Tensile Properties of A514 Steel Butt Joints Containing Cluster Porosity

Transcription

1 Tensile Prperties f 54 Steel Butt Jints Cntaining Cluster Prsity Tensile tests suggest weld inspectin specificatins fr tensile applicatins culd be relaxed fr wrking stress design, nt fr limit stress design BY E. M. HONIG, JR. ND K. W. CRLSON BSTRCT. The effects f cluster prsity n the tensile prperties f 54 steel butt weldments were assessed. Cluster prsity was measured bth in terms f the actual ttal area f pres and f the area f the cluster (including metal ligaments, r webs, that cnnect pres). Results shwed that the tensile strength f the welded metal is nt significantly reduced if the area f pres and the cluster area are belw critical sizes; hwever, ductility is rapidly reduced until these sizes are reached. Cnsideratin f the implicatins f tensile test data n bth wrking stress design and limit stress design indicate that relaxatin f welding cdes may be cnsidered fr certain tensile lading applicatins if the cdes are based n wrking stress design. This wuld be beneficial in terms f reduced prject csts fr weld rewrk. Relaxatin f cdes E. M. HONIG, JR. is Metallurgist, U.S. rmy Cnstructin Engineering Research Labratry, P.O. Bx 45, Champaign, IL 682 and K. W. CRLSON is Graduate Research ssistant, Dept. f Theretical and pplied Mechanics, University f Illinis, Urbana, IL 68. Paper t be presented at 57th WS nnual Meeting, St. Luis, MO, May -4, 976. based n limit stress design shuld nt be cnsidered fr the cnditins f prsity cvered in this study. Intrductin Prblem There is a need fr acceptance guide specificatins fr welds based n actual mechanical behavir f a structural weld in any given applicatin. T this end it is necessary t relate weld discntinuity characteristics such as size, type, shape, lcatin, rientatin, and spacing t mechanical prperties f the weld. Such a relatin is needed t replace the empirical apprach currently used in mst cases f specificatin fr structural welds an apprach that ften results in verdesigned r underdesigned welds. Objective arius types f weld discntinuities, such as cracks, prsity and inclusins, have been shwn directly respnsible fr premature brittle fracture f structural cmpnents. The purpse f this study was t evaluate the effect f cluster prsity n the tensile prperties f 54 steel weldments. Nndestructive tests and static tensile tests were used t establish the dependence f tensile prperties n cluster prsity size. This investigatin prvides a partial basis fr the evaluatin f inspectin criteria relating allwable discntinuity parameters t given levels f perfrmance. These criteria wuld fulfill part f the needs stated abve. Prcedure The prcedure cnsisted f fabricating the specimens, determining their tensile prperties, examining their fracture surfaces, and evaluating the radigraphs f the welds. Materials The base metal used was an TM 54 grade F structural steel. The electrde was ES.* The chemical cmpsitins f the base and filler metals are given in Table, and their mechanical prperties in Table 2. Fabricatin The tensile specimens were fabricated frm 6 x ft (.83 X 3.5 m) 'Minimum tensile and yield strengths and 98 ksi respectively, accrding t WS Structural Welding Cde, Table 4... irc's X was used, althugh ther prprietary bare wire electrdes are available. Table Chemical Cmpsitin f 54 Base Metal (% In. Thick) and ES Electrde (/6 In. Diam) Material 54 <a) ES (b) 5 84 Mn Si Cr M l.4.75 Zr.4 B Cu.5.2 (a) Data frm independent analysis (b) Data frm manufacturer WELDING RESERCH SUPPLEMENT! 3-s

2 54 steel plate stck. The plates were flame cut int X 36 in. (25.4 X 9.4 cm) blanks, and each f these was sawed in half. The sawed edges were machined t frm a duble- 6 deg grve. Befre welding, the beveled edges f the blanks were cleaned with acetne t insure il and dirt-free welding surfaces. F (38 C) preheat was used t remve any misture n the machined surfaces. ll welding was perfrmed using the gas metal-arc welding (GMW) prcess with an argn-2 percent xygen shielding gas mixture; the welding parameters are listed in Table 3. The prcedure cnsisted f the depsitin f tw weld passes, which cmpletely filled ne-half f the weld grve. The rt pass was then back grund using disc and carbide grinders, after which tw additinal passes were placed t cmplete the weld depsit. The weldments cntaining internal discntinuities were prduced Table 2 Tensile Prperties f Base Metal and Weld Tensile str., ksi Yield str., ksi Elng. in 2 in., % Red. in area, % Base metal (a) Weld metal" (c» (a) Prperties f base metal prvided by manufacturer (b) verage f three specimens taken frm weld metal (Ref. ) (c) Elngatin in 3/4 in. Table 3 Welding Parameters ltage, 22 Current, Travel speed, in./min similarly t the sund welds except that the flw f the shielding gas was interrupted during the placement f the third pass (Ref. 2). This created an unstable arc, which in turn prduced the clustered prsity. Interruptin f the gas flw was accmplished using a slenid-activated valve situated between the welding gun and the shielding gas tank; the shielding gas was reduced r turned ff fr varying lengths f time, depending upn the size f cluster desired. fter welding, the weld reinfrcement was remved by disc grinding and an initial radigraph was taken t determine the apprximate nature and size f the prsity cluster prduced. Using a template and shapecutting unit, the welded blanks were then flame cut t the test specimen cnfiguratin shwn in Fig.. The flame cut edges were filed and sanded in the directin f the lngitudinal axis f the specimen. The specimen faces were then milled flat and sanded t the same smthness as the edges. Each weldment was first examined by nrmal incidence radigraphy t determine the extent and lcatin f weld defects. Stereradigraphs, taken using the prcedure f Carlsn and Lawrence (Ref. 3), then prvided greater understanding f the character f each defect. Mechanical Testing The specimens were munted in a 6 x 5 lb capacity MTS tensile lad frame and laded directly t fracture Interpass and preheat temp, F 2 Heat input, kj/in. 44 Shielding gascmp, % 98r-2, at the cnstant crsshead rate f.3 in./min. ll tests were cnducted at rm temperature. The defrmatin ccurring in a in. (2.54 cm) gage length straddling the weld was measured with a linear variable differential transfrmer (LDT) extensmeter attached t the centerline f each specimen. The lad and LDT extensin signals were recrded thrughut the test with x-y pltters, and the lad and LDT signals were cnverted frm analg dc t digital signals and recrded n magnetic tape. The latter step was perfrmed t retain a separate permanent recrd f the data, especially if autgraphic pltting errrs shuld later be discvered. The percent strain was determined as the rati (times ) f the measured extensin t the gage length. The strain and stress at maximum lad were termed the unifrm strain, e u, and the ultimate (tensile) stress, - u, respectively. The yield stress, - y, was determined at.2% plastic strain ffset. Fracture Surface Examinatin fter testing, the ends f the specimen halves were sawed ff t permit easy measurement f the prsity clusters and strage f the fracture surfaces. Each fracture surface was examined t determine its nature and the actual dimensins f the cluster prsity. phtgraph, rughly 2X, was made f each pair f fracture surfaces, as shwn (smewhat reduced) in Fig. 2. Tw measures f the prus area, the area f the pres and the area f the regin enclsing the pres n the fracture surface, were determinedby using a plar planimeter directly n the phtgraph. Radigraphic Evaluatin The weld quality f each specimen was evaluated by using the radigraph f the specimen and military specificatin MIL-R-468 (ORD), Radigraphic Inspectin. 6.5 :i.25" 36 I.7 Fig. Tensile specimen cnfiguratin Results and Discussin The tw measures f the prus area mentined abve are the key independent variables in the analysis f this prblem. It is useful t cnsider the prjectin f the fracture surface n the plane transverse t the tensile axis, as in Fig. 2. The prus regin intercepted by the fracture surface is defined here as the least area, in this plane f prjectin, that cntains all the prjectins f the pres visible n the fracture surface. In this regin the greatest dimensin parallel t the 4-s I PRIL 976

3 weld axis is defined as the cluster length (fracture surface). The cluster width is the greatest dimensin, in this regin, transverse t the cluster length. Bth the cluster width and length are transverse t the tensile axis. The prus regin intercepted by a radigraph is defined here as the least area that cntains all the visible radigraphic images f the pres. In this regin, the greatest dimensin parallel t the weld axis is defined as the cluster length (radigraphic). This cluster length is the infrmatin an inspectr in the field wuld have available fr mst nndestructive weld examinatins. This dimensin is parallel but nt necessarily equal t the cluster length (fracture surface). This inequality fllws frm the fact that a cluster f prsities is three-dimensinal, f unequal extent amng the dimensins. Hence the prjectin f the cluster n the radigraphic plane is nt necessarily the same as the prjectin n any ther plane. Mrever, thrugh errrs f radigraphic technique, the radigraphic images (r prjectins) f sme pres may nt be discerned. Table 4 Undepicted Data Specimen S - CN Reductin in area, % 4.2 (b) Cluster wi dth, (fracture su rface) in Cluster width,< a ) (radigraphic) in (c) (a) This is greatest dimensin perpendicular t cluster length (radigraphic) in the radigraphic plane (b) Extensmeter capacity exceeded by extensin at maximum lad (c) Linear prsity (split cluster) Results Sme f the data, nt depicted, are given in Table 4. Figure 3 shws the rapid decrease f unifrm strain, e u, with pre area. Only abut.3 in. 2 pre area can be tlerated befre e u is reduced t less than 5%. Hwever, up t.2 in. 2 pre area can be tlerated fr which e u exceeds 3%. Similar results appear in Fig. 4 fr e^ as a functin f the area f the prus regin. This regin, the fracture surface prjectin defined abve, includes nt nly the pre area, but als the area f the cnnecting ligaments between the pres. If it is assumed that these ligaments rupture very early in the lading f the specimen, then the area f the cracked regin at large lads wuld be this area enclsing the pres. regin abut.9 in. 2 causes Eu t drp belw 5%, yet up t.62 in. 2 prus regin can be tlerated fr which e u exceeds 5%. Figure 5 shws a nearly linear dependence f the ultimate tensile strength, n u, n the area f the pres. Hwever, the scatter in the dependence f the yield stress, rj y, is brad, and n attempt has been made t pass a line r curve f crrelatin thrugh thse data. It is again clear that a pre area larger than abut.3 in. 2 causes a significant reductin in tensile prperties, while such an area up t abut.2 in. 2 des nt reduce <r y belw ksi. In Fig. 6, bth n u and <r y are fairly linearly related t the area f the prus regin. These tensile prperties de- Fig. 2 Typical fracture surface passing thrugh a cluster prsity crease sharply when the pres are distributed ver an area up t abut.9 in. 2 Hwever, ver ksi tensile strength can be maintained with a prus area up t.26 in. 2 Figures 7 and 8 shw fairly linear dependences f n u n the cluster length (fracture surface) and n the cluster length (radigraphic). These figures are related t Fig. 6 in that they pertain t the extent f the regin having pres. The tensile prperties are certainly affected fr prus regins lnger than abut.4 in. r radigraphic lengths larger than.6 in. < Discussin The quantitative effect f the pres can be measured in terms f their area. Figures 3 and 5 indicate that pre areas in excess f.3 in. 2 cause a strng reductin in tensile strength, while ductility is markedly reduced at this defect size. The quantitative effect f pre distributin can be measured in terms f the area f the regin ccupied by the pres. Such an area up t abut.9 in. 2 (Fig. 6) 5 r Fig. 3. X O U s? s -!_ I _U.2.3 RE OF PORES (IN.') Unifrm strain vs. area f pres WELDING RESERCH SUPPLEMENT! 5-S

4 6, 3 O.9 X O D s < ^ RE OF POROUS REGION (IN 2 ) Fig. 4 Unifrm strain vs. area f prus regin z b Z # jj i v in - 3;: 2 n IOO -e- x & a n O C <7 N s CN ?? RE OF PORES (IN 2 ) i D 3 Fig. 5 Yield stress and ultimate tensile strength vs. area f pres causes a significant reductin in tensile strength, while Fig. 4 shws that the same area range causes a sharp lss f ductility. The prus regin area f.9 in. 2 seems t crrespnd t a cluster length (fracture surface) f abut.4 in. 2 (Fig. 7), r an effective radigraphic length f abut.6 in. (Fig. 8). The freging analysis indicates the qualitative nature f analyzing welds frm radigraphic images alne since a single nrmal incidence radigraph cannt prvide infrmatin n the depth (in the plate thickness directin) f a defect. This in turn emphasizes the qualitative nature f accept/reject standards. Radigraphic inspectin f the specimens in accrdance with MIL-R- 468 shwed that specimens and -26 culd be accepted: -26 as "brderline, standard I" and as "brderline, standard III." This judgment was based n the bserva- q 2». I i H I2 n IOO ^ "» - +* X ++ t n ( n " b CN , RE OF POROUS REGION (IN 2 ) Fig. 6 Yield stress and ultimate tensile strength vs. area f prus regin 35 y LENGTH OF POROUS REGION INTERCEPTED BY FRCTURE SURFCE (IN) Fig. 7 Ultimate tensile strength vs. length f prus regin intercepted by fracture surface tin that the prsities mst nearly fitted the specificatin's "scattered cavities" classificatin. Specimen -26 was judged cmparable t radigraphic standard CI-2, while specimen was judged cmparable t standard CI-3. Relatin f Experimental Results and Cnstructin Design Criteria t Weld Specificatins Bradly speaking, there are tw cnstructin design philsphies, r criteria, cmmnly in use. The lder criterin is "wrking stress design," in which nly elastic strains are permitted in the structure. This design criterin is thus quite cnservative, since nly very small deflectins are permitted. The newer criterin is "limit stress design" in which (nt accunting fr factrs f safety) the structure is allwed t be laded t the yield pint as lng as a large amunt f ductility can be btained. Cnsequently, this design criterin is far less cnservative than the lder ne. lthugh these criteria are actually nt nearly as simple as stated, these statements d shw the rugh distinctins between the tw design philsphies. Welding cdes d nt explicitly state which f the abve criteria are reflected in weld specificatins. The WS Structural Welding Cde Dl.- 72 in discussing the permissible weld design stresses f new buildings and bridges, states that "the permissible stresses... fr cmplete jint penetratin grve welds... shall be thse allwed fr the same kind f stress fr the base metal" and that "the base metal stresses shall be thse specified in the applicable Building Cde" (Ref. 4), such as the cdes f ISC and HTO. The ISC cde, sectin.5.. (Ref. 5), states that the allwable tensile stresses in structural steel may be as large as.6 n ym (except at pin hles), but nt greater than.5 - um, where n ym and rr um are the minimum values f the yield and ultimate strengths respectively, fr the grade f steel f interest. The ISC cmmentary n the cde states that the basic wrking stress factr f safety is 5/3 with respect t and that, at the net sectin f axially laded members, the factr f safety is 2 with respect t rx um (Ref. 6). The HTO cde fr bridges states that the design stress fr axial tensin in members withut hles is.55 rt ym (Ref. 7). Frm the Steel Design Manual, - ym = ksi and n um = 5 ksi fr 54 Grade F steel in the size tested in this prgram (Ref. 8). Fr the ISC cde, the safety factr must be 2 and the design stress 57.5 ksi. Fr the HTO cde, the design stress must be 55 ksi. If these cdes were applied t these data, the respective design stresses wuld drp nly by abut 5 ksi t 54 ksi and 5 ksi, respectively. This suggests that, fr welding cdes based n wrking stress design, cluster prsity in the extent studied here may nt significantly degrade the quality f weldments under axial tensin. Thus, in these cases, relaxatin f cluster prsity restrictins in welding cdes may be cnsidered. This cncept is supprted by the results f the weld quality evaluatin perfrmed in this study in accrdance with MIL-R-468. pplicatin f this specificatin shwed tw f the welds in this study t be acceptable; Figs. 5 and 6 shw five welds with ultimate strengths exceeding ksi. It is quite plausible that these five welds wuld have sufficed fr a static tensile lad in a wrking stress design. Rejectin f these welds fr such an applicatin 6-8 I PRIL 976

5 prbably wuld accmplish nthing but increased prject csts. Less stringent radigraphic standards might thus be beneficial. Hwever, as Figs. 3 and 4 shw, a marked reductin in ductility f weldments ccurs when even small amunts f prsity are intrduced. This implies that relaxatin f welding cdes based n limit design cncepts wuld be undesirable, since the ductility required in that design is lst in prus weldments. One example f such a limit design applicatin is fund in Sectin III f the ME Biler and Pressure essel Cde (Ref. 8). In this cde, the algebraic difference between the largest and smallest principal stresses is defined as the stress intensity, S. Fr a given material, the largest S permitted is dented S m, and is usually - um /3. In the cde requirements fr Class cmpnents perating under nrmal and upset cnditins, sme cmbined lading cases allw a peak stress intensity f 3 S n, which is essentially the yield strength value. It is evident that n reductin in ductility r tensile strength culd be tlerated in such a critical design. cceptance f nly the mst ductile welds wuld be essential t the prper implementatin f a limit design. Thus, a reductin in the quality f radigraphic standards wuld nt be beneficial in a limit design. Summary Cnclusins. There is a cmbinatin f pre size (in terms f ttal area f pres) and distributin f prsity (in terms f area encmpassed by pres, i.e., the area f the prus regin) that is critical in the sense that the tensile strength is nt degraded until that pre size r prsity distributin is surpassed. In this study the critical pre area is abut.3 in. 2 and the critical area f the prus regin is abut.9 in There is a similar cmbinatin f pre size and distributin f prsity that is critical in that the ductility (as measured by e u) is sharply degraded until that pre size r prsity distributin is surpassed; beynd the critical pint the rate f degradatin is reduced. In this study the critical pre area is abut.3 in. 2 and the critical area f the prus regin is abut.9 in There is an effective radigraphic length f cluster prsity assciated with the critical cmbinatins in Cnclusins and 2. In this study, the tensile strength was unaffected fr radigraphic lengths less than.6 in., while the ductility drpped sharply until that length was reached. 4. In view f the relatin f tensile strength t critical cluster prsity size, relaxatin f weld inspectin specificatins may be pssible fr certain applicatins if the specificatin is based n wrking stress structural design criteria. Hwever, cnsidering the marked reductin in ductility with increasing prsity, relaxatin f weld specificatins based n limit design criteria shuld nt be cnsidered. Future Wrk The WS, ISC, and HTO cdes cntain specificatins fr the shear f weldments that are distinct frm thse fr the tensin f weldments. Data n the shear f weldments cntaining cluster prsity shuld be acquired t determine the extent f degradatin f the yield and ultimate shear stresses and unifrm shear strain. This will test the questin f whether wrking stress design may be unnecessarily cnservative in shear as well as in tensin f weldments. cknwledgments This investigatin was perfrmed fr the Office f the Chief f Engineers. U. S. rmy, as cvered in Ref.. The able assistance f Mr. F. Kisters f the Metallurgy Branch, CERL, is gratefully acknwledged. The views f the authrs d nt purprt t reflect the psitin f the Department f the rmy r f the Department f Defense. References. Lawrence, F. v., Jr., and Cx, E. P., "The Influence f Incmplete Jint Penetratin n the Tensile Behavir f 54 Steel Welds", Welding Jurnal, t be published. 2. Carlsn, K. W., Lawrence, F... Jr.. and Radziminski, J. B., "The Intrductin f Discntinuities in High Strength Steel LENGTH OF POROUS REGION INTERCEPTED BY RDIOGRPH (IN.) Fig. 8 Ultimate tensile strength vs. length f prus regin intercepted by radigraph Weldments," U. S. rmy CERL Preliminary Reprt M-27, December 972. D Carlsn, K. W., and Lawrence, F.., Jr., "The Examinatin f Discntinuities in Welds by Stereradigraphy." U. S. rmy CERL Technical Reprt M. September 972. D The merican Welding Sciety, WS Structural Welding Cde, WS D.-72, 972, pp 8 and The merican Institute f Steel Cnstructin. Specificatin fr the Design, Fabricatin, and Erectin f Structural Steel Buildings, 969, pp The merican Institute f Steel Cnstructin, Cmmentary n the Specificatin fr the Design, Fabricatin, and Erectin t Structural Steel fr Buildings, 969, pp The merican ssciatin f State Highway and Transprtatin Officials, Standard Specificatins fr Highway Bridges. th ed p United States Steel C., Steel Design Manual, 968, p The merican Sciety f Mechanical Engineers, "Nuclear Pwer Cmpnents," ME Biler and Pressure essel Cde, Sectin III. Subsectins N and NB, Hnig. E. M., Jr.. "Effects f Cluster Prsity n the Tensile Prperties f Butt- Weldments in T- Steel," U. S. rmy CERL Technical Reprt M-9, Nvember 974. D 4. The merican Welding Sciety publishes ver a hundred different Cdes, Standards, Recmmended Practices, Guides, Handbks and ther bks related t welding. If yu wuld like a cpy f ur Publicatins List, simply circle N. 5 n ur Reader Inf-Card and mail it. WELDING RESERCH SUPPLEMENT! 7-s