Papers by Keyword: Isothermal Martensitic Transformation

Abstract: This work investigates what phase transformations are taking place during a continuous heating as well as the influence of the solution temperature on the isothermal formation of martensite in a precipitation hardening semi-austenitic stainless steel. In previous studies in the stainless steel under investigation (12Cr-9Ni-4Mo-2Cu) only the isothermal mode of martensitic transformation has been experimentally detected. In this work it is shown that: 1) The A_F temperature is located around 1040 K; 2) The χ-phase present in the initial microstructure dissolves above ~1323 K; 3) above 1448 K the formation of delta ferrite is promoted at austenite grain boundaries; 4) the kinetics of isothermal martensite formation is strongly accelerated with increasing solution temperature. The kinetics has been monitored in-situ at room temperature by using high resolution dilatometry. A semi-empirical dilatometry model is used to convert the dilatometry signal into volume fraction of martensite transformed. The results are briefly compared with previous kinetic measurements under the influence of strong magnetic fields.

Abstract: We have investigated the effect of particle size on martensitic transformation by using single crystalline specimens of an Fe-30at%Ni alloy by scanning electron microscope (SEM) observation, X-ray diffraction(XRD), magnetic susceptibility and magnetization measurements. As a result, we have found that an athermal martensitic transformation changes to an isothermal martensitic transformation with decreasing particle size. Considering the result of the TTT diagram, we have estimated the driving force for the martensitic transformation and the cluster size for nucleation of the isothermal martensitic transformation based on the model previously proposed.

Abstract: We have investigated effect of magnetic field at cryogenic temperature on the stability of austenitic phase in a sensitized SUS304 stainless steel. The sensitization of the steel was made by heat-treatment in vacuum at 973 K for 100h after the solution treatment at 1323 K for 0.5 h. Isothermal holding of the sensitized specimens was made under a static magnetic field of 5 T in the temperature range between 60 and 260 K for various holding times. As a result, the TTT diagram under the magnetic field corresponding to the formation of 0.5 vol. % of -martensite shows a double-C curve with two noses. We found that by the application of magnetic field, the nose temperature and the incubation time of upper part of the C-curve, that is due to the    transformation, become lower and shorter, respectively. On the other hand, the nose temperature of lower part of the C-curve, that is due to the      transformation, does not change under the magnetic field although the incubation time shortens by the application of magnetic field.