Effect of microstructure on the impact toughness and temper embrittlement of SA508Gr.4N steel for advanced pressure vessel materials

Abstract

The effect of microstructure on the impact toughness and the temper embrittlement of a SA508Gr.4N steel was investigated. Martensitic and bainitic structures formed in this material were examined via scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and Auger electron spectroscopy (AES) analysis. The martensitic structure had a positive effect on both the strength and toughness. Compared with the bainitic structure, this structure consisted of smaller blocks and more high-angle grain boundaries (HAGBs). Changes in the ultimate tensile strength and toughness of the martensitic structure were attributed to an increase in the crack propagation path. This increase resulted from an increased number of HAGBs and refinement of the sub-structure (block). The AES results revealed that sulfur segregation is higher in the martensitic structure than in the bainitic structure. Therefore, the martensitic structure is more susceptible to temper embrittlement than the bainitic structure.Reactor pressure vessels (RPV) are generally considered to be the most critical components in the nuclear plant beside the reactor core 1,2 . The RPV should serve for 40 years under high-temperature, high-pressure, and neutron-irradiation conditions. Therefore, to ensure safe operation of the reactor, the RPV should be fabricated from material with high strength, high fracture toughness, and high resistance to irradiation embrittlement 3-5 .The RPV is typically fabricated from ASME SA508 Class 3 steel [6][7][8] . With the development of generation IV reactors, larger and higher-power nuclear power plants (compared with those housing generation III reactors) are being built. However, the low strength (620 MPa) of the SA508 Class 3 steel renders this material unsuitable for generation IV reactors and, hence, new materials must be developed. Many countries have considered SA508Gr.4N steel as a candidate material 9,10 .The RPVs are large forgings and with wall thickness of more than 200 mm 11 . Generally, the forged RPVs are quenched and tempered, as result, the through-thickness microstructure vary significantly because of the differential cooling rates during quenching. These differing microstructures will lead to heterogeneous mechanical properties along the thickness direction of the RPV. Therefore, an understanding of the microstructure and mechanical properties of RPV steels with different quenched microstructures is essential.The microstructure of the initial quenched martensite is a key factor in determining the performance of RPVs. Park et al. 12 reported that the strength of a quenched Ni-Cr-Mo low alloy steel with a fully martensitic microstructure is significantly higher than that of the quenched microstructure composed of bainite. Similarly, compared with bainite, tempered martensite is more susceptible to temper embrittlement. Raoul et al. 13 found that, in high-P (0.017%) A533 steel, P segregation in martensite is considerably higher than that in bainite. Furthermore, Lee et al. 14 dete...

Research on the Microstructures and Mechanical Properties of Bainite/Martensite Rail Treated by the Controlled-Cooling Process

“…The smaller the size of packet and block per unit area, the higher the fraction of the high-angle grain boundaries. The effective grain size of the P5 samples is smaller than that of the P15 samples due to the faster cooling-rate, which may improve the fraction of HAGBs [27]. The further transformation of retained austenite after the spray controlled-cooling process makes some filmy RA transform into filmy M/A constituents with more HAGBs.…”

The Relationship between Strength and Toughness in Tempered Steel: Trade‐Off or Invariable?

“…Lee and Takebayashi reported a linear relation between the carbide size and the critical distance for cleavage crack initiation, revealing that the coarse carbides are detrimental to the impact toughness. Several researchers thought that the distribution of carbides along the martensite lath and prior austenite grain boundaries causes embrittlement of materials, promoting rapid fracture. Cao et al pointed out that the local fracture stresses for cleavage fracture were determined by the morphology and size of the grain and carbide particles, illustrating that the coarse carbide and grain result in the inferior impact toughness.…”

Double-sided GTAW of nuclear grade steel: Mechanical and microstructure perspectives

“…The distribution of grain boundary misorientation-angle has a significant impact on mechanical integrity, especially for materials like polycrystalline. 31 The increase in tensile strength primarily depends on the Low Angle Grain Boundaries in the weldment zone when compared with SS321, while decrease in the fraction of High Angle Grain Boundaries reduced the impact strength, 32 and the theory behind this observation is reported by Kumar et al 27 The XRD pattern reveals the intensity of various phases present in the SS321 and weldment. The XRD plot for SS321 and weldment are shown in Figures 6 and 7.…”