Effects of grain size on fracture toughness in transition temperature region of Mn–Mo–Ni low-alloy steels
Introduction
In Mn–Mo–Ni low-alloy steels, widely used for pressure vessels, compressors, and steam generators in nuclear power plants, high strength and toughness are required to withstand the internal pressure and prevent unexpected failure [1], [2]. Excellent resistance to neutron irradiation embrittlement, in which upper shelf energy (USE) is reduced and the ductile–brittle transition temperature increases, due to high-speed neutrons radiating from nuclear reactors, is also needed [3]. Since neutron irradiation embrittlement determines the life of nuclear pressure vessels in particular, superior fracture toughness in the transition region is essential to achieve the sufficient life time, even though neutron irradiation embrittlement occurs in service.
Although various kinds of test methods, such as ASTM E399, E813, and E1290 standard test methods could be considered, they have some limitations for quantitatively analyzing fracture toughness in the transition region [4]. However, the ASTM E1921 standard test method [5], in which the variation of fracture toughness in the transition region is considered as a property of ferritic steels, can quantitatively evaluate the fracture toughness in the transition region from the probabilistic and statistical point of view. According to this test method, variations in fracture toughness as a function of temperature can be described using a master curve characterized by a reference temperature.
As Mn–Mo–Ni low-alloy steels have a bainitic microstructure to maintain strengths above a certain level, there have been numerous studies on the factors affecting fracture toughness in the bainitic structure [6], [7], [8], [9], [10], [11], [12]. However, quantitative analyses on the effect of prior austenite grain size (AGS) on fracture toughness in the transition region are scarce among these studies. Since the steels investigated in the previous studies was in general use, or developed for practical applications, the variation of grain size and alloying elements was simultaneously reflected in them. In this case, it is impossible to study the effect of grain size on fracture toughness in the transition region systematically. In order to avoid this problem, Mn–Mo–Ni low-alloy steels with different grain sizes and similar alloying elements were fabricated in the present study, and their fracture toughness in the transition region was evaluated in accordance with the ASTM E1921 standard test method. Based on the reference temperature that characterizes fracture toughness in the transition region, the effect of grain size was systematically investigated.
Section snippets
Experimental
In order to understand the effect of grain size on fracture toughness in the transition region of Mn–Mo–Ni low-alloy steels, grain sizes were controlled by utilizing the effect of grain boundary pinning by AlN without changing major alloying elements such as C, Mn, Mo, and Ni. Three kinds of steels with different grain sizes were fabricated by changing the contents of Al and N in the base of SA508 Gr.3 alloy, which is typically used as the low-alloy steel for nuclear pressure vessels. They are
Microstructure
Optical micrographs of the three alloys are shown in Fig. 2(a) through (c). All of them show a tempered upper bainitic structure, and the prior AGS were measured to be 35, 15, and 12 μm, respectively, for the A1-, A2-, and A3-alloys. Fig. 3(a) through (c) are TEM micrographs obtained from carbon thin film extraction replicas, showing that the size and aspect ratio of carbides decrease as the AGS decreases.
Quantitative analysis for carbides was conducted to study the effect of carbides on
Discussion
When the critical stress intensity factor, Kc, is applied to a flawed structure, initiation of a cleavage crack should occur and the cleavage crack initiated should be successfully propagated into a neighboring grain for cleavage fracture. This means that the probability of cleavage fracture in a structure can be represented as the multiplication of the probability of cleavage crack initiation and the conditional probability of cleavage crack propagation [22]. Based on such a cleavage fracture
Conclusions
In this study, three kinds of Mn–Mo–Ni low-alloy steels with different grain sizes were fabricated by varying the contents of Al and N, and the effect of grain size on fracture toughness in the transition region of these three alloys was interpreted from a qualitative and quantitative point of view.
(1) Reduction in the grain size increased the total number of carbides per unit area, but decreased the size and aspect ratio of carbides precipitated. The local fracture stress was determined by the
Acknowledgements
This work was supported by Korea Atomic Energy Research Institute. The authors would like to thank Professor Hu-Chul Lee and Dr Young-Roc Im of Seoul National University, Dr Jun Hwa Hong of Korea Atomic Energy Research Institute, and Professor Yong Jun Oh of Hanbat National University for their helpful discussion on fracture toughness analysis.
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