Effects of Q&P Processing Conditions on Austenite Carbon Enrichment Studied by In Situ High-Energy X-ray Diffraction Experiments

Abstract

Abstract:We report the first ultra-fast time-resolved quantitative information on the quenching and partitioning process of conventional high-strength steel by an in situ high-energy X-ray diffraction (HEXRD) experiment. The time and temperature evolutions of phase fractions, their carbon content, and internal stresses were determined and discussed for different process parameters. It is shown that the austenite-to-martensite transformation below the martensite start temperature Ms is followed by a stage of fast carbon enrichment in austenite during isothermal holding at both 400 and 450 • C. The analysis proposed supports the concurrent bainite transformation and carbon diffusion from martensite to austenite as the main mechanisms of this enrichment. Furthermore, we give evidence that high hydrostatic tensile stresses in austenite are produced during the final quenching, and must be taken into account for the estimation of the carbon content in austenite. Finally, a large amount of carbon is shown to be trapped in the microstructure.

Real-Time Investigation of Recovery, Recrystallization and Austenite Transformation during Annealing of a Cold-Rolled Steel Using High Energy X-ray Diffraction (HEXRD)

“…In situ studies of solid-solid phase transformations mechanisms under thermomechanical treatments. The 2D diffraction patterns serve to track the respective evolution of phase fractions but also their lattice parameters, which depend on their local compositions and internal stresses (Please refer, for instance, to References [15][16][17][18][19][20] for in situ thermal treatments or to References [13,21] for strain induced transformation during in situ tensile tests). It has been proven that the HEXRD experiment is the sole reliable method to measure carbon enrichment in austenite in real time during a Quenching and Partitioning treatment of steel thanks to a high acquisition rate (10Hz) and it allows the unambiguous deconvolution of the chemical and mechanical contribution in austenite lattice parameters' evolution [17].…”

Bainite and Martensite: Developments and Challenges

“…The present Special Issue includes one review paper [1], one technical note [2] and ten scientific papers [3][4][5][6][7][8][9][10][11][12]. In all of them, martensite and/or bainite are being studied and in some cases, in combination with other phases.…”

“…The response of martensitic and bainitic microstructures to more traditional treatments as nitrocarburising [9] or tempering [2,3,5] based on the initial microstructural characteristics. More fundamental studies on the ways and means of phase transformation in specifically designed alloys [6,8,12] and the mechanical response and relationships between microstructure and mechanical properties [2,7,10] are among the topics presented in this compendium.…”

In-Situ Characterization by High-Energy X-ray Diffraction of the Phase Transformations Leading to Transformation-Induced Plasticity-Aided Bainitic Steel

“…However, synchrotron-high-energy X-ray diffraction (HEXRD) equally offers a high penetration depth (several mm at an X-ray energy close to 100 keV), with the additional advantages of a very high time resolution (below 1 s), and a large number of available diffraction peaks (due to the small wavelength) [19]. Therefore, this technique has been increasingly used to study, e.g., austenitization, bainitic transformation, Q and P processing, or the TRIP effect during tensile loading [20][21][22][23][24][25][26][27]. In addition to the fraction of phases that can be obtained by Rietveld refinement, HEXRD can provide information on the carbon content of the phases through the precise measurement of the lattice parameters, which is of paramount importance when studying processes leading to the TRIP effect.…”