• Transactions of Materials Processing
• /
• v.32 no.2
• /
• pp.53-60
• /
• 2023

A wide range of grain size was achieved in a Fe-Cr-Mn austenitic stainless steel (STS) by cold rolling and reversion annealing. The tensile characteristics of the STS were analyzed in terms of the dependence of strain induced martensitic (SIM) transformation on the grain size. In the ultrafine grain regime, the steel showed a high yield strength over 1 GPa, a discontinuous yielding, and a prolonged yield point elongation followed by considerable strain hardening. By increasing the grain size, the discontinuous yielding diminished and the yield point elongation decreased. The microstructural examination revealed that these tensile characteristics are closely related to the suppression of SIM transformation with decreasing the grain size. Especially, the prolonged yield point elongation of the ultrafine grained STS was found to be associated with development of unidirectional ε martensite bands. Based on the microstructural examination of the deformed microstructures, the rationalization of the grain size dependence of SIM transformation was suggested.

• Journal of the Korean Society for Heat Treatment
• /
• v.9 no.3
• /
• pp.198-204
• /
• 1996

This work was carried out in order to investigate the effect of grain size on martensitic transformation temperature and morphology of Fe-27%Ni-0.27%C alloy. The martensitic transformation temperature was raised with increasing the austenitizing temperature within the range from $750^{\circ}C$ to $1200^{\circ}C$, owing to the grain growth, vacancy concentration. It was observed that the larger was the austenite grain, the higher was the martensitic transformation temperature. The influence of the austenite grain size was similar to that of the austenitizing temperature. The morphology of martensite in Fe-27%Ni-0.27%C alloy changed from lath to lenticular with the variation of grain size. From the above results, it was concluded that the martensitic transformation temperature and morphology was mainly dependent upon the austenite grain size.

• The Korean Journal of Ceramics
• /
• v.7 no.2
• /
• pp.70-73
• /
• 2001

Phase transformation of TiSi$_2$ confined in sub-micron area of which the size is around or smaller than the grain size of C49 TiSi$_2$ phase is studied. It has been known that the C49 to C54 phase change is massive transformation that occurs abruptly starting from C54 nuclei located at triple point grain boundaries of C49 phase. When the C49 phase is confined in sub-micron area, however, the massive phase transformation is observed to be hindered due to the lack of the triple point grain boundaries of C49 phase. Heat treatment at higher temperatures starts to decompose the C49 phase, and the resulting decomposed Ti atoms diffuse to, and react with, the underneath Si material to form C54 phase that exhibits spherical interface with silicon. The newly formed C54 grains can also trigger the massive phase transformation to convert the remaining undecomposed C49 grains to C54 grains by serving as nuclei like conventional C54 nuclei located at triple point grain boundaries.

• Proceedings of the KWS Conference
• /
• 2002.10a
• /
• pp.276-281
• /
• 2002

A metallurgical model for bainite transformation kinetics in the coarse-grained heat affected zone(CGHAZ) on the basis of an Avrami-type equation was studied. Isothermal transformation tests were carried out to obtain the empirical equations for incubation time and Avrami kinetic constants for C-Mn-Mo-Ni steel. The effect of prior austenite grain size(PAGS) on the reaction rate of bainite was also investigated. Compared with experimental transformation behavior of bainite, the predicted behavior was in good agreement. It was also found that a smaller grain size retard the bainite reaction rate, contrary to the classical grain size effect and this is considered to be caused by constraint of grain size to bainite growth.

• International Journal of Korean Welding Society
• /
• v.2 no.1
• /
• pp.11-14
• /
• 2002

A metallurgical model for bainite transformation kinetics in the coarse-grained heat affected zone(CGHAZ) on the basis of an Avrami-type equation was studied. Isothermal transformation tests were carried out to obtain the empirical equations for incubation time and Avrami kinetic constants for C-Mn-Mo-Ni steel. The effect of prior austenite grain size(PAGS) on the reaction rate of bainite was also investigated. Compared with experimental transformation behavior of bainite, the predicted behavior was in good agreement. It was also found that a smaller grain size retard the bainite reaction rate, contrary to the classical grain size effect and this is considered to be caused by constraint of grain size to bainite growth.

• Journal of Welding and Joining
• /
• v.28 no.3
• /
• pp.104-113
• /
• 2010

In this study, to predict the microstructure in weld HAZ of low alloyed steel, prediction model for the phase transformation considering the influence of prior austenite grain size and cooling rate was developed. For this study, six low alloyed steels were designed and the effect of alloying elements was also investigated. In order to develop the prediction model for ferrite transformation, isothermal ferrite transformation behaviors were analyzed by dilatometer system and 'Avrami equation' which was modified to consider the effect of prior austenite grain size. After that, model for ferrite phase transformation during continuous cooling was proposed based on the isothermal ferrite transformation model through applying the 'Additivity rule'. Also, start temperatures of ferrite transformation were predicted by $A_{r3}$ considering the cooling rate. CCT diagram was calculated through this model, these results were in good agreement with the experimental results. After ferrite transformation, bainite transformation was predicted using Esaka model which corresponded most closely to the experimental results among various models. The start temperatures of bainite transformation were determined using K. J. Lee model. Phase fraction of martensite was obtained according to phase fractions of ferrite and bainite.

• Korean Journal of Materials Research
• /
• v.26 no.6
• /
• pp.325-331
• /
• 2016

This paper presents a study of the tensile properties of austenitic high-manganese steel specimens with different grain sizes. Although the stacking fault energy, calculated using a modified thermodynamic model, slightly decreased with increasing grain size, it was found to vary in a range of $23.4mJ/m^2$ to $27.1mJ/m^2$. Room-temperature tensile test results indicated that the yield and tensile strengths increased; the ductility also improved as the grain size decreased. The increase in the yield and tensile strengths was primarily attributed to the occurrence of mechanical twinning, as well as to the grain refinement effect. On the other hand, the improvement of the ductility is because the formation of deformation-induced martensite is suppressed in the high-manganese steel specimen with small grain size during tensile testing. The deformation-induced martensite transformation resulting from the increased grain size can be explained by the decrease in stacking fault energy or in shear stress required to generate deformation-induced martensite transformation.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 1999.08a
• /
• pp.188-197
• /
• 1999

Grain refinement of the structural steels was selected as the most effective method to meet improvement of strength and toughness without heat treatment. So, the future research and developing direction of ultra fine grain steels are more and more required to response to the production of eco - materials(environmental consciousness - materials) In this paper, the product of surface fine grain steels by CRCT and Inverse Transformation Method by warm deformation of martensite is carried out in order to improve the production process of Dowel Bar. It is possible to obtain surface ultra fine grain steel, when warm deformation of martensite formed after quenching is carried out from 730$^{\circ}C$ to 800$^{\circ}C$ in the finishing rolling step. The characters of surface with ultra fine grain steel is showing the cementite particles inside the ferrite grain and fine ferrite grain of about 1.2$\mu\textrm{m}$ in size.

• Journal of the Korean Ceramic Society
• /
• v.42 no.11 s.282
• /
• pp.718-721
• /
• 2005

The high temperature tetragonal phase of zirconia containing $1.40{\~}1.60\;mol\%$ of yttria can be fully retained at room temperature by rapid cooling. The metastable tetragonal phase transforms into the monoclinic phase athermally upon subzero cooling. The transformation exhibited an athermal burst transformation. The effects of yttria content and grain size on the athermal martensitic transformation were studied in detail. The burst temperature linearly decreased with increasing yttria content or decreasing grain size. To consider the distribution of martensite nuclei, the Weibull modulus of the athermal martensitic transformation was evaluated from the distribution of the burst transformation temperature. From the Weibull analysis, the distribution of embryos appears to be more homogeneous than that of the defects responsible for the fracture of similar material.

• Journal of the Korean Society of Industry Convergence
• /
• v.25 no.6_2
• /
• pp.1055-1062
• /
• 2022

In this study, the mechanical properties of 80% cold-rolled austenitic 316L stainless steel were evaluated using specimens subjected to reverse transformation at 500-750℃ for 20 minutes and reverse transformation at 700℃ for 2-60 minutes. Also, for the elastic wave obtained from the tensile test, the dominant frequency according to the reverse transformation condition was investigated by time-frequency analysis. The SEM image of the 80% cold-rolled material was transformed into martensite and showed line and cross shapes. The TEM image showed that line shapes were shown at the grain, and grain boundary of martensite. The higher the heat treatment temperature and the longer time, the larger the grain. Tensile strength decreased as the heat treatment temperature and time increased, but elongation increased. Hardness was proportional to tensile strength. This is because the grain with different directions showed the same direction due to reverse transformation. The dominant frequency was decreased and then increased as the temperature and time increased. This is because the direction of the grain is different at a low temperature and the same direction is shown at a high temperature.