Study On Tempering Of Mn-Mo-Ni Steel For Large Forgings Of Nuclear Power

In recent years, nuclear power, as one of the best alternatives of traditional energyresources, has found its important position. Mn-Mo-Ni steel for large forgings ofnuclear power is a critical material for the safety of nuclear reactors, and, hence, itsheat treatment should be paid great attention. The present thesis is mainly focused onthe microstructure-property relationships, phase transformation kinetics and creepbehavior during tempering of this kind of Mn-Mo-Ni steel.To begin with, both isothermal and isochronal temperings of Mn-Mo-Ni steel inthe as-quenched state were carried out. After tempering, transmission electronmicroscope and scanning electron microscope were utilized to analyze themicrostructure evolution, including decomposition of martensite-austenite (M-A),precipitation and coarsening of carbides in both M-A and bainitic ferrite, andrecrystallization of bainitic ferrite. It is concluded that dominant transition in the steeltempered at low temperatures is decomposition of M-A, while considerable carbideprecipitation can be observed during high-temperature tempering. In order tocharacterize the mechanical property of the tempered specimens, Vickers hardnessmeasurements were applied, of which the results are reasonably correlated with themicrostructural analysis.In addition, tempering kinetics was also investigated by using JMAK equation asthe selected model. Meanwhile, the hardness measurements and differential scanningcalorimetry (DSC) were utilized to obtain the kinetic parameters. In terms of isothermaltempering, the definition of tempering ratio was introduced to build a kinetic equationpredicting the degree of tempering. The results show that transformations duringisothermal tempering are mainly controlled by carbon atom diffusion in ferrite. As forisochronal tempering, a kinetic equation of decomposition of M-A was established andits activation energy was found to be quite close to that of decomposition of retainedaustenite.Finally, stresses were introduced to the specimens undergoing high-temperaturetempering to study the creep behavior of Mn-Mo-Ni steel.Precipitate-coarsening-induced plasticity model was used to fit the measured data of creep strain. It is indicated that such a model could properly describe the creep behaviorof the steel during high-temperature tempering.