Cementite Layer Research Articles

A study of fatigue property enhancement of 1045 steel processed by surface mechanical rolling treatment with an emphasis on residual stress influence

Several factors contribute to the observed bulk mechanical properties enhanced by surface mechanical rolling treatment (SMRT), and it is important to distinguish the influences of each major factor. A commonly used engineering carbon steel, 1045 steel, was chosen for the current study because the material does not have stress-induced phase transformation. A fully reversed strain-controlled gradual decreasing amplitude loading spectrum (ENV) was employed to release the residual stresses induced by SMRT. Fatigue experiments were conducted on the 1045 steel specimens with and without ENV to assess the influences of grain refinement and the residual stress on fatigue. It was found that SMRT refined and recombined the microstructures in the surface layer through deformation and fragmentation of brittle cementite, and extrusion of ductile ferrite between adjacent cementite layers. In the high cycle fatigue regime, both the SMRT induced residual stresses and the refined/recombined microstructures contributed to the enhancement of fatigue strength and fatigue ductility but the gradient microstructure contributed significantly more than do the residual stresses. Unlike stainless steels processed by SMRT, the carbon steel does not display a kink point in the strain-life fatigue curve, and all the crack initiations were found to occur on the specimen surface.

Influence of post-deformation annealing conditions on microstructures and mechanical properties of hypereutectoid steel wires during cold drawing

Abstract This investigation delves into the complex influences of annealing conditions post-deformation, such as duration and heat levels, upon the microstructure, as well as the mechanical properties of hypereutectoid steel wires subjected to cold drawing. XRD and VSM analyses confirmed that higher annealing temperatures precipitated a notable increase in cementite volume, concurrently inducing the spheroidization of cementite lamellae. This transformation was evidenced by the expansion of cementite volume and the transition of cementite layers into a spherical morphology as the annealing temperature increased. The evolution of tensile strength in response to varying annealing temperatures and time revealed two distinct patterns. Initially, at lower annealing temperatures ranging between 200°C and 300°C, tensile strength exhibited an upward trajectory with increasing annealing time. This phenomenon can be attributed to age-hardening mechanisms, notably associated with solution hardening. As annealing time increases, the decomposition of cementite releases additional carbon atoms into the ferrite matrix. These carbon atoms act as effective barriers, hindering dislocation movement within the structure and consequently contributing to increased tensile strength. However, this increase in carbon content within the ferrite matrix also elevated the susceptibility to delamination phenomena. Conversely, at higher annealing temperatures of 400°C and 500°C, a contrasting trend was observed, characterized by a consistent decline in tensile strength post-annealing. This decline could be attributed to the phenomenon of aging softening, wherein the process of spheroidization and re-precipitation of cementite resulted in a reduction of dissolved carbon atoms within the ferrite matrix. Consequently, this led to a decline in tensile strength as well as a reduced incidence of delamination. In summary, the study explored how different annealing conditions affect the structure as well as the strength of hypereutectoid steel wires, revealing detailed mechanisms behind these changes.

Growth of Fe3C-VACNT surfaces by metal dusting under plasma carburizing: Fractional factorial study and correlation with morphological and structural aspects

This study investigates the development of surfaces that combine wear-resistance and solid lubrication properties using single low-temperature plasma carburizing treatments associated with the metal dusting phenomenon. The surfaces consisted of a regular cementite (Fe3C) layer and a film composed by vertically aligned carbon nanotubes (VACNT) grown over the carburized layer. For that, specimens of AISI 1005 steel were used as a substrate. A fractional factorial design of experiments was conducted to identify the best processing conditions to achieve the required Fe3C-VACNT surfaces. Morphological and structural aspects of the surfaces were assessed by scanning and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. The results show that the carburizing performed between 600 and 700 °C and with plasma power densities ≥0.30 W/cm2 were effective for developing the bilayer surfaces. A signature of that process was a temporal increase of the plasma current. According to a created statistical model, the most influential experimental parameters in the average power plasma density were output voltage, %CH4 in the gas mixture, and duty cycle. The surfaces were able to reduce the friction coefficient to values down to 0.08 and decrease the wear rate by 25 to 70 %, depending on their characteristics.

Galvanic effects induced by siderite and cementite surface layers on carbon steel in aqueous CO2 environments

Siderite (FeCO3) and cementite (Fe3C) layers develop naturally on carbon steel surfaces in aqueous carbon dioxide (CO2) environments. This study evaluates galvanic corrosion induced by such layers when coupled to bare carbon steel. In CO2-saturated, 50 °C, pH 5 conditions, the Fe3C-filmed carbon steel acted as the net cathode, significantly enhancing bare steel corrosion rates. Galvanic currents induced by the FeCO3-filmed steel were much lower, with FeCO3 removed from the surface as Fe3C was revealed concomitantly on bare steel. It is proposed that the presence of Fe3C amongst the FeCO3 layer is responsible for galvanic interaction, rather than FeCO3 itself.

Pearlite formation via martensite

When the coarsening of a second–phase particle in a solid–phase matrix is discussed, the microstructural evolution of the corresponding matrix phase is often unconsciously ignored. Present electron microscope observations have revealed that as–quenched high–carbon Fe–C martensite with a high density of twins first transforms into a pearlite structure upon tempering, followed by shrinkage of the cementite layers, in which ultra–fine θ–Fe3C particles are connected from a two–dimensional view. Two cementite layers form a loop or necklace–like morphology. Loop shrinkage or aggregation of the ultra–fine carbide particles results in carbide coarsening and pearlite structure development. The driving force for the aggregation of the fine carbide particles or the shrinkage of the cementite layers is believed to be the recrystallization of numerous α–Fe crystallites formed during the martensitic transformation. The thicker cementite layer in the pearlite structure is formed via a similar process (two or more cementite layers merging together) of detwinning behavior. The present experimental observations have revealed a new coarsening mechanism of cementite particles in carbon steels. A new mechanism for pearlite structure formation (twinned martensite can be treated as a precursor of pearlite) and coarsening has also been discussed.

Diffusion induced decomposition of cementite layers during gas nitriding of 4140 steel: thermodynamical and kinetical aspects

In this paper, some new aspects of the decomposition of cementite layers by diffusion of nitrogen during gas nitriding are investigated. Therefore, AISI4140 steel samples were heat-treated to harden by heating up to 1113 K for 120 min, followed by quenching in agitated mineral oil. Then samples were tempered at 863 K for 60 min. Nitriding was carried out for 1400 up to 21000 min. Samples were characterized by optical microscope, scanning electron microscope, x-ray diffractometer, and HSC thermodynamical software. Results showed that the rate of diffusion was higher in the early stages of nitriding. Due to nitrogen of diffusion, some nitrides are formed in grain boundaries and act as diffusion barriers. In this case, the diffusion mechanism from grain boundary diffusion changes to the volume diffusion or a combination of both mechanisms. Meanwhile, diffusion of nitrogen, in addition to the ferrite layers and grain boundaries, is also performed into the cementite layers. Although these layers are not the preferred areas of diffusion, volume diffusion of nitrogen through cementite layers leads to decomposition of them and turning them into iron nitrides and carbo-nitrides. Results show that almost 42 percent of cementite layers decomposed by nitrogen diffusion.

Structure evolution of the Fe3C/Fe interface mediated by cementite decomposition in cold-deformed pearlitic steel wires

Cold-drawn pearlitic steel wire is irreplaceably used in industry owing to its outstanding mechanical property which is dominated by the cementite/ferrite (Fe3C/Fe) interfaces in the material. However, the fine structures of the Fe3C/Fe interfaces in the deformed wires are less known to date. In this work, transmission electron microscopic investigation was performed on the atomic structures of the interfaces with the Isaichev orientation relationship (OR) in the wires with progressive deformation strains. In addition to the effect of the dislocation/interface interactions, this work revealed that the deformation-induced partial decomposition of cementite plays an important role in the interface reconstruction during deformation. The interfacial carbon vacancies generated by cementite decomposition and particularly, the amorphization of cementite layers in the sample with ε > 1 could effectively annihilated the interfacial dislocations and consequently relaxed the interfacial stress. The correlations between the interface structure changes and the mechanical properties of the wires were discussed.

Effect of low-temperature plasma carburizing on surface topography, mechanical and tribological properties of sintered iron and nitrided sintered iron

This study investigated the effect of a low-temperature carburized layer on the topography, mechanical and tribological properties of sintered and nitrided sintered iron. Two nitride layers (γ’-Fe4N and ε-Fe2–3N) were produced by plasma nitriding. Subsequently, low-temperature plasma carburizing was used to develop a cementite layer over the samples. The carburized layer increased H/E* and H3/E*2 of the sintered iron more effectively than the nitride layers. The cementite layer improved the wear resistance of the iron by 45%, statistically similar to the γ ´ nitride layer. Postcarburizing enhanced the mechanical properties of the nitrided iron due to the formation of a duplex layer. However, the presence of pores at the interfacial region of the duplex layers induced a brittle wear mechanism.

Microstructure and Tensile Properties of Graphite Ductile Iron Improved by Minor Amount of Dual‐Phased TiC–TiB2 Nanoparticles

Herein, the effects of dual‐phased TiC–TiB2 nanoparticles on the microstructure and tensile properties of graphite ductile irons are investigated. Aluminum master alloys containing the TiC–TiB2 nanoparticles are utilized in the production of novel graphite ductile irons. It is found that the graphite nodules become finer with increased nodularity, smaller average diameter, and incremental amount per unit area. The thickness of ferritic and cementite layers in pearlite is reduced and the orientation becomes more regular. More ferrite is produced in the matrix after adding the nanoparticles. In tensile tests, the yield strength, tensile strength, and ductility are simultaneously improved. Also, it is determined that the cleavage and semicleavage fractures are transformed into ductile fractures after engaging the nanoparticles, which could be associated with more dimples and ductile risers being formed to promote the plastic deformation of the graphite ductile irons.

Ultrasound effect on electrospark cementation process

The process of carburizing is well-studied and is one of the traditional processes of the thermochemical treatment of steel. The process has disadvantages such as high energy consumption, long exposure time, complicated processing of large products and surfaces. Electrospark deposition with a carbon electrode is proposed as an alternative process. The carburizing extent and the working layer depth can be increased when the process is used in the ultrasonic vibration field. A resonant vibration introducing circuit with a standing wave generated in the specimen tested is implemented to identify the influence of specific ultrasonic wave areas on the hardened layer formed. The highest carbon saturation of the surface layer was found in the standing ultrasonic wave oscillation node area, where cyclic stretching and compression of the medium contribute to the excess phases release. The carbon in the martensite estimated by the c/a ratio was 0.78% by weight. The highest cementite layer depth and transition zone size are in the ultrasonic wave antinode area, where the highest dynamic medium particles displacement is observed under the influence of vibrations. It is shown that the base metal structure component dispersiveness is responsible for the increased hardened layer depth.

Duplex surface treatment on microalloy steels by dehydrated paste pack boriding and pack carburizing

In this work, a novel duplex surface treatment is carried out on API X60 microalloy steels by boronizing and carburizing. Firstly, the dehydrated paste pack boriding (DPPB) on microalloy steels was carried out at a temperature of 1273 K for 8 h of exposure time. Then, the pack carburizing (PC) was conducted on borided steels at 1173 K temperature for four different exposure times. The layers formed by DPBP and DPBP+PC, were caracterized by X-ray difrraction, scanning electron microscopy, and energy dispersive spectroscopy. The results show that DPBP+PC promotes the formation of coatings outermost cementite layer with a range of depth (15.30–45.34 µm) over the boride layer. Hardness and Young's modulus of DPPB+PC were measured using the nanoindenter with a Berkovich diamond tip under load of 100mN. The higher average indentation hardness (HIT=1.53±0.25 GPa) and Young's modulus (EIT=212.70±2.4 GPa) were characteristic of carburized layer as well as Fe2B layer was characterized by HIT=20.56±0.80 GPa, and EIT=300.9±15.57 GPa, these parameters have an important influence on wear resistance to improve the performance in practical applications.

Graphitization behavior of medium-carbon high-silicon steel and its dependence on temperature and grain size

The graphitization behavior of medium-carbon, high-silicon steel (Fe-0.55C-2.3Si) is investigated with particular focus on the effects of heat-treatment temperature and initial grain size on the graphitization rate and mechanical properties. With the progress of heat treatment, pearlite gradually disappears because of the decomposition of the cementite layers and graphites are formed because of the diffusion and aggregation of carbon atoms; these occurrences lead to a gradual decrease in the hardness of the material. A higher heat-treatment temperature promotes the graphitization behavior, which consequently leads to a decrease in the time taken for completion of graphitization and an increase in the average graphite size. Graphites are dominantly formed at grain boundaries owing to the high energy and disordered atomic arrangements at these boundaries. Further, because grain boundaries act as graphite nucleation sites, a smaller grain size corresponds to faster graphitization as well as the formation of finer and more abundant graphites. The complete graphitization treatment causes a significant reduction in the tensile and compressive strengths of the material and a considerable improvement in its ductility and cold forgeability.

Atomic interactions between Si and Mn during eutectoid transformation in high-carbon pearlitic steel

The atomic interactions between Si and Mn during eutectoid transformation in high-carbon pearlitic steel were investigated. Atom probe tomography and first-principles calculations were applied to evaluate and analyze the atomic interactions at the ferrite/cementite interface. In the initial stage of eutectoid transformation, enrichment of Si and Mn occurred at the ferrite and cementite sides, respectively, of the interface. This interfacial segregation phenomenon gradually diminished as the transformation proceeded. Calculations of the cohesive energy and formation energy revealed a clear enhancement in the chemical bonding and stability of the pearlite system when the Si atom was moved from the ferrite layer to the cementite layer and the Mn atom was moved in the opposite direction. The interfacial segregation of the Mn and Si atoms was mainly responsible for the insufficient diffusion and high hybridization degree of Fe, Mn, Si, and C atoms. Furthermore, the partitioning ratio of Mn in high-Si steel was greater than that in low-Si steel, leading to greater partitioning of Mn into the cementite phase. Calculations of the electronic structure revealed that the enrichment of Si in the ferrite phase promoted the partitioning of Mn into the cementite phase owing to the strong repulsive force between Mn and Si at the pearlitic interface.

Duplex surface treatment of sintered iron by plasma nitriding and plasma carburizing at low temperature

The aim of this study is to investigate the combination of plasma nitriding and plasma carburizing at low temperature. Samples of sintered pure iron were nitrided at 500 °C for 3 h under three different atmospheres: (1% N2 + 99% H2), (10% N2 + 90% H2), and (90% N2 + 9% H2 + 1% CH4), all followed by plasma carburizing at 500 °C for 3 h under a mixed atmosphere of 2% CH4 + 98% H2. Microstructural characterization and chemical analysis of the layers were performed by scanning electron microscopy and energy-dispersive X-ray spectroscopy, respectively. Phase identification was carried out by grazing incidence X-ray diffraction. The layer hardness was verified by nanohardness and the case depth by Vickers microhardness. The results show that the post-carburizing treatment promotes the formation of a thin outermost cementite layer over the nitride layers, leading to a nitrided–carburized layer on the surface. The morphology and phase composition of the nitride layer influence the structural and mechanical properties of the nitrided–carburized layers. The best surface hardness improvement, approximately 72%, was achieved by the duplex treatment of nitriding with 1% N2 content followed by a plasma carburizing at low temperature. Moreover, duplex nitriding–carburizing leads to deeper hardening depths when compared to simply nitrided samples.

One-step annealing optimizes strength-ductility tradeoff in pearlitic steel wires

In this paper, the mechanical properties of a cold-drawn wire (ε=2.43) are modulated by simple annealing and the variation of its microstructure is characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and molecular dynamics (MD) simulation. The tensile ductility of the wire can be improved for about three times without compromising its strength when being annealed at 325 °C for 10–30 min. It is convinced that solid solution of carbon atoms from decomposed cementite lamellae improve the wire strength at low temperature annealing (up to 250 °C) and make the wire strength basically equal the as-drawn state even though cementite lamellae are weakened by cementite recrystallization at 325 °C. And reversely the weakening cementite layers lead to the great improvement of wire ductility at this time since it relaxes the restriction to the moving of dislocations. At higher annealing temperature, the wire strength decreases with the growth of cementite and ferrite grains. The appearance of nano-recrystallized cementite grains at a medium annealing temperature may be a critical factor governing the enhanced wire mechanical properties.

Microstructural dependence of anisotropic fracture mechanisms in cold-drawn pearlitic steels

The anisotropic fracture behavior of cold-drawn pearlitic steels was analyzed up to a true strain of 1.6. The structures were analyzed and quantified by scanning electron microscopy, electron backscatter diffraction and high-resolution electron microscopy. The aspect ratio Λ was defined to describe the fracture mechanism based on statistics pertaining to major axis and minor axis of pearlitic colonies, and the fitting formula was established as follows: Λ=0.4exp(2x)+0.65. At Λ ≤ 2.75, the fracture mechanism shifts from cleavage to quasi-cleavage fracture due to dislocation generation, as the measured parameters of interlamellar spacing (ILS) showed no obvious decrease. At values of Λ > 2.75, ductile fracture mechanism becomes dominant due to a drastic decrease in ILS. The <110> ferrite fiber microtexture formed during the cold drawing process and the component exhibiting a gradient distribution from the surface to the central regions increased gradually with drawing. The crack propagation path was then deflected and formed a ‘V’ shape at ε ≤ 1.5. However, as carbon migrated from cementite to the ferrite near the interphase, the fracture path deflected again. Furthermore, two reasonable models were formulated to explain fracture crack forming and anisotropic fracture behavior. In addition, this study illustrated that the relative crystallographic orientation of the ferrite and cementite components followed the Bagarytski relationship. With increasing strain, the cementite layers transformed from single crystals into nanostructured polycrystals and even evolved an amorphous structure at the interface at strains above 1.5.

In situ study of cementite deformation and its fracture mechanism in pearlitic steels

The deformation and fracture behaviors of cementite in pearlitic steels during tensile tests were investigated by in situ scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and electron backscatter diffraction (EBSD). The results showed that the deformed structure had three different types of shear bands: shear deformation in the pearlite colonies, at the interfaces of the pearlite colonies and at the phase boundary between ferrite and cementite. The orientations of the shear bands were nearly anisotropic with respect to the tensile axis. The path of shear deformation developed at the dislocation walls and was propagated by the offset of the cementite along the dislocation walls. As the dislocations cut perpendicularly through the cementite layers, the single-crystalline cementite platelets were divided into many nanometer-sized subgrains by the effects of the shearing stress. This process hindered dislocation movement and led to the accumulation of complex dislocations at the adjacent ferrite lamellae. Thus, the plastic deformation of the cementite layers was characterized by bulging until they fractured. The pearlitic texture component was evenly distributed, and the crystallographic axes of the colonies were randomly oriented. Hence, three different shear models characterized the crack propagation process, which occurred in a linear fashion and could be considered the pearlite colonies as a displacement unit for forward expansion.

AISI 1005 Steel Plasma Treated by Different Thermochemical Surface Treatments

To modify the surface structure of AISI 1005 steel and its properties without any dimensional loss, different plasma surface treatments were performed at low temperatures (500 oC) in this study. The samples were subjected to single plasma treatments including: nitriding (N5% and N3%), carburizing (CE) and ferritic nitrocarburizing (NC) and to duplex treatments of nitriding followed by carburizing (N5%+CE and N3%+CE) and ferritic nitrocarburizing followed by carburizing (NC+CE). The gas mixture used for these treatments was varied as follows: nitriding (5%N2+95%H2 and 3%N2+97%H2), carburizing (5%CH4+95%H2) and ferritic nitrocarburizing (5%N2+1.5%CH4+93.5%H2). A microstructural characterization of the samples was carried out using optical and scanning electron microscopy in addition to XRD analysis. Microhardness testing was also performed. The XRD analysis showed a stabilization of the outermost cementite layer for all of the carburizing treatments. The results show that a greater hardness increase was achieved for the nitriding treatment as well as a more regular compound layer. However, a greater depth of hardening was obtained in samples with NC+CE and N5%+CE, which extended to the hardened depth to 800 µm.

Nanoindentation study of cementite size and temperature effects in nanocomposite pearlite: A molecular dynamics simulation

We carry out molecular dynamics simulations of nanoindentation to investigate the effect of cementite size and temperature on the deformation behavior of nanocomposite pearlite composed of alternating ferrite and cementite layers. We find that, instead of the coherent transmission, dislocation propagates by forming a widespread plastic deformation in cementite layer. We also show that increasing temperature enhances the distribution of plastic strain in the ferrite layer, which reduces the stress acting on the cementite layer. Hence, thickening cementite layer or increasing temperature reduces the likelihood of dislocation propagation through the cementite layer. Our finding sheds a light on the mechanism of dislocation blocking by cementite layer in the pearlite.

Characterisation of the coke formed during the metal dusting of low alloy steels

The deposition of carbon from carbonaceous gas mixtures onto the surface of critical components is of particular concern to the petrochemical and chemical industry. In highly carbonaceous atmospheres, a type of corrosion occurs by which the metal (Fe, Ni or Co) disintegrates into a dust of fine metal-rich particles and graphite. This is called metal dusting.In this project, low alloy 1Cr 0.5Mo ferritic steels were exposed to a 50% CO and 50% H2 (vol.%) gas mixture at temperatures between 500°C and 650°C for times up to 100 hours. Metal dusting occurred on all the test samples. Analysis of cross-sections through the samples in a scanning electron microscope (SEM) showed the formation of a cementite layer together with a layer of coke/graphite deposited on the surface after short times. The amount deposited on the alloy’s surface increased with both time and temperature over the temperature range studied.High levels of sample coking were observed after 100 hour tests. Deposited carbon from these tests was characterised by SEM, X-ray diffraction, and (scanning) transmission electron microscopy, S(TEM). TEM of the carbon deposit/coke on selected samples showed a complex convoluted network of carbon filaments with small iron-rich particles at their tips. High resolution TEM was used to study these carbon filaments in greater detail and showed that there were two completely different orientation relationships between the catalyst particles and the graphite.