The current study focuses on the composition of the constituents that are obtained from ultra-fast heating and the composition of the parent austenite that was achieved at the peak temperature. In order to analyse the content and distribution of important elements such as carbon, manganese, chromium and molybdenum in the microstructure, simulation software was used. It is also found that the content of these elements shows a gradient in the microstructure in relation to the distance from the undissolved cementite. This leads to carbon-rich areas in the vicinity of cementite and carbon-depleted areas as the distance from the cementite grows. The former creates ideal conditions for the retainment of austenite due to the higher carbon content while the latter creates conditions that enable the formation of bainite in the microstructure.@en

The effect of ultra-fast heating on the microstructures of steel has been thoroughly studied over the last year as it imposes a suitable alternative for the production of ultra high strength steel grades. Rapid reheating followed by quenching leads to fine-grained mixed microstructures. This way the desirable strength/ductility ratio can be achieved while the use of costly alloying elements is significantly reduced. The current work focuses on the effect of ultra-fast heating on commercial dual phase grades for use in the automotive industry. Here, a cold-rolled, low-carbon, medium-manganese steel was treated with a rapid heating rate of 780 °C/s to an intercritical peak temperature (760 °C), followed by subsequent quenching. For comparison, a conventionally heated sample was studied with a heating rate of 10 °C/s. The initial microstructure of both sets of samples consisted of ferrite, pearlite and martensite. It is found that the very short heating time impedes the dissolution of cementite and leads to an interface-controlled α → γ transformation. The undissolved cementite affects the grain size of the parent austenite grains and of the microstructural constituents after quenching. The final microstructure consists of ferrite and martensite in a 4/1 ratio, undissolved cementite and traces of austenite while the presence of bainite is possible. Finally, it is shown that the texture is not strongly affected during ultra-fast heating, and the recovery and recrystallization of ferrite are taking place simultaneously with the α → γ transformation.@en

The current work focuses on complex multiphase microstructures gained in CrMo medium carbon steel after ultra-fast heat treatment, consisting of heating with heating rate of 300 C/s, 2 s soaking at peak temperature and subsequent quenching. In order to better understand the microstructure evolution and the phenomena that take place during rapid heating, an ultra-fast heated sample was analyzed and compared with a conventionally treated sample with a heating rate of 10 C/s and 360 s soaking. The initial microstructure of both samples consisted of ferrite and spheroidized cementite. The conventional heat treatment results in a fully martensitic microstructure as expected. On the other hand, the ultra-fast heated sample shows significant heterogeneity in the final microstructure. This is a result of insufficient time for cementite dissolution, carbon diffusion and chemical composition homogenization at the austenitization temperature. Its final microstructure consists of undissolved spheroidized cementite, nano-carbides and martensite laths in a ferritic matrix. Based on EBSD and TEM analysis, traces of bainitic ferrite are indicated. The grains and laths sizes observed offer proof that a diffusionless, massive transformation takes place for the austenite formation and growth instead of a diffusion-controlled transformation that occurs on a conventional heat treatment.

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In this study, UltraFast Heat Treatment (UFHT) was applied to a soft annealed medium carbon chromium molybdenum steel. The specimens were rapidly heated and subsequently quenched in a dilatometer. The resulting microstructure consists of chromium-enriched cementite and chromium carbides (in sizes between 5–500 nm) within fine (nano-sized) martensitic and bainitic laths. The dissolution of carbides in austenite (γ) during ferrite to austenite phase transformation in conditions of rapid heating were simulated with DICTRA. The results indicate that fine (5 nm) and coarse (200 nm) carbides dissolve only partially, even at peak (austenitization) temperature. Alloying elements, especially chromium (Cr), segregate at austenite/carbide interfaces, retarding the dissolution of carbides and subsequently austenite formation. The sluggish movement of the austenite/carbide interface towards austenite during carbide dissolution was attributed to the partitioning of Cr nearby the interface. Moreover, the undissolved carbides prevent austenite grain growth at peak temperature, resulting in a fine-grained microstructure. Finally, the simulation results suggest that ultrafast heating creates conditions that lead to chemical heterogeneity in austenite and may lead to an extremely refined microstructure consisting of martensite and bainite laths and partially dissolved carbides during quenching.

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A structural steel component that failed under fatigue was examined with the aim to identify the root causes of this failure. Fractographic examination revealed the presence of multiple beach marks; the position and arrangement of those signified the occurrence of fatigue fracture under the presence of combined loading conditions, involving torsion and bending stress components. Crack initiation was observed also at the corners of the steel plate where non-metallic inclusions were located. Stereo-microscopical examination of the fracture surface likely revealed the presence of casting inclusions, probably fluxes or slag particles, near the surface and in the interior of the component. These inclusions could be considered inherent—metallurgical stress raisers, behaving as locations of prominent crack nucleation under cyclic fatigue loading, stimulating subsequent crack propagation and final ultimate rupture.

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A comparative study on the microstructural changes after conventional (20 °C s⁻¹) and ultrafast (300 °C s⁻¹) heating is performed on a medium carbon steel in the soft annealed condition. Continuous-heating dilatometry experiments are carried out. The phase transformations and the volume phase fraction of austenite are determined among other microstructural changes. The microstructure is first observed using Optical Microscopy (OM), further characterized by Scanning (SEM), and detailed analyzed by Transmission Electron Microscopy (TEM). The effect of heating rate on the kinetics of cementite dissolution and austenite formation is rationalized. The experimental results are compared with Dictra calculations, and the possible effects on the kinetics of diffusion-controlled austenite formation are rationalized as well. Metallographic observations indirectly suggest the enhanced nucleation of austenite above A_m.

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The effect of heating rate on the formation and decomposition of austenite was investigated on cold-rolled low carbon steel. Experiments were performed at two heating rates, 150 ˚C/s and 1500 ˚C/s, respectively. The microstructures were characterized by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Experimental evidence of nucleation of austenite in α/θ, as well as in α/α boundaries is analyzed from the thermodynamic point of view. The increase in the heating rates from 150 ˚C/s to 1500 ˚C/s has an impact on the morphology of austenite in the intercritical range. The effect of heating rate on the austenite formation mechanism is analyzed combining thermodynamic calculations and experimental data. The results provide indirect evidence of a transition in the mechanism of austenite formation, from carbon diffusion control to interface control mode. The resulting microstructure after the application of ultrafast heating rates is complex and consists of a mixture of ferrite with different morphologies, undissolved cementite, martensite, and retained austenite.

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Heating experiments in a wide range of heating rates from 10 °C/s to 1200 °C/s and subsequent quenching without isothermal soaking have been carried out on a low carbon steel. The thermal cycles were run on two different cold rolled microstructures, namely ferrite+pearlite and ferrite+martensite. It is shown that the average ferritic grain size, the ferrite grain size distribution, the phase volume fractions and the corresponding mechanical properties (ultimate tensile strength and ductility) after quenching are strongly influenced by the heating rates and the initial microstructure. The ferrite grain size distribution is significantly modified by the heating rate, showing a markedly bimodal distribution after fast annealing. The rise of the heating rate has produced a change in the relative intensities of texture components, favouring those of the cold-deformed structure (RD fibre) over the recrystallization components (ND fibre).

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The initial microstructure, its local composition and phase morphology determines the final microstructure in ultra fast heat treatment processes (UFHT). In the present study, we designed and performed, via dilatometry, UFHT processes involving heating rates higher than 250°C/s, peak austenitization and helium quenching. This UFHT leads to a carbon gradient within the austenite, as confirmed by thermodynamic and diffusion calculations. With microscopic techniques we shed light on the microstructure evolution of steels with different compositions and initial microstructures. The most commonly achieved microstructure from UFHT in medium-carbon-steels is a mixture of fine bainite and martensite. Partially dissolved cementite, carbides and other microstructural features could be identified as similar to the original, even after UFHT.@en