ISSN 00360295, Russian Metallurgy (Metally), Vol. 2012, No. 10, pp. 890–897. © Pleiades Publishing, Ltd., 2012. Original Russian Text © A.I. Kovalev, D.L. Vainshtein, A.Yu. Rashkovskii, E.I. Khlusova, V.V. Orlov, 2011, published in Deformatsiya i Razrushenie Materialov, 2011, No. 7, pp. 41–48.
APPLIED PROBLEMS OF STRENGTH AND PLASTICITY
Effect of Phase Composition on the Micromechanism of Fracture of Strength Class K70 Pipe Steel A. I. Kovaleva, D. L. Vainshteina, A. Yu. Rashkovskiia, E. I. Khlusovab, and V. V. Orlovb, * a
Bardin Central Research Institute for the Iron and Steel Industry, Vtoraya Baumanskaya ul. 9/23, Moscow, 105005 Russia b TsNII KM Prometei, St. Petersburg, 191015 Russia *email:
[email protected] Received June 30, 2010; in final form, February 15, 2011
Abstract—The effect of the structural features of a highstrength (strength class K70) main pipeline steel with a ferritic–bainitic structure after thermomechanical treatment and quenching from rolling heating on the fracture micromechanism during static and dynamic loading is studied. These structural features are shown to affect the ductility margin, the fracture rate, and the crack nucleation and propagation energies. The frac ture micromechanisms detected during static tension of samples in the column of an electron microscope agree with the results of bending impact tests and dynamic tests of fullscale technological specimens. DOI: 10.1134/S0036029512100126
1. INTRODUCTION Steels with a ferritic–bainitic structure are now used to ensure the required set of mechanical proper ties of main line pipes, and the fraction and morphol ogy of bainite differ substantially as a function of the chemical composition and the process of production of steels of various strength classes. Both thermome chanical treatment (TMT) and quenching from roll ing heating (QRH) can be applied to make steels of strength class K70. The fracture resistance of a material is known to be determined by the resistance to crack nucleation and propagation, which is substantially related to the stress relaxation ability of the material near stress concentra tors in a plasticdeformation zone. At all stages of frac ture (such as crack start, stop, and propagation), a crack interacts actively with microstructural elements, which determine the configuration and distribution of the stress field before a crack front and affect the dis location mobility and dislocation multiplication and slip mechanisms. The role of these processes becomes important at relatively low crack growth rates, since they ensure a negative feedback and lead to a decrease in the stresses at a given load, i.e., stress relaxation. The energy released at a crack tip is eventually con sumed for the formation of two fracture surfaces and plastic deformation. The energetics of crack nucle ation and propagation can be estimated from the development of surface and signs of microplastic deformation. Fractographic investigation and an anal ysis of a microrelief in crack edges and a crack mouth at various stages of fracture make it possible to find the micromechanism of this process and to explain the effect of a steel microstructure on fracture. With such a physical picture, one can explain changes in the gen
eralized factors characterizing mechanical properties, such as impact toughness, stress intensity factor, yield strength, ultimate tensile strength, and relative elon gation. Researchers most often study the effect of the grain size (packet length in martensitic steels) and grain boundaries on the character of fracture [1, 2], and the effect of structural features, in particular, bainite of various morphology is poorly understood. It was found that, as compared to lath bainite, the fraction of low angle boundaries in granular bainite increases signifi cantly after plastic deformation [3], which can affect the fracture resistance [4]. The purpose of this work is to reveal the effect of structural features on the micromechanism of fracture of a highstrength pipe steel with a ferritic–bainitic structure during static and dynamic loading. 2. EXPERIMENTAL We studied highstrength (strength class K70) 06G2NDMFBT steel intended for main pipelines and containing (wt %) 0.06 C, 1.7 Mn, 0.9 Σ(Cr, Ni, Cu), 0.08 Σ(Ti, V, Nb), 0.0015 S, 0.007 P, 0.006 N, and 0.04 Al. This steel was investigated in the following two states: (i) after twostage TMT with rapid cooling to a temperature below the onset of bainitic transforma tion and (ii) after QRH upon cooling to room temper ature. The chosen rolling conditions ensured the for mation of small austenite grains after TMT and larger austenite grains after QRH. Deformation was com pleted near or below point Ar3 (onset of ferrite precip itation); in the case of QRH, the temperature of the end of rolling was ~50°C higher than that upon TMT.
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(a)
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Fig. 1. Structure of the 06G2NDMFBT steel after (a) TMT and (b) QRH.
The average cooling rate of rolled products during TMT and QRH was 5–15 and 15–30 K/s, respec tively. α phase transformation was analyzed on The γ a Dil 805 dilatometer, which provides compressive deformation and computerassisted temperature and strain control. The process parameters were as follows: heating to 1200°C (to form coarse grains) or 1000°C (to form fine austenite grains), holding at a chosen temperature for 5 min, deformation temperature of 920°C, strain of 25%, and cooling rate of 1–100 K/s. The deformation temperature was chosen below the recrystallization temperature of austenite in the steels of the given class, and the strain was as high as possible under industrial conditions. Thermokinetic diagrams were plotted using dilatometric curves and structural data. An Axiovert 25CA optical microscope equipped with a digital image analyzer was used to study the structure of the asdelivered steel (i.e., after TMT and QRH) on polished sections cut along the rolling direc tion and the steel structure after dilatometric investi gation. Impact bending tests of rolled products were per formed at –40°C on 10 × 10 × 55mm Charpy speci mens with a Vshaped notch cut according to State Standard GOST 9454. Fallingload tests (FLTs) were carried out on fullscale (23mmthick) specimens at a temperature of –20°C according to State Standard GOST 30456–97. Tensile tests were performed on a Zwick servohydraulic tensiletesting machine accord ing to State Standard GOST 1497 using fullscale specimens with a cross section of 23 × 30 mm cut across the rolling direction from the center of the sheet width and from a pipe in the transverse direction after samples were straightened by static bending. The structure of fracture surfaces was analyzed with a JSMU3 (JEOL, Japan) scanning electron micro scope (SEM). RUSSIAN METALLURGY (METALLY)
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Crack nucleation and development was studied during static tension of 8.0 × 0.1 × 30mm samples with lateral notches in the column of the JSMU3 electron microscope: as the tensile load increased, we analyzed the change in the sample surface microrelief in the notch zone and before a crack front after crack nucleation. 3. RESULTS AND DISCUSSION 3.1. Structure after Quenching from Rolling Heating and Thermomechanical Treatment After TMT, the steel structure consists of fine grained (grain number at most 13 according to State Standard GOST 5639) quasipolygonal ferrite and granular bainite. At the center of the sheet thickness, the steel structure changes: up to 5% lath bainite is present along with ferrite and granular bainite, which are the main structural constituents (Fig. 1a). After QRH, the steel structure mainly consists of lath α phase and ferrite is almost absent. A carbide phase precipitates along the boundaries of bainite regions in the form of dark irregular zones (Fig. 1b). These results agree well with the data of dilatomet ric investigations (Figs. 2, 3). Both the fragmentation of primary austenite grains and plastic deformation decrease the stability of supercooled austenite in the ferritic–pearlitic range and do not affect the tempera ture of the onset of bainitic transformation over a wide cooling rate range (see Fig. 2). When austenite with fine grains (which imitate the grain size after TMT) is cooled, a quasihomogeneous structure consisting of ferrite and granular bainite (up to 60–80%) forms (Fig. 3a). When austenite with coarse grains (which imitate the grain size after QRH) is cooled, the struc ture is represented by lath bainite α phase (Fig. 3b). The lath width depends on the cooling rate. The formation of lath bainite leads to higher strength characteristics of the steel after QRH as com pared to that after TMT (table). In this case, a smaller
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P
P
B B
400
50 30
200
0 10
10
101
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102
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Time, s
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Fig. 2. Thermokinetic diagrams of austenite transformation in (a) asdelivered 06G2NDMFBT steel and (b) after plastic defor mation: (fine grains) steel after TMT and (coarse grains) steel after QRH.
relative elongation and a lower fibrous component content in a fracture surface after FLTs are detected. 3.2. Fracture Micromechanism during Dynamic Tests The fracture surfaces shown in Figs. 4 and 5 illus trate the fracture surface microrelief in various crack development zones for steel samples cut from pipes after TMT and QRH. For ductile fracture in a crack nucleation zone, which is characteristic of both speci mens, the zone length is substantially different: in the steel specimen after TMT, the maximum length of the
zone with 100% ductile (cellular) fracture under a notch reaches 200 μm, and the crack nucleation zone in the steel specimen after QRH is substantially smaller (130 μm). The microscopic relief characteris tics, namely, the cellular structure dispersity and the dimple dept, are the same for both specimens. Thus, the smaller crack nucleation zone length in the steel specimen after QRH is caused by a smaller ductility margin, which is not related to the metal quality, and, hence, a lower crack nucleation energy. Certain differences are also observed in the crack propagation zone (Figs. 4a, 5a). The following three
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Fig. 3. Structure of strength class K70 after cooling at a rate of 10 K/s from (a) finegrained deformed austenite (heating to 1000°C) and (b) coarsegrained deformed austenite (heating to 1200°C), ×500.
B F
(a)
20 μm (b)
10 μm (c)
20 μm
Fig. 4. Fracture surfaces of the 06G2NDMFBT steel after TMT (tests at a temperature of –20°C): (a) quasicleavages along fer rite regions in the crack propagation zone (5%), F is ferrite, B is a bainite region; (b) large (deep) extended ductile fracture dim ples with nonmetallic inclusions in bainite regions in the crack propagation zone; and (c) quasicleavages on crystallographic planes in bainite regions near the final fracture zone.
types of regions can be distinguished in it: a region with a ductile smallcell relief, quasicleavage facets with a river pattern, and large flat or deep sells with signs of significant plastic deformation and an “orange skin” relief. Such a coarsecell relief is observed in bainite regions in steels with a bainitic structure. As a rule, bainite colonies nucleate on disperse excess phase particles, and these regions contain nonmetallic inclusions and coarse primary titanium and aluminum carbides or nitrides (Figs. 4b, 5a, 5b). In both cases, ductile fracture regions coexist with quasicleavage microvolumes in the crack propagation zone. The transverse size of quasicleavage facets is 40–80 μm (Figs. 4a, 5a). In these regions, a crack propagates according to a brittle mechanism, changes the vector of its motion many times, and forms a disperse faceted microrelief (Fig. 5c). Regions 1–6 in this fracture sur face correspond to brittle cleavage facets. The pres ence of a weakly pronounced river pattern on these RUSSIAN METALLURGY (METALLY)
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facets indicates brittle and fast crack propagation through a grain body. Most regions exhibit a disperse faceted structure and ductile transitions between facets. Therefore, we can conclude that, when moving, a crack consumed the accumulated energy for plastic deformation Mechanical properties of strength class K70 06G2NDMFBT steel Tech σy, MPa σu, MPa nology
δ5, %
KCV–40 J/cm2
B, %
TMT
600/620 710/724
20/18
290
100
QRH
660/627 770/727
17/21
340
95
Note: Numerator, average data for rolled sheets; denominator, aver age data for pipes. B is the ductile component content in a fracture surface during FLT at a test temperature of –20°C.
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2 2 3
1 6 (a)
20 μm (b)
10 μm (c)
4
20 μm
5
Fig. 5. Fracture surfaces of the 06G2NDMFBT steel after QRH (tests at a temperature of –20°C): (a) quasicleavages along fer rite regions in the crack propagation zone (15%), (b) ductile fracture regions in the crack propagation zone, and (c) fine faceted relief during crack propagation. Numerals 1–6 in (c) indicate brittle cleavage facets.
events. Such regions are associated with quasicleav age. The area occupied by quasicleavage regions in the fracture surface in the crack propagation zone does not exceed 5% in the specimen after TMT and reaches 15% in the specimen after QRH, which indicates higher brittle fracture resistance in the former case. This behavior also agrees with large plastic deforma tion of extended bainite regions. In this case, a coarse cell relief with a developed surface forms during crack propagation. A comparison of the specimens after TMT and QRH allows the conclusion that the cell extension in the former case is significantly larger and reaches crit ical values, which are accompanied by microcrak nucleation at the bottom of the voids. In the latter case, the cells are insignificantly elongated. This behavior indicates the formation of a more developed fracture surface in these regions in the former case and, hence, a higher energy consumed for the fracture of the specimens after TMT. In the zone close to final fracture, a crack moves at the maximum speed. In such cases, we usually did not detect any effect of micro structure heterogeneity on the fracture micromecha nism as compared to slow crack growth conditions. However, quasicleavages along the crystallographic planes of bainite colony growth are observed in this zone in the specimen after TMT (Fig. 4c). These fea tures point to the fact that slip on various planes occurs under these critical final fracture conditions; hence, significant relaxation processes develop before a crack front, and a high density of interfaces favors crack deceleration at all stages of crack development. In the specimen after QRH, the critical stresses before a crack front reaches the values at which shear fracture takes place in the final fracture zone at a smaller crack propagation zone area and absent signif icant relaxation processes.
With these data, we conclude that the rolled prod ucts made with TMT have a high ductility margin as compared to the specimens cooled from rolling heating. 3.3. Fracture Micromechanism during Static Tension Figures 6 and 7 show a series of images of steel specimen surfaces that were subjected to TMT and QRH and subsequent tensile tests in the column of an electron microscope. 3.3.1. Steel specimen after TMT. Figures 6a and 6b show the scheme of loading and the region of the beginning of crack development near a notch in the specimen after TMT. A 25 × 13μm plastic deforma tion zone, which is comparable with the nucleated crack length (20 μm), forms in front of the crack mouth related to the high plasticity of steel. An intru sion–extrusion relief forms on the specimen surface in the plastic deformation zone due to different plastici ties of the steel structural constituents and, corre spondingly, different strains over the specimen cross section. It was found that regions with a lower density of carbide precipitates undergo higher plastic deforma tion. At a given applied load, regions with a high den sity of carbide particles, which block dislocation prop agation, do not deform noticeably as compared to lowdensity regions. The trajectory of diffusion motion is curved (Figs. 6c, 6d), which provides significant energy dissi pation during crack propagation. Crack oscillations take place at the sites where a crack meets a structural constituent with a higher strength. The bulk plastic deformation band width increases substantially in the zone where a crack motion direction changes, which also increases the energy required for fracture. The crack motion step decreases to 3 μm near bend ing, which corresponds to the average grain diameter (Fig. 6d). A bulk plastic deformation zone forms near
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(a)
10 μm (c)
20 μm
100 μm (f)
10 μm
ΔL = 3 μm ΔL = 3 μm
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Fig. 6. Crack nucleation and propagation during static tension of a planar notched specimen made of 06G2NDMFBT steel after TMT in the column of a scanning electron microscope. The points in (b) and (d) indicate the configuration of the plastic defor mation zone in front of a crack mouth.
the edge of specimen in the crack propagation end zone. A crack branches in this zone. This behavior of the material also indicates its high ability to stress relax ation and, hence, a good ductility margin. Figures 6e and 6f show the appearance of a crack in a failed specimen. As follows from Fig. 6e, the crack moves and oscillates about a main direction. The non uniform deformation zone along the crack occupies a band 50–150 μm wide. The appearance of the crack edge corresponds to ductile fracture with quasicleav age regions, and the dimple diameter is several microns (see Fig. 6f). 3.3.2. Steel specimen after QRH. A narrow long plastic deformation zone is observed in front of a crack tip near the crack nucleation zone (Fig. 7a), which points to weak relaxation processes in the steel after QRH. The bulk plastic deformation zone width in front of the crack is 15 μm (Fig. 7b), which is substan tially smaller than in the steel specimen after TMT. A relatively narrow bifurcate plastic deformation zone forms in front of the tip of a moving crack (Fig. 7c), and the crack moves almost linearly. The plastic deformation zone in the final zone of its prop agation is also narrow and bifurcated (Figs. 7d, 7e). RUSSIAN METALLURGY (METALLY)
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The nonuniform plastic deformation zone contains regions with oppositely directed shear, which form under the action of a stress field near a concentrator, namely, a crack tip (Fig. 7e). The nonuniform plastic deformation zone width is 30 μm. Thus, the configuration of the plastic deformation zone changes noticeably at the early and late stages of crack growth. As the crack length increases, the variety of shear deformation directions is exhausted, and this zone contracts and is localized in a narrow zone along the main crack growth direction (Figs. 7c, 7d). After fracture, the crack walls are rather linear and vertical and have weak steps along the trajectory of crack development. Figure 7f characterizes the crack surface structure, which is represented by quasicleav age with ductile fracture regions containing small dimples (about 1 μm in size). 3.3.3. Fractal analysis of the cracks in the fracture surfaces of the steel after QRH and TMT. The high ductility margin of the steel after TMT leads to signif icant crack oscillation about the main crack develop ment direction and to crack branching on macro and microlevels (Fig. 8a). Since selfsimilarity crack devel opment is widely recognized in modern materials sci ence, we can apply fractal analysis of crack trajectories
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20 μm (c)
100 µm (b)
(a)
100 μm
30 μm 100 μm (e)
(d)
100 μm
10 μm
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Fig. 7. Crack nucleation and propagation during static tension of a planar notched specimen made of 06G2NDMFBT steel after QRH in the column of a scanning electron microscope. The points in (c) and (d) indicate the configuration of the plastic defor mation zone in front of a crack mouth.
during crack propagation [5]. With this analysis, we can reveal difference and similarity of images. To this end, we divided the microstructure images of a crack (a)
(b)
shown in Fig. 8 many times into squares with a side e = 2–300 μm and determined the number of the squares N containing a crack image in every case. Figure 9 shows the relationships between these parameters in loga rithmic coordinates. The slopes of the curves corre spond to fractal dimensions Df. In the case of Df = 1, 6.2 TMT QRH
ln(N)
5.2 4.2 3.2 2.2 100 μm
100 μm
Fig. 8. Crack propagation trajectory in 06G2NDMFBT steel after (a) TMT and (b) QRH. Reconstruction from SEM data (see Figs. 6, 7).
1.2 1.5
3.5 ln(e)
5.5
Fig. 9. Fractal dimension of the cracks formed in 06G2NDMFBT steel specimens after TMT and QRH during tension in a scanning electron microscope (see text). RUSSIAN METALLURGY (METALLY)
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fracture develops by cleavage with the formation of a smooth fracture surface. For the steel after TMT, we have Df = 1.19; for the steel after QRH, we have Df = 1.04, which means insignificant branching of a crack path and a low fracture energy in a first approximation. The deviation of the dependences from a straight line at low values of e means a low accuracy of estimating the crack relief on a macroscale, where the properties of the material correspond to its isotropic state. After TMT, the steel demonstrates an oscillating slope of the fractal dependence over a wide e range, which indi cates a multifractal crack propagation mechanism, i.e., the presence of ductile fracture and quasicleav age regions. Index H = 1/Df is applied as a universal fracture surface characteristic [6, 7]. In this work, we have H = 0.84 and 0.96 for the steel after TMT and QRH, respectively. According to [8], H ≈ 0.90 corre sponds to brittle fracture according to a quasicleavage mechanism and H ≈ 0.6–0.8 corresponds to ductile fracture. 4. CONCLUSIONS (1) After quenching from rolling heating, the sheets rolled from a highstrength (strength class K70) pipe steel are characterized by a lower ductility margin and undergo fracture at a lower level of the crack nucle ation and propagation energies as compared to the rolled sheets after TMT. (2) The results of fractographic investigation of the micromechanism of fracture of specimens in the col umn of an electron microscope were found to agree well with the results of bending impact tests and dynamic bending tests of technological specimens at a temperature of –20°C. (3) The favorable phase and structural state of the steel after TMT as compared to QRH promotes active stress relaxation in front of a growing crack and acti
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vates a large number of slip systems, which agrees with the general increase in the fracture energy. (4) As compared to lath bainite after QRH, TMT results in the formation of granular bainite and leads to a bulk state of stress (in contrast to a planar state of stress after QRH) during crack nucleation and prop agation, which eventually increases the fracture toughness. REFERENCES 1. V. I. Vladimirov, Physical Nature of Metal Fracture (Metallurgiya, Moscow, 1982). 2. V. M. Goritskii, “Fracture Criterion for the Steels Sen sitive to the Propagation of Brittle Microcracks along Crystallite Boundaries,” Probl. Prochnosti, No. 4, 37–43 (1987). 3. V. V. Rybin, E. I. Khlusova, E. V. Nesterova, and M. S. Mikhailov, “Formation of the Structure and Properties of LowCarbon and LowAlloy Steel during Thermomechanical Treatment under Rapid Cooling,” Vopr. Materialovedeniya 52 (4), 329–340 (2007). 4. L. I. Tushinskii, Structural Theory of Structural Strength of Materials (NGTU, Novosibirsk, 2004). 5. A. L. Horovistiz, L. M. F. Ribeiro, K. A. Campos, et al., “Quantitative Fractography of Profiles by Digital Image Processing: Analysis of Ti–4Al–4V at Different Microstructural Conditions,” Acta Microscopica 11 (1), 65–70 (2002). 6. K. J. Maloy, A. Hansen, E. L. Hinrichsen, and S. Roux, “Experimental Measurements of the Roughness of Brittle Cracks,” Phys. Rev. Lett. 68 (2), 213–215 (1992). 7. E. Bouchaud, G. Lapasset, and J. Planès, “Fractal Dimension of Fractured Surfaces: A Universal Value?” Europhys. Lett. 13 (1), 73 (1990). 8. R. H. Dauskardt, F. Haubensak, and R. O. Ritchie, “On the Interpretation of the Fractal Character of Fracture Surfaces,” Acta Metall. Mater. 38 (2), 143– 159 (1990).
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