Pearlitic Steel Research Articles

Frictional shear-induced nanolamellar oxidation and transformation to oxide nanoparticles during pearlitic steel sliding

The in situ formation of a tribolayer composed of oxide nanoparticles on the wear surface is an important method for suppressing wear in metals with lamellar structures. However, the complex interaction between microstructure evolution and oxidation in a tribolayer obscures the formation mechanisms of nanolamellar oxidation and transformation. In this paper, samples of tribolayers at different oxidation stages were prepared by a focused ion beam (FIB) lift-out method. The dynamic evolution process and distribution of oxides in tribolayers during the dry sliding process were analyzed by characterization and molecular dynamics simulation. The nanolamellar oxidation processes and the effects of oxide nanoparticles on self-lubricating behavior were investigated. The results showed that oxygen diffused into the nanolamellae, resulting in continuous oxidation and the formation of an oxide/metal interface. The nanolamellar oxides were broken and refined in the further friction process and eventually transformed into oxide nanoparticles. The oxidation process was influenced by frictional shear, and the growth of oxides proceeded along the direction parallel to the nanolamellae with high-density dislocations. The above results provide a theoretical basis for the next step to improve the wear resistance and corrosion inhibition of metals by designing lamellar structures.

Synergistic Effect of Alloying on the Strength and Ductility of High Carbon Pearlitic Steel

In this work, the effects of the micro-alloying of Mn, Ni, and Si on the microstructure and mechanical properties of high-carbon pearlite steels were investigated. The results indicated that the addition of solely Ni to high-carbon pearlitic steel can enhance the strength through the refinement of interlamellar spacing, but work-hardening in the ferrite of the pearlite colony may be delayed, leading to a reduction in area. The multiple additions of Ni and the increase in Mn and Si contents in high-carbon pearlitic steel were beneficial to obtaining a balance between ultimate tensile strength and reduction in area. Three-dimensional atom probe tomography results showed Si partitioning into ferrite and Mn and Ni elements partitioning into cementite. The addition of Si inhibited the formation of a continuous network of grain-boundary cementite, leading to high strength and high ductility through optimization of the microstructure.

Comprehending the correlation between microstructural features and wear performance in multiphase lightweight steels

This work investigates the wear behavior of three different Fe-Mn-Al-C steels using ball-on-disc reciprocating wear test and 3D optical surface profilometer. Further, the correlation between microstructural features and wear performance of these steels has been established. The experimental results indicate that the wear resistance of the studied steels is in the order: austenite and martensite (AM) > austenite and ferrite (AF) > ferrite and pearlite (FP) steels. In addition to higher hardness, the combined effects of finer width of martensite, larger proportion of high-angle grain boundaries, and higher dislocation density render excellent wear resistance in AM steel. Importantly, the formation of martensite phase during wear test significantly enhances the wear resistance of AM steel. In contrast, the absence of austenite phase reduces the wear resistance of FP steel.

On the Change in Hydrogen Diffusion and Trapping Behaviour of Pearlitic Rail Steel at Different Stages of Production.

To avoid hydrogen flaking in rail production, it is of crucial importance to understand the differences in hydrogen diffusion and trapping between different production steps. Therefore, as-cast unfinished material was compared with two finished rails, hot-rolled and head-hardened, using electron backscattered diffraction (EBSD), electrochemical permeation, and thermal desorption spectroscopy (TDS). A significant increase in dislocation density was in the head-hardened rail compared with the other material states. This leads to an effective hydrogen diffusion coefficient of 5.8 × 10-7 cm2/s which is lower by a factor of four than the diffusion coefficients examined in the other states. Thermal desorption spectroscopy analyses show a clear difference between unfinished and finished rail materials. While a peak in activation energy between 32 and 38 kJ/mol is present at all states, only as-cast unfinished material shows a second peak with an activation energy of 47 kJ/mol, which is related to microvoids. The results show that in the investigated material, the effect of increasing dislocation density has a stronger influence on the effective diffusion coefficient than the presence of a second active trapping site.

Assessing the influence of cyclic bending on pearlitic wire's microstructural evolution: a simulation of spiraling and armoring processing effects

Flexible pipelines are widely used offshore for oil and gas transportation and are subjected to several complex stress modes during service. Stress tensors cause different alterations in the macro and microstructure of the material. Therefore, studies involving the relationship between mechanical behavior and the crystallographic orientation of tensile armor wires need deep investigation. This work investigated the role of complex plastic deformation during the spiraling tensile armor on the microstructure and crystallographic texture of pearlitic steel with 0.74 wt%C. A heat-treated sample was prepared as a starting point. Further specimens were subjected to cold bending to investigate the strain accumulation during spiraling and armoring processing and then compared to an industrial twisted tensile armor sample. The microstructural and texture evolution was characterized by scanning electron microscopy, X-ray diffraction, and backscattered electron diffraction. Specimens were submitted to the Vickers microhardness and tensile tests to evaluate the mechanical properties. Results revealed that the bending deformation causes a reduction of (001)//ND fiber and increases the mechanical resistance of the material, which may prevent the onset and crack propagation. (111)//ND fiber increased while (110)//ND decreased, leading to stability in the slip systems. The formation of shear band structural changes resulted from the increase of (110)//ND with armoring and a decrease of (111)//ND fibers. Bending strains enhance the desirable texture components while diminishing the undesirable ones. Deformations to which the materials were subjected provide less mobility of dislocations, which causes an increase in mechanical strength. Little change in texture and microstructure was also observed.

Formation of plastic properties during deformation stretching of pearlitic steel

Under tensile deformation, the effect of pearlite fineness and strain hardening parameters under uniform and local deformation on the plastic properties of high‑carbon steel has been studied. The increase in the plasticity of pearlitic steel with a thin‑lamellar structure is due to an increase in the intensity of strain hardening in the region of concentrated deformation and an increase in the deformability of cementite plates. For the structures of thin‑lamellar pearlite, the ratio between the interlamellar distance and the value of the relative narrowing is given.

Pearlite Interlamellar Spacing and Vickers Micro-Hardness in the Necking Region of Cold-Drawn Pearlitic Steel Wires

The final aim of this paper is to study the microstructural changes in the necking region of progressively cold-drawn pearlitic steel wires by means of a thorough and detailed analysis of pearlite interlamellar spacing and Vickers micro-hardness in this special region. To this end, a set of progressively cold-drawn pearlitic steel wires belonging to a real manufacturing chain were subjected to standard tension tests, in such a manner that the tests were interrupted before the final fracture, i.e., the test development was aborted just at the necking instant. The microstructural changes during necking were evaluated by measuring the pearlite interlamellar spacing in the necking region, as well as the Vickers micro-hardness in the different points of it. The study of the afore-said microstructural changes preceding the final fracture was the final aim of the research, intending to determine the local areas in the necking region of the specimens in which microstructural changes are most evident, thereby affecting the local mechanical response of a specific cold-drawn steel at the moment of instability under load control during the standard tension test.

Microstructure and mechanical properties of Nb microalloyed high‑carbon pearlitic steels subjected to isothermal transformation

Niobium (Nb) is mainly added to low-carbon steels, in which the main microstructure is ferrite and/or bainite, to refine grains and improve their properties. However, the studies on Nb in high-carbon pearlitic steels are insufficient, and there are some debates regarding the existing studies. High-carbon pearlitic steel is widely used in bars, wires, and steel rails. The role of Nb in high-carbon steels may differ for different steel grades and under different heat treatments. Hence, further detailed studies are required to improve the properties of high-carbon pearlitic steels. In this study, two series of high-carbon pearlitic steels (68C and 75C series) were designed and subjected to isothermal pearlite transformation at different austenitization and isothermal transformation temperatures. The microstructures and mechanical properties of the steels were investigated by optical microscopy, field-emission scanning electron microscopy, electron backscatter diffraction, scanning transmission electron microscopy, and hardness tests. The results showed that the pearlite lamellae were refined by the addition of Nb to high-carbon pearlitic steels, which was attributed to the solute drag effect of Nb. The pearlite colony and nodule sizes also became finer with the addition of Nb because of the finer prior austenite grains and smaller pearlite growth rates. However, the refinement of the pearlite lamellae, nodules, and colonies by the addition of Nb was not significantly enhanced by increasing the Nb content from 0.014 wt% to 0.027 wt% in 75C series samples. In addition, the pearlite interlamellar spacing and colony and nodule sizes of the samples decreased with decreasing isothermal and austenitization temperatures. Moreover, the addition of Nb to high-carbon steels increased their hardness and yield strength. The main strengthening mechanisms of Nb in high-carbon steel are grain refinement strengthening and precipitation strengthening in the samples austenitized at 900 °C, whereas the improvement of strength is mainly attributed to grain refinement strengthening in the samples austenitized at 1180 °C. This study systematically characterized the pearlite morphology of Nb microalloyed high-carbon steels and analyzed the strengthening mechanism, providing a theoretical reference for the development of Nb-containing high-carbon pearlitic steels.

New insights to the metallurgical mechanism of niobium in high-carbon pearlitic steels

Two series of high-carbon pearlitic steels (68C-xNb and 75C-xNb) with different C and Nb contents were produced by hot-rolling. The effects of Nb on the microstructure and impact toughness of high-carbon pearlitic steels were investigated using scanning electron microscope, transmission electron microscope, electron backscattering diffraction and impact toughness tests. In particular, the nucleation and growth of pearlite were investigated through in-situ observations. It was found that the average pearlitic nodule was markedly refined from 10.2 μm to 6.0 μm with the addition of 0.026 wt% Nb in 68C steels, and it was refined from 13.7 μm to 6.7 μm with the addition of 0.014 wt% Nb in 75C steels. In situ observations determined that the refinement of the pearlitic nodule was due to the refinement of prior austenite grains and retardation of pearlite growth by Nb addition. The impact toughness of high-carbon steels was significantly improved by Nb addition without a decrease in strength because a finer pearlitic nodule was obtained. The maximum improvement ratios for 68C and 75C steels were 81% and 26%, respectively. In addition, according to the experimental results and theoretical calculations of the dissolution of Nb-containing precipitates, the refinement of prior austenite was mainly caused by the solute-dragging effect of Nb during isothermal holding at 1180 °C. Moreover, there was an optimum addition of Nb to 75C steels (less than 0.015 wt%); above this threshold, neither the prior austenite grains or pearlitic nodules were further refined, nor was the impact toughness further improved.

Hydrogen-Assisted Microdamage of Eutectoid Pearlitic Steel in the Presence of Notches: The Tearing Topography Surface

This paper studies the hydrogen-assisted microdamage (HAMD) in fully-pearlitic steel. A detailed analysis is provided of the HAMD region in axisymmetric round-notched samples of high-strength eutectoid pearlitic steel under hydrogen embrittlement environmental conditions. The microscopic appearance and evolution of the hydrogen affected region is analyzed from the initiation (sub-critical) to the fracture (critical) situations. The use of very distinct notched samples and their associated stress distributions in the vicinity of the notch tip allows for a study of the key role of the triaxial stress state on hydrogen diffusion and micro-cracking (or micro-damage). The microscopic appearance of the hydrogen-affected zone (the so-called tearing topography surface) resembles micro-damage, micro-cracking or micro-tearing at a micro- or nano-scale due to hydrogen degradation, thus affecting the notch tensile strength and producing hydrogen embrittlement. A micromechanical model is proposed to explain these hydrogen effects on the material on the basis of the lamellar micro- and nano-structure of the pearlitic steel.

Influence of Local Electropolishing Conditions on Ferritic–Pearlitic Steel on X-Ray Diffraction Residual Stress Profiling

Layer removal with electropolishing is a well-established method when measuring residual stress profiles with lab-XRD. This is done to measure the depth impact from processes such as shot peening, heat treatment, or machining. Electropolishing is used to minimize the influence on the inherent residual stresses of the material during layer removal, performed successively in incremental steps to specific depths followed by measurement. Great control of the material removal is critical for the measured stresses at each depth. Therefore, the selection of size of the measurement spot and electropolishing parameters is essential. The main objective in this work is to investigate how different electrolytes and electropolishing equipment affect the resulting surface roughness, geometry, microstructure, and consequently the measured residual stress. A second objective has been to establish a methodology of assessing the acquired electropolished depth. The aim has been to get a better understanding of the influence of the layer removal method on the accuracy of the acquired depth. Evaluation has been done by electropolishing one ground and one shot peened sample of a low-alloy carbon steel, grade 1.1730, with different methods. The results showed a difference in stresses depending on the electrolyte used where the perchloric acid had better ability to retain the stresses compared to the saturated salt. Electropolishing with saturated salt is fast and results in evenly distributed material removal but has high surface roughness, which is due to a difference in electropolishing of the two phases, ferrite, and pearlite. Perchloric acid electropolishing is slower but generates a smooth surface as both ferrite and pearlite have the same material removal rates but may cause an increased material removal for the center of the electropolished area. In this work, it is suggested to use perchloric acid electropolishing for the final layer removal step.

Fatigue Crack Propagation in Pearlitic Steel under Pressurized Gaseous Hydrogen: Influences of Microstructure Size and Strength Level

For the wall-thickness reduction of the components destined for pressurized gaseous hydrogen, widespread use of high-strength martensitic steels has long been desired. However, their strong susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG) is still limiting their proactive applications. Here, we instead focused on pearlite as another potential reinforcing agent for the development of new hydrogen-compatible steels with acceptable cost performance. Fatigue crack growth (FCG) behavior of three eutectoid steels with different microstructure sizes (i.e., ferrite/cementite interlamellar spacing, colony and block sizes) and strength levels was investigated in a 90 MPa hydrogen gas, an essential evaluation when attempting to perform a defect tolerant design of the components used for high-pressure gases.

Details of pearlite to austenite transformation in steel: Experiments and phase-field modeling

The austenitization of an initial pearlitic microstructure is simulated using the phase field model to achieve insight into White Etching Layer (WEL) formation in pearlitic railway steels. The simulations take into account the resolution of the cementite lamellae within a pearlite colony as well as the presence of pro-eutectoid ferrite. The austenite growth kinetics and morphology obtained via simulations are compared with dilatometry and microscopy observations. The influence of γ/θ and γ/α mobilities on the austenite growth morphology are studied. The simulations reproduce the microstructural features as well as the experimentally observed kinetics behavior of austenite formation, involving the correlation between mobilities and nucleation behavior.

Methodology to assess damage mechanisms of rail steels based on small-scale experiments

A new methodology to distinguish between the damage mechanisms wear only and RCF crack initiation in combination with wear in rolling contacts is proposed in this work. The new methodology distinguishes between the damage mechanisms by comparing the experimental crack depth against a critical crack depth which is governed by the surface roughness and by the plastic shear deformation depth in the near-surface layer. The new methodology is used to assess the results of twin disc experiments that have been conducted with three pearlitic rail steels at different normal contact pressures and different longitudinal creep values. The observed experimental results show agreement with the criterion and the threshold of the new methodology.

Microstructural Refinement and Strain Hardening Behavior in Low‐Density Ultrafine Pearlite Steel

Advanced low‐density steels that meet energy shortages and high safety requirements have become a hot research topic in materials science. The addition of light elements with medium‐Al content (5 wt%) to ultrahigh‐strength pearlite cable steel alloy reduces the material density and causes a microstructural change from traditional θ‐pearlite (ferrite + cementite) to θ‐pearlite and κ‐pearlite (ferrite + cementite + κ‐carbide). The results show that the moderate addition of Al and Mn (5 wt%) promotes the formation of κ‐carbide, which inhibits the diffusion of C and significantly delays the eutectoid transformation process of pearlite. Meanwhile, the transformation free energy of pearlite increases and obviously refines the lamellar spacing of the ultrafine pearlite by κ‐carbide. Simultaneously, the interaction between κ‐carbides and dislocations is initiated, which increases the strain hardening rate of ultrafine pearlitic steel.

Simultaneously improving memory effect and mechanical properties in Cu-based alloys by α phase spheroidization and Fe alloying: A CuAlMnFe as an example

The spheroidization of lamellar Fe3C in pearlite in steels can significantly increase the plasticity. Inspired by the fact, we proposed that to introduce ductile spherical α phase with precipitates of κ phase by Fe alloying can realize not only better shape memory effect (SME) but also higher mechanical properties than to introduce the blocky α phase does in Cu-based alloys. To confirm our idea, the effects of two annealings in two-phase region were studied on the spheroidization of α phase and precipitation of κ phase and the resultant SME and mechanical properties in a Cu8.5Al9.6Mn3.2Fe alloy. Our results showed that the two annealings could simultaneously spheroidize the α phase and precipitate the κ phase in the alloy. The alloy with the spherical α phase showed ∼1% higher recovery strain and ∼8% higher fracture strain than the alloy with the blocky α phase did. The spheroidization of α phase together with the precipitation of κ phase will be a direction for developing ductile Cu-based alloys with better SME and higher strength.

Microstructure, microtexture, and crack susceptibility in pearlitic steel during lab-simulated processes aiming tensile armor application in flexible pipelines

Microstructure, microtexture, and crack susceptibility in pearlitic steel during lab-simulated processes aiming tensile armor application in flexible pipelines

High-Temperature Hardness in Hypereutectoid Steels with Various Microstructures Measured by Using Small Ball Rebound Hardness Test

The temperature dependence of hardness in hypereutectoid steels with various microstructures was measured by using the small ball rebound hardness test, and the effect of carbon state on the high-temperature hardness was discussed. As-received and spheroidized cementite steels consisted of spheroidized cementite, graphite and ferrite matrix, and the volume fraction of graphite in the spheroidized cementite steel was larger than that in the as-received steel. Pearlite steel had ferrite and cementite lamellar structure without graphite. The characteristic hardening was detected above 700 K in all steels, suggesting that the solid solute carbon enhances the hardness even in hypereutectoid steels with high strength. The pearlite steel exhibited the highest hardness owing to the ferrite and cementite lamellar structure and no graphite. While the spheroidized cementite steel exhibited the lowest hardness, although the size of cementite was finer than that in the as-received steel. It was quantitatively demonstrated that the large volume fraction of graphite caused the low hardness in the spheroidized cementite steel.

SPD Deformation of Pearlitic, Bainitic and Martensitic Steels

SPD Deformation of Pearlitic, Bainitic and Martensitic Steels

Origin of tearing topography surface in hydrogen-charged pearlitic steel

In the present study, the formation mechanism of tearing topography surface (TTS), which is observed after slow strain-rate tensile tests of H-charged pearlitic specimens, was investigated in detail. The TTS always appeared at the subsurface of failed H-charged specimens, where H atoms were concentrated after room-temperature electrochemical H-charging. The TTS consisted of stepped flat surfaces and dimpled surfaces. The flat surface was caused by the coalescence of sharp micro-shear cracks, not micro-voids, of H-enriched cementite (θ) platelets in a pearlite colony, namely brittle transcolonial fracture. The regions surrounding the flat surface were H-depleted due to the migration of H atoms into neighboring micro-shear cracks during tensile deformation. As a result, the H-depleted regions were fractured by the typical coalescence of micro-voids, namely relatively ductile shear cracking, resulting in dimpled surfaces after tensile fracture. These results mean that the TTS region, strictly speaking, the flat surface region, which is generated by transcolonial fracture inside the TTS region, is an initial site of H-induced cracking of pearlitic steel. The ab-initio calculations of the energy value of H desorption from a C vacancy in θ supported the recent report that H atoms are concentrated inside the θ platelets after H charging.