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Carbide precipitation kinetics in model bainitic steels

  • Benarosch, Anna
Publication Date
Dec 16, 2021
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Large components of the primary circuit of nuclear reactors, made with 16 to 20MND5 low alloy bainitic steels, operate at 300°C under a pressure of 155 bars, undergoing thermal aging and irradiation, and must retain their mechanical properties throughout the plant life. A thorough understanding of the links between microstructure and mechanical properties is therefore crucial to be able to predict their evolution under operating conditions. Carbides are known to play an important role in explaining the mechanical properties of the steels. Their population, size and distribution, evolve mainly during temper heat treatments performed after a step of austenitisation and quenching. However, after quenching, chemical heterogeneities can appear, locally affecting the microstructure, the carbide population, hence the mechanical properties. Therefore, it is necessary to understand the impact of alloying elements on carbide populations to be able to predict the mechanical properties of these heterogeneous areas. Furthermore, understanding the impact of alloying elements on the carbide precipitation sequence can lead to an optimisation of the heat treatments, and the design of steels with enhanced mechanical properties, as well.Amongst the three main alloying elements (Mn, Ni, Mo) of 16 to20MND5 steels, molybdenum, and manganese to a lesser extent, contribute to define the carbide population. In this context, to isolate the contribution of each alloying element, high-purity model alloys, FeCMo, FeCMn and FeCMoMn are investigated, which compositions are chosen close to the industrial alloy.First, continuous cooling transformation diagrams are constructed to understand the impact of the alloying elements on as-quenched microstructure, as well as to be able to choose a starting bainitic microstructure for tempering. It is demonstrated that FeCMoMn is a good model for the industrial alloy. Furthermore, in bainitic microstructures, Mn seems to favor the presence of block boundaries with a misorientation of 59° around [433]. In contrast with bainitic microstructures, Mo or Mn has no impact on the misorientation angle/axis pairs nor on the misorientation angle distributions in martensitic microstructures. Eventually, investigations carried out in prior austenite grains reveal that there exists at this level a continuous evolution from martensite to slowly cooled bainite, with a decrease of block boundaries.Second, two tempering temperatures are explored, 650 and 700°C, for times of several seconds up to two months. The starting microstructures of the model alloys are selected to be as close as possible to the 16 to 20MND5 steel bainite. The carbide precipitation sequences are characterized, their volume fraction quantified by ex-situ synchrotron X-ray diffraction. In FeCMn, only cementite is present and it enriches in Mn, associated with a lattice volume contraction. In FeCMo and in FeCMoMn, it differs from what is reported in literature. In FeCMo, cementite, present at first, dissolves for tempering times of one month or more, while M2C and ξ carbide precipitation is observed. M2C are both present at lath boundaries and intralath with a needle shape. ξ carbides precipitate at block boundaries, with a misorientation superior to 50°. In FeCMoMn, M2C and ξ carbides precipitate as well, while cementite does not dissolve for long tempering times. The use of thermodynamic calculations indicates that in most cases the systems have not reached thermodynamic equilibrium yet.The evolution of as-quenched microstructure with chemistry and cooling rate as well as the characterization of the carbide precipitation sequences are useful for modelling the mechanical properties of the nuclear reactor component heterogeneous regions and, as a result, of the whole component.

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