DEFORMED AND HYDROGENATED HEAT-RESISTANT 15Kh2MFA STEEL ... to predict the influence of the PPD of steels on their mechanical properties [3, 4],.
Materials Science, Vol. 44, No. 3, 2008
MICROSTRUCTURE AND FRACTURE STRESSES OF PLASTICALLY DEFORMED AND HYDROGENATED HEAT-RESISTANT 15Kh2MFA STEEL P. V. Yasnii, V. B. Hlad’o, I. B. Okipnyi, and O. T. Tsyrul’nyk
UDC 620.187.3
The joint effect of hydrogenation and preliminary plastic deformation (PPD) by tension and combined tension at 623 K on the fracture stress of heat-resistant 15Kh2MFA steel is studied. Hydrogenation was performed both before and after the PPD. Using transmission electron microscopy it was established that, independently of the type of loading and hydrogenation scheme, the PPD decreases the distance between low-angle boundaries and increases the dislocation density in them and the disorientation of the bainitic structure. The hydrogenation after the PPD does not influence the fracture stress of the steel. The PPD by tension and combined tension of the preliminarily hydrogenated steel reduces its fracture stress proportionally to the increase in the dislocation density in low-angle boundaries.
Plastic deformation increases the hardness and strength of metals. However, the use of preliminary plastic deformation (PPD) with the aim to increase the serviceability of components and elements of structures causes anxiety in the case of hydrogen brittleness because factors facilitating the absorption of hydrogen by a metal (specifically, strain hardening) hinder its mechanical and operating characteristics [1]. It was shown in [2] that the hardening of steel 20 specimens by a shock wave extends substantially the low-cycle life in a hydrogenated environment. This is explained by changes in the microstructure that affect the selective interaction of the most stressed microvolumes of the metal with hydrogen. As a result of the PPD, in spite of the higher solubility of hydrogen, defects and, therefore, hydrogen are distributed more homogeneously, which levels the difference in properties between the grain body and grain boundaries, and defects are smoothed. Thus, in the presence of hydrogen, it is difficult to predict the influence of the PPD of steels on their mechanical properties [3, 4], including its influence of the static and cyclic crack resistance [5]. The aim of the work is to investigate the joint influence of hydrogenation and the PPD by tension and combined tension on the fracture stress of 15Kh2MFA heat-resistant steel and microstructure of the deformed material. Testing Technique We investigated the steel in the heat-treated state (henceforward referred to as the initial state), which simulates the radiation embrittlement of the material as a result of neutron irradiation in the middle of the of the service life of a body of a VVER-440 nuclear reactor. Its mechanical characteristics are as follows: σ 0.2 = 897 MPa, σu = 1000 MPa, δ = 15.8%, and Ψ = 39.2%. The chemical composition and heat treatment regimes were described earlier [6]. The fracture stresses σcl were determined by uniaxial tension at temperature of 77 K (in liquid nitrogen) in a servohydraulic machine using cylindrical specimens with a diameter of the working region of 5 mm preliminarily deformed by tension or combined tension (tension with application of a cyclic component) to a plastic strain ε pl = 1.0 and 3.0% at 623 K. To localize the fracture zone, a circular Pulyui Ternopil State Technical University, Ternopil, Ukraine. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 118 – 121, May – June, 2008. Original article submitted September 20, 2006. 1068–820X/08/4403–0441
© 2008
Springer Science+Business Media, Inc.
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concentrator with a radius of 0.6 mm was cut on the deformed specimens. We hydrogenated specimens by the following schemes: before the PPD (series 1); after the PPD (series 2); the PPD without hydrogenation (series 3). Parameters of combined tension, electrolytic hydrogenation, nickel plating, copper plating, and temperature exposure of specimens were described in detail earlier [7].
Fig. 1. Microstructure of 15Kh2MFA steel: (a) needle-like precipitates in bainite; (b) parallel dislocation subboundaries of bainite; (c) highly disperse precipitates in the bainitic–ferritic structure; (d) carbide precipitates on boundaries of structural elements.
The microstructure was studied by the method of thin foils with a PEM-125K transmission electron microscope. Object were cut in the longitudinal direction and thinned by jet electrolytic polishing in a 10% HClO4 + 90% CH3 COOH electrolyte. The dislocation density was calculated in low-angle boundaries by analysis of the azimuthal disorientation of reflexes on diffraction patterns [8]. Results and Discussion The steel has a bainitic–ferritic structure (Fig. 1). In the bainite, we detected a streaky dislocation microstructure with a system of parallel dislocation subboundaries (Fig. 1a, b), at an angle of ∼ 60° to which needlelike carbide precipitates are located (Fig. 1a). Besides them, in the bainitic–ferritic structure, highly disperse precipitates are present (Fig. 1c), and carbide precipitates are observed on boundaries of structural elements (Fig. 1d). The deformation to 1.0 – 3.0% changed insignificantly the morphology. Specifically, the distance between subboundaries decreased (Fig. 2a – c), the nonparalellism of dislocation subboundaries, fragmentation of the structure, and the angle of disorientation between subboundaries increased (Fig. 2d). The number of reflexes and their smear on the microdiffraction pattern indicates a decrease in the distance between low-angle boundaries and the continuity of the orientation (Fig. 2d). The hydrogenation and type of tension do not influence the morphology of the dislocation structure.
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Fig. 2. Microstructure of 15Kh2MFA steel after deformation: (a) – (c) bainitic structure; (d) microdiffraction.
Fig. 3. Dependence of the dislocation density in low-angle boundaries of hydrogenated and nonhydrogenated 15Kh2MFA steel on the PPD by tension and combined tension at 623 K. Hydrogenation, series 1: ( 䉫 ) tension; ( 䉲 ) combined tension; hydrogenation, series 2; ( 䉯 ) tension; ( 䉳 ) combined tension; hydrogenation, series 3: ( 䊊 ) tension, ( 䊏 ) combined tension.
As the plastic strain rises, the dislocation density ε pl in low-angle boundaries ρ s b increases (Fig. 3). The 14 14 – 2 substantial spread of the values of ρ s b (from 4.4 × 10 to 6.7 × 10 m ) can be associated with the locality of 14 plastic deformation. At ε pl = 3%, the dislocation density differs insignificantly, namely from 5.2 × 10 to 14 – 2 6.4 × 10 m , which is due to, on the one hand, the fact that the whole volume of the material rather than of only local regions deforms. On the other hand, at 623 K, thermal recovery, which consists in the homogenization of the structure, i.e., in the elimination of the nonequilibrium excess of vacancies and dislocations, occurs. Remaining dislocations form stable low-angle boundaries with approximately equal densities. Thus, structural changes caused by the PPD do not depend on the type of deformation and hydrogenation scheme.
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P. V. Y ASNII, V. B. HLAD’O, I. B. OKIPNYI,
(a)
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O. T. TSYRUL’NYK
(b)
Fig. 4. Dependence of the fracture stress of 15Kh2MFA steel after PPD and hydrogenation on the dislocation density in low-angle boundaries: (a) series 2 and 3; (b) series 1 (see designations in Fig. 3).
We established (Fig. 4a) that, independently of the type and level of loading, for specimens of series 2 and 4, as ρ s b rises, the fracture stress of the steel increases against that of the undeformed material (Fig. 4a). Thus, the hydrogenation of specimens after the PPD (series 2) did not affect the character of the σcl – ρs b dependence in comparison with the nonhydrogenated material (series 3). In the case where the PPD was performed after hydrogenation, σcl decreased (Fig. 4b). It is known that hydrogen absorbed by metals degrades the mechanical properties of steels only when, during loading, it migrates to certain local regions increasing many times its concentration. That is why it does not affect under low-temperature loading when its diffusion rate is very small and under rapid loading, e.g., in impact toughness tests, when it is not enough time for its accumulation. That is why in the test of the steel the hydrogenation of which after the PPD did not cause specific structural changes, we detected no effect of hydrogen on the fracture stress. The higher level of σ cl against that in the initial state is due to the hardening effect during the PPD. From the viewpoint of resistance to brittle fracture of the steel, the PPD of the preliminarily hydrogenated metal is dangerous. Though no effect of the hydrogenation regime on the morphology of the dislocation structure after plastic deformation was detected, the PPD of hydrogenated specimens reduces the rupture stress, which is evidently due to the development of the damageability of the steel during interaction of the metal with hydrogen. CONCLUSIONS Independently of the type of loading and hydrogenation scheme, PPD at 623 K reduces the distance between low-angle boundaries of 15Kh2MFA steel, facilitates an increase in the dislocation density in them, increases the disorientation of subboundaries, and, at a strain of 3%, as a result of thermal recovery, stable configurations of low-angle boundaries form. The PPD at 623 K, hardens the steel, which manifests itself in an increase in fracture stress proportionally to the increase in the dislocation density in low-angle boundaries. The hydrogenation after the PPD does not influence the fracture stress. Hydrogen preliminarily absorbed by the steel does not change the morphology of the dislocation structure after strain hardening as a result of he PPD but changes the fracture stress also proportionally to the increase in the dislocation density.
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