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The morphologies and the formation mechanisms of isothermally transformed Widmanstatten ferrite and bainite in various steels
were investigated. Widmanstatten ferrite often grew from the grain boundary ferrite allotriomorph formed by a diffusional
mechanism in the temperature range of the upper C-curve, whereas, bainite grew directly from an austenite grain boundary showing
its own C-curve in a TIT diagram at temperatures between Bs (bainite starting temperature) and Ms (martensite starting temperature).
Both structures accompanied well-defined surface reliefs of the invariant plane strain-type, and were in the shape of a lath
or a plate consisting of several parallel needle-like ferrite subunits with parallelogram cross sc:ctions. The crystallographic
properties of Widmanstatten ferrite and upper bainite were similar to those of lath martensite. Therefore, it was concluded
that the difference between bainitic ferrite and Widmanstatten ferrite existed only in the nucleation events, where both structures
grew by the same mechanism. 相似文献
15.
Myoung-Gyu Lee Sung-Joon Kim Heung Nam Han Woo Chang Jeong 《International Journal of Mechanical Sciences》2009,51(11-12):888-898
An automotive flex plate that is installed in the automotive engine and delivers a torque to a transmission is manufactured by hot press forming technique. By this technique, significant increase of strength through quenching of heated high carbon SK5 steel sheet and higher dimensional stability after forming through the press operation at high temperature can be attainable. The indirect method with a pre-forming step and direct oil quenching to attain uniform and fast cooling rate are employed considering cooling characteristics of the SK5 steel and large thickness of the flex plate. A new tool design is proposed for the hot press forming process, and an optimized heat treatment condition is determined by studying two frequently utilized heat treatments: austempering and quenching with tempering heat treatments. By introducing designed tools and selected heat treatment condition, the target product can be successfully manufactured, which satisfies two main manufacturer's specifications: high hardness and good dimensional accuracy. Moreover, finite element analysis, which considers transformation induced plasticity (TRIP) during phase transformations, is performed to understand the thermo-mechanical behavior of hot press formed sheet. The analysis verifies that phase transformations play significant roles in strengthening by transforming hard martensitic phase and in reducing dimensional change by additional plastic deformation during phase transformations. 相似文献
16.
Yi-Gil Cho Young-Roc Im Jae Kon Lee Dong-Woo Suh Sung-Joon Kim Heung Nam Han 《Metallurgical and Materials Transactions A》2011,42(7):2094-2106
It is well known that nonisotropic volume changes in dilatometry were observed during the phase transformation in steel. In
this study, a finite element (FE) model incorporating the transformation plasticity was adopted to describe the nonisotropic
dilatometric behavior during the phase transformation in steel. An implicit numerical solution procedure to calculate the
deformation during the dilatometric experiment was incorporated into the general purpose implicit FE program. The nonisotropic
dilatometric behavior could be successfully reproduced by using the FE simulation considering the transformation plasticity.
The transformation plasticity was caused by the small amount of stress that naturally developed in the specimen during the
dilatometric experiment. In conventional low carbon steel, the stress in the specimen mainly forms due to the very small external
force supplied to support it during the dilatometric experiment. As regards ultralow carbon steel, whose phase transformation
occurs within an extraordinarily narrow temperature range, the inhomogeneous phase transformation due to the temperature deviation
in the specimen was mainly responsible for the stress field in the specimen during the dilatometric experiment. 相似文献
17.
Keesam Shin Sunghak Lee Sung-Joon Kim Kyungmox Cho 《Advanced Performance Materials》1998,5(4):307-318
The powder metallurgy fabrication of 2XXX Al composites reinforced with SiC whiskers was studied by investigating the evolution of microstructure and its relation to the mechanical properties. In this study, SiC whiskers and gas-atomized aluminum powders were mixed by fluid zone mixer, consolidated by vacuum hot press, and then extruded. The optimum condition for consolidation was 620°C and 50 MPa, at which fully densified pore-free billets were obtained. The composites with relatively homogeneous microstructures were produced by extrusion at 450–500°C under the extrusion pressure of 700–1000 MPa. The mechanical properties of the extruded bars were found to be comparable with those of the composites processed by Advanced Composite Materials Corp. The optimum fabrication conditions have been proposed for producing composites of improved mechanical properties through elimination of coarse intermetallic particles, uniform distribution of reinforcements, and minimization of whisker breakage. The possibility of using particulates rather than whiskers, and the modification of the alloy matrices for high temperature applications are also discussed in relation to the distribution of reinforcements and the optimization of the consolidation temperature. 相似文献
18.
Dilatometry is a useful method to investigate the transformation kinetics of ferrous alloys. In the present study, a dilatometric
analysis procedure that considers the effect of non-isotropic volume change was applied to evaluate the kinetics of austenite
formation during the intercritical annealing of TRIP steels. Metallographic analysis was conducted to validate the dilatemetric
analysis results. The austenite fractions from the dilatometric analysis showed reasonable agreement with those from the metallographic
one. This verifies that the kinetics of austenite formation during heat treatment of low carbon TRIP steel can be reliably
analyzed with the proposed procedure. 相似文献
19.
Byung-Hak Choe Baek-Hee Lee Je-Hyun Lee Tae-Ho Lee Chang Gil Lee Sung-Joon Kim Yong-Tai Lee 《Metals and Materials International》2001,7(6):551-556
A phase resulting from quenching solution treated beta-Ti alloys has long been recognized as a metastable structure containing unusual phenomena, and is not well understood. Tweed structure consisting of the metastable phase was observed by transmission electron microscopy, and spot satellites inflicted by streaks were also observed in the lattice diffraction. The results of microstructural studies and diffraction pattern surveys have identified the metastable structure as the elastic strain zone caused by the supersaturation of solute atoms, which may be induced by the electron charge distribution and atomic bonding between solvent matrix and solute atoms, causing the tweed structure. This paper presents the main finding of a microstructural abnormality, and compares the results with ordering behavior such as premartensitic transformation, and reviews a new regularity generated in the metastable phase of the supersaturated state. 相似文献
20.
Cainian Jing Dong-Woo Suh Chang-Seok Oh Zuocheng Wang Sung-Joon Kim 《Metals and Materials International》2007,13(1):13-19
The effects of a P addition on the mechanical properties and austenite stability are investigated for 0.15C−1.5Mn−1.5Al TRIP-aided
cold-rolled steels containing 0.05 and 0.1 wt.% of P. The strength and retained austenite fraction are increased by an increment
of the P content. The strengthening of P-added TRIP-aided steel partially comes from the solid-solution hardening effect of
P, and a higher fraction of strain-induced martensite plays an important role as well. The elongation of steel containing
0.1 wt.% P is diminished compared with that containing 0.05 wt.% P. This is attributed to the lower mechanical stability of
retained austenite in TRIP-aided steel containing 0.1 wt.% of P, which inhibits persistent work hardening during deformation. 相似文献