This study is carried out to find the effect of nickel on the beginning of isothermal transformation of austenite in an iron-carbon-molybdenum alloy containing 0.55 carbon and 0.35 molybdenum. An introduction describes the iron-carbon equilibrium system, the products of slow cooling of austenite, the relation between slow cooling and isothermal transformation at temperatures below equilibrium, and a full discussion of terminology used. A literature review discusses theories of transformation and previous work on the effect of nickel on austenite transformation. Development of experimental technique in isothermal transformation and melting of pure alloys is discussed. The isothermal transformation diagrams are shown for beginning of transformation of austenite of base composition 0.55 carbon and 0.35 molybdenum, relatively free from impurities (silicon, manganese, etc.), showing the effect of nickel on the beginning of transformation. Nickel additions used are 0, 2.13, 3.69, and 5.31 percent. Isothermal transformation is shown by photomicrographs which are discussed fully. The effect of increasing nickel on the isothermal transformation of an alloy containing 0.55 carbon and 0.35 molybdenum is as follows: (1) the pearlite reaction is delayed appreciably: (2) ferrite formed at intermediate temperatures (880 to 1000 deg. F.) becomes more prominently acicular, the acicular ferrite reaction taking the place of the upper bainite reaction of low nickel alloys: (3) the acicular ferrite reaction is followed first by rejection of carbide particles, and later by agglomeration and growth of the carbide phase: (4) the acicular ferrite and feathery bainite reactions as represented on the isothermal transformation diagram become separated by the appearance of a bay in the isothermal transformation curve. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This study critically examines the additivity of the proeutectoid ferrite transformation. The study has been carried out in two parts with the first part involving experimental verification of the additivity of the proeutectoid ferrite transformation and the second part a theoretical assessment of the additive nature of the ferrite transformation with the aid of mathematical models. Austenite-to-proeutectoid ferrite transformation kinetics were measured under a number of isothermal, stepped-isothermal and continuous-cooling conditions for three plain-carbon hypo-eutectoid steel grades (AISI 1010,1020 and 1040) using a dilatometer and a Gleeble 1500 thermomechanical simulator. Isothermal transformation kinetics were characterized using the Avrami equation. The stepped-isothermal transformation tests were designed to experimentally assess the additive nature of the austenite to proeutectoid ferrite transformation by measuring transformation kinetics partially at one temperature and after a rapid temperature change to another temperature. Results on the 1010 and 1040 steels showed that the proeutectoid ferrite transformation with polygonal morphology is additive under changing temperature in that the ferrite transformation kinetics at the second temperature are quite similar to the isothermal kinetics at that temperature. Stepped-isothermal transformation measurements were made on the 1020 steel with the resulting ferrite morphology either remaining Widmanstatten at both temperatures or changing from allotriomorph to predominantly Widmanstatten at the two temperatures. In both cases the results showed additive behavior. However, the stepped-isothermal test in which the proeutectoid ferrite was transformed to an equilibrium amount and equilibrated at the first temperature and then rapidly changed to the second temperature was not additive. Characterizing the isothermal formation of proeutectoid ferrite in the three steels using the Avrami equation resulted in a reasonably constant value of n and the b parameter increasing with increasing transformation temperature. Early site saturation was evident in a number of test specimens. In the second part of the study, two mathematical models with planar and spherical interface geometries were developed to theoretically assess the additivity of the proeutectoid ferrite transformation. A finite-difference numerical technique was employed to describe the austenite to ferrite diffusion controlled moving-interface problem for a system having finite boundaries. A test of additivity of the proeutectoid ferrite transformation was made by predicting the ferrite growth kinetics and the associated carbon gradients under stepped-isothermal conditions. The predictions were consistent with the observed experimental additivity of the proeutectoid ferrite transformation in the 1010 steel. The spherical model predicted isothermal ferrite growth kinetics compared more favorably with the experimentally measured kinetics of the 1010 steel than the planar model. An unusual phase was detected in the 1020 steel. A number of tests were performed to measure the isothermal transformation kinetics of the new phase. Scanning electron microscopy (SEM) and scanning-transmission electron microscopy (STEM) were used to further investigate the structural details in this new phase. The results indicated that the new phase is a type of bainite but having some of the characteristics attributable to massive transformation products. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The pearlite-to-austenite reversion transformation kinetics under isothermal and continuous heating conditions in a eutectoid plain-carbon steel have been measured, using a dilatometric technique on a Gleeble 1500 Thermomechanical Simulator. The isothermal data was characterized in terms of the transformation start time at temperature for the onset of the P→ γ transformation, and in terms of the Avrami parameters n and b. Under the assumption that the P→ γ transformation was additive, the Scheil equation was applied to the measured isothermal transformation start data to predict the onset of the transformation on continuous heating, and the isothermal phase transformation kinetics were used to predict the continuous heating kinetics. It was found that the kinetic model significantly underpredicted the transformation start time during continuous heating. This was attributed to the large experimental error inherent in the estimation of the isothermal transformation start time, t[formula omitted]. The model's continuous heating kinetic predictions were excellent at low heating rates, but it tended to overpredict the kinetics at higher heating rates. The problem was traced to an observed difference between the measured temperature and the programmed temperature during the high heating rate tests. When the model was modified to incorporate the actual temperature profile, its prediction of the kinetics was considerably improved. Thus the austenite reversion transformation was concluded to be experimentally additive. An average Avrami n value of 2.2 suggested that austenite was nucleating on pearlite colony corners and edges. This conclusion was verified with optical and scanning electron microscopy. Previously published data, which indicated that the pearlite-to-austenite transformation is isokinetic, was found to be based on questionable assumptions. Metallographic information suggests, however, that the nucleation sites are saturated early in the reaction. Furthermore, the isothermal austenite formation data generated in this work was found to meet the effective site saturation criterion for additivity, implying that the austenitization process would be expected to be additive. The effect of starting microstructure was evaluated by performing isothermal and continuous heating tests on two different pearlitic microstructures. It was found that, in agreement with published results, the transformation rate varied in inverse proportion with the pearlite spacing and colony size. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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Twin induced plasticity (TWIP) steel is a new type of structural steel, which is characterized by both high strength and superior ductility. The key to twin formation lies in the control of the stacking fault energy (SFE) of the alloy. In this thesis, the effect of heat treatment on the tensile property and microstructure of a Fe-10Mn-3Al-0.6C alloy was studied. After annealing at 675¢J for different times, It was found that both austenite and ferrite phases existed. The volume fraction and composition of these two phases did not change significantly by the heat treatment conditions used. When annealed at 675¢J, increasing annealing time caused the carbides formed at grain boundaries gradually dissolved, and led to the higher ultimate tensile stress, 900MPa, and elongation, 40%. Deformation twins were formed in the austenite phase after tensile test in the annealed specimens, indicating that TWIP behavior occurred. The best tensile property of Fe-10Mn-3Al-0.6C alloy obtained by annealing at 675¢J is within the target property of the 3rd AHSS.

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Stress relaxation measurements were carried out on a plain carbon and four Ti steels over the temperature range 850 to 1050$ sp circ$C. The results show that the stress relaxation of plain carbon austenite after a 5% prestrain can be described by the relation $ sigma$ = $ sigma sb0$-$ alpha$ln(1 + $ beta$t). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is complete. As a result, this new mechanical method is suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures. / PTT diagrams were determined by the present technique for the steels containing 0.05, 0.11, 0.18 and 0.25% Ti. The PTT curves obtained are C shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the current values of the solubility product (Ti) (C). This phenomenon is attributed to the catalytic effect of trace amounts of dissolved N on the nucleation rate of Ti(CN) in austenite. / Changes in the size distribution and morphology of the precipitates during relaxation of the 0.25% Ti steel were followed by means of electron microscopy. The cube shaped Ti(CN) precipitates were heterogeneously distributed in either a chain-like or a cell-like manner. Electron microanalysis was additionally carried out to determine the compositions of the Ti carbosulphide-Mn sulphide inclusions. / Finally, the experimental results are compared with the predictions of classical nucleation theory and of the diffusion controlled particle growth theory. Good agreement was obtained between the predictions of the theories and the experimental results. It is demonstrated by means of the thermodynamic analysis of nucleation that the Ti(CN) precipitate/austenite interface is of a semi-coherent nature. It is shown that the interface between a critical nucleus and the matrix can be characterized by the newly introduced coherency loss parameter C.

Thesis (M.S.)--University of Wisconsin--Madison, 1966. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

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