Abstract
Microscopic stress and strain partitioning control the mechanical and damage behavior of multiphase steels. Using a combined numerical and experimental approach, local strain distributions and deformation localization are characterized in a carbide free bainitic steel produced by continuous cooling. The microstructure of the steel consists of bainite (aggregate of bainitic ferrite and thin film retained austenite), martensite and blocky retained austenite.Numerical simulations were done using a von Mises J2 plasticity flow rule and also a phenomenological crystal plasticity material model. The representative volume element (RVE) was created using a realistic 2D geometry captured through Electron Backscatter Diffraction (EBSD). These simulations describe the strain distribution and deformation localization in this steel. To validate the simulation results, local strain maps were obtained experimentally via in-situ tensile testing using micro digital image correlation (µDIC) in scanning electron microscopy (SEM). The information gained from numerical and experimental data gave valuable insight regarding the microstructural features responsible for strain partitioning and damage initiation in this carbide free bainitic steel. The results of the modelling show that martensite, martensite/bainitic ferrite interfaces, interface orientation with respect to tensile direction, bainitic ferrite size and phase composition influence the strain partitioning in this carbide free bainitic steel.
Highlights
Pearlitic steels are commonly used in railway applications
The retained austenite usually exists in two morphologies: (i) thin film retained austenite (TFRA) between bainitic ferrite (ii) blocky retained austenite (BRA), which is distributed between bainitic regions [8]
This study shows that large BRA islands are less stable compared to finer or nano-crystalline retained austenite islands during mechanical deformation
Summary
Pearlitic steels are commonly used in railway applications. Due to demands of future railway transport, there is a need for better materials that can endure the high contact stresses which over time lead to rolling contact fatigue [1,2,3] and wear [4,5] in rails. The conventional bai nitic microstructure contains a mixture of bainitic ferrite with fine cementite (carbide) laths (or particles) in between them. The presence of cementite laths can lead to void initiation or cleavage in the bainitic microstructure and is detrimental for the fatigue life of the steel [8]. The TFRA acts against crack propagation [8] and the aggregate it produces together with fine bainitic ferrite results in high strength, toughness and hardness [11,12,13]. These steels offer better rolling contact fatigue (RCF) characteristics compared to conventional pearlitic steels [14,15]. Carbide free bainitic steels could be a good candidate for railway applications
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