Examination of phase transformation kinetics during step quenching of dual phase steels

Isothermal Transformation Behavior and Microstructural Evolution of Micro-Alloyed Steel

“…At temperature 400°C, the microstructure consists of lower bainite and martensite with small amount of allotriomorphic ferrite (AF) around the austenite grain boundaries (Figure 3c). This microstructure is similar to allotriomorphic transformation process as reported for high temperature (at around 720°C) in the work of Ashrafi et al [14]. The several nucleation sites form from the austenite grain boundary and simultaneously hard impingement also occurs between sheaves of bainite inside austenite grains [15].…”

Unraveling the Effect of Martensite Volume Fraction on the Mechanical and Corrosion Properties of Low‐Carbon Dual‐Phase Steel

“…Most of the works on the effect of volume fraction of martensite on tensile properties are dedicated to the intercritical annealing (IA) of ferrite–pearlite microstructures (IA route) or the IA of the martensitic microstructures (intermediate quenching, IQ, route) . However, for the step quenching (SQ) route (austenitization, holding at the IA temperature, and quenching), there are very few systematic works on the relation of martensite content and tensile properties . In contrast to the IQ samples, Bag et al reported that, with increasing martensite content in SQ samples, the yield stress (YS) increases, the ultimate tensile strength (UTS) remains approximately constant, the uniform elongation decreases, and the total elongation remains constant.…”

Effect of Step Quenching Heat Treatments on the Kinetics of Ferrite Formation and Quenching & Partitioning Modeling for a Commercial C-Mn-Si Steel

“…It should be noted that: a) the used austenitizing conditions were the same as those employed by Magalhães et al 11 , i.e., 950°C for 3 minutes; b) the specimen cooling rate starting from its austenitizing condition to step temperature, 15°Cs -1 , were selected as being the lowest possible rate that would guarantee, for the three evaluated step temperatures, that the austenite decomposition in ferrite would only start during isotherm and not during continuous cooling 11 ; c) the time interval of 10 minutes was selected to give sufficient time for a quasi-equilibrium state to be achieved, which was successful in terms of the formed ferrite fraction, as it will be presented and discussed in the results 14 ; d) the cooling rate of 200°C s -1 was defined as a cooling rate at which the non-occurrence of bainitic transformation was guaranteed during the final specimen cooling, and the final microstructures consisted of different ferrite, martensite, and retained austenite fractions 11 .…”

“…To evaluate the kinetics of ferrite formation at T α temperature, the JMAK equation was fitted to the experimental data. The JMAK model shows that for isothermal phase transformations, the fraction of the new formed phase can be predicted as a function of time as Equation 1presents, where x is the new phase fraction, k is the temperature-dependent kinetic constant, n is the Avrami exponent, and t is time 14,29,30 .…”

“…The isothermal treatment is performed at a strategically chosen temperature and during a well-planned time interval, in order to obtain the desired ferrite and austenite fractions. Next, the steel is cooled at high enough rate to promote the transformation of a part of the remaining austenite into martensite, maintaining a fraction of austenite, enriched in carbon, retained in the microstructure 13,14 . Some authors have reinforced that for the successful fine-tuning of the microstructure and the precise control of mechanical properties, it is necessary to understand and to predict the kinetics of ferrite formation in SQ cycles [12][13][14] .…”