OF PLASTICALLY DEFORMED HEAT-RESISTANT STEEL .... mechanical properties: (i) the formation of a bainite striated dislocation structure and dislocation ...
Strength of Materials, Vol. 39, No. 4, 2007
RELATIONSHIP BETWEEN RUPTURE STRESS AND MICROSTRUCTURE PARAMETERS OF PLASTICALLY DEFORMED HEAT-RESISTANT STEEL P. V. Yasnii, V. B. Glad’o,
UDC 620.187.3
and I. B. Okipnyi The paper addresses the influence of plastic prestraining in tension and combined tension at 150 and 350°C on the rupture stress in 15Kh2MFA steel upon heat treatment that simulates irradiation embrittlement of materials in a WWER-440 type reactor towards the end of its lifetime. The dependence of rupture stress on the dislocation density in the material upon plastic prestraining is studied. It is found out that as the plastic prestraining in tension and combined tension grows, the dislocation density within small-angle boundaries increases and thus results in a larger rupture stress. An increase in rupture stress in the case of prestraining at 350°C is more intensive than that at 150°C. We analyze the microcrack nucleation and growth micromechanisms in 15Kh2MFA steel during tensile plastic deformation and discuss the effect of the material substructure on the microcrack arrest. Keywords: rupture stress, plastic prestraining, dislocation density, crack nucleation and growth micromechanisms. Introduction. It is well known that the elastoplastic prestraining (EPS) can either increase or decrease rupture stress σ k in 15Kh2MFA steel depending on the loading mode (quasistatic, cyclic), type (tension, compression), and parameters [1–3]. Generally, there are two factors responsible for an increase in rupture stress in the prestrained structure of 15Kh2MFA steel [2]: (i) the formation of an intragranular substructure that serves as extra barriers to a microcrack propagation, and (ii) the growth of disorientation of boundaries of structural constituents, which exist in the material in its initial state. Plastic deformation gives rise to a cellular dislocation structure with subsequent reduction of the cell dimensions to some limit value; at the same time, dislocations tend to accumulate at the boundaries of the structural constituents and thus decrease the size of an effective structural block. A rise in the stress needed for a microcrack to propagate through the boundaries of the cellular structure and structural constituents is due to a decrease in the cell size and an increase in the number of the boundaries [2]. It should be mentioned that the increase in rupture stress was observed at relatively small number of cycles. With a larger number of cycles and thus greater fatigue damage accumulated, the rupture stress decreases in comparison to an intact material [3]. It has been found out that as the strain amplitude is raised the number of pits grows and so does the brittle component of fracture, while the fracture mechanism changes from intragranular to intergranular one [3]. Microstructural observations have revealed that the accumulation of fatigue damage in the form of voids during the elastoplastic cyclic prestraining results in a reduced net cross-sectional area, and the presence of numerous microcracks makes it easier for a macrocrack to arise at the existing structural flaws and further propagate along subboundaries of the striated dislocation structure with a limiting density of dislocations, thus reducing the rupture stress [4]. Troshchenko et al. [1] demonstrated that a tensile EPS at a temperature above the ductile–brittle transition led to a noticeable increase in the microcleavage stress; however, they provided no data as to how this phenomenon could be associated with microstructural features of the material. Ivan Pulyui Ternopol State Technical University, Ternopol, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 19 – 30, July – August, 2007. Original article submitted June 14, 2006. 0039–2316/07/3904–0349 © 2007 Springer Science + Business Media, Inc.
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Since microstructural parameters of a material upon EPS have a significant effect on its mechanical characteristics, rupture stress included, it is necessary to get a deeper insight, on the microstructural level, into the influence of EPS temperature and cyclic component on the structural parameters of 15Kh2MFA steel for the purpose of physically substantiating the EPS mechanism. The objective of the present work has been to study, by means of transmission electron microscopy, the microstructural parameters of 15Kh2MFA steel upon EPS and to reveal a relationship between these parameters and the rupture stress. Experimental. The material under consideration was 15Kh2MFA steel subjected to a heat treatment (hereinafter called the initial state) that simulates the material embrittlement due to neutron irradiation by the end of the service life of a WWER-440 type reactor vessel. The heat treatment conditions to simulate the irradiation embrittlement were the following: oil quenching at 1000°C and single tempering for 6 h at 600°C. Mechanical characteristics and rupture stresses were measured by uniaxial tensile tests on smooth cylindrical specimens of two dimensional types: test portion diameters of 8 and 5 mm and test portion lengths of 20 and 10 mm, respectively. The tests were carried out on a Mod. STM-100 servohydraulic machine (manufactured by Antonov Aircraft Company, Kiev, Ukraine) controlled by an IBM PC/AT type computer, following the procedure outlined earlier [5]. Specimens were prestrained in tension and combined tension (i.e., tensile loading combined with cycling) up to plastic strain levels ε pr = 0.5, 1.0, and 3.0% at 150 and 350°C. The range of stress Δσ in the case of combined tension was 90, 110, and 220 MPa, the loading frequency was 25 Hz, where Δσ = Δσ max − Δσ min , σ max and σ min are the maximum and minimum stresses, respectively. The main parameters of the EPS conditions are summarized in Table 1. The rupture stress σ k of the prestrained material was measured at −196°C in a heat-insulated chamber filled with liquid nitrogen. Only those test results which were obtained on the specimens whose brittle fracture occurred within their test portion were considered valid. The rupture stress σ k was determined by the formula σ k = Pk S k ,
(1)
where Pk is the fracture force and S k is the cross-sectional area of the broken specimen, S k = πd 2 4, d is the broken specimen diameter. The microstructure of 15Kh2MFA in the initial state, upon tensile and combined tensile EPS was examined by transmission electron microscopy (TEM). For this purpose, at least five microsections were cut longitudinally from each specimen’s test portion. Following the procedure [6] the microsections were prepared by the thin-foil method for the TEM observations under a Mod. PEM-125K microscope. Final thinning of the foils was achieved by electrolytic polishing in a 10% HClO4 + 90% CH3COOH solution at 140 V and 90 mA. Considering that the majority of dislocations are accumulated at subboundaries, we calculated the density of dislocations within small-angle boundaries of the dislocation structure using the results of analysis of the azimuthal disorientation of reflections on the microdiffraction patterns. According to the secant method, the scalar density of dislocations is one order of magnitude smaller and difficult to calculate because the dislocations cannot be distinguished individually; therefore, it would be reasonable to analyze merely the density of dislocations within small-angle boundaries. The density of dislocations with the Burgers vector b within the subboundaries of the dislocation structure was calculated by the mean size between the subboundaries d and their disorientation angle Θ [7]: ρb =
KΘ , bd
where K is the coefficient that depends on the shape of subgrains. 350
(2)
TABLE 1. Elastoplastic Prestraining Conditions for 15Kh2MFA Steel Ò, ° Ñ 350 350 350 150 150 150 150 350 350
ε pr , % 1.07 1.25 0.53 1.12 1.33 0.48 2.17 3.40 3.00 Intact material
Δσ, MPa – 110 – – 90 90 90 – 220
Symbolic notation of EPS conditions £ £ r s s s £ ¢
The small-angle boundary disorientation angle Θ was calculated based on the analysis of microdiffraction patterns, using the relationship Δr Θ= , (3) R hkl where Δr is the trace length or the distance between the reflexes hkl and R hkl is the distance between the hkl reflection and the central reflection. Results and Discussion. Steel 15Kh2MFA is a heat-resistant steel of pearlitic class. When in the initial state, which corresponds to the commencement of operation of a WWER-440 reactor pressure vessel, this steel has a ferrite-bainite microstructure [8]. Upon heat treatment that simulates embrittlement of the material due to neutron irradiation towards the end of the reactor vessel lifetime the steel has a bainite-martensite microstructure with a mean grain size of about 100 μm (Fig. 1). The bainite structure was found to contain a striated dislocation structure in the form of a system of parallel dislocation subboundaries (Fig. 1a, b). The acicular carbide precipitations are arranged in the same direction at an angle of about 60° to the dislocation subboundaries (Fig. 1a). The fact these precipitations are tilted at 60° to the crystal axis enables this structure to be identified as bainite unlike tempered martensite where platelet precipitations shows up in three orientation at the same time [9]. The martensite areas in the structure are made up of dislocation-type lath martensite. The parallel martensite laths are combined to form clusters. The structure of the clusters is represented by a system of variously sized parallel laths (Fig. 1c, d). Inside the martensite laths there is a high density of dislocations of spotty contrast, which are almost indistinguishable individually. This contrast makes the dislocation lines look blur and is apparently attributable to a considerable concentration of impurity atmospheres at the dislocations. Thus, the image features point not only to a significant density of dislocations but also to the fact that the dislocations are to a noticeable extent fixed by impurity atmospheres. The areas of martensite laths with a lower density of dislocations look brighter in the microphotographs. The diffraction image contrast is governed not only by the structure nature but also by the crystallographic orientation with respect to the electron beam. If some local areas appears darker at various foil orientations, this suggests that this contrast reproduces a real distribution of dislocations. Carbide precipitations are observed at the boundaries of the martensite laths (Fig. 1d). Fine dispersed carbide phase precipitations are also revealed inside the laths. In this case, however, the image contrast due to carbide particles is difficult to distinguish from that of dislocations because of their high density. The density of dislocations in between the dislocation subboundaries of the bainite structure is somewhat lower than that inside the martensite laths (Fig. 1a, b). In addition to acicular carbide precipitations arranged at an angle to 60° to the crystal axis, the bainite structure contains also some finely dispersed carbide precipitations at the boundaries of the structural constituents (Fig. 1b). 351
a
b
c
d
Fig. 1. Bainite (a, b) and martensite (c, d) microstructure in 15Kh2MFA steel (×30, 000). Thus, the heat treatment of 15Kh2MFA, which simulates embrittlement of the material due to neutron irradiation towards the end of the reactor pressure vessel lifetime, causes the following changes to the steel’s mechanical properties: (i) the formation of a bainite striated dislocation structure and dislocation boundaries of lath martensite; (ii) a significant density of dislocations in the laths and between subboundaries of the bainite structure; (iii) pile-up of dislocations inside martensite laths; (iv) dislocations fixed by impurity atmospheres; (v) finely dispersed precipitations; (vi) dislocation motion blocked by inclusions. The electron microscopic observations of specimens subjected to tensile and combined tensile loading have revealed the following slight changes in morphology of the dislocation structure, considering that the longitudinal plastic strain was 0.5–3%: (a) the dislocation subboundaries become increasingly nonparallel; (b) the distance between bainite structure subboundaries is reduced (Fig. 2a), and (c) the fragmentation of laths of the dislocation martensite increases (Fig. 2b). Another finding is that the disorientation angle between the bainite structure subboundaries and the dislocation martensite laths grows in comparison to the material’s initial state, as evidenced by microdiffraction patterns (Fig. 2c, d). The number of reflections and their blurred appearance in a microdiffraction pattern (Fig. 2d) suggest a reduction in the distance between small-angle boundaries and the existence of a continuous disorientation. Based on the analysis of microdiffraction patterns, we calculated the change in the density of dislocations within small-angle boundaries in 15Kh2MFA steel specimens under the conditions of tensile and combined tensile deformation. We analyzed at least five microdiffraction patterns from each foil, with at least five foils prepared from each specimen. Thus, a total of 25–30 microdiffraction patterns were analyzed for each point. Figure 3 illustrates the dependence of the density of dislocations within small-angle boundaries on EPS in tension and combine tension at 150 and 350°C. Under these deformation conditions, the 15Kh2MFA steel specimens exhibit a growth of the density of dislocations within small-angle boundaries with increasing plastic strain ε pr . No influence of the EPS cyclic 352
a
b
c
d
Fig. 2. Bainite (a) and martensite (b) microstructure in prestrained 15Kh2MFA steel (×30, 000); (c), (d) microdiffractions from the areas of bainite and martensite structures, respectively. component on the density of dislocations within small-angle boundaries was revealed. Whatever the EPS type (tension or combined tension), the density of dislocations within small-angle boundaries tend to grow by similar relationships. The only point in Fig. 3a, where the density of dislocations within small-angle boundaries was found to decrease, is an exclusion from the general tendency; however, it is attributable to a lower density of dislocations in the material in the initial state rather than to EPS. The dependence of the density of dislocations within small-angle boundaries in 15Kh2MFA steel on the EPS temperature is shown in Fig. 4. It was mentioned above that as the plastic strain ε pr increases, the density of dislocations within small-angle boundaries grows, an this growth is more intensive at the EPS temperature of 150°C than that at 350°C. The plastic deformation at a higher temperature (350°C) is apparently accompanied by thermal reversion processes responsible for some elimination of a nonequilibrium excess of point defects and transformation of the dislocation structure. During spontaneous structural transformation, free energy in the crystal is reduces because as a result of the thermal reversion the density of dislocations decreases and the remaining dislocations make up stable configurations in the form of small-angle boundaries. Also, we studied micromechanisms of microcrack nucleation and growth in 15Kh2MFA steel during tensile EPS. Nucleation of microcracks is known to be a joint effect in dislocation structures, which is associated with transformation of these structures and implies conversion of the energy accumulated in them into the surface energy of a microcrack [10]. Thus, microcracking, whatever the nucleation mechanism, results from evolution of the material’s dislocation structure. The present research has revealed that microcracks and voids in 15Kh2MFA steel under tensile and combined tensile EPS arise along parallel subboundaries of the striated dislocation pattern in the bainite structure (Fig. 5a) as well as along martensite laths in the martensite structure (Fig. 5b). Microcracks and voids occurring along 353
ρ b ⋅ 10 −14 , m −2
ρ b ⋅ 10 −14 , m −2
ε pr , % a
ε pr , % b
Fig. 3. Density of dislocations within small-angle boundaries as a function of EPS in tension and combined tension at 150 (a) and 350°C (b). (For symbolic notation of EPS conditions see Table 1.) ρ b ⋅ 10 −14 , m −2
ε pr , % Fig. 4. Density of dislocations within small-angle boundaries in 15Kh2MFA steel as a function of ESP temperature: () T = 150°; (£) T = 350°C; (¢) intact material. grain boundaries or at the boundaries of structural constituents are initiated mainly by inclusions (Fig. 5c). Some microcracks and voids tend to arise also in the vicinity of massive inclusions (Fig. 5d). The microcracks near such inclusions originates at and propagate along the dislocation structure. Figure 5d shows large voids formed due to massive inclusions as well as a microckack that connects these voids; the inclusions themselves were etched out during electrolytic polishing of foils. We have also found the formation of voids on acicular carbide precipitations due to detachment of inclusions from the matrix (Fig. 5e). The emergence of voids at inclusions in the process of loading is usually attributed to the development of dislocation clusters capable of breaking the cohesive bond between the inclusions and the matrix [11]. 354
a
b
c
d
e
f
Fig. 5. Microcracks and voids in 15Kh2MFA steel upon EPS: (a, d, e, f) ×30,000; (b) ×15,000; (c) ×80,000. Generally, microcracks in 15Kh2MFA steel arise and propagate along parallel dislocation subboundaries and martensite laths. As this takes place, the boundaries of structural constituents, in particular grain boundaries, and dislocation subboundaries present an efficient barrier to the microcrack growth. Figure 5f shows a microcrack which initially propagated parallel to the subboundaries but had to stop when trying to cross the boundary perpendicular to the crack growth direction. Another thing studied was the dependence of rupture stress σ k at a temperature of −196°C on density of dislocations within small-angle boundaries in steel 15Kh2MFA upon EPS in tension and combined tension (Fig. 6). As the density of dislocations within small-angle boundaries increases due to plastic deformation irrespective of the EPS conditions (loading type, temperature, plastic strains), rupture stress grows, the growth being more significant upon EPS at 350°C. This is evidently associated with the fact that during EPS at a higher temperature, with the thermal reversion processes involved, there arise the dislocation structures which, with the density of dislocations within small-angle boundaries being the same, tend to form an intragranular substructure that provides additional barriers capable of arresting the microcrack. 355
σ k , MPa
ρ b ⋅ 10 −14 , m −2 Fig. 6. Rupture stress σ k vs. density of dislocations within small-angle boundaries in steel 15Kh2MFA upon tensile and combined tensile EPS: (1) T = 350°C; (2) T = 150°C. (For symbolic notation of EPS conditions see Table 1.) Thus, the EPS effect on the microstructural level consists in raising the density of dislocations within small-angle boundaries, disorientation of subgrains in the bainite structure, fragmentation of martensite laths, and reducing the distance between small-angle boundaries (the size of structural constituents); furthermore, at 350°C it gives rise to stable configurations in the form of small-angle boundaries due to the thermal reversion processes. The above-mentioned factors are responsible for increasing the stress needed for a microcrack to cross the boundaries of structural constituents, resulting in a higher rupture stress in 15Kh2MFA steel. We have also revealed an increase in rupture stress in 15Kh2MFA steel upon EPS. In one case, this is attributed to a decrease in the density of dislocations within small-angle boundaries (see the minimum value of the density of dislocations in Fig. 3a and the related decrease in rupture stress in Fig. 6, a left-hand point). In the other case, we have found a large number of microcracks and voids (Fig. 5e) which reduced the net cross-sectional area in the specimen. These numerous microcracks facilitated the formation of a macrocrack from the existing structural defects, which resulted in a decreased rupture stress (Fig. 6, the lowermost point). CONCLUSIONS 1. We have studied the influence of plastic prestraining in tension and combined tension at 150 and 350°C on microstructural changes in 15Kh2MFA steel upon heat treatment that simulates embrittlement of the material in a WWER-440 type reactor pressure vessel towards the end of its lifetime. The density of dislocations within small-angle boundaries has been found to grow with increasing EPS, irrespective of the loading type and test temperature. 2. The microcrack nucleation and growth micromechanisms in 15Kh2MFA steel during the tensile EPS and the effect of the material substructure on the microcrack arrest have been studied. The majority of microcracks arise at and propagate along parallel dislocation subboundaries and martensite laths, while the boundaries of structural constituents, in particular grain boundaries, and dislocation subboundaries serve as an efficient barrier to the microcrack propagation. 3. As the density of dislocations within small-angle boundaries increases and the structural constituents become smaller in size due to tensile and combined tensile EPS, rupture stress grows, the growth being more significant upon EPS at 350°C in comparison to that at 150°C. This is associated with the fact that during EPS at a 356
higher temperature, with the thermal reversion processes involved, there arise the dislocation structures which, with the density of dislocations within small-angle boundaries being the same, tend to form an intragranular substructure that provides additional barriers capable of arresting the microcrack. REFERENCES 1.
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