UDK 539.42:621.791.05:669.14.018.298 ISSN 1580-2949 Original scientific article/Izvirni znanstveni članek MTAEC9, 40(6)263(2006) FRACTURE TOUGHNESS OF A HIGH-STRENGTH LOW-ALLOY STEEL WELDMENT ŽILAVOST LOMA ZVARA VISOKOTRDNEGA MALOLEGIRANEGA JEKLA Jelena Turna1, Nenad Gubeljak2, Borivoj Šuštaršic1, Borut Bundara3 1Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia 2University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia 3Institute of Metals Constructions, Mencingerjeva 7, 1000 Ljubljana, Slovenia jelena.tumaŽimt.si Prejem rokopisa – received: 2006-05-17; sprejem za objavo - accepted for publication: 2006-11-15 The use of high-strength low-alloy steels for high-performance structures, e.g., pressure vessels and pipelines, requires often high-strength consumables to produce an overmatched welded joint. This globally overmatched welded joint contains local mis-matched regions, which can affect the unstable fracture behaviour of the welded joint and the welded structure itself. If local mis-matched regions are present in the vicinity of a crack tip, then the fracture toughness of the weld metal can be significantly lower than that of the base metal. In this paper, the influence of the weld-metal microstructure on the fracture behaviour is estimated enabling an evaluation of the resistance to stable crack growth through different microstructures. The lower bound of the fracture toughness for different microstructures was evaluated using a modified Weibull distribution. The results, obtained using specimens with a through thickness crack front, indicated a low fracture toughness, caused by the strength mis-matching interaction along the crack front. In the case of through-the-thickness specimens, at least one local brittle zone (LBZ) or a local soft region is incorporated into the process zone in the vicinity of the crack tip. Hence, an unstable fracture occurred with small stable crack propagation, or without it. Despite the fact that the differences between the impact toughness of the weld metal and the base metal can be insignificant, the fracture toughness of a weld metal can be significantly lower. Key words: fracture mechanics, welded joint, crack-tip opening displacement, resistance curves Uporaba visokotrdnih malolegiranih jekel za zelo obremenjene strukture, npr. posode pod pritiskom in cevovode, zahteva uporabo varilnega materiala, ki ustvari zvar z večjo trdnostjo. Taki zvari vsebujejo lokalna področja z mešano trdnostjo, ki lahko vplivajo na nestabilno lomno vedenje zvara in zvarjene strukture. Če mešana področja ležijo v bližini vrha razpoke, je lahko žilavost loma pomembno manjša kot pri osnovnem materialu. V tem delu je ocenjen vpliv mikrostrukture zvara na vedenje pri lomu, kar omogoča oceno odpornosti proti stabilnem širjenju razpoke skozi različne mikrostrukture. Nižja vrednost žilavosti loma je bila ocenjena za različne mikrostrukture z modificirano Weibullovo porazdelitvijo. Rezultati, ki so bili doseženi pri vzorcih z razpoko preko debeline, so pokazali nizko žilavost loma zaradi različne trdnosti vzdolž čela razpoke. V primeru vzorcev z razpoko preko debeline je vsaj eno lokalno krhko področje (LBZ) ali lokalno mehko področje vključeno v procesno področje v bližini vrha razpoke. Zato se je stabilna propagacija razpoke izvršila z majhnim stabilnim širjenjem ali brez njega. Čeprav so majhne razlike med udarno žilavostjo zvara in osnovnega materiala, je lahko žilavost loma zvara pomembno manjša. Ključne besede: mehanika loma, zvarni spoj, premik vrha odprtja razpoke, krivulje odpornosti 1 INTRODUCTION Strength-overmatched welded joints are designed to ensure the safe service of a welded structure by keeping the flaws, e.g., planar defects, in an elastic weld metal, while the base metal starts to yield. Such an approach ensures that a welded structure can sustain local plastic deformation, important when temporary overloading or geometrical changes occur. These changes can be caused by temperature variations during a structures service life. The strength-overmatching requirement presents no special problems for steel with yield strength of less than 600 MPa 1, but in case of steels with higher yield strengths, e.g., above 700 MPa, locally undermatched regions can occur. Such an overmatched weld joint is quite sensitive to planar cracks developing from defects. Thus, a higher stress concentration around the planar defects in a weld metal in locally undermatched regions can cause unstable fracture behaviour2. In this case a significant range of experimentally fracture-toughness values is obtained. It is possible, however, to overcome this problem by determining the lower bound fracture toughness, which can ensure the structural integrity and a safe service life. The lower-bound fracture-toughness value represents the value where the crack propagation occurred. If the stress intensity factor (caused by applied load) is lower than the lower-bound fracture toughness, than crack propagation does not appear. This paper presents a procedure for determining the lower-bound fracture toughness of laboratory specimens cut from a critically overmatched weld joint. The influence of the weld-metal microstructure on the fracture behaviour is estimated, enabling an evaluation of the resistance to stable crack growth through different microstructures, as well as an evaluation of the relevant value of the lower bound of the fracture toughness. Reasons for the range of experimentally measured fracture-toughness values are also presented. MATERIALI IN TEHNOLOGIJE 40 (2006) 6 263 J. TUMA ET AL.: FRACTURE TOUGHNESS OF A HIGH-STRENGTH LOW-ALLOY STEEL WELDMENT 2 MATERIALS AND WELDING The base metal is a high-strength low-alloy steel, corresponding to grade HT80. The steel, with a thickness of 40 mm, was delivered in quenched-and-tempered (Q + T) condition. Different mechanical properties can be obtained for such a steel by using different tempering temperatures (600-700 °C). The microstructure of the steel of tempered martensite and lower bainite provides a high strength and a high impact toughness. The welding was done on plate samples (500 × 250 × 40) mm and (1000 × 250 × 40) mm using the flux-cored arc-welding (FCAW) process. The edge preparation was X-shaped, Figure 1, as is usual for the welding of steel plates with a thickness of 40 mm. The consumables were filled wires ( 12 s) reduce the toughness due to the formation of a martensite-austenite constituent3. The welding parameters are given in Table 1. The first passes of the welded joint were made using preheating at 120 °C 4. Table 1: Welding parameters for each weld pass Tabela 1: Varilni parametri za vsak varek Welding pass Current A Voltage V Speed of welding cm/min Interpass temperature °C Heat input kJ/cm At8/5 s Weld joint region 1 155 24.5 11.5 120 19.813 10.61 2 185 23.5 13.7 135 19.040 10.82 root 3 250 24 16.0 30 22.500 8.78 4 250 24 15.0 65 24.000 10.47 5 240 23 10.5 105 31.543 15.65 6 240 23 11.1 120 29.784 15.66 7 220 23 15.9 65 19.130 8.43 8 220 23.5 14.6 85 21.203 9.96 filler 9 210 23 14.2 120 20.481 10.95 part 10 220 23 13.9 125 21.763 11.83 11 210 24 15.4 80 19.662 9.11 12 250 24 14.8 110 24.342 12.43 13 220 25 15.8 95 20.899 10.18 14 230 25 19.5 125 17.692 9.70 average 210 24 14.0 88 21.338 10.17 root average 226 24 15.0 103 22.648 11.39 filler Table 2: Chemical composition of the base metal, the pure weld metal and the actual weld metal of the filler and the root regions Tabela 2: Kemična sestava osnovnega materiala, čistega vara, realnega vara v območju polnitve in korena v masnih deležih w/% Material C w/% Si w/% Mn w/% P w/% S w/% Cr w/% Ni w/% Mo w/% CE/% Base metal 0.09 0.27 0.25 0.015 0.004 1.12 2.63 0.25 0.366 Weld metal (pure) 0.06 0.35 1.43 0.011 0.008 0.86 3.01 0.56 0.448 WMfill (filler part) 0.07 0.33 1.27 0.008 0.006 0.86 2.21 0.47 0.404 WMroot 0.08 0.32 0.78 0.012 0.007 0.99 2.50 0.35 0.388 Figure 1: The "X" shaped groove used in this investigation Slika 1 : Žleb z X-obliko, uporabljen pri tej raziskavi The chemical compositions of the base metal (BM) and the different weld regions are listed in Table 2. These compositions indicate a more pronounced alloying effect from the BM in the root region than in the filler regions. Local tempering or quenching caused by reheating and cooling during the deposition of subsequent passes is also present in the root and filler weld regions. This is the main reason why the local mis-matching through the weld thickness varied, even in the case of a homogeneous weld. 264 MATERIALI IN TEHNOLOGIJE 40 (2006) 6 J. TUMA ET AL.: FRACTURE TOUGHNESS OF A HIGH-STRENGTH LOW-ALLOY STEEL WELDMENT The content of carbon is very low in the base metal and the weld metals and the temperature of martensite transformation is higher. Thus, the time interval for self-tempering from the temperature of martensite transformation up to room temperature is larger. In this case a brittle hard microstructure does not appear. This is the reason for the high toughness of the microstructure. Table 2 presents the change of the carbon equivalent (CE) during the welding process. The low CE and the low strength hinder the hydrogen-assisted cold cracking. The mechanical properties of the welds were determined using round tensile specimens (05 with the lower bound, J l.b., as the toughness for a failure probability of zero. The term J can be replaced with CTOD = ô by using the relation between ô and J 14: A J d =-------- m ¦ g„. m ¦ R* -0111 + 0.817 —+1.36-R*; W 500 -n 2.718 or R* - 5m (3) The terms m and R* were introduced by Kirk and co-workers Š14] on the basis of finite-element analysis results. The lower bound fracture toughness can be derived by considering the condition of continuity for Pf. As a result, the lower bound is simply obtained from the mean value by the expression: with <5l.b. = 0.26-,3-<5c,mean ß = 1 + 2.737p-2.327p 2 + 12580p3 (4) CTOD /mm Figure 6: Failure probability with lower bound values classified by microstructures WM(A) and WM(B) Slika 6: Verjetnost preloma pri spodnji vrednosti za mikrostrukturi WM(A) in WM(B) where p is the fraction of data that is rejected by the size criterion. The results of this analysis are two curves of Weibull distribution that are in good agreement with the experimental results for each individual microstructure, as shown in Figure 6. The lower-bound fracture-toughness value is represented by the CTOD value at the intersection point of the Weibull distribution curve with the x-axis. Although the Weibull distribution curves are for different microstructures, it is worth pointing out that the fracture-toughness lower-bound value is low for both of them 7. (2) 6 CONCLUSION In spite of the fact that the differences between the impact toughness of the weld metal and the base metal are insignificant, the fracture toughness of the weld metal can be significantly lower. The overmatched weld metal exhibited unstable crack propagation, while the base metal is ductile at the same test temperature. From the analysis performed on the B x B specimen it can be concluded that the critical value of the fracture toughness and the fracture behaviour of the weldment as a whole, depend on the crack depth and the microstructure at the crack tip, and also on the microstructure toward which the crack is growing. The influence of these parameters is reflected in pronounced differences in the experimentally obtained values. The higher reliability for estimating the fracture-toughness lower bound was achieved by using a modified Weibull distribution with the CTOD parameter. The B x 2B specimens also indicated low critical CTOD values, but with lower scatter. The reason for this is the increased constraint, since the ligament profile is of square shape, and also the fact that the stress state at the crack tip causes an interaction between strength-mis-matched microstructures, which are inevitably crossed by the crack front. In the case of through-the-thickness specimens at least one local brittle zone (LBZ) or a local soft region is incorporated in the process zone in the vicinity of the crack tip. Hence, the unstable fracture occurred with small stable crack propagation, or without. The statistically determined lower-bound fracture toughness takes account of this effect, which causes an increased scatter of experimental results. 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