Cementite Lamellae Research Articles

Plasticity, localization, and damage in ferritic-pearlitic steel studied by nanoscale digital image correlation

The evolution of deformation from plasticity to localization to damage is investigated in ferritic-pearlitic steel through nanometer-resolution microstructure-correlated SEM-DIC (µ-DIC) strain mapping, enabled through highly accurate microstructure-to-strain alignment. We reveal the key plasticity mechanisms in ferrite and pearlite as well as their evolution into localization and damage and their relation to the microstructural arrangement. Notably, two contrasting mechanisms were identified that control whether damage initiation in pearlite occurs and, through connection of localization hotspots in ferrite grains, potentially results in macroscale fracture: (i) cracking of pearlite bridges with relatively clean lamellar structure by brittle fracture of cementite lamellae due to build-up of strain concentrations in nearby ferrite, versus (ii) large plasticity without damage in pearlite bridges with a more “open”, chaotic pearlite morphology, which enables plastic percolation paths in the interlamellar ferrite channels. Based on these insights, recommendations for damage resistant ferritic-pearlitic steels are proposed.

Fine structure of low-carbon steel after electrolytic plasma treatment

Abstract This work shows the results of research of the fine and dislocation structure of the transition layer of 18CrNi3Mo low-carbon steel after the influence of electrolytic plasma. Conducted research has shown that the modified steel layer, as a result of carbonitriding, was multiphase. Quantitative estimates were made for carbonitride М23(С,N)6 in various morphological components of α-martensite and on average by material in the transition layer of nitro-cemented steel. It was established that α-phase is tempered martensite after nitrocementation. Released martensite is represented by batch, or lath and lamellar low-temperature and high-temperature martensite. Inside the tempered martensitic crystals, lamellar cementite precipitates are simultaneously present, and residual austenite is found along the boundaries of the martensitic rails and plates of low-temperature martensite. It was determined that inside the crystals of all morphological components of α-martensite there are particles of carbonitride М23(С,N)6.

Position artifacts in 3D reconstruction of plate‐shaped precipitates in steels depending on the analysis direction of atom probe tomography

Atom probe tomography (APT) analysis often produces artifacts in atomic position owing to the difference in evaporation fields in materials. The atomic density profiles of grain boundaries, plate‐shaped low‐field precipitates (cementite), and high‐field precipitates (vanadium carbide) in steels were investigated via two analyses in directions perpendicular and parallel to the boundaries and platelets. Although the perpendicular analysis indicated better quantitative performances in the composition analyses of segregating atoms at the grain boundary and the constituent atoms of the precipitates, an abnormal atomic density was observed around the interfaces of the precipitates. Geometric calculation using a two‐radius model based on the local magnification effect suggested that a spike structure at the front interface and a dip structure at the rear interface for lamellar cementite correspond to an overlap and a gap in the evaporated atoms from the cementite and matrix, respectively. This indicates that even in the perpendicular analysis, the precipitate constituent atoms and matrix atoms mixed in the depth direction. We discuss the origin and impact of the position artifacts in the perpendicular analyses of low‐ and high‐field plate‐shaped precipitates and also discussed the application limits of the model.

Microstructural evolution and mechanical property of ultrafine-grained pearlitic steel by cold rolling: The influence of cementite morphology

Microstructural evolution of a pearlitic steel during cold rolling, which was heat treated by four different routes to obtain different volume fractions of granular cementite (GC) and lamellar cementite (LC) including full GC, GC-dominated, LC-dominated and full LC samples, was investigated. It was found that ferrite grains and pearlitic colonies were elongated along rolling direction and LC particles become curled and bent shaped during cold rolling. Ultrafine ferrite grains were obtained by heavy cold rolling (HCR). At the early stage of cold rolling, GC greatly enhanced the generation and multiplication of dislocations. Sub-grain boundaries and high angle grain boundaries were gradually evolved from dense dislocation walls in ferrite. The decomposition of carbide occurred during HCR process, owing to the interaction of high-density dislocations with carbon atoms. The maximum yield strength and ultimate tensile strength were obtained in full LC sample by HCR.

Elucidation of void defects by soft reduction in medium carbon steel via EBSD and X-ray computed tomography

In this work, void formation in medium carbon ferrite-pearlite steel is investigated with soft reduction (SR) technology as regards its initiation sites and governing factors: prior austenite grain size, grain boundary ferrite (GB-α) precipitation, pearlite block size and sub-grain boundaries misorientation. The small and tightly packed pearlite colonies attached with GB-α and irregular broken cementite lamellae provide an easier path for void initiation at prior austenite grain boundary (PAGB) and pearlite colony boundary respectively. 3-dimensional void morphology and void size including porosities with different SR parameters were characterized via X-ray computed tomography (XCT). The total number of voids, their sizes, and shape complexity are increased by increasing reduction amount, and serious void defects are evident without the implementation of SR technology. The size of shrinkage void increases with increasing reduction amount, which is twice without implementation of SR technology as compared with the high reduction amount of 6 mm. The total number of voids and their sizes was less along with porosities at minimum reduction amounts of 2 mm and 4 mm, which are small enough to compensate the solidification shrinkage voids.

Under-stoichiometric cementite in decomposing binary Fe-C pearlite exposed to rolling contact fatigue

In this study, we investigate the fundamentals of deformation-driven cementite decomposition during rolling contact fatigue in a binary Fe-0.74C (wt.%) steel with pearlitic microstructure. To reveal the involved nano-scale mechanisms, we apply transmission electron microscopy and atom probe tomography, as well as the combination of both techniques on the same probe volume for correlative structural and chemical analysis. After ~32,500 individual ball contacts under a nominal Hertzian contact stress of ~1,250 MPa, cementite lamellae are consistently depleted in carbon (C) down to ~20 at.%, which is a reduction of 20% in comparison to the original stoichiometric composition of 25 at.% before deformation. By electron diffraction on atom probe tip volumes, we show that this under-stoichiometric cementite still maintains its crystal structure. The potential effect of temperature increase during rolling contact fatigue on cementite decomposition is addressed by carrying out annealing experiments at 150°C and 250°C for 30 minutes after rolling contact fatigue. Our results show that slip transmission across cementite lamellae is primarily responsible for the observed C-depletion in cementite. We quantitatively discuss slip transmission from ferrite to cementite using slip trace analysis and correlate the amount of slip activity with the observed C-depletion. In this way, we explain the formation of under-stoichiometric cementite by combined slip transmission and solute drag by dislocations, where C is transported out of cementite by dislocation gliding through cementite lamellae. Only afterward, cementite decomposition in terms of phase dissolution is suggested to occur by the dislocation-shuffle mechanism.

Effect of Al on Pearlite Transformation and Spheroidization of High Carbon Steel

Normalizing and isothermal spheroidizing annealing tests were carried out on two types of high-carbon steel specimens with different aluminum (Al) contents. The effects of Al content on the phase transformation and spheroidization of high-carbon steel were analyzed. It was found that addition of Al element can increase the pearlitic eutectoid transition temperature of high-carbon steel, making the eutectoid point move toward the high-carbon direction, and inhibiting the precipitation of proeutectoid cementite during pearlitic transformation. The higher the Al content, the greater the undercooling degree during pearlitic transformation and the finer the pearlite lamellar spacing would be after transformation. In addition, the finer pearlite lamellar spacing allowed the test steel to achieve more phase interfaces in the same crystal range and more dislocations and other defects on the interface, thus providing more nucleation centers for lamellar cementite during isothermal spheroidizing annealing. In addition, more defects provided more diffusion channels for carbon during spheroidizing annealing and smaller interlamellar spacing to shortening the diffusion distance of carbon atoms, thus accelerating carbon diffusion rate and spheroidizing the lamellar cementite into granular one. The addition Al element can improve the machinability of steel and facilitate the commercial application of pearlitic steel.

Strain localization and deformation behavior in ferrite-pearlite steel unraveled by high-resolution in-situ testing integrated with crystal plasticity simulations

This paper attempts to study the microstructural stress-strain evolution and strain localization in the ferrite-pearlite steel by high-resolution experimental-numerical integrated testing. Ferrite crystal orientations measured by electron backscatter diffraction (EBSD) were mapped onto precise scanning electron microscopy (SEM) micrographs of ferrite and cementite lamellar morphologies. Furthermore, in-situ SEM tensile testing was employed to map strains during the deformation using digital image correlation (DIC) at high spatial resolutions. Finally, spectral solver-based crystal plasticity (CP) simulations loaded by the local SEM-DIC boundary conditions were performed and compared to the experimental data. Crucially, all microstructure data was carefully aligned, and strains were filtered to the same spatial resolution, measured to be as small as ~75 nm. This integrated methodology yields new insights in the high degree of plastic strain partitioning between ferrite grains and pearlite colonies, and ferrite and cementite lamellae inside pearlite. One-to-one experimental-numerical comparison augmented with the numerical variation of the ferrite constitutive model, pearlite interlamellar spacing, and lamellar orientation reveals how the ferrite-cementite strain partitioning and the onset of strain localization depend on the morphology and distribution of ferrite grains and pearlite colonies. Importantly, this comparison showcases the limitations of state-of-the-art simulations in their capability to predict specific mechanisms and correct degrees of strain heterogeneity.

Hardening of Steel Through High‐Voltage Low‐Current Energy Input

A novel approach of high‐voltage low‐current energy input is applied for hardening of plain carbon eutectoid steel. Initial fine lamellar pearlitic structure disintegrates into four characteristic regions: lamellar pearlite often containing nucleated cementite spheroids (Region‐I), fragmented cementite lamella in α‐ferrite matrix (Region‐II), submicroscopic cementite particles/clusters dispersed in α‐ferrite matrix (Region‐III), and supersaturated α‐ferrite (Region‐IV). At a particular applied voltage, structural refinement and matrix supersaturation (evolving martensite) progress concomitantly up to 5 min, followed by a reverse trend of coarsening and degeneration of martensite. The refinement effect and martensite‐peak‐broadening effect are augmented with increasing voltage up to 75 kV at which the highest hardness (429 HV) of the steel is achieved with treatment duration of 5 min. While primary hardening effect arises from the martensite region of stratified plate morphology (Region‐IV), a secondary effect of hardening is resulted from the region containing dispersed submicroscopic cementite particles/clusters in α‐ferrite matrix (Region‐III). In addition, in the specimen exhibiting maximum hardening effect (at 75 kV, 5 min), as a unique feature, nanosized cementite particles also appear in Region‐IV being dispersed in martensite matrix so as to provide further effect of dispersion hardening along with martensitic hardening.

The synergy between cementite spheroidization and Cu alloying on the corrosion resistance of ferrite-pearlite steel in acidic chloride solution

In this work, the corrosion behavior of medium-carbon steels (45, 45Cu and 45Cuq steels) in acidic chloride environment was investigated. The results indicated that the micro-galvanic effect between the anodic ferrite matrix phase and the cathodic cementite secondary phase notably affected the corrosion resistance of the three steels. For 45 steel, serious pitting corrosion happened in and around the pearlite regions, and a large number of lamellar cementite was fixed in the corrosion pits. Meanwhile, the continuously increasing superficial area of cathodic cementite enhanced the micro-galvanic corrosion, resulting in a rapidly increase in corrosion rate with time. While for 45Cu and 45Cuq steels, macroscopic uniform corrosion occurred, and the cementite accumulation was markedly reduced as compared with 45 steel, thus the micro-galvanic effect was weakened and the corrosion rate was decreased accordingly. Among these, 45Cuq steel showed the most stable and excellent corrosion resistance during long-term corrosion, indicating the occurrence of a synergistic effect between cementite spheroidization and Cu alloying, thereby significantly improving the corrosion resistance of 45 steel.

Correlation between Microstructures and Ductility Parameters of Cold Drawn Hyper-Eutectoid Steel Wires with Different Drawing Strains and Post-Deformation Annealing Conditions

The relationship between microstructures and ductility parameters, including reduction of area, elongation to failure, occurrence of delamination, and number of turns to failure in torsion, in hypereutectoid pearlitic steel wires was investigated. The transformed steel wires at 620 °C were successively dry-drawn to drawing strains from 0.40 to 2.38. To examine the effects of hot-dip galvanizing conditions, post-deformation annealing was performed on cold drawn steel wires (ε = 0.99, 1.59, and 2.38) with a different heating time of 30–3600 s at 500 °C in a salt bath. In cold drawn wires, elongation to failure dropped due to the formation of dislocation substructures, decreased slowly due to the increase of dislocation density, and saturated with drawing strain. During annealing, elongation to failure increased due to recovery, and saturated with annealing time. The variation of elongation to failure in cold drawn and annealed steel wires would depend on the distribution of dislocations in lamellar ferrite. The orientation of lamellar cementite and the shape of cementite particles would become an effective factor controlling number of turns to failure in torsion of cold drawn and annealed steel wires. The orientation and shape of lamellar cementite would become microstructural features controlling reduction of area of cold drawn and annealed steel wires. The density of dislocations contributed to reduction of area to some extent.

Optimization of strength, ductility and wear resistance of low-carbon grade A shipbuilding steel by post-ECAP annealing

Effect of equal channel angular pressing (ECAP) and subsequent annealing on the microstructure, mechanical properties and tribological behavior of grade A shipbuilding steel was investigated. Coarse-grained microstructure of initial sample is eliminated during the ECAP at 375 °C and more refined microstructure is achieved. Post-ECAP annealing at 575 °C results in some grain growth, and also the cementite lamellae in the pearlite colonies is broken and got spheroidised during the annealing treatment. ECAP increases strength and hardness of steel significantly while it decreases the ductility. On the other hand, post-ECAP annealing treatment brings about an increase in ductility and diminishes the hardness, as expected. Strain hardening capacity, ductility, hardness and oxidation rate were found to be the main factors affecting wear resistance of the grade A steel. It was found that high hardness and strength, good wear resistance with sufficiently high ductility can be achieved in the grade A steel by applying ECAP + annealing processes.

Acceleration of spheroidisation in a medium carbon steel processed by equal channel angular pressing

A medium carbon alloy steel was processed using equal channel angular pressing (ECAP) with a view to accelerating the softening process during subsequent annealing. The steel was pressed at 600°C for up to four passes. The original well-aligned lamellar cementite became bent and discontinuous with increasing number of passes. Complete spheroidisation was achieved after annealing at 720°C for only 30 minutes, resulting in the same hardness as that obtained after 72 hours without ECAP. This can potentially lead to significant cost reduction and environmental benefit for the industry.

Deformation and Microstructural Evolution of Nanostructured Pearlite under Tension versus Torsion

In the current work, nanostructured pearlitic steels in bulk with interlamellar spacing of 86 and 168 nm, respectively, are developed through suitable alloying combined with appropriate heat treatment. The specimens extracted from the steels are tested separately in tension and torsion. Under tensile extension, finer pearlite shows a higher yield strength and ultimate tensile strength but fractures at a lower value of strain. However, under torsion testing, finer pearlite deforms to higher shear stress while fracturing at marginally larger shear strain than the coarser pearlite. Under torsion loading, the fractured surface of the finer pearlite presents a larger length of ductile crack propagation (DCP) extending from the circumference toward the center before initiation of cleavage fracture. Scanning electron microscopy (SEM) images of the DCP region are used to characterize the deformation of cementite lamellae, whereas electron backscattered diffraction (EBSD) maps reveal the misorientation changes in ferrite due to torsion. A more frequent and irregular change in ferrite misorientation for coarser pearlite in the DCP region is observed compared with finer pearlite. Bending and fragmentation of cementite lamellae are observed to be higher for fine pearlite, suggesting better strain accommodation. Thus, decreasing the lamellar spacing in nanostructured pearlite improves the torsional response.

Evolution of the Microstructure and Properties for 2000 MPA Bridge Cable Steel

In this paper, the microstructure, texture and mechanical properties of pearlite steel wires produced using different processes were investigated by mechanical testing and electron microscopy techniques including combining SEM, TEM and XRD to reveal the evolution of the microstructure and properties of 2000 MPa bridge cable steel. The experiments were carried out under industrial conditions. The microstructural observations indicate that the orientation of the lamellar pearlite in the steel rod is random and that patenting produces a fully pearlitic microstructure with a fine pearlitic interlamellar spacing. The pearlitic lamellae of the wires tilt and stretch toward the drawing direction during cold drawing. Although the cementite of the cold drawn wires retains a laminar shape, partial fragmentation of the cementite lamella occurs. A large number of SBs appear in the galvanized steel wire, and fragmentation of the cementite lamella is more obvious. After cold drawing, the orientation of the 〈110〉 texture intensifies and becomes dominant. However, the intensity of the 〈110〉 texture decreases slightly after galvanizing. The results from the mechanical experiments show that the hardness and tensile strength of the steel wires increase rapidly after cold drawing before reaching a peak. However, the hardness and tensile strength of the galvanized steel wire decrease slightly.

Formation of Cu-rich Nanoprecipitates in Cu Containing Pearlitic SGI

The presence of Cu-rich nanoprecipitates in a pearlitic spheroidal graphite (ductile) cast iron alloyed with 0.82 wt%Cu was studied. The size and distribution of the precipitates were examined by transmission electron microscopy at different locations of the pearlitic matrix. Some areas were nearly free from precipitates, while other regions showed precipitates at the cementite and ferrite lamellae and at the ferrite/cementite interface. Calculation of the thermodynamic equilibrium under stable and metastable conditions using Thermo-Calc led to the identification of the conditions controlling the formation of Cu-rich nanoprecipitates along three different stages depending on the Cu concentration. Together with a differential scanning calorimetry test and elemental diffusional calculations, thermodynamic predictions supported the observation of Cu-rich precipitates despite the low concentration of Cu of the alloy investigated and allowed the authors to explain the observed heterogeneity in the distribution of precipitates as resulting from the heterogeneous distribution of Cu in the alloy caused by microsegregation during solidification. The knowledge gained is relevant for the design of strengthening strategies in SGI based on the dispersion of Cu-rich nanoprecipitates.

Cyclic deformation behavior of steels with a nanolamellar microstructure and tensile strength of 1500 MPa

An attempt is made to obtain a nanolamellar microstructure in common metallic engineering materials, including austenite, pearlite, bainite, and martensite steels, by alloying and tailoring process parameters. The tensile strength of all four steels is 1500 MPa. On the basis of the tensile and impact deformation behaviors, the cyclic deformation behaviors of the steels with nanolamellar microstructures are studied using the crystallographic phase as the critical factor. Results indicate that the cyclic stress amplitude of the steels with a nanolamellar microstructure depends on the yield and tensile strengths. The change in the dislocation mobility of the steels with a nanolamellar microstructure results in cyclic hardening during the initial cyclic deformation stage. The fatigue life, which is not indicated by the tensile properties, is mainly affected by the initiation and propagation of fatigue cracks. The hard, brittle cementite lamellae accelerate the initiation and propagation of fatigue cracks, thus deteriorating the fatigue properties of the steels with a nanolamellar microstructure. In contrast, film-like retained austenite effectively suppresses the initiation and propagation of fatigue cracks, extending the fatigue life.

Granular Carbides-Assisted Ultrafine-Ferrite Fabrication in the Pearlitic Steel Without Severe Plastic Deformation and Annealing

The evolution of ferrite grain and cementite lamella during cold rolling in a granular carbide–pearlite steel has been investigated. Particular attention has been given to a quantitative characterization of changes in the ferrite grains. Electron back-scattered diffraction and transmission electron microscopy observations show that the ultrafine ferrite (~ 388 nm) can be produced through low equivalent strain cold rolling without severe plastic deformation (SPD) and annealing. The average grain size of ferrite depends on the volume fraction, shape and distribution of granular carbides as well as interlamellar spacing of pearlite. A general explanation of granular carbides-assisted grain refinement is that the embedded carbides between natural barrier will significantly facilitate dislocation nucleation during cold rolling. Dislocation reaction occurs more drastically and quickly near these granular carbides. Such reactions promote the formation of high-angle grain boundaries. The formation of ultrafine ferrite grains and subgrains in steel after cold rolling to ε = 1.4 strain makes the strength and ductility increased simultaneously compared with ε = 0.6 cold-rolled steel. The results suggest a new material design strategy to obtain ultrafine-grained structure via the granular carbides assistance.

Effect of V/Mo ratio on the evolution of carbide precipitates and hydrogen embrittlement of tempered martensitic steel

Hydrogen embrittlement (HE) behavior of tempered martensitic steels subjected to V and Mo addition was investigated. V and Mo were added to a Base steel in two V/Mo wt.% ratios of 3:1 (3V1Mo) and 1:6 (1V6Mo). Addition of V and Mo improved the HE resistance, as a result of breakage of lamellar cementite and of hydrogen trapping by V/Mo carbides. The steel with high V/Mo ratio had better HE resistance than the steel with low V/Mo ratio, mainly because the high V content formed a large amount of V carbides, and that also refined prior austenite grains.

Effects of Wire Drawing and Annealing Conditions on Torsional Ductility of Cold Drawn and Annealed Hyper-Eutectoid Steel Wires

The effects of microstructural features on torsional ductility of cold drawn and annealed hyper-eutectoid steel wires were investigated. The patented wire rods were successively dry drawn to ε = 0.79 (54.7%) ~ 2.38 (90.7%). To examine the effects of hot-dip galvanizing conditions on torsional ductility, steel wires with ε = 1.95 were annealed at 500 °C for 30 s for ~1 h in a salt bath. In cold drawn wires, the number of turns to failure increased steadily, showing the maximum peak, and then decreased with drawing strain. During the post-deformation annealing at 500 °C, torsional ductility of steel wires decreased with annealing time, except for the rapid drop due to the occurrence of delamination for 10 s annealing. The decrease of the number of turns to failure would be attributed to the microstructural evolutions, accompanying the spheroidization and growth of cementite particles and the recovery of ferrite in cold drawn steel wires. From the relationship between microstructural evolution and torsional ductility, it was found that among microstructural features, the shape and orientation of lamellar cementite showed the significant effect on torsional ductility of cold drawn and annealed hyper-eutectoid steel wires.