Weld Nugget Development and Integrity in Resistance Spot Welding of High-Strength

Cold-Rolled Sheet Steels

The relationship between sheet steel microstructure and welding cycles is investigated

BY Z. HAN, J. E. INDACOCHEA, C. H. CHEN AND S. BHAT

ABSTRACT. Spot welds were produced using a high-strength cold-rol led sheet steel 0.08-in. (1.9-mm) thick. The weld­ing parameters were systematically var­ied to examine their effects on nugget formation, microstructure, and mechan­ical properties. Following welding, sev­eral spot welds were cross-sectioned and mechanically polished for metallo­graphic examination. Other spot welds were submitted to shear tensile tests.

It is observed that welding current and weld time are more significant parame­ters than applied electrode force in af­fecting expulsion. In all the spot welds examined, it was found that solidif ica­tion cracking could be reduced but not eliminated, and it was observed that for this particular steel, a weld cycle of 1 8 and a holding cycle of 15 resulted in the least amount of solidification cracking. Nugget displacement to one side of the faying surface, also known as "stuck we ld , " was observed in some samples that showed banding from rolling. Prior to melting, it appears that solid-state metallic bonding occurs during nugget development.

Introduction

Resistance spot welding is a joining process in which coalescence of the

Z. HAN andj. E. INDACOCHEA are in the Civil Engineering, Mechanics and Metallurgy Dept., University of Illinois, Chicago. C. H. CHEN and S. BHAT are with Inland Steel Co. Research Laboratories, East Chicago, Ind.

Paper presented at the 71st AWS Annual Convention, held April 22-27, 1990, in Ana­heim Calif.

metal sheets is produced at the faying surface by the heat generated at the joint by the resistance of the work to the flow of electric current. This process has wide application in industries of mass produc­tion, where lengthy production runs and reliable conditions are maintained. The automotive industry is the major user, and it is also utilized by industries fabri­cating products with thin-gauge metals.

It is well known that high-strength cold-rol led sheet steels obtain their strength from a combination of fine grain size, solution strengthening and precipi­tation strengthening by means of mi­croal loying additions of t i tanium, nio­bium and vanadium (Refs. 1, 2). Exten­sive studies, both by automotive and steel companies, have demonstrated that these steels can be easily formed into the desired shapes with only moderate changes in press-shop practice. In addi­tion to their formability, it is imperative that these steels be weldable by resis­tance welding since this is the major

KEY WORDS

Weld Nugget Resistance Spot Weld HSLA Cold-Rolled

Sheet Steel Microstructure Mechanical Properties Expulsion Welding Current Weld Time Nugget Displacement

manufacturing process used on automo­tive assembly lines for joining sheet steel parts together. Since low-carbon steel is readily spot welded, it is therefore the standard by which all other materials wil l be judged. It is reasonable to expect that the closer the welding characteris­tics of the high-strength cold-rolled sheet steels are to low-carbon steel, then the smaller the changes in accepted weld­ing schedules.

The thermomechanical process of re­sistance spot welding is a complicated phenomenon which involves mechani­cal, electrical, thermal and metallurgi­cal factors. These factors in combination with the welding parameters have a sig­nificant influence on weld nugget devel­opment and final geometry. In order to consistently produce sound weld nuggets, it is necessary to understand these complicated phenomena and eval­uate the role of the major metallurgical factors and processing parameters.

Many high-strength steels are known to have narrow welding current ranges. Sometimes, this limited weldabil i ty is a consequence of the interfacial failure of the weld nugget (Ref. 3), producing an apparently smaller fusion zone. High-strength sheet steels have also been shown to be more susceptible to expul­sion because of their higher electrical resistivity (Refs. 4-6), hence, lower weld­ing currents must be used with the higher resistivities to avoid expulsion in spot weld production (Ref. 7). The physical variables of the metal may include not only the composit ion of the steels, but also the surface condi t ion. Surface ef­fects have been studied (Refs. 8, 9) and found to have noticeable effects on spot weldabil i ty. Likewise, chemistry varia-

WELDING RESEARCH SUPPLEMENT I 209-s

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13 18 28 33 38

Fig. 1

Weld Current (KA)

- Lobe curve for the sheet steel used in this investigation.

Weld Cvcles

Fig. 2 — Weld nugget size as a function of welding time (cycles) for two welding currents.

t i ons have been f o u n d to have n o t i c e ­ab le ef fects o n spot w e l d a b i l i t y (Refs. 9 -11 ) .

The o b j e c t i v e of th is invest iga t ion is to assess the ef fect o f p rocess ing char ­acteristics and sheet steel microst ruc ture on nugget deve lopmen t and integr i ty.

E x p e r i m e n t a l P r o c e d u r e

Spot w e l d s w e r e p r o d u c e d us ing h igh -s t reng th c o l d - r o l l e d sheet steel coupons o f d imens ions 4 X 1 X 0.08 in . (102 X 25.4 X 1.93 mm) . The steel c o m ­pos i t ion and mechan i ca l proper t ies are s h o w n in Tab les 1 and 2. The steel coupons were used in the as-rol led c o n ­d i t ion and we re c leaned tho rough ly w i t h a c e t o n e . Several spot w e l d s w e r e p ro ­cessed w h e r e o n e w e l d i n g var iab le w a s m e t h o d i c a l l y va r ied as the others w e r e he ld f i xed . A Sciaky press-type 100 kVA s ing le -phase res is tance w e l d i n g m a ­ch ine was used. The e lect rodes u t i l i zed w e r e a Class II c o p p e r a l l oy , of a c o m ­posi t ion of 9 9 . 2 % Cu and 0 . 8 % Cr. The t ips h a d a c o n t a c t d i a m e t e r o f 0 .31 in (7.9 m m ) and a 45 -deg t runca ted c o n e

Table 1 — Steel Composition, wt-%

Mn •\i

0.12 0.27 1.42 0.003 0.023 0.053 0.010

nose in a c c o r d a n c e to the manu fac tu r ­ing standards o f the Ford M o t o r Co. (Ref. 12).

F o l l o w i n g w e l d i n g , t he spot w e l d s were cross-sect ioned and mechan i ca l l y p o l i s h e d fo r m e t a l l o g r a p h i c e x a m i n a ­t i o n . A s o d i u m t r y d e c y l b e n z e n e su l -fonate plus p ic r ic ac id etchant was used to reveal the so l id i f i ca t ion structure, and 2 % nital was used to b r ing ou t the phase m ic rocons t i t uen t s . A LECO 3 0 0 m e t a l ­l og raph w i t h an image ana l yze r was used for the micros t ruc tura l eva lua t ion .

Results a n d Discussion

Effect of Welding Parameters on Nugget Characteristics

In t h e c o n t i n u e d des i re t o increase p r o d u c t i v i t y , it has been demons t ra ted that t he m a x i m u m w e l d i n g speed that can be a t t a i ned increases as the sheet thickness is reduced (Ref. 1 3). However , this increase in w e l d i n g speed has to be c o m p r o m i s e d w i t h the possib i l i ty o f ex­p u l s i o n , w h i c h occurs at h igh cur ren ts (Ref. 14). This expu ls ion is a result of the

Table 2 — Steel Tensile Properties

Yield Strength: 80 ksi Ultimate Tensile Strength: 90 ksi Elongation: 18"o

excessive heat ing and of the mo l ten and plast ic de fo rmat ion o f the meta l , next to the nugget , ex tend ing b e y o n d the e lec­t rode c o n t a c t z o n e w h i c h is i nade ­qua te l y f o r g e d . Th is p h e n o m e n o n has generated conce rn a m o n g manu fac tu r ­ers because o f the poss ib le c o n n e c t i o n w i t h lack of w e l d s t rength and rap id e l e c t r o d e d e t e r i o r a t i o n (Ref. 15) . Th is has led to the d e t e r m i n a t i o n o f opera t ­ing ranges over w h i c h accep tab le spot w e l d s are o b t a i n e d o n p l a i n c a r b o n sheet steel.

A g raph ica l representat ion of ranges of w e l d i n g var iables over w h i c h accept­able spot we lds are fo rmed on a speci f ic mater ia l w e l d e d w i t h a preselected elec­t rode force is k n o w n as a "spot w e l d lobe c u r v e " (Ref. 16) . C h a r a c t e r i z a t i o n o f such cu rves mus t be m a d e i f e f fec t i ve u t i l i z a t i o n o f w e l d i n g parameters is sough t to cons i s ten t l y p r o d u c e s o u n d spot we lds . The spot we lds de f in ing the l ower l im i t o f the lobe cu rve are sma l l , and in some cases unders i ze , resu l t ing in w e a k e r nuggets. O n the o ther h a n d , the spot w e l d s at the upper l i m i t o f the c u r v e are larger, and q u i t e f r e q u e n t l y , t hey e x p e r i e n c e e x p u l s i o n , w h i c h d e ­creases w e l d s t rength and increases e lect rode wear . Figure 1 shows the lobe curve deve loped for the steel used in this invest igat ion.

T h e c ross -sec t ion areas o f t he spot w e l d spec imens used to c o n s t r u c t the

2 1 0 - s I M A Y 1 9 9 3

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Fig. 3 — Cross-sectional versions of spot welds with (left) and without (right) expulsion.

been plotted as a function of the weld cycle in Fig. 2 for two welding currents (10 k A a n d 1 2 kA) at a constant elec­trode force of 1 600 Ib (727 kg). As ex­pected, the tendency is for the nugget to increase in size with longer weld cycles up to a point, beyond which it may ei­ther remain constant or experience ex­pulsion, causing the normal weld nugget to be thinner since the rest of the liquid weld metal is expelled beyond the usual nugget containment. The spot welds pro­cessed with a 10-kA welding currentsus-tained a gradual increase in nugget size up to a weld cycle of 23. Note that the nugget size of the spot welds corre­sponding to weld times of 28 and 33 cy­cles remained constant and did not ex­perience expulsion. The 12-kA spot welds increased in size at a faster rate, but experienced expulsion which re­sulted in a thinner nugget thickness as seen in Fig. 2. Figure 3 shows two typi­cal spot welds corresponding to weld cycles of 1 8 and 23 for an applied cur­rent of about 12 kA. Note the occurrence of expulsion and deep indentation for the 23-cycle spot weld specimen. It is apparent that external indentation of the steel sheet is a consequence of nugget expulsion.

The effect of applied electrode force on nugget characteristics was monitored also in terms of nugget size and whether expulsion occurred, as shown in Fig. 4. The increase of electrode force is ac­companied by a small but continuous reduction in nugget size, clearly ob­served for the 10-kA weld samples. This small decrease in the weld nugget cross-section area could be explained, in part, to be due to the fact that the increase in applied force would result in a minor decrease in the sheet thickness and in a larger increase in contact area at the fay­ing surface in view of the deformation of the asperites. Both factors w i l l con­

tribute to a small reduction of the total resistance of the system, leading to a re­duction in the amount of Joule heat. The 1 2-kA spot welds all showed expulsion, and no consistent relationship could be established between cross-section area size and applied electrode force. The weld time used for processing these last spot welds was 23 cycles. Note that pre­viously it was determined that such weld time at 12 kA resulted in expulsion (Fig. 2). Examining the data presented in Fig. 4 for the same welding time and several applied electrode forces and for two welding currents, expulsion only oc­curred in those spot welds processed at the higher welding currents. Such results indicate that expulsion is not dependent only on the applied force. In the case of the 10-kA current, no expulsion was found even when the applied force was as low as 1400 Ib (636 kg). On the other hand, for the spot welds produced at 12 kA, expulsion was found even in those welds produced at 2200 Ib (1000 kg). It is apparent then, that the welding cur­

rent and time are the most sensitive welding parameters to cause expulsion.

Weld Nugget Microstructure and Mechanical Properties

The microstructure of all these spot welds is martensitic and their solidifica­tion structure is influenced by the de­gree of constitutional undercooling. The solidif ication structure is cellular near the fusion line, followed by a columnar dendritic structure and equiaxed den­drites if expulsion occurs at the faying surface, as seen in Fig. 5. It was estab­lished that the microhardness difference between the cellular, columnar and equiaxed structures is more dependent on the amount of carbon available than on the magnitude of the cool ing rates. The carbon content should be low for the cellular structure because solidifica­t ion begins at this location and conse­quently the solute segregation is low. The martensite corresponding to the columnar dendrites would have a higher

20

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Fig. 4 — Effect of elec­trode force on nugget size for two applied currents.

1000 1200 1400 1600 1800 2000 2200

Electrode Force (Lbs.)

W E L D I N G RESEARCH SUPPLEMENT I 211-s

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Heat-affected Zone,VHN 427

Echant: Sodium Trydecy[benzene Sulfonate

Weld Paremeters: P=1600Lbs.. 1=12.49 KA, t=23 Cycles

X400 Cokimrwr Grains. VHN 482

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msm x400

Faying Interface

Fig. 5 — Solidification microstructures observed in a weld nugget of the high-strength cold-rolled sheet steel.

and a more uniform carbon distribution, because of the steady-state solidif ica­tion. The highest microhardness was en­countered in the equiaxed region, ap­parently, the result of the largest carbon segregation, which is expected in this region as a result of the overlapping of the opposite solidification fronts.

A defect frequently present in the weld nugget is solidif ication cracking. Several factors affect its occurrence, e.g., impurity segregation, cooling rate and processing stresses. In this investigation, the effect of cool ing rate on solidif ica­tion cracking was studied by varying the holding cycle and by maintaining the weld current at 12 kA, the electrode force at 1 600 Ib, and the weld time at

18 cycles. Several spot weld macro­graphs corresponding to different hold­ing cycles are shown in Fig. 6. It is ob­served that by decreasing the holding time from 60 to 1 5 cycles there was a decrease in the extent of solidif ication cracking. However, as the holding time was reduced below 1 5 cycles, there was again an increase in solidification crack­ing, at the faying surface, probably caused by the relaxation of the system (withdrawal of the electrodes) before a large portion of the liquid metal solidi­fied. A relative measurement of the total crack length on these samples was per­formed and plotted as a function of hold­ing cycles, as shown in Fig. 7. Note that the amount of solidification cracking for

th=60Cycles th=15Cycles th=10Cycles

th = 7Cycles th = 5Cycles th = OCyeles

Fig. 6 — Effect of holding time on solidification cracking in spot welds.

holding times lower than 1 5 cycles was greater than for the commonly used 60 cycles. Holding cycles lower than 1 5 apparently do not al low sufficient time for the weld nugget to solidify thor­oughly before the electrodes are wi th­drawn, and consequently, cracking re­sults upon release of the electrodes.

Tensile shear tests were performed on these spot weld samples to establish the effect of solidif ication cracks on shear strength. These results, presented in Fig. 8, show no change in tensile shear strength among the spot welds processed with holding times of 60 and 30 cycles. But, there was a slight drop in strength for the welds processed wi th holding times of less than 1 5 cycles, yet it was not significant enough to be correlated with the degree of solidification crack­ing. However, it was established that weld time was most influential of nugget strength as shown in Fig. 9. The tensile shear strength of the spot welds in­creased with weld time, consistent with an increase in nugget size, as shown in Fig. 2. Furthermore, the spot welds with expulsion show lower strengths, as ex­pected, because of the smaller nuggets compared to the spot welds with no ex­pulsion and due to the effect of inden­tation.

Despite the lack of correspondence between solidification cracking and spot weld shear strength, it would be ex­pected that this type of cracking can have an impact on the soundness of the nugget under more dynamic testing,

212-s I M A Y 1993

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Holding cycles Fig. 7 — Relative solidification crack lengths in spot welds as a func­tion of welding cycles.

0 15 30 45 60

Holding Cycles

Fig. 8 — Effect of holding cycles on the tensile shear strength of spot welds.

such as fatigue. Fatigue tests wi l l be con­ducted in the follow-up investigation.

Weld Nugget Displacement

In assessing the influence of resis­tance welding processing parameters on nugget characteristics, it was observed that, occasionally, the weld nugget was displaced to one side of the faying sur­face. This type of spot weld is sometimes known as "stuck weld" (Ref. 1 7), which is most frequently observed when weld­ing sheet metals of dissimilar thick­nesses.

Figure 1 0 shows several macrographs of weld nuggets arranged according to variations in weld time for two welding currents, and also as a function of cur­rent for a fixed weld time of 18 cycles. A constant electrode force of 1 600 Ib was used in producing these spot welds. It was observed for the series of spot welds processed with a 12-kA current that expulsion was eliminated and the nugget size increased when the weld time was reduced from 23 to 1 8 cycles. Further decrease of the weld time to 1 3 cycles, resulted in smaller nuggets be-

100

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3

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25

Welding Parameters: P=1600Lbs., I=12KA, and th=60Cyls.

4

• a Pull-out failure A a Faying surface failure

» A Expulsion

Fig. 9 — Effect of welding cycles on the tensile shear strength of spot welds.

13 18 23

Weld Time, Cycles

28

WELDING RESEARCH SUPPLEMENT I 213-s

Fig. 10 — Effect of welding parameters on weld nugget soundness and appearance.

. ' V2u"un

Fig. 11 — Micrograph composite showing "banding" in the sheet steel and the nugget dis­placement.

cause of the lower heat per time avail­able due to the shorter current flow time. This same set of spot welds, however, showed no significant displacement of the weld nugget. In the case of the 10-kA spot welds, no expulsion occurred, and also, a decrease in nugget size ac­companied the reduction of weld time. Most significant, though, was the shift­ing of the spot weld to one member of the steel sheets for the shortest weld cycle, as seen in Fig. 10. Similar results were found for the welds processed at the fixed weld time of 18 cycles and three weld currents. The greatest dis­placement of the weld nugget occurred for the lowest weld current of 8.5 kA.

In investigating the possible causes of nugget displacement, the sheet thick­nesses were measured and found to be uniform in all cases. The copper elec­trodes were also examined for any pos­sible irregularities that could affect the nugget formation, but were found to comply wi th specified dimensions, in accordance to the Ford Motor Co. man­ufacturing standards.

A microstructure feature observed in the base metal, "banding" shown in Figs. 11 and 1 2, is speculated to contribute to the nugget displacement. Resistivity is an intrinsic material characteristic which depends on the composition and microstructure of the material. The re­sistivity of a metallic conductor is known to increase with the concentration of a solute. Banding, on the other hand, in­cludes not only solute segregation, but also pearlite lamination, both of which can disrupt the continuity of the ferrite and consequently increase its resistiv­ity. Melting would then occur at a band­ing region if the resistance at this loca­tion is larger than that at the faying sur­face.

Figure 11 shows a displaced weld nugget cross-section that includes band­ing in the base metal. Note that band­ing is observed on both steel sheets of the spot weld, and the microstructural gap that this banding causes seems to be wider than the gap found at the fay­ing surface. The nugget is displaced to­ward the bottom sheet steel, because ap­parently this microstructural irregularity ran continuously across the portion of the material compressed between the copper electrodes. The top steel sheet also shows banding, but this does not run all the way across the material squeezed between the electrodes.

A second possibility for the nugget displacement might be the presence of low-melting-temperature compounds such as iron sulfide and manganese sul­fide with the banding microstructure. These compounds have melting points well below the melting temperature of iron, thus, despite the temperature at the

214-s I M A Y 1993

faying surface being several degrees higher, melting wi l l commence at these binding regions because the local tem­peratures are already above the melting points of such compounds.

Figure 12 shows the weld nugget cen­tered at the faying surface, despite ex­tensive banding on the two sheet mem­bers; however, no banding lines were found running continuously across the material between the copper electrodes.

Formation of the Weld Nugget

The electrode force produces a local deformation at the common interface to seat the workpiece properly and to es­tablish good electrical contact before current flows. This local deformation at the faying surface mechanically breaks down the surface films, oxides and as-perites. It is understood that, during the welding cycle, the electrode load also helps to maintain proper electrical con­tact. The contact areas at the faying sur­face are dependent on the stress distri­bution and local deformation, thus, re­gions exposed to the largest stresses would be in tighter contact since the as-perite gaps become reduced resulting in lower resistances. As the current starts f lowing, the temperature at the faying surface increases causing further defor­mation and greater contact that could lead to solid-state bonding in some places. It should be noted that the cur­rent distribution at the faying surface is not uniform, since it also depends on the electrode geometry.

Figure 1 3 shows the state of the fay­ing surface prior to any melting. Regions B and D clearly show metallic bonding in these areas without melt ing taking place, whi le region C shows a gap. It is apparent that with time melting wil l ini­tiate at the center and w i l l spread out­ward. These results, shown in Fig. 13 are consistent with a nonuniform but sym­metrical stress distr ibution. Neid (Ref. 18) determined the stress distribution at the faying surface for a truncated elec­trode, similar to the electrode used in this investigation, where the stresses are highest at the regions corresponding to the edge of the electrodes and lowest at the center. Furthermore, because of the electrode geometry, the current f low lines would be more constricted near the edges, implying larger current den­sities at these edge locations than at the center of the electrode, as schematically represented in Fig. 14.

Based on the combination of these two factors, it could be speculated that, at the onset of welding, heat would be produced at a faster rate at the edges of the forming nugget, and because of the higher stresses at these locations, defor­mation wi l l occur more rapidly, leading

Fig. 12 — Banding in the sheet steel with nugget centered at faying sudace.

to solid-state bonding. As a result of this bonding, the contact resistance drops to zero at the edges, but a minor gap or high-resistance interface still remains at the center of the faying surface which does not completely stop the current f low. Melting could then initiate at this center spot and expand outward to the edges.

Conclusions

1) In addition to composition and sur­face condition, the microstructure of the steel is an important physical variable in resistance spot welding.

2) Welding current and welding time are the most sensitive parameters to con­trol expulsion. Tighter control of either parameter w i l l improve weld nugget soundness and reduce expulsion, and thus improve electrode wear.

3) Holding cycles influence the ex­tent of solidif ication cracking. In this study, it was found that holding cycles smaller or greater than 15 produced weld nuggets with larger amounts of so­l idif ication cracks. Despite the small changes in room-temperature tensile shear strength found in this study, it is expected that fatigue and toughness wil l

Fig. 13 — Microstructural characterization ot different regions of the faying surface prior to the formation of the weld nugget.

W E L D I N G RESEARCH SUPPLEMENT I 215-s

Fig. 14 — Schematic representation of the

current distribution between electrodes.

Electrode

Steel sheets

be af fec ted by the a m o u n t o f th is t ype of c rack ing .

4) It appears that sol id-state b o n d i n g at the edge o f the c o n t a c t sur face , f o l ­l o w e d by m e l t i n g at t he cen te r are the first steps in nugget deve lopment .

Acknowledgments

The authors a c k n o w l e d g e the f i n a n ­c ia l suppor t o f In land Steel C o . W e are also grateful to Dan Ca l ihe r for car ry ing o u t the spot w e l d s at I n l a n d Steel Re­search Laborator ies.

References

1. Greday, T., and Lamberigts, M. 1975. The combined effect of microalloying steels

with columbium and vanadium. Microalloy­ing 75, Proceedings, p. 172. Washington, D.C.

2. Vlasov, N. N. 1975. Microal loy ing of carbon steels with vanadium and cobalt. M i ­croalloying 75 Proceeding, p. 188. Washing­ton, D.C.

3. Sawhi l l , J. M., and Baker, J. C. 1980. Spot weldability of high-strength sheet steels. Welding Journal 59:19-s to 30-s.

4. Pollard, B. 1974. Spot welding charac­teristics of HSLA steel for automotive appl i­cation. Welding Journal 53(8)'343-s to 350-s.

5. Schumacher, B. W. 1982. Resistance spot weld ing of LC and HS steels. Metal Progress, pp. 30-36.

6. Yamanchi, N., and Taka, T. 1980. ITW Doc. 111-644-80.

7. Sawhil l , J. M., Watanabe, H., and

Mitchell, J. W. 1977. Spot weldability of Mn-Mo-Cb, V-N, and SAE 1008 Steels. Welding Journal 56:21 7-s to 224-s.

8. Savage, W. F., Nippes, E. F., and Was-sell, F. A. 1978. dynamic contact resistance of series spot welds. Welding Journal 57(2):43-s to 50-s.

9. Dickinson, D. W., HaserJ. M.,and Ries, G. D. 1975. Spot weldabil i ty comparison of selected HSLA steels. Republic Steel Research Report 12055-8.

1 0. Johnson, K. I. 1973. Quality c o n t r o l -resistance weld ing qual i ty—control tech­niques. British Welding journal 20:1 76-1 8 1 .

11 . Han, Z., Orozco, J., Indacochea, J. E., and Chen, C. H. 1989. Resistance spot weld­ing: a heat transfer study. Welding journal 68: 363-s to 371-s.

12. Ford Laboratory Test Methods. 1980. Schedule BA 13-4.

1 3. Yamamoto, T „ and Okuda, T. 1 974. Study on seam welding phenomena and their application to speed up of seam welding. IIW Doc. 111-504-74.

14. Welding Handbook, Vo l . 18th edi ­tion, American Welding Society, pp. 373.

1 5. Kirnchi, M. 1 984. Spot weld proper­ties when welding with expulsion — a com­parative study. Welding Journal 63:58-s to 63-s.

1 6. Metals Handbook, Vol. 6, 9th edition, ASM International, Materials Park, Ohio, pp. 478.

17. Dix, F. ). 1968. Metallurgical study of resistance weld nugget formation and stuck welds. British Welding Journal 15(11:7-16.

1 8. Nied, H. A. 1984. The finite element modeling of the resistance spot welding pro­cess. Welding journal 63:123-s to 1 32-s.

WRC Bulletin 369 December 1991

Nitrogen in Arc Welding — A Review

By IIW Commission II

In 1983, Commission II of the International Institute of Welding (IIW) initiated an effort to review and examine the role of nitrogen in steel weld metals. The objective was to compile in one source, for future reference, the available informa­tion on how nitrogen enters weld metals produced by various arc welding processes, what forms it takes in these welds, and how it affects weld metal properties.

This bulletin contains 13 reports and several hundred references related to Nitrogen in Weld Metals that has been pre­pared as a review to show the importance nitrogen has in determining weld metal properties.

Publication of this report was sponsored by the Welding Research Council, Inc. The price of WRC Bulletin 369 is $85.00 per copy, plus $5.00 for U.S. and $10.00 for overseas, postage and handling. Orders should be sent with pay­ment to the Welding Research Council, Room 1301, 345 E. 47th St., New York, NY 10017.

216-s I MAY 1993