Spot Weldabi l i ty of High-Strength Sheet Steels

Two low-cost high-strength sheetsteels are satisfactorily resistance spot welded with slight modifications of

welding practice for plain carbon steels

BY j . M. SAWHILL, JR., AND J. C. BAKER

ABSTRACT. Rephosphorized steels and stress-relieved annealed (SRA) steels have recently been introduced in the automotive industry to permit thickness reductions when replacing plain carbon steels and thus reduce weight and improve fuel economy. The rephosphorized steels are strengthened by phospho.rus in solid solution and are typically used in parts requiring good formability. The SRA steels are plain carbon steels that achieve even higher strengths in com­bination wi th moderate formabil ity by control of the cold-rol l ing and recov­ery/annealing process. Since neither group of steels requires elaborate heat treatments or costly alloying, they are low in cost; therefore, a strong incen­tive exists for establishing the spot weldabil ity of these steels.

Our tests showed that, compared with plain carbon steel, the rephos­phorized steels have greater weld ten­sile shear strengths proportionate to their higher base-metal strength. Weld direct tension strengths were found to be 0.4 to 0.6 times the tensile shear strength, in line with most high-strength low-alloy (HSLA) steels. Static and dynamic weld toughness proper­ties proved to be similar to those of plain carbon steel welds, and fatigue properties at 10s cycles were also found to be no different than those of plain carbon steel welds.

In all tests, samples failed by a pull-out mode, although some interfacial fracture frequently preceded pull-out. There was no evidence that interfacial fracture, before pull-out, affected me­chanical properties provided an ade­quate-sized weld was obtained. By slightly increasing weld t ime and elec­trode face diameter, all rephospho­rized steels tested could be welded

over a wide current range and meet stringent quality control requirements on button size in a peel test.

Commensurate with their higher base-metal tensile strength, spot welds in the SRA steels also had higher ten­sile shear strengths than plain carbon steel welds. The direct tension strength was l imited by a soft region in the heat-affected zone and was con­sistently 0.4 times the tensile shear strength. The performance during peel or chisel testing indicated that these steels could be welded with the same parameters used for plain carbon steel and still meet the most severe automo­tive requirements.

Introduction

In the automotive industry there has been a significant increase in the use of high-strength steel sheet to permit reductions in thickness and thus in vehicle weight. The substitution of high-strength steels for thicker plain carbon steels helps to lower weight and meet federally mandated im­provements in fuel economy.

In addition to satisfying strength, formability, weldabil i ty, and paintabil-ity requirements, steels for high-vol­ume applications should be low in cost. Both the rephosphorized and stress-relieved annealed (SRA) steels recently introduced in the automotive industry meet this requirement, be­cause neither uses the complex heat

Paper presented at the AWS 60th Annual Meeting held in Detroit, Michigan, during April 2-6, 7979.

/. M. SAWHILL, JR., and I. C. BAKER are with the Research Department of Bethlehem Steel Corporation, Bethlehem, Pennsylva­nia.

treatments or costly alloying required for high-strength low-alloy (HSLA) and dual-phase steels.

Since resistance spot welding is the principal method of joining automo­tive sheet steels, this study was con­ducted to determine the spot welda­bility of both rephosphorized and SRA steels. Specifically, our objectives were threefold:

1. To determine the ease of produc­ing sound spot welds as determined by a conventional quality control test.

2. To evaluate the serviceability of spot welds in these steels under a range of loading conditions.

3. To relate the weld properties to geometric and metallurgical factors.

The mechanical tests and the quali­ty-control peel test showed that, wi th little or no modifications in standard welding procedures, the ease of obtaining a sound weld and the level of mechanical properties obtained for these two high-strength steel grades are equal to or better than those in plain carbon steel welds. Thus, •from the standpoint of weldabil i ty both the rephosphorized and SRA steels are suitable as weight-reducing substi­tutes for plain carbon steels.

Experimental Program

The experimental program com­prised:

1. Conventional peel tests to deter­mine the opt imum conditions for pro­ducing a sound weld.

2. Static, dynamic, and cyclic tensile tests to determine weld serviceability.

3. Metallographic examinations to relate the metallurgical factors and the mechanical properties.

Various mil l-produced steels from the two grades, rephosphorized and

W E L D I N G RESEARCH SUPPLEMENT I 19-s

SRA, were selected to permit evalua­tion of a wide range of chemistries and strengths.

Materials

The rephosphorized steels evaluated in this paper are cold-rolled and batch-annealed sheet steels that contain phosphorus for solid-solution strengthening. Table 1 gives the com­position and strength ranges of the steels evaluated. They were all killed steels produced in gages below 0.04 in. (1 mm) for body sheet. Some steels were mil l-produced in experimental trials. Rephosphorized steels produced to a specification for high-strength applications typically have 30 to 60% higher yield strength than plain carbon steels. In spite of their higher strength, the good formabil ity of the rephos­phorized steels permits them to be substituted for plain carbon steels in most applications.

The stress relieved annealed (SRA) steels are usually produced in heavier thicknesses and at higher strengths. They are plain carbon steels, typical of an AISI 1008 composit ion, that gain their strength through control of the cold-roll ing and annealing processes to produce a recovered and partially annealed microstructure. The SRA steels are produced in yield strengths ranging from 50 ksi (345 MPa) to above 80 ksi (550 MPa) in combination wi th

Table 1—Chemistry and Tensile-Property Range of Rephosphorized Steels Evaluated

Chemistry Range

C: Mn: Si: S: P: Al: N: Tensile Property

Yield strength:

Ult imate tensile strength:

Total elongation:

0.03-0.09% 0.3-0.6% 0.01-0.30% 0.015-0.030% 0.04-0.12% 0.01 -0.09% 0.005-0.025%

Range

30-45 ksi (200-300 MPa)

50-65-ksi (350-450 MPa)

30-36%

moderate formability. Because of their properties, the SRA

steels are more suitable for thicker parts, such as auto door bars, that require higher strength. In contrast, the rephosphorized steels are used for thinner parts requiring better formabil­ity, such as body panels.

Procedures

Welding. Spot Welding was per­formed wi th a pedestal machine using conditions that would be practical during high-volume, high-speed pro­duction. Welding current was moni­

tored wi th a Duffers meter, and elec­trode force was measured wi th a load cell and digital force indicator. Actual weld and hold times, as opposed to the controller's settings, were checked by monitoring voltage across the elec­trodes w i th a high-speed recorder.

Hold time is the t ime between cur­rent cutoff and lift ing of the electrodes from the sheet. A longer hold time produces a faster cooling rate in a spot weld and, therefore, higher hard­nesses. In highly alloyed steels, spot welds produced with a long hold t ime may undergo a degradation in proper­ties. We deliberately selected long hold times, i.e., 30 to 60 cycles, a range recognized by automotive companies as representing the most severe condi­tions for spot welding high-strength steels.

Tensile Testing. The specimens used for tensile tests are illustrated in Fig. 1. Static strength was measured using both conventional tensile-shear and direct-tension (U-tension) specimens. Specimen details and testing methods conformed to AWS practice.1

Fatigue Testing. The fatigue proper­ties of spot welds were determined using the same tensile-shear and direct-tension specimens employed for static testing—Fig. 1. Constant-load-amplitude tests were performed according to ASTM recommended practices'23 at a frequency of 5000 cycles/min using tensile/tensile load-

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A. TENSILE SHEAR

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0 (25)—•• All dimensions in inches(mm)

B. DIRECT TENSION

Fig. 1—Specimen configuration for static and fatigue loading

All dimensions in inches (mm)

Fig. 2—Spot-weld impact specimens B. DIRECT-TENSION IMPACT SPECIMEN

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KNIFE EDGE

mm

Fig. 3—Instrumented peel test

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Carbon Steel

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DISTANCE FROM FUSION LINE,inch Fig. 5—Hardness traverses in a spot weld between rephosphorized and plain carbon steel

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Iftu/m

Fig. 4—Base metal microstructures for killed plain carbon steel and rephosphorized steel: A-0.07C; B-0.07G-0.10P. 2% nital etch. xlOOO (reduced 50% on reproduc­tion)

ing wi th a stress ratio R of 0.1, i.e., minimum load is 10% of maximum load. The end of the test was reached when complete separation of the specimen occurred.

Welding conditions were adjusted to achieve the same weld diameter wi th in ± 0.005 in. (0.12 mm) for steels of the same thickness. Duplicate spec­imens at the start and end of each test series were sectioned and etched to allow measurement of the fused-area diameter and verify that it was wi th in the desired range.

Toughness Testing. Impact proper­ties were also determined both parallel and perpendicular to the sheet sur­face—Fig. 2. For both types of speci­mens strongbacks 0.25 in. (6 mm) thick

were used to minimize bending of each piece during the test.

Tests were also performed using a newly developed technique for mea­suring toughness during peeling. A peel specimen (Fig. 3) was bent at the weld and equipped with knife edges to which a clip gage was attached to monitor displacement adjacent to the weld. The specimen was then pulled to failure in a tensile machine, giving a plot of load vs. displacement.

This instrumented peel test allows one to observe better what happens during the peel test, as wi l l be demon­strated in the results. It also permits comparison of the energy absorption for different steels during peeling since the area under the curve gives the energy absorption.

Peel Tests. Conventional peel tests were performed on two 1 X 4 in. (25 x 100 mm) sheets. Welding condi­tions were varied beyond those typi­cally used for plain carbon steel in order to determine the specific condi­tions that wou ld produce wide current ranges based on the peel test.

In some cases, the failure mode dur­ing the peel test was a combination of interfacial fracture into the weld metal and pull-out. Although in these cases the total fused diameter was larger, evaluation of current range in the peel test was based on button diameters alone as in normal production situa­tions.

Metallography and Hardness Tests. Untested weld specimens as well as tensile, fatigue and peel specimens unloaded just after crack initiation were sectioned and prepared for me-tallographic examination. All speci­mens were observed optically, and some were subjected to various elec­tron microscopic techniques including scanning (SEM), scanning transmission (STEM), and Auger microscopy.

Vickers hardness traverses of the weld were obtained using a 1.0 kg (2.2 lb) load and oriented parallel to the interface between the sheets. Weld metal hardnesses were averaged from at least five impressions performed along a diagonal in a weld metal cross section.

W E L D I N G RESEARCH SUPPLEMENT I 21-s

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F/g. 6—Weld-metal microstructures of plain carbon steel and rephosphorized steel for different hold times. A-0.07C, 10 cycle hold; B-0.07C, 30 cycle hold; C-0.06C-0.09P, 10 cycle hold; D-30 cycle hold. 4% nital etch. xlOOO (reduced 46% on reproduction)

Results and Discussion

A more detailed discussion is de­voted below to the welding results for the rephosphorized steels, since in contrast wi th the SRA grade they are not identical in composit ion with the plain carbon steels.

Rephosphorized Steels

Microstructures and Hardness. M i ­

crostructures of the base metals in a 0.07% C plain carbon steel and in a 0.07% C, 0.10% P steel are compared in Fig. 4. Both base metals contain poly­gonal ferrite grains wi th carbides and some inclusions that, in the case of the rephosphorized steels, would of course contain somewhat higher phosphorus. Since most of the phos­phorus is dissolved in the ferrite, it causes an increase in the base metal hardness from 100/110 HV for the

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plain carbon steel to 140/170 HV1.0 for the rephosphorized steel.

Hardness traverses shown in Fig. 5 were performed on opposite sides of the interface in the same spot weld joining a rephosphorized to a plain carbon steel. This technique ensured that both steels would be subjected to the same thermal cycle. Throughout the intercritical region of the heat-affected zone toward the weld metal, more martensite was observed in the rephosphorized steel than in the plain carbon steel. However, the main cause of the hardness difference was the solid solution strengthening by the phosphorus. At the fusion line, where both steels were martensitic, the trans­formation strengthening prevailed over the difference in solid solution strengthening and reduced the hard­ness difference between the two steels to about 30 HV1.0 as compared to the' 60-70 HV1.0 difference in the base metal.

Microstructures of rephosphorized and plain carbon steel spot welds (Fig. 6) were similar in the center of the fusion zone, except that the plain car­bon steel had slightly more proeutec-toid ferrite at the grain boundaries. Both steels had a significant degree of proeutectoid ferrite when 10 cycles hold time was used, but the center of the columnar grains was primarily mar­tensitic. The average hardness of the weld metal in both steels was about 250 HV1.0 with the short hold time. With the 30 or 60 cycle hold time, the ferrite veining was nearly el iminated, and both steels exhibited an average hardness of 340-360 HV1.0.

In general, the phosphorus content played only a secondary role in weld metal hardness, and only carbon and hold time were significant variables. Results by Jurkowski' also showed lit­tle effect of phosphorus on weld metal hardness, which was approximately 38 HRC (about 370 HV) for 0.05 C-0.08 P steels welded with longer hold times.

At higher levels of phosphorus,

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0.02 inch \ 1

0.5 mm Fig. 7—Cross section of spot weld in 0.11 P steel near outer perimeter of weld. X52

I Opm

Fig. 8—Typical phosphorus-rich particles from 0.11 P steel spot welds shown in Fig. 7. 4% nital etch. XI000 (reduced 50% on reproduction)

22-s I JANUARY 1980

above 0.10%, some small grain-boun­dary particles (indicated by arrows in Fig. 7) were occasionally observed in the weld metal near the fusion boun­dary of spot welds. The particles could be observed only at the outside peri­meter of the spot weld near the fusion line in the vicinity of the interface.

Figure 8 shows a higher magnifica­tion view of typical particles. They were determined by energy-dispersive analysis to be phosphorus-rich. No particles were observed in welds of less than 0.10% P. Extensive STEM and Auger analyses of welds having up to 0.10% P failed to reveal any partit ion­ing of phosphorus above average com­position to the grain boundaries. Opt i ­cal examination of many welds throughout the program failed to reveal any microfissures or hot cracks in this region, even up to 0.12% P.

Tensile Strength. Single spot-weld tensile tests give a measure of joint efficiency during static loading. Equa­tions were developed for tensile-shear loading, and strength was also evalu­ated in direct tension. Tests were also performed on welds in zincrometal-coated rephosphorized steels and on welds joining plain carbon to rephos­phorized steels.

1. Tensile-Shear Strength: During tensile shear tests all welds failed by pull-out fracture—in some instances preceded by up to about 0.05 in. (1.2 mm) of interfacial fracture into the weld metal. The tensile shear strength (TSS) of a rephosphorized steel spot weld was found to be directly propor­tional to the tensile strength of the base metal (UTS), diameter (D) of the weld button, and thickness (Tk) of the sheet, as follows: TSS = F X Tk X D X UTS (1) where F = 3.1 wi th a standard devia­tion of 0.3.

Chemistry was a factor only insofar as it affected base metal strength.

Equation (1) was obtained from an analysis of data taken from 27 mil l-produced rephosphorized steels.with the range of chemistries and proper­ties listed in Table 1. The equation has the same form as that reported by Heuschkel" in the early 1950's for plain carbon steel spot welds.

Heuschkel found that the F factor varied from the 2.5-3.0 range for steels of low carbon equivalent to below 2.0 for steels having high carbon equiva­lents, e.g., C + (Mn/20) = 0.2. Our comparative killed plain carbon steel of 0.07% C and 0.31% Mn had an F factor of 3.0, indicating that the increase in weld tensile-shear strength of rephosphorized steel over that of plain carbon steel was proportional to the increase in base-metal tensile strength. Welds in rephosphorized steels were about 40-50% stronger in tensile shear than equivalent-size

MO- Welding Conditions Weld time 15 cycles Hold time 30 cycles Electrode force 660 lb (2.9 kN) Electrode diameter 0.25 inch(6.4mm)

O 0.07 C Plain Carbon £, 0.06 C-0.05 P A 0.07 C-0.09 P

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CYCLES TO I0e I07

FAILURE Fig. 9—Fatigue results for rephosphorized and plain carbon steels. All steels 0.033 in. (0.84 mm) thick and all welds 0.23 in. (5.8 mm) diameter

welds in plain carbon steel. The F factor can be used to estimate

the width of base metal that the weld can support. The maximum load that can be applied to the base metal is simply its tensile strength (UTS) times the width (W) times the thickness (Tk). Making that load equal to the tensile-shear strength of the weld (equation 1), we obtained the fo l low­ing:

UTS X W X Tk = F X Tk x D x UTS (2)

Cancelling out UTS and Tk, we get F = W / D .

Since the F factor thus equals the ratio of sheet width to weld diameter for equivalent base metal and weld strength, a rephosphorized weld

WELD FUSION

ZONE

loaded in tensile shear wi l l support a sheet width about three times its diameter.

The direct dependence of weld ten­sile-shear strength on weld diameter is consistent w i th the findings of other studies on high-strength steels where pull-out fractures occur.67 In this study, pull-out fractures were ob­served even when the diameter dropped below 0.15 in. (3.8 mm)—the minimum for this thickness and strength according to Vanden Bossche's analyses6 for ensuring that pull-out wil l occur in tensile shear. Thus, in accordance wi th our equation and the findings of other investigators, the rephosphorized steels exhibit pre­dictable behavior in tensile shear.

2. Direct-Tension Strength: The di-

FATIGUE CRACK INITIATION

NTERFACE

0.02inch 0.5 mm

Fig. 10— Location of fatigue fracture in typical rephosphorized steel tensile-shear specimen unloaded before fracture. 4% nital etch. X50 (reduced 35% on reproduction)

tf) UJ or

W E L D I N G RESEARCH SUPPLEMENT I 23-s

rect-tension strength of rephospho­rized steel spot welds was found to be 0.4-0.6 times the strength in tensile shear. This ratio, sometimes called the ducti l i ty ratio, is lower than the 0.5-0.9 range for plain carbon steel welds but is typical of the range for most HSLA spot welds. Strength in direct tension was little influenced by base-metal strength or chemistry. However, some correlation was observed between increasing direct-tension strength and increasing thickness and diameter of the weld.

In contrast wi th direct-tension strength, chemistry did affect the duc­til ity ratio. Phosphorus and, to a lesser extent, carbon increase the base-metal strength and therefore the tensile-shear strength. As a result, the ratio of direct-tension strength to tensile-shear strength decreased as these elements were added. However, the mean duc­til ity ratio for steels having less than 0.06% C and 0.09% P was greater than 0.4. As in tensile shear, failures were always by pull-out.

3. Effect of Zincrometal Coating: Tensile properties of welds in Zin-crometal-coated rephosphorized steels were found to be identical wi th those predicted from the uncoated steel for the same weld sizes. Hence, a Zincrometal coating has no effect on these properties in rephosphorized steels.

4. Spot Welds between Plain Car­bon and Rephosphorized: The tensile properties of spot welds between plain carbon steel and rephosphorized steel were also evaluated. For these studies a 0.07 C plain carbon killed steel 0.038 in. (0.97 mm) in thickness was welded to thinner rephosphorized steels 0.028-0.030 in. (0.71-0.76 mm) in thickness. The rephosphorized steels

contained 0.05 to 0.06% C and 0.05 to 0.10% P.

Despite their smaller thicknesses the rephosphorized steels are capable of supporting approximately the same load as the thicker plain carbon steel. The tensile shear strength was found to be a funct ion of the weld diameter, the average base-metal strength and average thickness, as shown in the previous analyses. In this case, the average F factor, calculated from aver­age strength and thickness values, was 2.9. The practical point here is that tensile properties of welds between plain carbon and rephosphorized steels can be estimated using equation (1) and average properties.

Ducti l i ty ratios for the welds be­tween carbon steel and rephophorized steel were 0.6-0.7 compared to 0.5-0.6 for homogeneous welds in the rephosphorized steels tested in this series and 0.77 for welds in the plain carbon steel. Thus, the ducti l i ty ratios of welds between rephosphorized and plain carbon steels were approximate­ly equal to the average of homoge­neous welds in the separate steels.

Fatigue. Although tensile strength data are important in the case of large overloads, resistance to cyclic loading is the controll ing factor for serviceabil­ity of spot welds in many automotive structures. A typical measure of ser­viceability by the industry is the behavior at 10* cycles.

Tensile shear fatigue results for rephosphorized and plain carbon steels are presented in Fig. 9. The 0.07 C-0.09 P steel exhibited a small amount of interfacial fracture before pull-out during the peel test, whereas the other steels exhibited no interfa­cial fracture during this test. All steels, however, failed by pull-out in tensile-

O 0.032 inch(0.8lmm) 0.07 C -O.I I P

• 0.034 inch (0.86 mm) 0.07 C PLAIN CARBONO

5 0

O.I 0.2 BUTTON DIAMETER ,inch

Welding Conditions Electrode 0.25 inch (6.4 mm) dia. Force 660 lb (2.9kN) Weld Time 10 cycles Hold Time 30 cycles

1 0.3

Fig. 11—Tensile-shear impact tests of rephosphorized and plain carbon steels

shear fatigue.

All steels exhibited about the same fatigue strength of about 270-280 lb (1.20-1.24 kN) at 106 cycles. The plain carbon, 0.05 P, and 0.09 P steels of Fig. 9 exhibited average tensile-shear strengths of 1140, 1240 and 1445 lb (513, 558 and 650 kg), respectively. Therefore, the fatigue strengths of welds in the highest-strength rephos­phorized steels are a lower portion of their tensile-shear strengths than is the case wi th the lower-strength steels. These results essentially duplicate those reported for HSLA steels."•" This fact points up the importance of designing high-strength-steel welds as well as other areas of sharp discon­tinuity away from areas of high cyclic stesses.

Further fatigue tests were performed on the highest-phosphorus steel and the plain carbon steel using direct-tension specimens—Fig. 9. At 10" cycles, the fatigue strength in direct tension is 60-70 lb (0.27-0.31 kN) or about 0.24 times that in tensile shear. Thus, the anisotropy in fatigue strength is even greater than that observed in static tests. The orienta­tion of the buil t- in notch around the spot weld has a stronger effect in cyclic then static loading as would be expected.

In tensile shear, fatigue fracture always initiated at the specimen-cen-terline intersection wi th the weld pe­rimeter (Fig. 10) and propagated out­side the fusion zone. Initiation oc­curred at this point because the stress concentration was highest there—6.1 by Kan's model.1" Likewise, in the case of direct-tension fatigue, crack intit ia-tion occurred at this point. However, both plain carbon and rephosphorized steel direct-tension specimens that failed at greater than 10:> cycles exhib­ited some interfacial fracture. During fatigue loading of a spot weld, severe geometric factors overrode metallurgi­cal and strength factors, and as indi­cated by these results, fracture loca­tion and fatigue strength are essential­ly the same for spot welds of the plain carbon and high-strength rephospho­rized steels.

Although the 0.09 P steel performed differently in peel tests by failing par­tially in the weld metal, all steels exhibited the same fracture location and long-life strength in cyclic load­ing. Thus, the kind of fracture that occurs in the peel test cannot be used to predict fatigue properties.

Toughness. A spot weld should be able to absorb energy during an over­load situation so as to safely transmit static and dynamic loads through the structure.

1. Dynamic Toughness: The tensile shear impact toughness of a 0.09

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80

70

50

4 0

2 30

20

10

mm 3 4 2 0 2.5

O 0.032 inch(0.8lmm) 0.09 C - 0.1 I P • 0.034 inch (0.36 mm) 0.07 C Plain Carbon

0.1 • 0.2

BUTTON DIAMETER,inch

-100

-90

- 8 0

-70

- 6 0

- 5 0 -

- 4 0

- 3 0

- 2 0

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0

2 5 0 -

2 0 0 -

150

100

1000

750

500

250

Fig. 12—Direct-tension impact tests of rephosphorized and plain carbon steel

0.05 DISPLACEMENT, inch

Fig. 13—Instrumented peel tests of rephosphorized steel and plain carbon steel of 0.028 in. (0.71 mm) thickness

C-0.1-1 P r e p h o s p h o r i z e d steel and a 0.07 C r i m m e d p la in c a r b o n steel are c o m p a r e d in Fig. 1 1 . The scatter in th is t ype o f test is large fo r b o t h steels, b u t t h e mean i m p a c t toughness is s l igh t ly larger fo r the r e p h o s p h o r i z e d steel . O t h e r tests p e r f o r m e d to —75°F (—60°C) d i d n o t reveal a d u c t i l e - t o -b r i t t l e t r ans i t i on in e i ther t he p la in ca rbon or r e p h o s p h o r i z e d steel spot w e l d s . The smal l th i ckness o f these steels was insu f f i c ien t t o cause t rans i ­t i on t e m p e r a t u r e p r o b l e m s .

D i r e c t - t e n s i o n spec imens tested in i m p a c t are c o m p a r e d in Fig. 12. Aga in t he scatter is very large fo r b o t h steels, and the scatter bands fo r b o t h steels over lap . In th is case, t he t oughness o f t h e p la in ca rbon steel may be s l igh t l y h igher . H o w e v e r , in sp i te of t he rela­t i ve ly h igh c a r b o n a n d p h o s p h o r u s c o n t e n t o f t h e r e p h o s p h o r i z e d s tee l , there is no m a r k e d d i f f e rence b e t w e e n this steel and l o w - c a r b o n steel in te rms of d i r e c t - t e n s i o n i m p a c t t o u g h ­ness. A l l i m p a c t spec imens , w h e t h e r in shear or d i rec t l o a d i n g , fa i l ed by a p u l l - o u t m o d e w i t h a s l igh t a m o u n t o f in te r fac ia l f r ac tu re p r e c e d i n g t h e p u l l -ou t .

2. Static Toughness : W e l d impac t tests are represen ta t i ve o n l y o f h i gh l y rest ra ined w e l d s backed by heavy plates, and the scatter in these tests (Figs. 11 and 12) makes a c o m p a r i s o n o f steels d i f f i cu l t . F u r t h e r m o r e , d y ­namic l o a d i n g is o f l i t t l e va lue here, because it is imposs ib l e even at very l o w t empe ra tu res to observe a d u c t i l e - ' t o - b r i t t l e t r ans i t i on . Since a stat ic test is m o r e c o n t r o l l a b l e and pee l i ng is t h e most severe t ype o f l o a d i n g , t h e ins t ru ­m e n t e d pee l test was d e v e l o p e d , as i l lus t ra ted p rev ious ly in Fig. 3.

Fig. 14—Typical peel-test specimen of rephosphorized steel showing small amount of interfacial fracture (arrow) before pull-out

Typica l curves o b t a i n e d f r o m th is test (Fig. 13) d e m o n s t r a t e d i f fe rences in t h e f rac tu re b e h a v i o r o f c a r b o n steel and r e p h o s p h o r i z e d steel peel spec i ­mens. The c a r b o n steel cu rve has a smal l d r o p in load near t he start, w h e r e the d i f f u s i o n - b o n d e d hea t -a f fec ted z o n e separated and the u n b o n d e d edge m o v e d f r o m ou t s i de t he heat -a f fec ted z o n e u p t o t h e f u s i o n l ine . In t he r e p h o s p h o r i z e d s tee l , m o r e d is ­c o n t i n u i t i e s w e r e obse rved . W h i l e t h e first i n t e r r u p t i o n in t he cu rve m i g h t have resu l ted f r o m some hea t -a f fec ted z o n e u n b o n d i n g , succeed ing d i s c o n ­t inu i t i es o c c u r r e d w h e n the f rac tu re p ropaga ted i n t o t he o u t e r edge o f t h e w e l d me ta l . Th is f rac tu re l o c a t i o n was

ve r i f i ed by e x a m i n a t i o n o f spec imens u n l o a d e d be fo re m a x i m u m load . The load c o n t i n u e d to rise af ter w e l d - m e t ­al f r ac tu r i ng up un t i l t he p o i n t w h e r e p u l l - o u t c o m m e n c e d .

Figure 14 w h i c h is a p h o t o g r a p h of a typ ica l b u t t o n p u l l e d f r o m a peel spec­i m e n i l lust rates t h e smal l a m o u n t o f in ter fac ia l f rac tu re (a r row) that can occu r be fo re p u l l - o u t . This t ype of f rac tu re has also been repo r ted for o the r h i gh -s t r eng th steels a n d even for p la in c a r b o n s tee ls . " In ou r w o r k , t he i n c i d e n c e of in ter fac ia l f rac tu re was observed t o increase at t he h ighest levels o f ca rbon and phospho rus . SEM e x a m i n a t i o n o f the f rac tu re surfaces o f peel spec imens h igh in these e l emen ts s h o w e d that the in te r fac ia l p o r t i o n was p r ima r i l y in te rg ranu la r , a l o w -energy t ype o f f rac tu re .

N o in te rg ranu la r i nc lus ions observa­b le op t i ca l l y or on t h e SEM w e r e asso­c ia ted w i t h th is f rac tu re , b u t i n te rg ran ­ular c o n c e n t r a t i o n s o f l o w - m e l t i n g cons t i t uen t s may have reduced the i r f rac ture resistance. By con t ras t , t he p u l l - o u t p o r t i o n of the f rac tu re was a lways d i m p l e d , charac te r is t i c o f a h igh -energy , m i c r o v o i d - c o a l e s c e n c e m o d e . The fact that t he smal l i n te r fa ­cial p o r t i o n of t he f rac tu re a lways o c c u r r e d be fo re m a x i m u m load shows tha t t h e w e l d p roper t ies w e r e c o n ­t ro l l ed by t he d u c t i l e p u l l - o u t p o r t i o n . The o n l y p rac t ica l e f fec t o f t h e smal l in ter fac ia l f rac tu re is tha t t he b u t t o n size is s l igh t ly smal ler . A c c o r d i n g l y , a fabr i ca to r may w a n t to ad jus t w e l d i n g c o n d i t i o n s to p r o d u c e a s l igh t l y larger w e l d . W e l d - s i z e c o n t r o l w i l l be cov ­ered later in t he d iscuss ion o n c o n v e n ­t i ona l peel tes t ing .

Results f r o m these types o f tests can

W E L D I N G R E S E A R C H S U P P L E M E N T I 25 -s

0.06 C - 0.08 P

0.07 C PLAIN CARBON

0.10 0.15 0.20 0.25 BUTTON DIAMETER, inch

Fig. 15—Instrumented peel tests of a rephosphorized steel that exhibits slight interfacial fracture before pull-out and a plain carbon steel that exhibits no interfacial fracture. Welding conditions same as in Fig. 9

Fig. 76 (right)—Peel-test data showing effect of in­creasing electrode face diameter

0.028 inch(0 7lmm) 0 05 C- 0.09 P

0.25inch (6.4mm) Electrode

0.3l2inch (79mm) Eg • Electrode J r

Welding Conditions Weld Time 15 cycles Electrode Pressure I3ksi Hold Time 30 cycles

• I

) I0 CURRENT, kA

12 13 14

be p l o t t e d as a f u n c t i o n o f w e l d d i a m ­eter to g ive a p l o t of energy a b s o r p t i o n versus b u t t o n d i ame te r , as i l lus t ra ted in Fig. 15 fo r p la in c a r b o n and rephos ­p h o r i z e d steels. A l l of t h e w e l d s in th is r e p h o s p h o r i z e d steel e x h i b i t e d some in ter fac ia l f rac tu re be fo re p u l l - o u t . The k i l l ed p la in ca rbon steel w e l d s exh ib i t ed no in te r fac ia l f rac tu re be fo re p u l l - o u t bu t in th is case ac tua l l y e x h i b ­i t ed a l o w e r energy a b s o r p t i o n . F rom several o f these curves fo r var ious steels w e o b t a i n e d t h e energy abso rp ­t i on for a 0.150 in . (3.8 m m ) d i ame te r b u t t o n , t he m i n i m u m b u t t o n size c las­s i f ied as sat is factory by A W S s tan­dards.12

As seen in Tab le 2, t h e r e p h o s p h o ­r ized steels e x h i b i t e d b o t h h igher a n d l o w e r energy a b s o r p t i o n t h a n p la in ca rbon steel d u r i n g pee l i ng . T h e steels w i t h a smal l a m o u n t o f in te r fac ia l f rac­tu re be fo re p u l l - o u t had b o t h h igher and l owe r energy a b s o r p t i o n t h a n t h e steels f ree of in te r fac ia l f rac tu re . Thus , there was no co r re la t i on o f e i ther p h o s p h o r u s c o n t e n t or f rac tu re m o d e w i t h energy a b s o r p t i o n .

D u r i n g pee l i ng w e l d s w e r e d e ­f o r m e d ex tens ive ly and to a b o u t t h e same degree; t hus , the ma jo r e f fect o n energy a b s o r p t i o n was the m a x i m u m load . As m i g h t be e x p e c t e d , a c o m p a r ­ison of these results w i t h stat ic d i rec t -tens ion data s h o w e d reasonab ly g o o d co r re l a t i on . As in d i r e c t - t e n s i o n tests,

energy a b s o r p t i o n increased w i t h an increase in sheet th i ckness or b u t t o n d iamete r . H o w e v e r , as o p p o s e d to t e n ­si le-shear resul ts, t he e f fec t o f g e o m e t ­ric parameters o n energy a b s o r p t i o n was no t large and no t d i r ec t l y p r o p o r ­t i ona l .

Conventional Peel Tests. Peel tests we re c o n d u c t e d to d e t e r m i n e the range of w e l d i n g c o n d i t i o n s over w h i c h adequa te -s i ze w e l d s c o u l d be p r o d u c e d . Da ta o b t a i n e d in the peel test are i l l us t ra ted in Fig. 16 for a typ ica l r e p h o s p h o r i z e d steel . The use­fu l cu r ren t range is t he d i f f e rence b e t w e e n the m a x i m u m cu r ren t w h e r e expu l s ion w i l l no t oc c u r o n the second of t w o peel samp le w e l d s and the m i n i m u m cu r ren t w h e r e a m i n i m u m -size w e l d is o b t a i n e d . The m i n i m u m b u t t o n d i a m e t e r se lec ted , 0.150 in . (3.8 m m ) , c o r r e s p o n d s t o t h e A W S sat isfac­tory w e l d size.1'- As s h o w n in Fig. 16, t he m i n i m u m of the scatter b a n d was a lways used t o d e f i n e cu r ren t range.

If o i l was c o m p l e t e l y c l eaned f r o m peel spec imens or if w e l d e d peel spec­imens w e r e de layed fo r an h o u r b e f o r e tes t ing , w e l d meta l f rac tu re c o u l d be p reven ted in some steels tha t w e r e bo rde r l i ne b e t w e e n p u l l - o u t and par­t ial w e l d meta l f rac tu re . These results i nd ica te tha t h y d r o g e n p r o b a b l y assists w e l d meta l f rac tu re and the ef fect is revers ib le. A l l spec imens in th is inves­t i ga t i on w e r e tested in t he o i l e d c o n d i -

Table 2- lns t rumented Peel Tests

Steel""

Plain carbon Plain carbon'131

0.05 P 0.09 P 0.08 P

Mean energy for 0.15 in. (3.8 mm)

but ton, J

1.2 1.1 1.4 0.94 1.7

Fracture mode

Pull-out Slight interfacial Pull-out Slight interfacial Slight interfacial

' "All steels 0.05/0.07 C and 0.028 in. (0.71 n,m) thick. ""Unkilled, others killed.

t i on w i t h no de lay , ensu r i ng tha t t he results w o u l d be conserva t i ve .

Cur ren t range is s ign i f i can t l y i m ­p roved by us ing a larger e l e c t r o d e face d iameter—Fig . 16. The larger e l e c t r o d e causes the cu r ren t density to c h a n g e less for a g iven change in cu r ren t ; t he re fo re , t he s lope of w e l d size vs. cu r ren t is decreased. A l so , t he larger e l ec t rode pe rm i t s a larger w e l d be fo re expu l s i on thus g i v i n g an a d d i t i o n a l i n c remen t of cu r ren t range at t h e h igh e n d .

The s t rong e f fec t o f e l e c t r o d e face d iame te r o n cu r ren t range is d e m o n ­strated in Fig. 17. In fact , e l e c t r o d e face d i ame te r is m u c h m o r e i m p o r t a n t t h a n chemis t r y fo r c o n t r o l l i n g cu r ren t range. The p la in ca rbon steel has an adequa te cu r ren t range at t he smal le r e l ec t rode sizes n o r m a l l y e m p l o y e d fo r these steels, bu t w i t h a s l igh t l y larger e l ec t rode face d i a m e t e r the rephos ­p h o r i z e d steels have t h e same cu r ren t range.

Regression e q u a t i o n s fo r c h e m i s t r y ef fects s h o w e d that c a r b o n and p h o s ­phorus w e r e co r re la ted w i t h cu r ren t range. A l t h o u g h the t rends p l o t t e d in Fig. 18 s h o w some scatter i n d i v i d u a l l y , the data g ive us a l o w e r b o u n d o n t h e expec ted cu r ren t range. The curves i l lust rate tha t a minimum c u r r e n t range o f 1500 A was a lways o b t a i n e d w h e n us ing at least a 0.28 in . (0.71 m m ) e lec t rode .

The 0.28 in . (7.1 m m ) e l e c t r o d e d iame te r p rov ides an o p t i m u m ba l ­ance b e t w e e n the r e q u i r e m e n t s fo r h igh cu r ren t range and m i n i m i z i n g t he p o w e r r equ i r emen ts . The cu r ren t level w h e n us ing the 0.28 in . (7.1 m m ) e lec ­t r o d e is a b o u t 1000 A h i g h e r t h a n tha t o b t a i n e d fo r t h e s tandard 0.25 in . (6.4 m m ) size. H o w e v e r , w h e n p la in car­b o n steel is rep laced by t he rephos ­p h o r i z e d steel , t he cu r ren t level is v i r tua l l y t he same, because the h igher resist iv i ty o f a r e p h o s p h o r i z e d steel reduces t h e cu r ren t level and c o m p e n -

2 6 - s l J A N U A R Y 1 9 8 0

4000

3000

mm 7

Welding Conditions Weld Time 15 cycles Electrode Pressure 13 ksi Hold Time 30 cycles —i—i— i— i—i—i—i—i I i i_

0.15 0.2 0.25 0.30 0.35 ELECTRODE DIAMETER,inch

CURRENT RANGE: EXPULSION TO 0.15 INCH( 3.8 mm) BUTTON

Fig. 17— Effect of electrode face diameter on current range

0.10

% C + % P

Fig. 18— Effect of carbon and phosphorus on current range

sates for t he larger e l ec t rode con tac t area. Thus, no increase in overa l l e lec­t r ode size or p o w e r w o u l d be requ i red w h e n rep lac ing p la in c a r b o n steels w i t h r e p h o s p h o r i z e d steels.

A n o t h e r va r iab le tha t a f fects cu r ren t range is w e l d t i m e . If several cu r ren t

range tests are m a d e at d i f f e ren t w e l d t imes , p lo ts of t h e m a x i m u m and m i n ­i m u m cur ren ts p r o d u c e a w e l d i n g l obe such as t he o n e in Fig. 19 for a 0.031 in . (0.79 m m ) r e p h o s p h o r i z e d steel . A t 10 cyc les w e l d t i m e , t h e n o r m a l t i m e fo r w e l d i n g p la in ca rbon steel , th is

r e p h o s p h o r i z e d steel has a smal l cur ­rent range. H o w e v e r , at 15 cycles and longer w e l d t imes the cu r ren t range is fa i r ly w i d e — a b o v e 1500 A. Some steels w e r e no t as s t rong ly a f fec ted by w e l d t i m e , b u t t he behav io r o f m a n y steels was typ ica l of tha t s h o w n in Fig. 19. It

30

o> 2 0 -

z: o _ i UJ 1 0 -

1

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1

1 1

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\ M in imum \ C u r r e n t

V

1 1

1 1

1

Maximum C u r r e n t

I

\

Welding Conditions

Electrode 0.25 inch (6.4mm)

Force 6 6 0 lb(2.9 kN) Hold Time 30 cycles

1 1 10 I I

CURRENT, kA

10 II CURRENT, kA

Welding Conditions Electrode 0.25 inch (6.4mm) dia Force 660 lb (2 9 k N I Weld Time 15 cycles Hold Time 30 cycles

_ l _ 13 14 15

Fig. 20—Peel-test results for a zincrometal-coated steel containing 0.07C and 0.08P

Fig. 19 (left) —Welding lobe showing conditions necessary to produce a satisfac­tory size button without expulsion in the peel test oi a 0.07C-0.09P steel

W E L D I N G R E S E A R C H S U P P L E M E N T , 27-s

2 0 g y . 240 HVIO

F/g. 21—Base-metal microstructure of SRA steel 4% nital etch. X500 (reduced 52% on reproduction)

3 0 0

o 2 5 0 -

Q or < Z

-

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0.5 F

1

WELD METAL

i i

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TRANSFORMED HAZ

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or o UJ

N _ l _) £ CO

> u UJ or

/

1.0 1 5 1 | 1

STRAIN-AGED | B A S E REGION METAL

/ ^K^ /

1 Welding Conditions

/ E lect rode 0 28 inch Dia (7.0mm 1 Force 8 5 0 lb

Weld Time 11 cycles Hold T ime 6 0 cycles

i 1 il i I

2 0 1

) "

-

•

02 01 0 2 .03 0 4 .05 06 07 DISTANCE FROM FUSION LINE, inch

Fig. 23-Hardness traverse of spot weld in SRA steel 0.050 in. (1.3 mm) thick

. <M*~"

ZOfixri

Fig. 22—Microstructure of SRA weld metal. 4% nital etch. X500 (reduced 52% on repro­duction)

is important, therefore, to increase weld time slightly for the complete range of rephosphorized steels.

Electrode force is usually increased when joining high-strength steels in order to compensate for higher spring-back forces when parts are poorly aligned. Although about 25% higher force than for plain carbon steel was used in line wi th industrial practice, it was found that force does not measur­ably affect current range.

Long hold times result when using large press machines wi th mult iple electrodes that fire sequentially. Therefore, to cover all possible condi­tions, only long hold times (e.g., 30 cycles) typical of the most severe situ­ation were employed. However, most production welds are made using shorter hold times, i.e., less than 10 cycles, and consequently would be produced under conditions that result in wider current ranges.

Zincrometal-Coated Steels. When one-sided Zincrometal-coated rephos­phorized steels were peel-tested, an unusual effect was observed. If a Zin­crometal coating was located at the interface, the current range became much wider, as illustrated in Fig. 20. This steel, containing 0.05 C and 0.09 P, exhibited a current range of 2200 A when the one-sided coating was fac­ing the electrodes. However, when the

coating was facing the interface between the sheets, the current level to reach expulsion was increased and a lower slope of the button diameter vs. current was observed.

The result was to increase the cur­rent range to 4000 A, an effect similar to that shown for increasing electrode size. Other work showed that the zinc-rich coating melts and is pushed out to the perimeter of the weld during the early stages of the weld cycle. This ring of molten coating acts to increase the contact area at the interface and, as when using a larger electrode, lower the change in current density for a given change in current.

Tip-life tests of the Zincrometal-coated rephosphorized steel were conducted wi th the coating facing the electrodes. The initial welding condi­tions were set up to produce a maxi­mum-size weld on peeling, and the welds were repeated until a minimum button size was reached—in this case over 4000 welds, indicating that the coated rephosphorized steels have a tip life equivalent to that of a coated plain carbon steel.

Discussion of Rephosphorized Steels. The tensile-shear strength of welds in rephosphorized steels is related only to the base-metal tensile strength and the geometric parame­ters, thickness and weld diameter. The direct-tension strength and toughness are affected to a smaller extent by thickness and weld diameter but not by base-metal tensile strength. During these tests, a small amount of interfa­cial failure occurs at the perimeter of the weld fusion zone. However, pul l-out failure at a higher load always follows the interfacial port ion. Conse­quently, the only effect of partial inter­

facial fracture on strength or tough­ness is to reduce button size slightly. The reduced button size can be com­pensated for by increasing electrode face diameter by about 10%—a desir­able modif ication for opt imum current range in the peel test.

Weld fatigue properties are unaf­fected by chemistry, strength or the occurrence of partial interfacial frac­ture. The equivalency in spot weld fatigue properties between plain car­bon steel and high strength steel is well documented,55 '13 but the large difference in fatigue strength wi th loading direction is less well known. Because of this strong orientation effect, joint design has a much larger effect on cyclic weld strength than steel chemistry or strength.

Stress-Relieved Annealed Steels

The stress-relieved annealed (SRA) steels are plain carbon, 1008-type, steels that achieve their high strength by process control to produce an incompletely recrystallized structure.

Metallography and Hardness. The recovered and partially annealed mi­crostructure presented in Fig. 21 is typical of an SRA steel base metal. In contrast, a typical weld-metal micro-structure, illustrated in Fig. 22, consists of low-carbon martensite wi th some upper bainite and blocky ferrite.

The SRA steels are normally used in greater thicknesses than rephospho­rized steel. For this reason the center of the spot welds in ' these steels is slightly farther from the water-cooled electode and therefore cools at a slow­er rate. Consequently, the weld-metal microstructures are slightly softer. The average weld-metal hardness when

28-s I JANUARY 1980

TRANSFORMED! RECRYSTALLIZED 1 STRAIN-AGED REGION — REGION — REGION

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I80HV

Fig. 24—Microstructure near outside edge of heat-affected zone in SRA steel spot weld. 4% nital etch. X500 (reduced 32% on reproduction)

20

10

WELDING CONDITIONS

Electrode 0.25 inch (6.4 mm;

Force 850 lb (3.8 kN)

Hold Time 60 cycles

V

10 I I CURRENT,

12

kA Fig. 25-Welding lobe for 0.06C capped SRA steel 0.048 in. (1.2 mm) thick

using 30 cycles hold t ime is between 270 and 330 HVI.Oin a 0.07 C SRA steel compared to about 350 HV for the thinner-gage 0.07 C steel previously discussed.

The hardness traverse in Fig. 23 reveals a softened region of the heat-affected zone in a typical SRA spot weld. This soft region is surrounded by the predominantly martensitic weld metal of about 300 HV and the cold-worked and recovered base metal. The small increase in hardness of the untransformed region adjacent to the base metal is evidence of strain aging at the outer extremity of the heat-affected zone.

The softened region of the heat-affected zone was produced by both recrystallization and transformation reactions. The microstructure of the outer region of the softened region is illustrated in Fig. 24. In the strain-aged region of the base metal, shown in the right side of Fig. 24, no obvious change from the base-metal microstructure has occurred. In the center port ion, recrystallization of the cold-worked and recovered structure has occurred causing softening. On the left side of Fig. 24, the matrix has transformed, on heating, to austenite plus recrystal-lized ferrite and, on cooling, to poly­gonal ferrite grains plus carbides. The partially transformed region on the left is soft because the polygonal ferrite has no cold-work or transformation strengthening. The total softened re­gion is 0.015 to 0.020 in. (0.4 to 0.5 mm) wide. Closer to the weld metal (not shown), the peak temperature and cooling rate increase, producing mar-tensite on cool ing. The microstructure gradually approaches the martensitic content and hardness of the weld met­al (Fig. 22).

Mechanical Test. The tensile-shear strength of spot welds in the SRA steels was not found to be adversely affected by the soft region. As with the rephosphorized steels, the tensile-shear strength was a function only of sheet thickness, weld diameter and base-metal ultimate tensile strength. In this case, the F factor was approxi­mately 2.5. Compared to plain carbon steel, the increase in tensile-shear strength of welds in the SRA steel was equal to 85% of the increase in base-metal tensile strength, and all fractures were by pull-out.

The direct-tension strength of SRA welds was 0.35 to 0.45 times the ten­sile-shear strength and virtually unaf­fected by composit ion or welding con­ditions. Fractures during direct-tension tests were by a pull-out mode outside the weld metal and were located in the vicinity of the softened zone. Dur­ing direct tension this narrow zone was more favorably oriented to play a role in the fracture process and accounts for some of the difference between the strength in direct tension and tensile shear. However, the speci­mens failed by a gross plastic overload, and the fracture process necessarily involved a larger area than could be accommodated by the softened region alone.

As in all high-strength steels, the lower direct-tension strength (com­pared to the tensile-shear strength) must be accounted for in the design of parts that include welds highly loaded normal to the sheet surface. The strength of such welds in direct ten­sion is still substantial. For example, a typical SRA steel 0.068 in. (1.7 mm) in thickness with a weld 0.31 in. (7.9 mm) in diameter has an average direct-tension strength of 1630 lb (7kN).

During peel testing, pull-out frac­tures could be obtained over a wide range of welding conditions, as shown in Fig. 25 for an 0.048 in. (1.2 mm) thick SRA steel. The wide current range is what one would expect from a steel that is basically a Type 1008 steel. The current range is smaller for nonkilled grades above 0.07 C and for killed grades above 0.08 C Below these car­bon levels, the SRA steels met the most severe automotive requirements for peel and chisel test performance. Since the softened region of the heat-affected zone provided an easier frac­ture path than the weld metal, pull-out fractures during peel testing were ensured throughout the normal car­bon range of the SRA steels.

Conclusions

To sum up the results of mechanical and quality-control tests as well as the metallographic evaluations of a wide range of rephosphorized and stress relieved annealed steels, we found that:

1. Welds of adequate size, strength, and toughness can be produced over a practical range of welding conditions in both grades of steel. Equations for estimating weld strengths are pro­vided.

2. In peel tests, high phosphorus steels can exhibit a small amount of weld-metal fracturing. However, the weld mechanical properties such as tensile strength, fatigue strength, and toughness are equal to or greater than those measured in plain carbon steel.

3. To ensure adequate button size over a large operating current range for rephosphorized steel chemistries up to 0.2% total carbon plus phosphorus, electrode face diameters and welding

W E L D I N G RESEARCH SUPPLEMENT I 29-s

t imes need to be o n l y s l igh t l y larger t h a n those n o r m a l l y e m p l o y e d for p la in c a r b o n steels.

4. SRA steels requ i re no a d j u s t m e n t in c o n v e n t i o n a l ca rbon-s tee l w e l d i n g pract ices to o b t a i n w i d e cu r ren t ranges.

5. A n a r r o w so f tened reg ion in t he recrys ta l l i zed a n d t r a n s f o r m e d regions o f SRA hea t -a f fec ted zones p r o m o t e s p u l l - o u t f rac tu re in peel and chisel tests. Fu r the rmore , in tens i le shear, SRA w e l d s are s ign i f i can t l y s t ronger than ca rbon steel w e l d s by near ly t he f u l l a m o u n t o f the i r increase in base-meta l tens i le s t reng th .

Acknowledgments

The au thors express the i r apprec ia ­t i o n to R. L. Kiefer fo r p e r f o r m a n c e o f expe r imen ta l w o r k , E. C. Poel t l and R. Gruver for m e t a l l o g r a p h y , W . A. Beve-r idge fo r p repa ra t i on o f i l l us t ra t ions , and B. S. M i k o f s k y fo r t echn i ca l ed i t ­

ing.

References

1. Recommended Practices for Resistance Welding, AWS C1.1-66, 1966, American Welding Society, pp. 79-85.

2. Standard Recommended Practice for Constant Ampl i tude Axial Fatigue Test of Metallic Materials, ASTM E466-76.

3: Standard Recommended Practice for Verification of Constant Ampl i tude Dy­namic Loads in an Axial Load Fatigue Test­ing Machine, ASTM E467-76.

4. Jurkowski, J. J., and Gonzales, M., Jr., "Product ion Forming and Welding Applica­tions wi th Higher Strength Cold-Rolled AK Rephosphorized Steel," SAE Paper 790210 (1979).

5. Heuschkel, J., "The Expression of Spot-Weld Properties," Welding journal, 31(10), October 1952, Research Suppl., pp. 931 -s to 943-s.

6. Vanden Bossche, D. J., "Ul t imate Strength and Failure Mode of Spot Welds in High Strength Steels," SAE Paper 770214 (1977).

7. Sawhill, Jr., J. M., Watanabe, H., and

Mitchel l , J. W., "Spot Weldabi l i ty of M n -Mo-Cb, V-N, and SAE 1008 Steels," Welding journal 56(7), July 1977, Research Suppl. pp. 217-s to 224-s.

8. Pollard, B., "Spot Weld ing Characteris­tics of HSLA Steel for Automot ive Applica­tions," Welding journal 53 (8), August 1974, Research Suppl., pp. 343-s to 360-s.

9. Pollard, B., and Coodenow, R., H., "Spot Weldabi l i ty of Dual-Phase Steel," SAE Paper 790006 (1979).

10. Kan, Yih-Renn, "Fatigue Resistance of Spotwelds—An Analytical Study," Metals Engineering Quarterly (November 1976), pp. 26-36.

11. Fine, T. E., and Fostini, R. V., "Spot Weldabil i ty of High-Strength Cold-Rolled Steels, SAE Paper 790005 (1979).

12. Specification for Automotive Weld Quality—Resistance Spot Welding, AWS D8.7-78/SAE HS J-1188.

13. Cappelli, P. C , Castagna, M., and Ferrero, P., "Fatigue Strength of Spot-Welded Joints of HSLA Steels," Welding of HSLA (Microalloyed) Structural Steels, Con­ference Proceedings (Nov. 1976) ASM, pp. 734-749.

The Advisory Subcommittee on Welding Stainless Steel

for the Welding Research Council

. . . is pleased to announce that it will sponsor two important sessions on the welding of austenitic stainless steels during the 61st Annual Meeting of the American Welding Society in Los Angeles, California, during April 14-18, 1980. The two sessions—each featuring experts of world renown—are:

• Fully Austenitic Stainless Steel Weld Metals-Part I, Tuesday morning, April 15: Hot Cracking Problems—Flux Cored Wire for Cryogenic Service—Weld Metal Cracking of 25Cr-20Ni—Nitrogen Fissuring in Weld Metal and Means for Control.

• Fully Austenitic Stainless Steel Weld Metals—Part II, Tuesday afternoon, April 15: Cracking Resistance— Weldability of Nitronic Steels—Cryogenic Toughness of Welds—Welding Variables and Microfissuring.

Full details are contained in the tentative program for the 61st Annual Meeting technical sessions that appeared in the December 1979 issue of the Welding Journal (Sessions 6 and 9 on page 72).

30-sl JANUARY 1980