Deformation-induced α΄-martensite generally forms at shear bands in the coarse-grained austenite, while it nucleates at grain boundaries in the ultrafine-grained (UFG) austenite. The available kinetics models are related to the nucleation on the shear band intersections, and hence, their application to investigating the kinetics of α΄-martensite formation for the UFG regime cannot be justified. Accordingly, in the present work, the general Johnson–Mehl–Avrami–Kolmogorov (JMAK-type) model was implemented for comparing the kinetics of α΄-martensite formation in the UFG and coarse-grained regimes using an AISI 304L stainless steel. On the experimental front, the X-ray diffraction (XRD) patterns and the electron backscattered diffraction (EBSD) maps were used for phase and microstructural analyses, respectively. It was revealed that the simple JMAK-type model, by considering the dependency of the volume fraction of α΄-martensite on the strain, is useful for modeling the experimental data, predicting the nucleation sites based on the theoretical Avrami exponents, and characterizing the transformation kinetics at low and high strains.

In the present work the two microalloyed steel (X65 and X70) used in oil and gas transition pipeline, was obtained as a hot rolled plate with accelerated cooling. First, weld heat affected zone thermal cycles, according to four-wire tandem submerged arc welding process were analyzed. The Baehr 805A/D dilatometer was used for weld heat affected zone thermal cycles’ simulation. The thermal cycles simulated process for heated region involved heating the steel specimens to the peak temperatures of 950, 1150 and 1350 ° C and transformation behaviour and microstructure is investigated. By analyzing the dilatometry results, continuous heating diagram, austenite grain growth and austenite formation kinetics were investigated. Austenite formation modeling was done using Johnson-Mehl-Avrami-Kolmogorov (JMAK) classic equation. The parameter n was found to be relatively independent on temperature (or heating rate); While the parameter k is strongly dependent to temperature, phase fraction transformed and austenite grain growth.

In the current study, the recrystallization behavior of 75% cold-rolled Fe-22Mn-10Al-1. 4C steel during annealing at 750, 770, 790, 810, and 830° C was studied. X-ray diffraction patterns and optical microscopy were used to characterize microstructures. The Vickers Micro-hardness test was used to characterize recrystallization kinetics during annealing. Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was used to evaluate the experimental data. The ashomogenized microstructure illustrated only austenite with a high fraction of annealing twins, and austenite to martensite phase transformation was not observed after quenching at a high temperature and also until high thickness reduction. Avrami exponent was decreased from 0. 76 to 0. 42, with increasing the annealing temperature from 750 to 830° C. The activation energy value was determined to be ~175 kJ/mol, which was slightly higher than the diffusion activation energy of carbon in austenite.

In this study, the influence of cold work on precipitation behavior in Ni Span C902 super alloy is considered to investigate. The samples were homogenized at 1000oC, and then samples were subjected to different amount of cold work of 30, 40 and 60%. Precipitation hardening treatments were performed at 450, 550, 650 and 750oC for 1 to 6 hours. Microstructural observations were done by optical microscope and precipitation hardening behaviors were investigated by Vickers hardening test. The results indicated that precipitation hardening at the temperatures of 450 and 550oC could not provide proper conditions for formation of strengthening precipitations (g’); while hardness increased significantly by increasing the aging temperature. The maximum hardness has been achieved for the homogenized and the 60% cold worked samples at 750 and 650oC, respectively. The results of empirical observations of the experiments in the present study are in well consistent in comparing to Johnson-Mehl-Avrami equations.

In this study, the kinetic of Au-based BMG (Au50Cu25. 5Ag7. 5Si17(at%)) at high temperature in supercooled liquid region was investigated. Crystallization kinetics of this amorphous alloy under non-isothermal (continuous heating) and isothermal conditions were investigated by differential scanning calorimetry (DSC). The results show that crystallization in this bulk metallic glass has one stage crystalline precipitation process during continuous heating. It was found that glass transition and crystallization kinetics are the function of continuous heating rate. Under non-isothermal conditions, activation energies corresponding to the characteristic temperatures were estimated by Kissinger equation. The calculated activation energies of glass transition, onset of crystallization and crystallization peak temperature are 246, 183 and 161 kJ/mol respectively. These activation energies revealed that the energy barrier for the glass transformation is higher than that for crystallization. The crystallization mechanism under isothermal condition was investigated by using Johnson– Mehl– Avrami (JMA) equation. The Avrami exponent is mainly in the range of 1 to 1. 6, which indicates heterogeneous nucleation with significant volume at the beginning of the transformation.

One of the most promising ways to produce a grain-refined microstructure in some steel materials is the thermomechanical processing route of subcritical recrystallization annealing of a cold-deformed martensite structure. In the present study, the microstructural evolutions and the associated recrystallization kinetics under various subcritical annealing heat treatment conditions are explored in an API X120 grade, advanced, High-Strength, Low-Alloy (HSLA) steel with an initial cold-deformed martensite microstructure. The steel sheet was the subject of a conventional cold rolling process for moderate true strain of 60% followed by isothermal recrystallization for different temperature-time combinations. Optical microscopy and scanning electron microscopy were used to characterize the microstructural evolutions, and the recrystallization kinetics was evaluated by hardness measurements with the aid of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) relationship. The experimental results indicated that annealing at 948 K (675 ° C) for 18 h is the optimum condition to achieve a grain-refined ferrite microstructure with an averaged grain size of 5. 2 μ m. The slow kinetics of recrystallization was also revealed by JMAK model as the Avrami exponent was calculated around one for all of the experiments. These observations are rationalized in part by the possible formation of microalloying elements carbides during the annealing process in association with the existence of the inhomogeneously deformed initial microstructure. This results in the appearance of a continuous regime for the recrystallization nucleation along with the sluggish movement of recrystallization fronts.

In this research, graphitization transformation of a commercial hypereutectoid steel called CK100 was studied by the dilatometric experiments at the range of 600–700 oC from prior martensitic structure. Also the effect of quenching media on the initial graphitization time and completion of transformation has been discussed. Also, graphitization transition from the different prior microstructures was studied using microscopic observations. Analyzing dilatometric data acquisitions confirmed good following results from Johnson–Mehl–Avrami equation. By calculating Avrami exponent, following of transformation kinetic from diffusion controlled nucleation and growth was resulted. Also, the minimum time for completion graphitization transformation was concluded about 45 hrs relative to the water quenched specimen after annealing at 670 oC.

In this study, the hot deformation and dynamic recrystallization behavior of low carbon steel containing 21 ppm boron was investigated. After homogenizing the samples at 1250 ℃ for 1-hour, hot compression tests were conducted at temperatures ranging from 850 ℃ to 1150 ℃ and strain rates from 0.01 to 10 s⁻¹, resulting in strain-stress flow curves. Following corrections, calculations and modeling were performed based on Arrhenius equations. Among them, the hyperbolic sine relationship provided the most accurate estimate and was selected as the valid model for the applied strain range. According to this model, the deformation activation energy (Q), was determined to be 293.37 KJ/mol. Additionally, critical and peak stress and strain values were obtained for each temperature and strain rate, and power relationships were established to describe their variation with respect to the Zener-Hollomon parameter (Z). Recrystallization fractions were derived by comparing the hypothetical recovery curves with the material flow curves, and the results were successfully modeled using the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation. The Avrami exponent was measured at approximately 2, indicating that nucleation predominantly occurred at grain boundaries. Microstructural analysis revealed that at higher Z values, recrystallization occurred along with a fraction of elongated grains, while lower Z values resulted in a greater fraction of equiaxed dynamic recrystallization (DRX) grains. The average grain sizes after compression tests at 950 ℃, 1050 ℃, and 1150 ℃ were measured as 21.9 µm, 30.4 µm, and 33.6 µm respectively at a strain rate of 0.1 s⁻¹, and 17.7 µm, 28.7 µm, and 31.3 µm at 1 s⁻¹. The overall microstructure displayed a more uniform grain size distribution with increasing deformation temperature.

In present research, the graphitization behavior of the hypereutectoid steel CK100 with the initial martensitic microstructure has been studied. The optical and scanning electron microscopic observations showed that the graphite particles were formed within the ferritic matrix due to the dissociation of carbides during annealing of the martensitic steel. The graphitization behavior of the steel was examined at various temperatures using the dilatometric tests, and the results obtained from these tests were consistent with those predicted by the Johnson-Mehl-Avrami equation. Moreover, the TTT diagram corresponding to the graphitization phenomenon was obtained from the results of dilatometric tests and then, the optimum temperature of the graphitization was determined.

This study was launched to investigate the effects of heating rate and aging parameters on the kinetic of precipitation reactions in a high alloy high strength steel having Ni, Co, Mo and Ti. For this purpose, as quenched specimens were subjected to three types of aging methods with different heating rates. These methods consisted of aging in Pb bath, salt bath, and furnace at different aging cycles. The kinetic of precipitation in each method was studied by hardness measurements and was described adequately by the Johnson-mehl-Avrami equation. Remarkable increase in hardness and its rate is observed when the rate of heating increases. The substantial increase in hardness of the specimens aged rapidly in salt & Pb baths, compared with those aged normally in furnace, seemed to be due to the formation of thermo elastic stresses during sudden expansion of the substance subjected to rapid heating. According to the results obtained in this research, increase in the Avrami constants, n & k, and decrease in the start time of transformation, ts, are associated with heating rate increasing. Analysis of the observed and calculated data for hardness using Arrhenius equation, shows that for the same amount of volume fraction of precipitates, the activation energy of precipitates decreased for f=25 and 50%, while at f=90 % it increased by increasing heating rate.