| Summary: | 碩士 === 義守大學 === 材料科學與工程學系 === 90 === The purpose of this study is to discuss the influence of soaking time on the d-ferrite content and morphology variation in austenitic stainless steels as well as their high temperature mechanical properties. Results of this study can help the understanding of the effect of different Cr/Ni equivalents on high temperature mechanical properties of austenitic stainless steels.
The experiments in this research showed that d-ferrite content is increased by the increasing of Cr/Ni equivalent ratio. By heat treatment of samples during different soaking time the d-ferrite is concentrated in early stage of heating. Then d-ferrite is dissolved into austenitic matrix if the soaking time increasing. Additionally, there exits a maximum value of d-ferrite content in early stage during heat treatment. Then d-ferrite content is decreased with the increasing of soaking time.
The analysis of mechanism of Cr and Ni diffusion in d、d/γ andγ by EDS, show the phase transformation from d-ferrite to austenite by way of solid state diffusion of Cr and Ni. After heating for a period of time, Cr is piled up at d/γ interface because of the slow diffusion rate of Cr in γ phase in comparison with d phase.
In the aspect of high temperature mechanical properties, tensile strength of materials decreased with the increasing of temperature because dynamic recovery and dynamic recrystallization happen. The study on 310 stainless steel shows the excellent behavior of reduction of area (R.A.) in hot tensile test. Besides, d-ferrite content influences the reduction of area (R.A.) during hot working process. The reason is that the deformation stress concentrates at the interface of d/γ that can cause fracture of material.
Therefore, the knowledge about the kinetics of d/γphase transformation can help the understanding of hot workability of high alloyed stainless steels.
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