Qualitative Investigation of Damage Initiation at Meso-Scale in Spheroidized C45EC Steels by Using Crystal Plasticity-Based Numerical Simulations

This research uses EBSD data of two thermo-mechanically processed medium carbon (C45EC) steel samples to simulate micromechanical deformation and damage behavior. Two samples with 83% and 97% spheroidization degrees are subjected to virtual monotonic quasi-static tensile loading. The ferrite phase is assigned already reported elastic and plastic parameters, while the cementite particles are assigned elastic properties. A phenomenological constitutive material model with critical plastic strain-based ductile damage criterion is implemented in the DAMASK framework for the ferrite matrix. At the global level, the calibrated material model response matches well with experimental results, with up to ~97% accuracy. The simulation results provide essential insight into damage initiation and propagation based on the stress and strain localization due to cementite particle size, distribution, and ferrite grain orientations. In general, it is observed that the ferrite–cementite interface is prone to damage initiation at earlier stages triggered by the cementite particle clustering. Furthermore, it is observed that the crystallographic orientation strongly affects the stress and stress localization and consequently nucleating initial damage.

High Throughput Quantitative Metallography for Complex Microstructures Using Deep Learning: A Case Study in Ultrahigh Carbon Steel

We apply a deep convolutional neural network segmentation model to enable novel automated microstructure segmentation applications for complex microstructures typically evaluated manually and subjectively. We explore two microstructure segmentation tasks in an openly available ultrahigh carbon steel microstructure dataset: segmenting cementite particles in the spheroidized matrix, and segmenting larger fields of view featuring grain boundary carbide, spheroidized particle matrix, particle-free grain boundary denuded zone, and Widmanstätten cementite. We also demonstrate how to combine these data-driven microstructure segmentation models to obtain empirical cementite particle size and denuded zone width distributions from more complex micrographs containing multiple microconstituents. The full annotated dataset is available on materialsdata.nist.gov.

Ferrite-to-Austenite and Austenite-to-Martensite Phase Transformations in the Vicinity of a Cementite Particle: A Molecular Dynamics Approach

We used classical molecular dynamics simulation to study the ferrite–austenite phase transformation of iron in the vicinity of a phase boundary to cementite. When heating a ferrite–cementite bicrystal, we found that the austenitic transformation starts to nucleate at the phase boundary. Due to the variants nucleated, an extended poly-crystalline microstructure is established in the transformed phase. When cooling a high-temperature austenite–cementite bicrystal, the martensitic transformation is induced; the new phase again nucleates at the phase boundary obeying the Kurdjumov–Sachs orientation relations, resulting in a twinned microstructure.

Effects of Mn, Si and Cr Addition on the Spheroidization of Cementite in Hypereutectoid Fe-1mass%C Steel

Effect of Mn, Si and Cr on spheroidization of cementite in Fe-1mass%C steel has been investigated over a range of austenitizing temperatures. In Fe-1C steel, a fully spheroidized structure is obtained but some large cementite particles are formed. The addition of 1.5 mass% Si or Cr accelerates spheroidization of cementite. An addition of Cr remarkably refine the cementite particle size, but the influence of Si addition on the cementite particle size is not remarkable. A fully spheroidized structure fails to develop in steel with the addition of 1.5% Mn under the condition used in present study. Some lamellar cementite still exist in the 1.5Mn steel. The pearlite-promoting effect of Mn is possibly attributed to the inhomogeneous distribution of cementite particles during the intercritical austenitization.

Microstructure Evolution of Pearlitic Lamella Impacts at Ultra-High Strain Rates

The microstructure evolution of eutectoid steel with lamellar pearlite was investigated by SEM and TEM during ultra-high strain rate loading. The results indicate that ultrafine microduplex structure (ferrite + cementite) with the grain size to sub-micrometer level was observed at the surface of eutectoid steel after single pass ultra-high strain rate loading. Equiaxed ferrite grain was about 0.8 μm and the cementite lamella was spheroidized fully, and the diameter of the cementite particle was about 50 nm. The bent or fractures can occur at the edge of shock wave. Ultra-high strain rate shocking induced severe plastic deformation at the surface of materials and the cementite lamella has better plastic deformation capacity.

X-Ray Stress Estimation of Carbide in Spheroidized JIS SK5 Steel

The low volume fraction of carbide phase in carbon steel determines that it is difficult to estimate the stress state in it by diffraction method. In the present study, different from the studies before, we improve the technique of surface treatment on specimen and have successfully finished the stress estimations of carbide phase in carbon steels by X-ray diffraction method under normal conditions. Moreover, we investigate the affection of spherical cementite particle size on the residual stress distribution in both phases during the plastic deformed steels. We observed that the steels with small-sized cementite particles showed higher stress states than the steels with relatively large-sized cementite particles.