Yeon-Chul Yoo

Dynamic recrystallization behavior of AISI 304 stainless steel

Abstract The dynamic recrystallization (DRX) of AISI 304 stainless steel was studied with torsion test in the temperature range of 900–1100°C and the strain rate range of 5.0×10 −2 –5.0×10 0 s −1 . The initiation and evolution of DRX were investigated with microstructural analysis and then the critical strain for DRX initiation could be confirmed by analysis of flow stress. The volume fraction of DRX ( X DRX ) as a function of processing variables, such as strain rate ( e ), temperature ( T ), and strain ( e ) were established. For the exact prediction of X DRX , the variation of flow stress with strain was analyzed in detail. The calculation of grain size was based on X DRX and the investigated microstructure at a few points. It was found that the calculated results agreed with the microstructure of the alloy at any deformation conditions.

Effects of nitrogen flow rates on the growth morphology of TiAlN films prepared by an rf-reactive sputtering technique

Abstract Over the last decade titanium aluminum nitride (TiAlN) has been under active investigation as a promising alternative to binary coating materials for cutting and forming tools because of its superior oxidation resistance and lower thermal conductivity. However, the effects of nitrogen flow rates on the reactive sputtering of TiAlN films remain relatively untapped despite their importance in studying the growth morphology to acquire valuable knowledge for the selection of optimum coating conditions. In this paper we report the results of our study regarding the implications of nitrogen flow rate on the growth morphology of the TiAlN films reactively sputtered from a stoichiometric target of TiAl. Scanning and transmission microscopy (SEM, TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM) have been used for film characterization, the latter of which is yet to be fully exploited as a tool for analyzing the effects of N 2 as a sputtering parameter.

Hot rolling simulations of austenitic stainless steel

The dynamic, static and metadynamic recrystallization behavior of austenitic stainless steel during hot rolling was analyzed. In this approach, each of those recrystallization behaviors is described by appropriate kinetics equations. The critical strain for dynamic recrystallization was determined so that a distinction could be made between static and metadynamic recrystallization; then the amounts of strain accumulation compared with the critical strain each pass. The effects of grain size on the fraction recrystallized and of the latter on the flow stress were evaluated for each type recrystallization behavior. In this way, the dependence of the mean flow stress (MFS) on temperature could be analyzed in terms of the extent and nature of the prior or concurrent recrystallization mechanisms. Finally, an example is given of an industrial process in which DRX/MDRX can play an important role. More grain refinement can be achieved by increasing the strain rate, decreasing the interruption time and lowering the temperature of deformation.

Prediction of dynamic recrystallization condition by deformation efficiency for Al 2024 composite reinforced with SiC particle

Hot torsion test has been carried out for Al 2024 composite reinforced with 8 μm SiCp (15 vol.%) to suggest optimum hot working condition for dynamic recrystallization (DRX) at the temperature range of 320 to 520 °C and strain rate range of 0.1 to 3.0/sec. Flow curve and deformed microstructure have been analyzed to identify the hot restoration mechanism of DRX. Processing map showing the variation of the deformation efficiency expressed by [2m/(m + 1)], where m is the strain rate sensitivity, with temperature and strain rate has been described for the composite. The characteristics of domain of DRX and peak efficiency of the composite have been analyzed by observing deformed microstructure. The composite showed 40–50% efficiency at the DRX domain (370–460 °C, 0.1–0.5/sec). Also, the variation of deformation efficiency with Zener-Hollomon parameter (Z = έ exp(Q/RT)) were discussed to find out optimum hot working condition for DRX of the composite. It is found that the optimum temperature and strain rate condition for DRX of the composite is 430–450 °C and 0.5/sec.

Hot-deformation behaviour of AA2124 composites reinforced with both particles and whiskers of SiC

Abstract The hot-deformation behaviour of AA2124 composites reinforced with both whiskers and particles of silicon carbide (15 vol% SiC w + SiC p ) was investigated by means of hot torsion tests in the temperature range 300 to 500 °C over a strain rate range of 1.32 × 10 −3 to 2.38 s −1 . The composites with various SiC w : SiC p ratios of 1:1, 1:2 and 1:3 were fabricated by the powder-metallurgy route. From the flow curves and hot-deformed microstructures, the hot recovery mechanism of composites deformed at 300–400 °C at 1.32 × 10 −3 s −1 strain rate was found to be dynamic recrystallization, while at temperatures higher than 450 °C dynamic recovery was responsible for the hot recovery mechanism of the composites. It was found that the composite with SiC w :SiC p = 1:1 showed the highest flow stress (52 MPa) and failure strain (375%) at 450 °C and 7.94 × 10 −2 s −1 .

The effect of SiC whiskers on the hot-deformation behavior of SiCw/AA2124 composites

Abstract The hot-deformation behavior of composites consisting of an AA2124 matrix reinforced with 15 vol% of SiC whiskers has been studied over a temperature range of 200 to 530 °C and a strain rate range of 1.32 × 10 −3 to 2.38 s −1 . Detailed analyses of flow curves and deformed microstructures were made to identify the hot-recovery mechanism of the composites. The critical strain, e c , for dynamic recrystallization of the composites was also evaluated by the work-hardening rate, θ, and effective stress, σ, from the flow curves. It was found that the hot-recovery mechanism of the composites was dynamic recrystallization and the addition of SiC whiskers promoted nucleation of the dynamic recrystallization. The relationship between the critical strain, e c , and peak strain, e P , of the composites was e c ≈ 0.52 e P .

Static recrystallization kinetics of 304 stainless steels

AbstractStatic restoration mechanism during hot interrupted deformation of 304 stainless steel was studied in the temperature range from 900 to 1100°C, various strain rate from 0.05 to 5/sec and pass strain of 0.25–3 times peak strain. It was clarified that the static recrystallization was happened after 3–10 seconds at first deformation. The static restoration was depended on the pass strain, deformation temperature and strain rate and fractional softening (FS) values increased with increasing strain rate, deformation temperature and pass strain. Recystallization kinetics was explained with Avrami equation and Avrami constant was 1.113. This value was independent of deformation variables significantly. The time of 5, 50, 95% recrystallization was evaluated using such equations: t0.05 = 2.9 × 10−12 ∈−1.17

Continuous dynamic recrystallization of AISI 430 ferritic stainless steel

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