Bainite Formation in Medium-Carbon Low-Silicon Spring Steels Accounting for Chemical Segregation

Abstract: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.

Section: Phase Transformations During Quenchingmentioning

confidence: 99%

Study of Carbide Dissolution and Austenite Formation during Ultra-Fast Heating in Medium Carbon Chromium Molybdenum Steel

Papaefthymiou

Bouzouni

Petrov

2018

“…Cu (Copper) -Copper precipitation with displacive mechanism in bainite reaction. N (Nitrogen) -Enrichment in nitrogen tends to slow down bainite transformation [35].…”

Section: Mn (Manganese) Lower Bsmentioning

confidence: 99%

Bainite Transformation-Kinetics-Microstructure Characterization of Austempered 4140 Steel

Zhu

Sun

Barber

et al. 2020

Bainite transformation is a kinetic process that involves complex solid diffusion and phase structure evolution. This research systematically studies the bainite transformation of austempered 4140 steel in a wide range of isothermal temperatures, in which four bainite phases structures were generated: upper bainite; mixed upper bainite and lower bainite; lower bainite and mixed lower bainite and martensite. The kinetics of bainite transformation has been described with a linear trend using an Avrami n-value. It was found that the bainitic ferrite sheaves grow with widthwise preference. The sheaves are stable when half-grown and are variable in length, due to austenite size limit or soft/hard impingement, or autocatalytic nucleation, or these conditions combined. The full-grown upper/lower bainite sheaves were found to be 1.9 µm/1.2 µm in width under the conditions of this study. Each individual bainite sheave is lath-like instead of wedge-like. The upper bainite sheaves mostly appear as broad-short-coarse lath, while the lower bainite sheaves appear as narrow-long-fine lath. The overall bainite transformation activation energy ranges from 50-167 kJ/mol. The mechanism of austenite to bainite transformation has been described by three theories: displacive theory, reconstructive or diffusional theory [3,4], and a combination of the theories [5]. The bainite transformation volume fraction increases with isothermal holding time. This is the kinetic aspect of the microstructure phase changes. Numerous efforts have been carried out to establish kinetic models that will agree with actual experimental data, but so far, there is not one model that all researchers accept and apply. However, the Johnson-Mehl-Kolmogorov-Avrami equation (JMKA) [6] is still popular 70 years after it was first reported. This kinetics model, or variations of this model, is the most commonly used model for transformation kinetics calculations.The bainite transformation mechanisms, nucleation, and growth kinetics are often linked to the Gibbs free energy concept [3], which describes the thermodynamics of the bainite reaction. The thermodynamically induced physical-chemical driving force behind the reaction is expressed well by the Arrhenius equation [7], which can be applied to evaluate the phase transformation, and to assess newly proposed kinetic models. The Arrhenius equation also has been applied for more than 70 years since it was published. It remains popular and important in phase transformation evaluations.4140 steel is widely used in industry for structures, tooling, and some key automotive components. This research addresses the kinetics, morphology, and activation energy characterization of austempered 4140 steel. The knowledge acquired in this work can help develop improved mechanical properties in terms of strength, ductility, hardness and toughness due to the microstructures achieved.

“…The local chemical composition of the material after the ultrafast process is a crucial issue, because the local chemical compositions could affect the rate and temperature, by which phase transformations occur. In this way, the phases or the microstructural constituents of the final microstructure are created as a consequence of different diffusion rates of the alloying elements as discussed by Goulas et al [44] and Hidalgo et al [45].…”

Section: Effect Of Rapid Heating On the Phase Transformation Evolutionmentioning

confidence: 99%

Ultrafast Heating and Initial Microstructure Effect on Phase Transformation Evolution of a CrMo Steel

Papaefthymiou

Karamitros

Bouzouni

2019

Main target of the present work is to elucidate the effect of both initial microstructure and heating rate on phase transformations that occur during ultrafast processing. For this purpose, two initial microstructures, a ferritic-pearlitic and a soft-annealed microstructure were considered. We applied different heating rates (10 °C/s, 200 °C/s, 300 °C/s) up to the peak austenitization temperature, θ ≅ 900 °C. The evolving microstructure is analysed via SEM and EBSD, whereas the carbide dissolution and austenite formation is simulated with Thermocalc® and DICTRA software. Data obtained in this research proves that, when the heating rate increases, the carbide dissolution rate is disseminated. Compared to a conventional heating rate, where the local chemical composition homogenizes as a result of diffusion, rapid reheating leads to intense segregation of the substitutional atoms at the cementite/austenite interface and turns diffusion to a sluggish process. This fact, combined to the infinitesimal time for diffusion, forms an inhomogeneous carbon distribution along the microstructure. This inhomogeneity is further enhanced by the presence of increased carbides’ size present in the initial microstructure. Due to rapid heating, these carbides cannot be decomposed since the diffusion distance of alloying elements increases and the diffusion of alloying elements is impeded during ultrafast heating, thus, remain undissolved at peak austenitization temperature. Their presence and effect in heterogeneous ferrite nucleation restrict austenite grain growth. Consequently, fine austenite grains in conjunction with their chemical heterogeneity lead to the coexistence of fine martensite, bainite laths and undissolved carbides in the final microstructure after quenching.