Precipitation Behaviors And Its Effect On Mechanical Properties In Reduced Activation Steels

Reduced activation ferritic steels are selected as the most promising structuralmaterial for fusion reactors. Long-term exposures of these steels under serviceconditions of high temperatures and strong magnetic fields lead to microstructuralchanges. The microstructure evolution can severely influence the mechanicalproperties of the steels and hence the safety of the fusion reactors. The aim of thiswork is to systematically and quantitatively investigate the influence of such serviceconditions on the precipitation behavior of carbides in reduced activation steels andhow the size and distribution of these carbides influence the steels mechanicalproperties, in order to further understand the service behaviors of the steels and toimprove the safety of fusion reactors.This work first focuses on adjusting the austenitizing processes based on a betterunderstanding of the dissolution mechanism of TaC. When the austenitizingtemperature is raised above1150℃, TaC particles disappear in the matrix, and thesize of homogenous grains abruptly increases. A thermodynamic model for thedissolution of TaC particles during austenitizing is applied to interpret these results.Subsequently, the precipitation behavior of these carbides in the steels is investigatedby means of experimental investigations and theoretical modeling. A continuousmodel based on the Langer-Schwartz theory for nucleation, growth and coarsening isdeveloped for both homogeneous and heterogeneous precipitation. The effects ofdifferent thermodynamic parameters on the evolution of the precipitates in the steelsare discussed.Mechanical properties of the steels melted by vacuum induction melting （VIM）and vacuum induction melting followed by consumable electrode remelting（VIM+ESR） are studied. The impact and creep properties of the steels are noticeablyimproved by the VIM+ESR process. In addition, thermal ageing treatments arecarried out at550℃to simulate in-service condition and at600℃toprovide heavyover-ageing up to5000h. The relationship between precipitation behavior and fracture is thus evaluated. From these results, some processing procedures areproposed that can improve the steels mechanical properties by controlling itsprecipitation behavior. In particular, a special intermediate heat treatment is exploredin order to obtain a dispersed distribution of fine M₂₃C₆carbides in the steels. Resultsindicate that TaC rather than M₂₃C₆precipitates first formed with an intermediate heattreatment at850℃, which lead to a reduced mean size of M₂₃C₆from150nm to70nm. Increasing the Ta content in the steels can also result in the formation of TaC anda decreased M₂₃C₆mean size; in that case, most of the carbon in the steels are presentin TaC, so the carbon concentration decreases in the carbon-supersaturatedmartensitic matrix, resulting in the decrease of the M₂₃C₆mean size. The purpose ofthese studies is to master the precipitation behavior in the steels in order to improveits mechanical properties, and eventually provide a theoretical foundation for theoptimization of the mechanical properties and lifetime of the reduced activationsteels.Lastly, the influence of high temperature along with a high magnetic field on theprecipitation behavior of carbides and the subsequent mechanical properties of thesteels is studied. As-quenched steels are tempered at650℃for3h with and withouta10T magnetic field. Applications of the Weiss molecular field theory to calculatethe difference in interfacial energy caused by the high magnetic field, and of theLanger-Schwartz theory to model the metal carbide （TaC） precipitation behaviorunder the magnetic field are described. Results indicate that the density of TaC isdecreased by nearly an order of magnitude and its mean size increased by40%owing to an increase of0.03J/m²of the carbide/ferrite interfacial energy. Moreover,an improvement of the formula that describes the relationship between the yieldstrength of the steels and the mean size of the precipitates is made. The newlydeveloped model is able to predict the effect of precipitate coarsening on the yieldstrength of reduced activation steels.