In Situ Observation of Bainite Transformation and Simultaneous Carbon Enrichment in Austenite in Low-Alloyed TRIP Steel Using Time-of-Flight Neutron Diffraction Techniques

Abstract

An in situ neutron diffraction experiment during austempering of low-alloyed transformationinduced plasticity steel, Fe-1.48Si-1.52Mn-0.15C, in wt pct was conducted. In this study, timeof-flight neutron diffractometer with a large detector coverage, iMATERIA at J-PARC MLF, was employed. The phase fraction and carbon concentration in austenite could be quantitatively determined with a time resolution 1 minute although considerable textures existed for both ferrite and austenite. The carbon concentration in austenite during austempering was found to be inhomogeneous, resulting in a bimodal concentration distribution. The low-carbon region was consumed by bainite transformation whereas the high-carbon austenite slightly increased and even survived the final cooling to room temperature, forming a retained austenite. The rate of bainite transformation was affected by the state prior to the start of austempering. Consequently, different morphological features of the retained austenite were formed. More block-shaped austenite was observed in the case of slower bainite transformation, and it was determined that film-shaped austenite could also exist. The average carbon concentration was similar to that of high-carbon austenite during austempering. Hence, the film and block shapes of the retained austenite do not necessarily reflect the difference in carbon concentration.

Phase Transition Kinetics in Austempered Ductile Iron (ADI) with Regard to Mo Content

“…Note that, in our earlier work ( [24] and more recently also [46]), we showed that different enrichment of austenite with C during the ausferrite transformation process is visible in asymmetries of the (111) reflection. This peak asymmetry could be described with two separate peaks.…”

In Situ Heating Neutron and X‐Ray Diffraction Analyses for Revealing Structural Evolution during Postprinting Treatments of Additive‐Manufactured 316L Stainless Steel

“…An earlier report describes the detail furnace and instrument set‐up for in situ neutron experiments. [ 37 ] A heat cycle for the present in situ experiments includes holding at room temperature for 10–15 min, heating from room temperature to 670 and 1240 K for the as‐built and after HPT specimens, respectively, at a heat ramping rate of 4 K min −1 , holding the targeted maximum temperatures for 10 min, and subsequent fast furnace cooling towards room temperature.…”

Effect of Carbon Partitioning and Residual Compressive Stresses on the Lattice Strains of Retained Austenite During Quenching and Isothermal Bainitic Holding in a High‐Silicon Medium‐Carbon Steel

“…The RA phase leading to the transformation‐induced plasticity (TRIP) effect in these advanced multiphase steels contributes to the improved tensile ductility with a concomitant increase in tensile strength. [ 4–7 ]…”

“…The RA phase leading to the transformation-induced plasticity (TRIP) effect in these advanced multiphase steels contributes to the improved tensile ductility with a concomitant increase in tensile strength. [4][5][6][7] The latest development of ultrahigh strength steels has adapted to the new concept of microstructural design involving multiphase, and multiscale microconstituents. [8][9][10] In the past decade, a few novel heat treatment processes, such as quenching and partitioning (Q&P), [11][12][13] quenching-partitioning-tempering (Q-P-T), [14] and lowtemperature bainitic transformation (close to or even below M s temperature), [15][16][17] have been designed to achieve an excellent combination of high/ultrahigh strength combined with good low temperature toughness and reasonable ductility.…”