Effects of grain size on fracture toughness in transition temperature region of Mn–Mo–Ni low-alloy steels

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Abstract

An investigation was conducted into the effect of grain size on fracture toughness in the transition temperature region of Mn–Mo–Ni low-alloy steels used for nuclear pressure vessels. Three kinds of steels with different austenite grain sizes (AGS) were fabricated by varying the contents of Al and N, and their microstructures and mechanical properties were examined. Elastic–plastic cleavage fracture toughness, KJc, was determined by three-point bend tests of precracked Charpy V-notch (PCVN) specimens according to ASTM E1921 standard test method. When the AGS decreased, the total number of carbides increased, while the size and the aspect ratio of carbides decreased. Local fracture stresses, estimated from a theoretical stress distribution in front of a crack tip, were found to be mainly determined by the 92nd% size of carbides. Cross-sectional areas beneath fracture surfaces were observed to understand microstructural features to affect the cleavage crack propagation behavior. The results showed that measured cleavage fracture units were smaller than AGSs, indicating that packet boundaries as well as austenite grain boundaries played an important role in the cleavage crack propagation. Based on the electron back-scatter diffraction (EBSD) results, the cleavage fracture units could also be matched with the effective grain sizes determined by the misorientation tolerance angle of 25°.

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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