Carbon Distribution In Austenite Research Articles

Effect of Carbon Distribution During the Microstructure Evolution of Dual-Phase Steels Studied Using Cellular Automata, Genetic Algorithms, and Experimental Strategies

The development of ferrite-martensite dual-phase microstructures by cold-rolling and intercritical annealing of 0.06 wt pct carbon steel was systematically studied using a dilatometer for two different heating rates (1 and 10 K/s). A step quenching treatment has been designed to develop dual-phase structures having a similar martensite fraction for two different heating rates. An increase in heating rate seemed to refine the ferrite grain size, but it increased the size and spacing of the martensitic regions. As a result, the strength of the steel increased with heating rate; however, the formability was affected. It has been concluded that the distribution of C during the annealing treatment of cold-rolled steel determines the size, distribution, and morphology of martensite, which ultimately influences the mechanical properties. Experimental detection of carbon distribution in austenite is difficult during annealing of the cold-rolled steel as the phase transformation occurs at a high temperature and C is an interstitial solute, which diffuses fast at that temperature. Therefore, a cellular automata (CA)-based phase transformation model is proposed in the present study for the prediction of C distribution in austenite during annealing of steel as the function of C content and heating rate. The CA model predicts that the carbon distribution in austenite becomes more inhomogeneous when the heating rate increases. In the CA model, the extent of carbon inhomogeneity is measured using a kernel averaging method for different orders of neighbors, which accounts for the different physical space during calculation. The obtained results reveal that the 10th order (covering 10-µm physical spaces around the cell of interest) is showing the maximum inhomogeneity of carbon and the same effect has been investigated and confirmed using auger electron spectroscopy (AES) for 0.06 wt pct carbon steel. Furthermore, the optimization of carbon homogeneity with respect to heating rate has been performed using a bi-objective genetic programming (BioGP) strategy for the steel composition varying from 0.06 to 0.12 wt pct carbon along with other parameters like average austenite grain size and time of heating as the input variables. The analysis of the results obtained from BioGP suggests that the homogeneity of carbon increases with the increasing carbon concentration of steel. This is corroborated by analyzing the AES results obtained for 0.28 wt pct carbon steel using the same technique as that used for 0.06 wt pct carbon steel.

New Model for Carbon Distribution in Austenite and Steel Transformation Products

A model is proposed for the carbon distribution in austenite and steel transformation products. The model is founded on the approximate equality of lattice periods for austenite and diamond. Due to this equality the centered tetrahedron from the diamond structure can substitute the iron tetrahedron in the FCC austenite lattice. An octahedron in the FCC structure is reconstructed into the trigonal prism; this reconstruction is accompanied by the decomposition of the carbon tetrahedron onto five separate atoms. Each carbon atom is inserted into the interior of the trigonal prism and stabilized it. The net result is the linear chain of three-capped trigonal prisms, i.e. the {113} twin of the austenite lattice. Repeating twinning of the BCC structure by {112} plane generates metallic network of the cementite. Thus, the martensite atomic structure can be represented as an alternation of thin ferrite interlayers and parallel stacking of trigonal prisms stabilized by centering carbon atoms.

New experimental evidence on the incomplete transformation phenomenon in steel

In this work, the carbon distribution in austenite during isothermal bainite formation and the incomplete reaction phenomenon was analyzed by means of X-ray diffraction and atom-probe tomography in high-silicon, manganese-alloyed steels. The results provide new evidence on the temporary cessation of bainitic ferrite formation at abnormally low transformation temperatures.

Phase-Field Simulation of Cooperative Growth of Pearlite

Cooperative growth of pearlite is simulated for eutectoid steel using the multi-phase field method. This allows to take into account diffusion of carbon not only in γ phase, but also in α phase. The lamellar spacing and growth velocity are estimated for different undercoolings and compared with experimental results from literature and theoretical results from analytical models. It is predicted, that diffusion in ferrite and growth of cementite from the ferrite increase the kinetics of pearlitic transformation by a factor of four as compared to growth from austenite only, which is assumed by the classical Zener-Hillert model. Further on the effect of stress due to inhomogeneous carbon distribution in austenite and due to transformation strain is discussed shortly.

Determination of local carbon content in austenite during intercritical annealing of dual phase steels by PEELS analysis

Parallel electron energy loss spectroscopy has allowed to analyse and quantify local variations in the carbon concentration of austenite islands transformed during the intercritical annealing treatment of commercial dual-phase steels. These changes in the carbon content of different austenite regions are responsible for the different volume fractions of tempered martensite, martensite and retained austenite obtained after intercritical annealing and overaging treatment. This technique reveals how carbon distribution in austenite evolves as the transformation process advances.

Analysis of the γ → α transformation in a C-Mn steel by phase-field modeling

This article deals with the austenite (γ) decomposition to ferrite (α) during cooling of a 0.10 wt pct C-0.49 wt pct Mn steel. A phase-field model is used to simulate this transformation. The model provides qualitative information on the microstructure that develops on cooling and quantitative data on both the ferrite fraction formed and the carbon concentration profile in the remaining austenite. The initial austenitic microstructure and the ferrite nucleation data, derived by metallographic examination and dilatometry, are set as input data of the model. The interface mobility is used as a fitting parameter to optimize the agreement between the simulated and experimental ferrite-fraction curve derived by dilatometry. A good agreement between the simulated α-γ microstructure and the actual α-pearlite microstructure observed after cooling is obtained. The derived carbon distribution in austenite during transformation provides comprehension of the nature of the transformation with respect to the interface-controlled or diffusion-controlled mode. It is found that, at the initial stage, the transformation is predominantly interface-controlled, but, gradually, a shift toward diffusion control takes place to a degree that depends on cooling rate.

Modelling of Nodular Cast Iron Graphite Growth in Solid State

A micro-model of the graphite and ferrite solid growth from austenite was developed for the nodular graphite cast iron and a simulation was performed. The model allows for a relationship which exists between the carbon diffusion coefficient and temperature, taking also into account the transient carbon distribution in austenite. A stationary diffusion flux of carbon through ferrite envelope was assumed. The simulation shows the effects of the eutectic grain radius and of the cooling rate on the kinetics of eutectoid transformation and cooling curve.

Determination of Carbon Content in Bainitic Ferrite and Carbon Distribution in Austenite by Using CBKLDP

Convergent beam Kikuchi line diffraction patterns taken with a 10nm-diameter electron beam have been used to determine the lattice parameter and hence the carbon concentration in both ferrite and austenite. The experimental results show that bainitic ferrite is supersaturated in carbon and that, during ageing of austenite prior to the precipitation of cementite, the original carbon distribution across a grain becomes very nonuniform with distinct regions of both carbon enrichment and carbon depletion.

SH-CCT diagrams for welding and continuous cooling transformation behaviour for ductile cast iron

The purpose of this study is to clarify the continuous cooling transformation behaviour of the HAZ (Heat-Affected Zone) of a ductile cast iron according to JIS FCD700. For this purpose, two SH-CCT diagrams (Continuous Cooling Transformation Diagrams for simulated HAZ) for welding were prepared at a maximum temperature of 1223 K corresponding to a region of the HAZ near the base metal, and of 1323 K corresponding to a region of the HAZ near the fusion line. Ferrite and pearlite transformation regions in the SH-CCT diagram at 1323 K occurred after a longer time than those at 1223 K. A Bainite transformation region existed only in the SH-CCT diagram of 1223 K. Martensite transformation temperature is about 530 K for both of the simulated HAZs. These effects will be because of non-homogeneous carbon distribution in austenite near the graphite caused by the dissolution and growth of graphite during the heating and cooling in a welding thermal cycle.The range of cooling times when the hardness drops rapidly was about 10s to 60s in both simulated HAZs. It seems that the hardenability of the simulated HAZ prepared at 1323 K is a little greater than at 1223 K.

Cems study of the carbon distribution in austenite

CEMS resolution allows a careful study of Fe−C pure austenite spectra with high carbon content. Three Fe environments are detected which are ascribed to Fe atoms with one carbon first nearest neighbour and zero carbon second nearest neighbour and two environments with no carbon first nearest neighbour but zero carbon second nearest neighbour and one to four carbon second nearest neighbours respectively. This confirms the repulsive interaction between carbon interstitials and the tendency towards Fe8C ordering is suggested.

多層盛溶接熱影響部の組織分布と靭性に関する研究 Ｖ 溶接熱影響部における低焼入性域の組織生成機構

In the previous paper, it was shown that carbon distribution in austenite was not uniform in low hardenability zone in HAZ. This heterogeneous distribution of carbon was estimated to be the main factor for the formation of the microstructure in the low hardenability zone.In this paper, mechanism of the formation of the microstructure in the low hardenability zone was investigated on the basis of these results.Main results obtained were as follows.(1) The amount of high carbon region formed during the transformation to austenite decreased as peak temperature increased. This decrease in the amount of high carbon region was found to correspond with the increase in hardenability.(2) On cooling, polygonal ferrite was nucleated at low carbon region in austenite, when the peak temperature was within the low hardenability region (that is, high carbon region still remained). As temperature decreased, ferrite grew and the remaining austenite (including high carbon region) became M-A constituent.(3) As the results, resultant microstructure in the low hardenability zone was composed of polygonal ferrite and M-A constituent. The distribution of M-A constituent in this zone was similar to that in the zone heated to austenite-ferrite two phase temperature range.