Ferrite-cementite Steel Research Articles

Cross-scale method of MD-FE for modeling mechanical damage behaviors of ferrite-cementite steels

Inspired by the experimentally observed ferrite-cementite interface damage and ferrite matrix damage, this work develops a cross-scale method using the cohesive zone model (CZM) as a bridge to study the mechanical damage behavior of ferrite-cementite steel at the micro-nano scale. The CZM parameters for the damage region of ferrite-cementite steel were obtained by molecular dynamics (MD) simulations, which were then applied to cohesive elements in the finite element (FE) framework, thus enabling crack initiation and propagation at the micro-scale. Simulation results show that the ferrite-cementite interface is more susceptible to damage resulting in cracking compared to the ferrite matrix, which is consistent with the experimental observation that there are extensive debonded voids due to interface damage. The implementation of this work provides a new way to study the micro-nano scale mechanical behavior of heterogeneous materials containing both interface damage and matrix damage.

Investigation of damage mechanisms related to microstructural features of ferrite-cementite steels via experiments and multiscale simulations

Spheroidized ferrite-cementite steel (SFC) is susceptible to micro-defects owing to the mismatched deformation between the ferrite and cementite during cold forming, which seriously deteriorates the fatigue life. However, the damage mechanism related to microstructural features in the SFC steel during cold deformation has not been fully investigated, hindering control of the microstructure and forming process. This study presents a method coupled with experiments and multiscale simulations to research the damage mechanisms and the dependence of microstructure features under uniaxial tension in SFC steel. In situ tensile test revealed three damage mechanisms in SFC steel under uniaxial tension: cementite cracking, ferrite/cementite interface debonding, and ferrite cracking. In particular, the final fracture was mainly dominated by the ferrite/cementite interface debonding. In multiscale simulations, the macroscale tensile and nanoindentation simulations were carried out to obtain the strain history and mechanical properties of the mesoscale simulations, respectively. The mesoscale IP-based RVE and CP-based unit cells simulations were performed to capture the driving forces from the three damage mechanisms and the dependence of microstructural features, respectively. The nanoscale molecular dynamics simulation was conducted to identify the model parameters of cohesive zone model (CZM) in the mesoscale simulations. The numerical results indicated that particle-cracking void occurred when the internal von Mises stress exceeded the ultimate strength of the cementite particles. Interface-debonding void nucleated when the load on the particle was insufficient to resist its strength. Matrix-cracking should be attributed to the stress concentration caused by the dislocation pile-up or the crossing of multiple slip bands. In addition, interfacial debonding were more likely to occur in the stable orientations and high-angle grain boundaries of ferrite matrix. Compared with a circular cementite particle, an elliptical particle was more prone to cause interfacial debonding. Also, the ferrite/ferrite grain boundary showed earlier interfacial debonding perpendicular to the loading direction than parallel to the loading direction.

Effect of Annealing Time on Microstructure Stability and Mechanical Behavior of Ferrite-Cementite Steel with Multiscale Lamellar Structure

Effect of Annealing Time on Microstructure Stability and Mechanical Behavior of Ferrite-Cementite Steel with Multiscale Lamellar Structure

Continuous and discontinuous yielding behaviors in ferrite-cementite steels

The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macro-scale deformation examinations. The continuous yielding behavior was observed in heat-treated specimens with a lamellar or cementite-spheroidal structure even after strain-aging treatment. However, the discontinuous yielding behavior accompanied by the Lüders elongation appeared in ferrite-cementite steel that was recrystallized after cold rolling. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.

Continuous and Discontinuous Yielding Behaviors in Ferrite-Cementite Steels

The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macro-scale deformation examinations. The continuous yielding behavior was observed in heat-treated specimens with a lamellar or cementite-spheroidal structure even after strain-aging treatment. However, the discontinuous yielding behavior accompanied by the Luders elongation appeared in ferrite-cementite steel that was recrystallized after cold rolling. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.

The Influence of Microstructural Characteristics on Austenite Formation Kinetics in a Plain Carbon Steel

Since the condition of austenite phase formed during intercritical annealing treatment has a crucial impact on the final microstructure and mechanical properties of dual-phase (DP) steels, detailed investigations with regard to austenite formation in this heat treatment process need to be done. In this study, the effects of different microstructural features, such as ferrite grain size, cementite particle size, and pearlite morphology, on austenite formation in a plain carbon steel (0.165 wt pct C, 1.15 wt pct Mn) during isothermal intercritical annealing treatment have been evaluated. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model was used for modeling the kinetics of austenite formation in this steel during isothermal annealing treatment. The volume fraction of austenite (martensite at room temperature) at different intercritical annealing holding times was calculated by this model using the corresponding results obtained from the experiments. It was found that the starting steel microstructure from which austenite phase is formed has a significant effect on both austenite nucleation and growth. The effect of microstructural parameters on the kinetics of austenite formation in ferrite-cementite steel microstructures was more significant than that in ferrite-pearlite (F-P) steels. An increase in the average cementite particle size or ferrite grain size in ferrite-cementite steels caused a significant decrease in the rate of austenite formation. In F-P steels, on the other hand, pearlite morphology exhibited a small effect on the kinetics of austenite formation while ferrite grain size had a pronounced effect on the rate of austenite formation at the later stage of intercritical annealing, i.e., ferrite to austenite transformation stage.

Reduction of Cracks During Punching Process by Cementite in High Tensile Strength Steel Sheets

An experimental study on high tensile strength steel sheets with ferritic and ferrite–cementite microstructures was performed with the objective of determining the crack formation and propagation mechanism in the punching process. In addition, the effect of dispersed cementites was also investigated. SEM, EPMA, and EBSD analyses confirmed that voids initiate at the ferrite-inclusion/precipitate (TiN, TiS, and cementite) interface, and the voids then propagate into the ferrite matrix by cleavage. EPMA, EBSD, and TEM/EDS demonstrated that cracks which formed within and near a center segregation area propagate along the Mn center segregation by intergranular fracture due to Mn segregation at the grain boundary. In addition to TiN and TiS, observation of the number of cracks, the length of cracks in the punched surface, and the number density of voids in tensile fracture parts indicated that dispersed cementites are also void formation factors, and cementites decrease the number of cracks and average crack length in the punched surface of ferrite–cementite steel sheets by reducing the stress applied to the voids.

A microstructure-based macro-micro multi-scale fine-blanking simulation of ferrite-cementite steels

The fine-blanking process of ferrite-cementite steels was simulated with a macro-micro multi-scale approach. In the macroscopic simulation, the effects of material and blanking clearance were investigated by evaluating the damage work density of the material in the center of the fine-blanking shearing zone. A series of microstructure-based models with different particle fraction and distribution were generated as sub-models for the microscopic simulations. Microscopic damage in the sub-models was predicted with the damage work model. The simulation results indicated that an increase in particle fraction and the existence of carbide banding accelerated the microscopic damage of the material in the shearing zone during fine-blanking. Micro cracks appeared and extended along the carbide band. In addition, an increase in blanking clearance speeded up the microscopic damage accumulation at the microscale and caused early fracture of the material in the shearing zone at the macro scale. The SEM observations of various fine-blanking specimens substantiated the predictions of the simulations and suggested that severe carbide banding would change the macro crack path in the sheets, adversely affecting the quality of the blanking surface.

Stabilized uniform deformation in a high-strength ferrite-cementite steel with multiscale lamellar structure

In order to ameliorate the ductility and stabilize the uniform deformation of ultrafine-grained (UFG) ferrite-cementite steel while retaining its high strength, a multiscale lamellar structure was developed and compared with the UFG structure. Microstructures were characterized and tensile behaviors were analyzed in the samples with multiscale lamellar and UFG structures; the mechanisms underlying the different strain-hardening capabilities were studied. Plastic instability dominated during the tensile deformation of spheroidized UFG carbon steel, because of its low strain-hardening owing to the uniform UFG structure with high dislocation density. However, the steel with multiscale lamellar structure exhibited a marked improvement in the balance between strength and uniform deformation, ascribing to the enhanced strain-hardening capability. This result was primarily attributed to the extra strain-hardening generated by the strain gradient and stress state change caused by the multiscale lamellar structure.

超微細粒鋼の塑性加工限界までの真応力－ひずみ関係におよぼす試験片サイズの影響

超微細粒鋼の塑性加工限界までの真応力－ひずみ関係におよぼす試験片サイズの影響

Effect of particle size, fraction and carbide banding on deformation and damage behavior of ferrite–cementite steel under tensile/shear loads

Deformation and damage behavior of ferrite–cementite steel was investigated using microstructure-based representative volume element (RVE) methodology. A series of automatically generated 2D RVEs with ferrite matrix and globular cementite particles were generated as representative of various microstructures. The geometrically necessary dislocation (GND) accumulation at the ferrite–cementite interphase was also studied by introducing an intermediate layer around the cementite particles. Damage mechanisms such as ductile fracture in ferrite matrix, brittle fracture in cementite, and decohesion at ferrite–cementite interphase were considered to study the fracture modes. The relationships between interface strength and particle size were estimated according to the modified Argon criterion and showed satisfactory agreement with related works. The influences of microstructural features, such as particle size, particle fraction and carbide banding, on deformation and damage evolution were investigated under tensile and shear loads. Simulation results indicated that small particle size and particle fraction could postpone the initial decohesion under both tensile and shear loads, while carbide banding can lead to early fracture due to local stress concentration, which has potential to cause the loss of ductility and premature failure. These adverse effects become more severe when more cementite particles remain in the band or the gather density of the cementite particles in the band becomes higher.

On the yield point phenomenon in low-carbon steels with ferrite-cementite microstructure

Low-carbon steels with a ferrite-cementite microstructure represent a discontinuous yielding resulting in several industrially undesirable problems. Considering the thermal contraction mismatch between the ferrite and cementite phases during the cooling stage to room temperature, which is comparable with the volume expansion due to austenite to martensite transformation, one may expect that these steels, similar to dual-phase steels, exhibit a continuous yielding behavior. In order to examine this phenomenon, different microstructures including ferrite-cementite, ferrite-cementite-martensite and ferrite-martensite were produced using a low-carbon steel and yielding behavior of steel samples with these microstructures were evaluated during uniaxial tensile loading. In addition, the effect of volume expansion due to austenite to martensite transformation and thermal contraction mismatch between ferrite and cementite phases on each microstructure was also evaluated. It was found that for the case of ferrite-cementite steels with very small cementite particles, continuous yielding cannot be observed. Finally, yielding behavior of different steel samples were explained based on the theoretical results obtained.

Effect of Particle Size and Carbide Band on the Flow Behavior of Ferrite-Cementite Steel

A microstructure-based representative volume element (RVE) method is employed for simulating the mechanical properties of the alloy steel 42CrMo4, a typical ferrite–cementite steel with ferrite (α) and spheroid cementite (θ) phases. A series of 2D RVEs with ferrite matrix and globular cementite particles are generated according to various microstructures, among which the geometrically necessary dislocation (GND) accumulation at the α–θ interphase is introduced as a coating layer around the cementite particles. Satisfactory agreement between the simulation and experimental flow curves can be obtained when the ratio of GND layer thickness to particle diameter is 0.25. On this basis, the influences of some modeling parameters such as element type and boundary conditions, as well as some microstructure features like particle size and carbide band, on the mechanical properties are investigated. The results show that small particle size can increase the strength of the material. Carbide band leads to the microstructure inhomogeneity and early finish of uniform elongation, which is potential to cause the loss of ductility and premature failure. These adverse effects become more severe when more cementite particles remain in band or the cementite particles gather denser in carbide band.

Prediction model on cleavage fracture initiation in steels having ferrite–cementite microstructures – Part II: Model validation and discussions

The proposed prediction model on a cleavage fracture initiation in ferrite–cementite steels is applied to the fracture toughness tests using notched specimens of steels with various sizes of ferrite grain and cementite particle. The prediction results of fracture toughness and fracture initiation site showed good agreement with the experimental results for all the steels and temperatures tested. The results of parametric studies using hypothetical steels successfully show the influence of the sizes of microstructures on fracture toughness. The results also show that the transition of bottleneck process in the fracture initiation with increasing temperature.

Prediction model on cleavage fracture initiation in steels having ferrite–cementite microstructures – Part I: Model presentation

A prediction model on a cleavage fracture initiation in ferrite–cementite steels is presented. The model requires only information of microstructures without any adjustable parameters. The model is characterized in a multiscale framework based on the Monte Carlo method. A fracture process of three stages is assumed; (I) crack nucleation at cementite particle; (II) crack propagation into ferrite matrix; (III) crack propagation across grain boundary. An active zone is divided into volume elements where microstructures are assigned. Cleavage fracture is assumed to initiate at the time when all the fracture conditions are simultaneously satisfied in any one of the volume elements.

Effect of strain rate on true stress–true strain relationships of ultrafine-grained ferrite–cementite steels up to plastic deformation limit

The true stress (σ)–true strain (ε) relationships up to the plastic deformation limit of low-carbon ferrite–cementite (FC) steels with ferrite grain sizes between 0.5 and 34μm were estimated at strain rates between 3.3×10−1 and 5.0×10−4s−1 to investigate the effect of the strain rate on the σ–ε relationship. Both σ and ε increased with increasing strain rate for each of the FC steels considered, and larger work-hardening rates were observed with increasing strain rate in the ultrafine-grained FC specimens with average ferrite grain sizes less than 1μm. Increasing the strain rate was found to be effective in improving σ and ε up to the plastic deformation limit. The important factors in obtaining good σ–ε relationships in FC steels are the loads and the radius of curvature of the neck profile after the maximum load point. On the basis of experimental results that both σ and ε can improve under high-speed deformation such as at 10−1s−1 (as in static tensile tests) and that the decrease in ductility with increasing strain rate was small, grain refinement of up to 0.8μm was concluded to be effective in improving the tensile deformation behavior of the low-carbon FC steels considered in this study.

Observation and Quantification of Crack Nucleation in Ferrite-Cementite Steel

Observation and Quantification of Crack Nucleation in Ferrite-Cementite Steel

Quantitative Prediction of Cleavage Fracture Toughness of Ferrite Steel without Adjustable Parameters

This study presents a numerical model to quantitatively predict a cleavage fracture toughness of ferrite-cementite steel without any adjustable parameters. The model is constructed in framework of multiscale analysis. The macroscopic analysis is an elasto- plastic finite element analysis. A microscopic analysis is based on the Monte Carlo method considering microscopic fracture process of the three stages; Stage-I: formation of fracture origin by cementite cracking, Stage-II: propagation of the cementite crack into ferrite matrix and formation of a cleavage crack, and Stage-III: propagation of the cleavage crack across ferrite grain boundary. The predicted results of fracture toughness show good agreement with the experimental results including their scatter. Influences of microstructures on fracture toughness can be estimated by parameter studies. According to the results, the validation and the effectiveness of the proposed model are found out.

Effect of ferrite grain size on the estimated true stress–true strain relationship up to the plastic deformation limit in low carbon ferrite–cementite steels

Abstract

フェライト・セメンタイト鋼におけるき裂核生成の観察と定量化

フェライト・セメンタイト鋼におけるき裂核生成の観察と定量化