UDK 539.42:539.55:621.791.05:669.14.018.298 Izvirni znanstveni članek ISSN 1580-2949 MATER. TEHNOL. 35(3-4)89(2001) I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED ... THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED WELD JOINTS PROVIDED ON HSLA STEELS POJAV LOMA V GLOBALNO/LOKALNO TRDNOSTNO NEENAKIH ZVARNIH SPOJIH, IZVEDENIH NA VISOKOTRDNOSTNIH JEKLIH Inoslav Rak, Nenad Gubeljak, Zdravko Praunseis University of Maribor, Faculty for Mechanical Engineering, Welding Laboratory, Smetanova 17, 2000 Maribor, Slovenija Prejem rokopisa - received: 2001-01-11; sprejem za objavo - accepted for publication: 2001-04-10 The fracture behaviour of thick-section high-strength steel weldments that contain soft root passes has been studied. Two different weld consumables with different strength-mismatch (1>M>1) and fracture-toughness properties in the WM have significantly increased the complexity of the mismatch effect and the failure behaviour of weld joints, depending upon the crack location and the thickness of the soft root layer. The aim was to explain the effect of strength heterogeneity between the BM and the WM, and between different regions in the WM (global/local mismatching). R-curves of the WM and the HAZ regions were also discussed. The conclusion is that the application of a welding procedure with a two-pass soft root layer introduced for the purpose of reducing or even omitting preheating, can be recommended for mismatched weld joints on HT80 steel. Nevertheless, the alloying from BM and the tempering effect of the subsequent weld passes have to be taken into account. They can cause a reduction in the root-region ductility and affect the local mismatch in the WM and the HAZ. The deterioration when providing a soft root layer can probably be reduced by choosing a particular consumable and a proper welding procedure. The final conclusion is that the application of a mismatched weld joint with a soft root layer can be recommended only if high root toughness can be provided. Key words: toughness, ductility, fracture toughness, mismatch effect, soft root layer, weld joints, high-strength steel Raziskovan je bil pojav loma v zvarnih spojih, izvedenih na visokotrdnostnem jeklu, ki so vsebovali mehke korenske varke. Dva različna dodajna materiala z različno trdnostno neenakostjo (1>M>1) in lomno žilavostnimi lastnostmi v strjenem zvaru sta izrazito povišala kompleksnost vpliva trdnostne neenakosti in pojave loma v zvarnih spojih. Navedeni vpliv je bil odvisen od položaja razpoke in debeline mehkega korenskega sloja. Namen je bil raziskati vpliv trdnostne heterogenosti med osnovnim materialom in strjenim zvarom in med različnimi področji v strjenem zvaru (globalna/lokalna trdnostna neenakost). Poleg tega so bile obravnavane odpornostne krivulje področij v strjenem zvaru in toplotno vplivanem področju. Zaključek je, da je uporaba varilne tehnologije z dvovarkovnim mehkim slojem, ki je vnesen zaradi znižanja ali celo preprečitve predgrevanja, priporočljiva za trdnostno neenake zvarne spoje pri jeklih tipa HT80. Pri tem pa je treba upoštevati nalegiranje, ki izhaja iz osnovnega materiala, in vpliv popuščanja od naslednjih varkov. To lahko povzroči zmanjšanje duktilnosti v področju korena in vpliva na lokalno trdnostno neenakost v osnovnem materialu in toplotno vplivanem področju. Poslabšanje lastnosti z uporabo mehkega korenskega sloja je verjetno lahko znižati z uporabo ustreznega dodajnega materiala in izbrano varilno tehnologijo. Sklepamo, da je uporaba trdnostno neenakega zvarnega spoja z mehkim korenskim slojem priporočljiva le, če je korenska žilavost visoka. Ključne besede: žilavost, duktilnost, lomna žilavost, trdnostna neenakost, mehki korenski sloj, zvarni spoj, visokotrdnostno jeklo 1 INTRODUCTION Substantial differences in the strength properties (mismatch) of thebasematerial (BM), theweld metal (WM) and the heat affected zone (HAZ) are found in the weld joints of high-strength-steel constructions. It is common practice in various engineering constructions to deposit WM which has a higher strength (overmatching) than the steel. In this case, the higher strength of theWM compared to theBM can providethebest weld-joint performance by shielding short cracks or other planar faults from the applied strains. It is however, rather difficult to deposit WM with a strength level which over-matches the high-strength micro-alloyed or low-alloy steel, and simultaneously fulfils the codes which prescribe the level of impact toughness. Sometimes due to the weldability limitation (sensitivity to coarse-grained HAZ-CGHAZ cold cracking for instance) the deposition of WM which has a lower strength (under-matching) is used with the aim of reducing or even eliminating the preheating. On the other hand, the fused part of the BM can effect, because of its additional alloying, the degree of weld under-matching which can be shifted towards the properties of the BM. Nevertheless, higher under-matched WM with lower toughness can be a potential danger for structural integrity. As a result it is necessary to have a higher level of toughness than that of the BM. It is known, however, that a fused-weld impact-toughness energy of over 50 J at -10 °C, or lower, is difficult to attain for Q+T steels with a yield strength above 700 MPa. Thefailurebehaviour of such a mechanically heterogeneous weld joint has to be influenced by the strength levels of the neighbouring zones associated with the existent defect. Both the strength and toughness of MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 89 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED the defective weld region will control the brhaviour of the welded structure. The aim of this research work was to estimate the CTOD fracture toughness using standard procedures 1,2,3,8 of over- and under-matched X-grooved multi-pass weld joints. Deeply notched standard SENB specimens (a/W=0.5), with thenotch positioned at thecentrelineof the WM, in the through-thickness direction were used. Wealso wanted to takeinto account theeffect of the weld width (2H) on the CTOD behaviour. The weld width varies across thethickness dueto theX weld groovepreparation and is theshortest at theroot region. Weintended to provethat if theweld width (2H) is shorter than the uncracked ligament, W-a, than the local brittlezone(LBZ) can beforced to appear early and the standard procedure for estimating JIC and ?c remains inaccurate 4. In order to determine the fracture toughness of the HAZ, steps were taken to hit the fusion boundary by notching theCG- or ICCGHAZ at two points to receivea so-called "compositenotch". Sincetheplastic deformation associated with the crack tip is not symmetrical in this case, the reduction of the HAZ toughness for over-matched weld joints as a result of increasing constraint dueto therestriction in plastic deformation on the WM side was expected 5. Dueto plastic deformation on theWM sidefor th under-matched weld joints an apparent increase in the HAZ toughness is expected, but only in the case when the WM is tough enough and its strain hardening takes place. Special treatment is needed during fatigue-crack preparation on a CTOD specimen taken from the weldments. If one uses the prescribed procedures (valid for uniform materials) 6,7,8, then the crack-tip front will not be straight. This phenomenon is caused by the residual stresses distributed in the specimen, through-thickness and heterogeneous hardness distribution along thecrack tip 9,10. To overcome this problem the modified version of High R-ratio, the so-called "Step Wise High R-ratio (SHR) method", is used 1,11. This method was improved recently by introducing the model of straight-fatigue-front prediction, which takes into account themagnitudeand distribution of theresidual stresses intensity factor at two levels and thus the maximum forcefor thepre-crack loading calculation. With this model the prediction of the equilibrium stage of fatigue-crack propagation is possible by finding the optimum fatigue- loading regime12. So, using this improved method, and taking into account the highest valueof theWM yield stress, straight crack-tip fronts can be achieved for the as-welded specimen. Using the a procedure in BS 7448, Part 2, such as Local compression as an alternative method, was shown to be inappropriate for global/local mismatched weld joints with a soft root layer. A series of Charpy impact toughness specimens taken from different weld-joint regions, and BM and a 90 series of SENB specimens extracted from thick steel weldments prepared for welding by using X-groove, were made of under- and over-matched WMs. The CTOD values on the SENB specimens which were perpendicularly notched and prefatigued in the WM and the HAZ, achieving the ratio a/W=0.5, are presented and the fracture behaviour of over- and under-matched weld joints with and without a soft root layer are compared . The differences in the mechanical properties among the different weld regions affected the strain distribution around the crack tip during the fracture-toughness tests and consequently affected the measured CTOD fracture toughness values of the under- and over-matched weld joints. 2 TESTING PROCEDURES AND RESULTS 2.1 Welding Procedures High-strength-low alloyed (HSLA) steel in a quenched and tempered condition, corresponding to grade HT 80, was used. For the welding of steel plates the FCAW procedure, and two tubular wires were selected so as to produce weld joints in over- and under-matched conditions. In Tables 1 and 2 the mechanical properties and the chemical compositions of the BM and the measured mechanical properties and the chemical compositions of the all-weld metals for the selected consumables are given. The strength mismatched factors M (M=WM yield stress/BM yield stress) were 0.76 for under- and 1.08 for over-matched weld joints. Table 3 shows the welding procedure and all the welding data used. In Table 4 thechanges of the chemical composition among the cap and root regions of thehomogeneous WM and theWM of thesoft root layer for an under-matched and over-matched weld joint are presented. Figure 1a and Figure 1b show a cross-section of the multi-pass homogeneous weld joint and theweld joint with thesoft root layer for on over-matched weld joint. 2.2 Testing of mechanical properties Mechanical properties were determined with specimens taken from welded plates, as shown in Figure 2. Only the results of testing the CTOD Bx2B specimens are presented. In Table 5 the mechanical properties obtained with full-thickness flat tensile specimen testing of over- and under-matched weld joints are shown. It can be seen that all the flat over-matched specimens failed in the BM and did not show any of the effects expected from a soft root layer. In the under-matched specimen the effect of the soft root layer was pronounced because it failed in the WM, while the under-matched specimen of the homogeneous weld joint failed in the BM. The mechanical properties of the WM obtained with round tensile bars for different WM regions are shown in Table 6. It can be recognisis clear that the under- and MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED Table 1: Mechanical properties of base metals and all weld metals for under- and over-matched weld joints Tabela 1: Mehanske lastnosti osnovnega materiala in čistega strjenega zvara v zvarnih spojih z nižjo in višjo mejo tečenja od osnovnega materiala Designation Y.S U.T.S. Elongation Charpy imp. Designed mis-match (MPa) (MPa) (%) toughness (J) factor M=GyWM/OyBM HT 80 used for 693 830 19.6 79, 78, 64 um weld joit at -10 °C - HT 80 used for om weld joint 711 838 19.6 158, 130, 158 at -50 °C Filler wire 403 466 32 100, 215,145 0.58 um, 0.56 om WM 3-B370 at -40 °C Filler wire 542 591 23 47, 70, 71 0.78 um, 0.76 om WM 2-B 575 at -40 °C Filler wire 770 845 16 59,55, 60 1.11 um, 1.08 om WM 1-B 800 at -40 °C Table 2: Composition of materials and all-weld metals (wt%) Tabela 2: Sestava materialov in čistih strjenih zvarov (mas.%) Chemical composition C Si Mn P S Cr Ni Mo Cu Al HT 80 um HT 80 om 0.10 0.09 0.68 0.27 0.75 0.25 0.020 0.015 0.003 0.004 0.79 1.12 0.09 2.63 0.032 0.25 0.24 0.037 0.020 Filler wire B370 0.05 0.25 0.61 0.011 0,008 0.06 0.07 0.03 - - Filler wire B 575 0.05 0.04 1.52 0.011 0.008 0.08 1.45 0.66 - - Filler wire B 800 0.06 0.35 1.43 0.009 0.008 0.86 3.01 0.56 - - HT 80um-base material used for under-matched, HT 80om-base material used for over-matched weld joints Table 3: Welding procedure Tabela 3: Postopek varjenja FCAW Welding procedure (80%Ar+20%) Under-matched weld joint Over-matched weld joint Filer consumable, root pass/other passes B575/B575 Hom. joint B370/ B575 Heter. joint B800/B800 Hom. joint B575/B800 Heter. joint Preheat temp. °C 120 - 102/113 - Heat input kJ/cm 16.5/16-23 10.1/16-23 18.14/19.63 18.06/18.78 Calculated At8/5, s 9.5/10.7 7.1/10.3 9.70/10.17 10.18/11.39 Measured At8/5, s 8.9/9.2 6.7/8.6 9.08/9.78 7.11/9.32 Interpass temperature 135/135 50/135 135 50/135 Postheating temperature, 200 °C/2 hours 200 -/- 200 -/- Number of passes 2/14 2/16 2/14 2/17 over-matched weld properties that were designed were not achieved in the two homogeneous weld joints. This might bedueto theweld pool alloying from themolten surrounding, thelocal quenching during cooling, or tempering as a consequence of additional weld bead deposition. The alloying effect is more pronounced in the root region of the under-matched weld joints than in thecup region. This is thereason why theWM under-matching properties in the through thickness did not appear. For the over-matched weld joints the designed mismatch properties were also exceeded. The alloying effect in the root region is actually less pronounced, but the absence of tempering effects in the cup region results in an increase in strength. The use of a lower strength filler consumable to produce a softer root layer produced a real under-matched condition in the designed under-matched weld joint, whilst the soft root layer in the designed over-matched weld joint did not provide the desired under-matched properties. 2.3 Hardness measurements The WM hardness measurements on both the under-and over-matched weld joints, and the yield stress calculation using theformula ?yw= 3.15HV-168 1,13, give even higher mismatch differences than the tensile-test MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 91 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED Table 4: Chemical composition of under- and over-matched weld metals of homogeneous and heterogeneous (soft root layer) weld joints Tabela 4: Kemijska sestava homogenih in heterogenih zvarnih spojev (mehki korenski sloj) z nižjo in višjo mejo tečenja od osnovnega materiala Chemical composition Composition (wt%) Under-matched weld joint C Si Mn P S Cr Ni Mo Cu Al WM - cap 0.04 0.44 1.48 0.010 0.009 0.12 1.63 0.49 0.12 WM - root 0.10 0.33 0.89 0.013 0.008 0.73 1.11 0.42 0.13 WM - soft root layer 0.12 0.41 0.78 0.015 0.006 0.40 0.10 0.17 0.16 Over-matched weld joint WM - cap 0.07 0.36 1.27 0.008 0.015 0.86 2.21 0.47 - 0.004 WM - root 0.08 0.32 0.78 0.012 0.013 0.99 2.50 0.35 - 0.014 WM - soft root layer 0.08 0.40 1.12 0.007 0.013 0.49 1.75 0.44 - - Table 5: Mechanical properties of full-thickness flat tensile specimens Tabela 5: Mehanske lastnosti ploščatih preizkušancev iz celotne debeline Weld joint type Rm (MPa) RA(%) Failure mode om-homog.w.j. 862 18.0 BM om-soft root w.j 849 19.5 BM um-homog.w.j. 804 22.6 BM um-softrotw.j. 792 20.5 WM Table 6: Mechanical properties of under- and over-matched weld joints of homogeneous and heterogeneous (soft root layer) weld joints Tabela 6: Mehanske lastnosti homogenih in heterogenih zvarnih spojev (mehki korenski sloj) z nižjo in višjo mejo tečenja od osnovnega materiala Designation Y.S. (MPa) U.T.S. (MPa) Elongation (%) Charpy (J) at -10 °C DesignedM Achieved M Under-matched weld joint - filler material B575 WM - cap 687 804 22.3 110, 104, 102 0.76 0.99 WM - root 730 803 21.8 72, 38, 50 0.76 1.05 Under-matched weld joint - soft root filler material B370 + B575 WM - soft root layer | 567 | 625 | 19.7 | 35,17,34 0.56 0.81 Over-matched weld joint - filler material B 800 WM - cap 861 951 11.7 56, 46, 66 1.08 1.21 WM - root 807 905 15.3 55, 56, 55 1.08 1.14 Over-matchedweld joint - soft root filler material B575 +B800 WM - soft root layer 769 818 17 71 0.76 1.08 0.82-0.96* HAZ notch WM notch position position HAZ notch Direction of microhardness measuring Figure 1: Weld joints cross-sections Slika 1: Prerez skozi zvarna spoja Figure 2: Specimens sampled in welded plates Slika 2: Vzorčenje preizkušancev v zvarjenih ploščah 1. Series of CTOD specimens Bx2B, with notch tip (through thickness) completely in weld metal - 2. Series of CTOD specimens Bx2B, with notch tip (through thickness) partly in the base metal - 3. Full thickness flat bend specimen, (normal bending) - 4. Full thickness flat bend specimen (transverse bending) - 5. Full thickness flat tensile specimen - 6. Series of the Charpy-V tests specimens with machined notch in the weld metal and in HAZ - 7. Series of CTOD specimens BxB, with surface notch tip completely in the weld metal - 8. Series of CTOD specimens BxB, with surface notch tip completely in the HAZ -9. Series of round tensile specimens - 10. Sample for metalographic sectioning. 92 MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED ... Low hardeness in overmatched weld WMmn WMlfjH+raot WMifii+WNW 10 15 20 25 30 35 40 Distance through thickness, mm 1.2 1 ¦s S 0.8 1.2 1 0.8 WMaill+root --------WMM+WMsraot / Č \ č ;' \SČS \_V ČX 0 5 10 15 20 25 30 35 40 D Distance through thickness, mm Figure 3: Mis-match distribution across the midthickness of a) over--matched, b) under-matched weld joint Slika 3: Porazdelitev trdnostne neenakosti skozi debelino a) visoko-trdnostnega b) nizkotrdnostnega zvarnega spoja results. Hardness measurements with a distance of 1 mm between two hardness indentations in the WM through thickness direction have shown factor M deviations over thewholeWM thickness. Local mismatching distributions calculated from the hardness values are plotted in Figure 3a and 3b. They can becompared with thosein Table6. It is evident that the determination of the real weld-joint mismatching properties is a complex task, due to the presence of different alloying and dilution mechanisms acting during thewelding in thewelding pool and theinfluenceof different bead-quenching and tempering effects on the strength. Local mismatching distribution determined by the hardness measurement in the through thickness direction across the WM, CGHAZ and BM gives even higher local deviations. The local mismatching along the prefatigued crack-tip line will certainly play an important rolein crack initiation and propagation. Therefore, the analysis of the appearance and origin of brittlecrack initiation reveals thelocal brittlezones (LBZs). Thus, the mechanical properties shown in Table 6 represent only the average properties of the exact areas where the tensile specimens were taken from and cannot givetheexact mismatching condition of thewholeweld ?NS- -40 -30 -20 -10 Temperature, °C -50 -40 -30 -20 -10 D Temperature, "C Figure 4: Impact toughness transition curves for a) over-matched b) under-matched weld joint Slika 4: Krivulje udarne žilavosti za a) visokotrdnostni b) nizko-trdnostni zvarni spoj 0.8 0.7 0.6 1 0.5 Š 0.4 O 0.3 0.2 0.1 1 WMifiii WMiroot WM1fi|| WMifiii WMifiii WM2root WM3root WMiroot °*u ÖDfflČffl D ÔU • 5m 8c* ¦ ou* WMifiii WM2root en Figure 5: CTOD(Ô5) values for specimens with the crack front through the HAZ and WM in the over-matched weld joints, measured at -10 °C Slika 5: CTOD(Ô5) vrednosti za preizkušanec z fronto razpoke skozi TVP in zvar v visokotrdnostnih zvarnih spojih, izmerjenih pri -10 °C MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 93 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED Aajump X>< ČP't Different portion T of base metal in the crack front d ? j-r HAZ WM o o WMlfilHroot ? ? WMiiiii+WM2root 1.2 1.4 1.6 Figure 6: CTOD-R resistance curves for specimen with the crack front through theHAZ (crack depth a/W=0.5) and WM in theover-matched weld joints Slika 6: CTOD-R odpornostnekrivuljeza preizkušancez fronto razpokeskozi TVP (globina razpokea/W=0,5) in zvar za visokotrd-nostnezvarnespoje joint. It seems that the local mismatching, which is extremely pronounced in the narrow CGHAZ region and often involved in the mechanism of the appearance of LBZs, can beestablished by theavailabledata on micro-hardness measurements. 2.4 Impact toughness measurement TheV notch of theCharpy impact toughness specimen was introduced into the BM, CGHAZ, WM cup layers (WM-C) and the WM root layers (WM-R), to plot thetransition curve. In Figure 4a thetransition curves are plotted for the over-matched weld joint, whereas in Figure 4b the same curves are presented for the under-matched weld joint. In both figures, the V-notch position in the weld joint is marked. For theover-matching condition (Figure 4a) the lowest impact toughness is measured in the weld-joint cap area. The alloying elements Mn, Ni and Mo due to the melting of the lower alloyed BM, compared to filler metal B800, reduced slightly (higher Y.S.), whereas the reduction of Mn, Ni and Mo in the root region is more pronounced (lower Y.S.). A higher Ni content improves slightly theimpact toughness of theWM-R. The upper-shelf impact toughness is approximately the same for WM-C and WM-R. Theimpact toughness for BM and thearea wheretheV-notch is sampling theWM, the fusion lineand theHAZ is, in thesinglespecimen approximately the same for all the tested temperatures. Theimpact toughness of thesoft root layer is dueto the alloying of C, Si, Cr, and Ni and because of to the reducing of Mn and Mo it is not the lowest. It is between theBM and theWM-R/WM-C values. For the under-matched condition the lowest impact toughness is obtained where the yield stress is the highest, see WM-R from Table 6 and Figure 4b. Dueto alloying elements such as C, Si, Cr and Mn, Ni, Mo reducing because of the dilution and alloying of this Figure 7: The crack growth path deviation of cleavage crack propagation on the macro-etch sectioning 8 mm below of fatigue pre-crack front, as the consequence of global/local mis-matching Slika 7: Sprememba smeri krhkega širjenja razpoke na odaljenosti 8 mm pod utrujenostno fronto, kot posledica globalno/lokalne trdnostne neenakosti region from the BM and the filler metal. The whole transition curve is considerably lower than the transition curve of the BM and the upper-shelf toughness is found to be the lowest. This indicates the position where, due to the lowest toughness and more pronounced plane strain condition, the potential danger of LBZ appearance can exist. Better impact toughness can be recognised in the cap layers. This is the consequence of Mn and Mo reducing and Ni alloying. The upper- shelf impact toughness for theBM is lower than that in thearea wheretheV-notch is sampling theWM, thefusion line and theHAZ in thesinglespecimen. Theimpact toughness of the soft root layer is very low. The alloying of C, Mn, Cr and Mo can be recognised. The yield stress and the strength are increased considerably and the ductility and the impact toughness are reduced, see Table 1 and Table 6. 2.5 CTOD fracture toughness According to the BS 7448 codes, all specimens should be fatigue precracked. The consequence of the usual prefatigueprocedureis theappearanceof a 0.8] WM2AH WM2fi|| 0.7 WM2root WM3poot WM2root ¦ 5c ¦ 8u 0.6 » 8o* " Su* | 0.5 WM2fi|| §. 0.4 BM WM3root O O 0.3 0.2 0.1 i ¦¦ 4 ¦ ? o D D O ¦ ¦ ¦ >• »— u*u aa > n Figure 8: CTOD(?5) values for specimens with the crack front through the HAZ and WM in the under-matched weld joints, measured at -10 °C Slika 8: CTOD(?5) vrednosti za preizkušanec z fronto razpoke skozi TVP in zvar v nizkotrdnostnih zvarnih spojih, izmerjenih pri -10 °C 94 MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED ... Figure 9: Comparison of R curves for HAZ and WM of homogeneous and heterogeneous undermatched weld joints Slika 9: Primerjava med R krivuljami za TVP in zvar za homoegene in heterogene nizkotrdnostne zvarne spoje non-straight crack-tip line, because of the non-uniform through-thickness residual stress distribution in the weld joint. To overcome this problem the Step-Wise High-R-ratio method for precracking was used for the rest of the specimens 1,11. When a crack initiation and growth of about 1 mm R=0.1 is used then the stress ratio is increased to R=0.7 with an allowable maximum load to therequired a/W ratio. Theallowablemaximum load for both ratios was calculated using the Ref. 1 equations. Some of specimens treated with the described R-ratio method were also invalid. The reason is probably the unknown effect of the distribution and magnitude of the residual stresses. The R-ratio method was improved 15, on the basis of determining the residual stress-intensity factor through theweld-joint thickness in theplaneof thefatiguecrack propagation, and so, thelimit conditions for theR-ratio procedureunder thestandard requirements for maximum precracking load were determined. For CTOD testing a single-specimen method was used. The geometry of the SENB specimen was Bx2B (B=40 mm) and the through-thickness notches were positioned in theWM and in thevicinity of themid thickness CGHAZ (CTOD composite notch specimen) as shown in Figure 1. The testing temperature was -10 °C. During theCTOD tests theDC potential-drop techniquewas used for monitoring thestablecrack growth 14. Theload-linedisplacement (LLD) was also measured with a reference bar to minimise the effects of possibleindentations of therollers. TheCTOD values were calculated in accordance with BS 7448, Part 2, and designated (SBS) 1 and also directly measured with a GKSS developed 85 clip gaugeon thespecimens side surfaces at thefatiguecrack tip over a gaugelength of 5 mm 3. The HAZ and WM CTOD data measured on the SENB specimen extracted from the multipass overmatched weld joints are given in Figure 5. For all 0.4 E E 0.3 "/ CTOD(BS), o ho /• ¦ 5c ¦ 8u ¦ / • 5m 0.1 ČmČ ° 5c* " 5u* 0 /,, , , , , , , , , , , , , , 0.1 0.2 0.3 CTOD(S5), mm 0.4 0.5 0.5 0.4 E E 0.3 0.2 0.1 - : D : ¦ / un/ ¦ / ¦ / ¦ /a Sc o / */ * ¦ / ¦ Su ° Sc* : / ¦ Su* '/...,....,. . . I ' ' ' ...... 0.1 0.2 0.3 CTOD(S5), mm 0.4 0.5 Figure 10: Comparison of directly measured (85) and calculated (5bs) CTOD fracture toughness values in homogeneous and heterogeneous a) over-matched b) under-matched weld joints Slika 10: Primerjava direktno izmerjenih (85) in izračunanih (Ôbs) CTOD vrednosti lomne žilavosti v homogenih in heterogenih a) visokotrdnostnih b) nizkotdrnostnih zvarnih spojih specimens except the clear BM specimens, where M is equal to 1, after some of slow crack growth (8c/8u values) the "pop-in" event appeared as a consequence of the LBZ presenceand as thebrittlefracturehad interrupted the testing. Nevertheless, the measured WM and HAZ CTOD fracture-toughness values of the homogeneous over-matched weld-joint conditions were different in magnitude, the HAZ fracture toughness being higher than theWM toughness 16. This can be seen from Figure 6 which represents the R-curves for the HAZ and the WM comparing the homogeneous weld joints and the weld joint with a soft root layer 12. But thefracture-toughness magnitude when comparing the differences between the two weld joint was approximately the same in theHAZ and theWM. Thefracturepath in thecaseof thenotch position in theHAZ was not affected by the softer root layer, shown in Figure 7b. This was not, however, when using, for example a soft root layer MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 95 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED which can locally reducethefracturetoughness as shown in Figure 5 and Figure 7c. The HAZ and WM CTOD data measured by the SENB specimen extracted from the multipass under-matched weld joints are given in Figure 8. Dueto the dilution of BM in the root region the over-matched condition appeared in the homogeneous WM despite an under-matched filler metal being used as can be seen from Figure 3b. This was the reason that the specimen failed in the softest regions which were placed among WM cap and root passes 16. Obviously thelocal mismatching of thethrough-thickness in theWM and the CGHAZ had affected the yielding behaviour along the crack tip. TheHAZ fracturetoughness was thehighest, this was also proven when testing impact toughness, see Figure 4b. The reason was the over-matched condition in theroot region and thefracturewas deviated into the BM as in thecaseof theover-matched weld joint. But theHAZ fracturepath changed when using a soft root layer 17. Dueto thelower root strength theinitiating fracture was deviating into root WM where "pop in" appeared as a consequence of the presence with very low toughness, see Figure 4b. Similarly thesamelocal effect was recognised in the specimens which were notched in the WM. Both affects can be clearly seen on Figure 9. 3 DISCUSSION It is a dilemma to decide whether to choose a high-loaded weld-joint under-match or over-match condition. When choosing the over-matched condition the most difficult problem is to prevent cold cracking in the WM even after 48 hours where the possibility of delayed cracking could appear. One can overcome this problem by introducing pre- and/or post-heating. The problem of the required WM toughness which is officially prescribed by the codes is still unsolved. The use of the recently introduced ETM 18,19 offers a good solution when the WM moderate toughness compared to the higher BM toughness is prevailing in the weld joint. The ETM concept does not incorporated the affect of residual stresses which could probably cause some additional solutions and which should beincorporated into it for proper usage. When choosing an under-matched condition the use of a preheating procedure is reduced or even omitted, but due to the softer WM strength properties, a high toughness is needed locally which should prevent the occurrence of brittleness when strains areintroducing into theWM. So, thepurposeof this research work was to answer the question whether the soft root layers in over/under-matched weld joints for a high-loaded condition can be used. To get the proper solution a careful analyses of the results should be carried out. 96 3.1 Fracture properties of an over-matched weld joint CTOD specimens taken from over-matched weld joints show WM CTOD values which are lower than thosefor theHAZ wherea compositenotch is used. Due to the lowest 2H/(W-a) ratio and the presence of higher stress intensity where the constraint is the highest (the conditions aresimilar to thoseby thenarrow bi-metal mismatch) the initiation point was expected in the weld root region of the homogeneous weld joint (H is half of theweld width). But dueto thehigher root hardness shown in Figure 3a which is the consequence of the BM dilution, two LBZ on both sides of the root regions appeared, where the lowest mismatched properties were revealed. When the WM soft root layer was introduced, a so-called heterogeneous weld joint was appeared. The soft root layer did not changethefractureproperties, as can be seen from Figure 5 and Figure 6, compared to the homogeneous weld joint. This is probably the consequence of the not-too-low undermatching in the root region compared to the over-matched remaining WM portions (M?0.9). The HAZ composite notch specimen was sampled WM-C (see Figure 1) on both sides, the CGHAZ at two points and theBM at themid thickness section. At the fatigue-tip front a pronounced crack blunting and strain hardening over the whole mid thickness appeared in the BM with high fracturetoughness dueto th over-matched WM properties of the homogeneous weld joint. The first unstable event appeared in the CGHAZ region as a small pop-in with the crack deviation from thefusion linetowards theBM. Becauseof thelower WM fracture toughness two LBZs appeared later where thecrack tip is sampling theWM. This is the consequence of the WM matching condition attained due to theblunting and strain hardening of thesofter BM. The crack tip was no longer protected, whereas a ductile fractureof themid section followed in theplanewhich deviates due to the overall weld-joint over-matching condition towards theBM. In thecaseof thesame heterogeneous weld-joint HAZ sampling, the soft root layer did not change the fracture properties either. This can be seen from Figure 7b. CGHAZ at thefusion line was again theposition of theLBZ spreading into less tough over-matched WM. The reason was the same as described for the WM fracture. The under-match was not low as to affect thefracturepath as can beseen in the case when using lower undermatched properties, M<0.8, and the fracture is directed by a soft root layer as shown in Figure 7c. 3.2 Fracture properties of an under-matched weld joint For the WM specimens taken from the under-matched homogeneous weld joints "pop-ins were expected at the LBZ's weld root where the 2H/(W-a) ratio is the smallest. These expectations were not fulfilled and wereprobably dueto theover-matched MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED ... shielding effect which is evident in the root region and can be clearly seen in Figure 3b. Theweld width (2H-30 mm) at the weld cap is approximately the same as thesizeof theuncracked ligament (W-ač35-40 mm). Therefore, the brittle-fracture initiation points appeared in the softest WM region where 2H is much smaller than the uncracked ligament. The LBZs appeared in the areas of thelowest M value(see Figures 3b arrows). After the initiation the main brittle-fracture event appeared at the root as a consequenceof a sudden risein thestress intensity and lower toughness. For the WM specimen taken from the under-matched heterogeneous weld joint with soft root layer the "pop-in" appeared at the LBZ's weld root where it was expected, due to the softest weld-joint portion and due to the lowest tough region as the consequence of theb BM dilution. The CTOD fracture toughness is slightly lower compared to the homogeneous WM as can be seen from the Figures 8 and 9. A fractureexamplefor both weld joints CTOD(Ô5) versus Aa is given in Figure 9. The behaviour of the HAZ specimen taken from the homogeneous under-matched weld joint precracked by thecompositenotch through WM-C, HAZ, WM-C was a result of higher HAZ hardness and over-matched conditions in theroot region as theappearanceof visible LBZ in the remaining WM with the lowest M. After the LBZ crack initiation a risein thestress intensity of the tempered HAZ and the remaining BM took place. A final ductilefractureacross theHAZ with a deviation into thetougher BM in themiddleof thespecimen was the consequence. CTOD(Ô5) versus Aa for theHAZ composite notched SENB specimen is also given in Figure 9. The behaviour of the HAZ specimen taken from the heterogeneous under-matched weld joint with soft root layer was quite different. After the LBZ crack initiation at theCGHAZ and theremaining WM thesoft root layer was thecauseof thefracture-path transformation into the soft layer with poor toughness. Thelowest CTOD fracturetoughness is the consequence, which can be seen from Figure 8 and Figure 9. Finally, in Figure 10, theCTOD(BS) and CTOD(55) values are compared for homogeneous and heterogeneous over- and under-matched weld joints. Discrepancies in the CTOD data shown, also suggested by other research 9,20, can be recognised. This is probably the consequence of local strength inhomogeneities, and no general rule is available at present on how to use the proper value of the yield stress in CTOD standard formulations. A local CTOD(85) technique offers a potential advantage when mismatched weld joints are tested. This is especially important in the case presented this paper where local mismatch is the dominating mechanical effect because of the low WM toughness level21. Theabovediscussion is based on detailed metallographical examinations (LM and SEM) by using the sectioning method to determine the LBZ initiation points, fatiguecrack tip microstructureat theinitiation point, and thefracturedeviation nature. The results were achieved on weld joints made of the Q+T HT80 type, which were used frequently for high-pressure penstocks with large diameters and thick walls. We are convinced that the presented philosophy of soft-root-layer use can also be applied to pressure vessels steel weld joints which are recently made of lower strength steels as the HT 80 steel. 4 CONCLUSIONS An experimental programme to compare the strength and toughness properties of a mismatched weld joint produced on quenched and tempered HT 80 steel has been carried out. The results can be summarised as follows: 4.1 It was found that besides the global mismatch defined as the ratio of the average WM and BM strengths, very distinctive strength differences in the through thickness exist (local mismatch) in over- and undermatched homogeneous weld joints for the WM and HAZ. This can be clearly shown by measuring the micro-hardness. In the under-matched WM the local mismatch is mostly the consequence of dilution and alloying from the BM, whereas in the overmatched WM local quenching and tempering is moreimportant. Thelocal mismatch can bethe dominating mechanical effect controlling the fracture behaviour in areas with low toughness. 4.2 Introducing the soft root layer is beneficial for the overmatch weld joint if the root mismatching is not lower than M=0.9 and the heterogeneous weld joint behaves as a homogeneous joint. This means that lower preheating or even its omission is possible. Introducing the soft root layer into the undermatched weld joint is dueto theroot overmatching condition, the obligation to prevent sensitivity to cold cracking and to reduce or even to omit the preheating. 4.3 Charpy impact toughness values of both the undermatched and overmatched weld joints were similar in theHAZ but in theWM they werelower for the over-matched condition. The lowest toughness of the under-matched WM was observed in the root region, whereas for the over-matched WM it was in the cap region. A soft root layer for the over-matched weld joint has to be selected by the filler metal not lower than M=0.9. The better root portion impact toughness compared to the remaining WM portions is the consequence. For the under-matched weld joint the soft root layer introduction was not beneficial and due to the formation of the M/A constituents as an affect of the root layer alloying, the lowest weld-joint impact toughness appeared. MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4 97 I. RAK ET AL.: THE FRACTURE BEHAVIOUR OF GLOBAL /LOCAL MIS-MATCHED 4.4 The CTOD fracture toughness values measured on the SENB-Bx2B specimens (a/W=0.5) with the notch in the WM were lower for both mismatched homogeneous weld joints than those with the HAZ composite notch. LBZs appeared in both mismatched homogeneous weld joints in the CGHAZ, followed by a more-or-less brittle fracture spread into the less tough WM and at theend theductilefracture followed through the BM. In the under-matched WM a local root over-matching condition prevented a fractureinitiation at theroot despiteits lower toughness and the brittle-fracture initiation (LBZ) has appeared in the local lowest strength WM area. An introduction of thesoft root layer did not changethe HAZ and WM fracture behaviour of the overmatched weld joint. Because the heterogeneous over-matched weld joint with soft root layer behaves like the homogeneous joint, the recommendation for reducing or even omitting the preheating can be expressed by a suggested welding procedure. This is not the case when using a soft root layer for an under- weld joint. Thepoor root toughness of theHAZ and theWM precracked specimens and the deviation of the fracturepath into thelowest tough root LBZs is the consequence. The use of a soft root layer to prevent cold cracking in an undermatched weld joint cannot be recommended, unless high toughness can be provided in this region. 4.5 The direct measurement of local CTOD(?5) provides CTOD values for which no material properties are required for the toughness calculation. This is particularly important in the case when treated crack-tip plastic zone involves different local microstructures of mismatched weld joints where the strength and toughness can vary substantially. Acknowledgements Theauthors wish to acknowledgethefinancial support of the Slovenian Foundation of Science and Technology and the Forschungszentrum Jülich GmbH. Thanks also to COR-WEL-TEC Fügetechnik GmbH and Jesenice Steelworks for the supplied materials and the welding execution. A special thanks is expressed to dr. M. Koçak and his staff at the GKSS Research Centre and to Dr. B. Petrovski for his help with the instrumented specimen CTOD testing. 5 REFERENCES 1 BS 7448, 1997 Fracture mechanics toughness tests. Part 2. Method for determination of KIC, critical CTOD and critical J values of welds in metallic materials. 2 ASTM E 1290-93: Standard Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement. 3 GKSS-Forschungszentrum Geesthacht, "GKSS-Displacement Gauge Systems for Applica-tion in Fracture Mechanics", 1991. F. M. Burdekin, M. Koçak, K-H. Schwalbe, R. 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