Austenite to ferrite transformation kinetics during continuous cooling

The austenite decomposition has been investigated in a hypo-eutectoid plain carbon steel under continuous cooling conditions using a dilatometer and a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary systems Fe-C-Mn. The austenite to ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates. The formation of ferrite nuclei takes place with equilibrium composition on austenite grain boundaries. The nuclei are assumed to have a pill box shape in accordance with minimal interfacial energy. For higher cooling rates, early growth has to be taken into account to describe the transformation start. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium; i.e. only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute drag effect on the boundary movement. Thus, growth kinetics is controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements

Phase transformations in the silver-aluminum system

The formation of grain boundary precipitates of the high temperature β phase from the supersaturated ɤ phase has been examined in Ag-5.64 wt.% aluminum alloys at 688°C . Large grained samples were used and the boundary misorientations were determined by X-ray diffraction. At low angle boundaries only primary sideplates formed while above a misorientation of 17° lenticular precipitates were dominant. Precipitate growth was studied on individual grain boundaries using a statistical technique. The lengthening and thickening rates were independent of the grain boundary misorientation indicating that grain boundary diffusion was not significant under these conditions. The precipitates grew with constant shape, with both the length and thickness increasing parabolically with time. By approximating the shape of the precipitate to that of an oblate spheroid growing with constant shape, an equivalent diffusion coefficient was calculated. The value obtained was in good agreement with measurements obtained from diffusion couples. The nature of the quenched β phase was also examined using optical and electron microscopy. The βphase transformed rapidly on cooling, forming a massive [symbol omitted] product or an acicular martensite at higher quenching rates. The structures were very similar to those reported for the Cu-Ga and Cu-Al systems. Many grain boundary precipitates showed unequal growth into the two matrix grains. Measurements of the matrix habit plane suggested that a possible orientation relationship existed between the precipitate and that grain into which no development occurred. In this case the precipitate nucleated in one grain but grew into the opposite grain. Precipitates which developed equally into both grains exhibited no apparent habit relationship with either grain.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat

The relationship of interfacial energy to graphite shape in the Fe-C system.

The relationship between surface energy and precipitated graphite form in Fe-C alloys was examined in this thesis.Surface tension and contact angle data were obtained using the sessile drop technique. Carbon saturated, puron iron crucibles were melted on pyrolytic graphite, the effect of time, temperature (1500-1600°C) and additions of Ni, Mn, S or Ce being examined. The graphite form was established by metallographic examination. An average ƔLV of 1152 dynes/cm was determined for the Fe-C alloys (4.6% C) at approximately 1300°C, the average contact angle being 128°. No significant change occurred with additions of Ni ( 0.85%) and Mn ( 1.65%). Additions of S lowered the surface energy and increased the equilibrium contact angle. Ce additions had a similar effect although a direct comparison with the Fe-C alloys could not be made as different temperatures were used. However, the interfacial energy difference apparently increased with increasing Ce content, implying an adsorption of Ce to the graphite-melt interface. The change from the flake to the nodular form was accomplished in several transition stages, the interfacial energy differences being small, indicating a marked dependence on the solidification and growth conditions.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat

Study of Fractional Softening in Multi-stage Hot Deformation

In this paper, the softening behaviour of a medium-carbon steel under hot working conditions in multi-stage compression is presented. Continuous and interrupted compression tests were performed in the temperature range 800–1100 degrees Celsius at strain rates of 0.1, 1 and 8 s1. The interrupted deformations were conducted with delay times varying between 1 to 20 s after achieving a strain of approximately 0.2 to 0.4 in the first stage. The fractional softening has been predicted by three different methods, namely the offset-stress method, the back-extrapolation stress method and a currently proposed strain-recovery method. It has been found that the back-extrapolation stress method normally predicts higher reloading stress and hence a lower fractional softening than that of the offset-stress method. The strain-recovery method yields consistent results in the estimation of fractional softening that can be used in predicting the deformation mechanism

Predicting the onset of transformation under noncontinuous cooling conditions. Part 2: Application to the austenite pearlite transformation

A detailed review of the additivity principle with respect to the incubation of the austenite decomposition was summarized in Part 1 of this two-part series and led to the concept of an ideal time-temperature-transformation (TTT) diagram. This curve is characteristic of the chemistry and austenite grain size in the steel and allows nonisothermal behavior to be described assuming additivity holds. The derivation of mathematical relationships between the ideal and experimental cooling data was presented in the first article. In this second article, an ideal curve for the austenite-to-pearlite transformation was derived from cooling data. The applicability of the ideal TTT curve for predicting the start of transformation under continuous cooling conditions was assessed for a range of cooling rates. Experiments were conducted under both isothermal and varying temperature conditions, including an industrial cooling schedule, using a Gleeble Thermal Simulator. Reasonable agreement was found between the predictions and the observed transformation start temperatures; predictions were consistent and compared favorably against other methods which have been frequently used to estimate the transformation start temperature for nonisothermal conditions