Weldng," Weldng Journal, 44 (11), Nov. 6, Res. Suppl., pp 489-s to 496-s. 74 Revsons to Structural Weldng Code The 74 Revsons to Structural Weldng Code (WS Dl.l-Rev 2-74) contans the second set of authorzed revsons to the Structural Weldng Code, Dl For convenence and overall economy n updatng exstng copes of the Code, 88 pages of the Code have been reprnted, 9 of whch have been revsed to ncorporate changes. (The remanng pages are not changed but appear on the reverse sde of revsed pages.) To fulfll the needs of all Code purchasers, the 74 revsons are avalable as a bound book and as ndvdual looseleaf sheets. These are the prncpal changes n Code requrements: SMW fllet weldng of studs s now permtted. The prequalfed status of jonts welded by short-crcutng transfer GMW has been removed. Camber tolerances of welded members have been revsed. SNT qualfcaton of all NDT operators s now requred. ddtons and deletons have been made to the lsts of prequalfed steels for buldngs, brdges, and tubular structures. Brdge desgn crtera relatng to fatgue stress have been elmnated. Prces Dl.1-72 Structural Weldng Code $16.00 Dl.l-Rev Revsons to Structural Weldng Code $6.00 Dl.l-Rev Revsons to Structural Weldng Code $6.00 Dscounts: 2% to and B members; % to bookstores, publc lbrares and schools; 1% to C and D members. Send your orders to the mercan Weldng Socety, 1 NW 7th Street, Mam, FL Florda resdents add 4% sales tax. Be sure to specfy whether you want a looseleaf or a bound copy. 32-s JNURY 7