Showing papers on "Pearlite published in 2021"

Sensitivity to hydrogen induced cracking, and corrosion performance of an API X65 pipeline steel in H2S containing environment: influence of heat treatment and its subsequent microstructural changes

Abstract: In this investigation, the effect of microstructural changes and phase equilibria on corrosion behavior and hydrogen induced cracking (HIC) sensitivity of an API X65 pipeline steel was studied. For this purpose, heat treatment was performed at 850 °C, 950 °C, 1050 °C and 1150 °C to engineer the desired microstructure of this pipeline steel. Then, the microstructural evolution was performed by optical microscopy, and Field Emission Scanning Electron Microscopy (FE-SEM) equipped with Energy Dispersive X-Ray Spectroscopy (EDS). Corrosion properties were evaluated in H2S environment by open circuit potential (OCP), Potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS). As well, HIC sensitivity of the API X65 pipeline steel was assessed by hydrogen charging of the cathode and immediately conducting the tensile test. Microscopy analyses showed that the microstructure of the steel is ferritic-pearlitic together with the islands of martensite/austenite constituents. Increasing the heat treatment temperature reduced the amount of pearlite and increased ferrite grain size. It also stabilized the ferrite content. Corrosion results indicated that no active layer was formed on the surface of this pipeline steel. Also, increasing the heat treatment temperature increased the corrosion resistance and reduced sensitivity to micro-galvanic localized corrosion. As well, results suggested that the sensitivity to HIC in the API X65 pipeline was substantially increased with increasing the amount of pearlite and reducing the amount of ferrite; i.e. at lower heat treatment temperature.

Role of inclusion and microstructure on corrosion initiation and propagation of weathering steels in marine environment

Abstract: The present research examined the effects of inclusions and microstructure on the initial marine corrosion and evolution of corrosion products in weathering steels by using first-principle modeling and various highly-sensitive analytical techniques including in-situ scanning vibrating electrode technique (SVET), scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS), x-ray diffraction (XRD), and electrochemical workstation. The results demonstrated that CaS in the (Al, Mg)Ox-CaS inclusion formed in both Q500qE and Q370qE steels preferentially dissolved and triggered the initial corrosion. The acidic environment created between the inclusions and the iron matrix further promoted the dissolution of the inclusions. Moreover, due to the discrepancy in corrosion tendency, galvanic couples generated between the bainite ferrite (BF) phase and martensite/residual austenite (M/A) island in the Q500qE steel as well as the ferrite phase and pearlite phase in the Q370qE steel, accelerating the initial corrosion. In addition, pearlite facilitates a faster spread rate of local corrosion compared to bainite. Furthermore, with prolonged exposure, Q500qE steel exhibited more uniform and dense structure of the corrosion products layer, demonstrating a higher corrosion resistance than Q370qE steel. Finally, the mechanistic model was established to illustrate the influence of inclusions and microstructure on corrosion initiation and propagation of weathering steels.

Plastic Behavior of Ferrite–Pearlite, Ferrite–Bainite and Ferrite–Martensite Steels: Experiments and Micromechanical Modelling

Abstract: In this work, low carbon low alloy steel specimens were subjected to suitable heat treatment schedules to develop ferrite–pearlite (FP), ferrite–bainite (FB) and ferrite–martensite (FM) microstructures with nearly equal volume fraction of hard second phase or phase mixture. The role of pearlite, bainite and martensite on mechanical properties and flow behaviour were investigated through experiments and finite element simulations considering representative volume elements (RVE) based on real microstructures. For micromechanical simulation, dislocation based model was implemented to formulate the flow behaviour of individual phases. The optimum RVE size was identified for accurate estimation of stress–strain characteristics of all three duplex microstructures. Both experimental and simulation results established that FM structure exhibited superior strength and FP structure demonstrated better elongation while FB structure yielded moderate strength and ductility. The von Mises stress and plastic strain distribution of the individual phase was predicted at different stages of deformation and subsequent statistical analyses indicated that hard phases experienced maximum stress whereas, maximum straining occurred in soft ferrite phase for all three structures. Micromechanical simulation further revealed that strain accumulation occurred at the F–P and F–B interfaces while the same was observed within the martensite particles apart from the F–M interfaces for FM. These observations were further substantiated through the identification of void and crack initiation sites via subsurface examinations of failed tensile specimens.

Microstructure evolution and mechanical property enhancement of high-Cr hot work die steel manipulated by trace amounts of nano-sized TiC

Abstract: Simultaneously increasing the strength and ductility of hot work die steels is of significant interest for broad applications. The in-situ development of a nano-phase in steels is a promising method for achieving this objective. However, little success has been reported so far. In this study, trace amounts of TiC nanoparticles were successfully introduced into a high-Cr hot work die steel (HHD) through an innovative method using a nano-TiC/Al master alloy. Upon adding 0.02 wt% nano-TiC, an incompletely recrystallized ferrite phase was observed in the HHD containing nano-TiC. Moreover, more globular pearlite was distributed at the ferrite grain boundaries in the HHD with nano-TiC than in the unmodified HHD. The added nano-TiC provided nucleation sites for γ-Fe dendrites and prevented γ-Fe growth during solidification and an austenitizing heat treatment. Furthermore, it noticeably increased the precipitation of more uniform nanoscale chromium carbides and significantly refined the carbides in the HHD during quenching and tempering. Finally, adding nano-TiC simultaneously enhanced the strength and toughness of the HHD. After quenching at 1353 K and tempering at 833 K, the yield strength, tensile strength, tensile strain, and impact toughness of the HHD with nano-TiC were 1330 MPa, 1620 MPa, 14.4 % and 460 J/cm2, respectively, which are all higher than those of the HHD without nano-TiC.

Study on thermal fatigue behaviors of two kinds of vermicular graphite cast irons

Abstract: Due to the good thermal stability and mechanical properties, vermicular graphite cast iron is widely used to make industry components. Many of them are inevitably impacted by alternating temperature load. Therefore, two kinds of vermicular graphite cast irons, RuT400 and RuT450, were employed to study the features of thermal fatigue crack initiation and propagation in different temperature ranges. And the relationship between the length of thermal fatigue crack and number of cycles were also analyzed. It can be found that the thermal fatigue properties between the two materials are obviously different because of different microstructures. At present testing temperatures, RuT450 has higher pearlite content, lower thermal crack growth rate, higher transition temperature. The grain boundary slide temperature of ferrite and transition temperature of pearlite have important influences on the thermal fatigue cracks of two kinds of vermicular graphite cast irons.

On Microstructure and Mechanical Properties of a Low-Carbon Low-Alloy Steel Block Fabricated by Wire Arc Additive Manufacturing

Abstract: In this study, wire arc additive manufacturing process is employed to fabricate a low-carbon low-alloy steel block, using an ER70S-6 solid wire. Three sets of samples with different orientations, including perpendicular (Vertical), parallel (Horizontal), and 45° (45-degree) relative to the deposition plane, were prepared in order to investigate the anisotropy in mechanical properties and microstructure of the fabricated part. Both Horizontal and 45-degree samples showed a uniform microstructure containing mostly ferritic grains with a small volume fraction of pearlite at their grain boundaries. Differently, a periodic microstructure was detected in the Vertical sample, consisting of a combination of acicular ferrite, bainite, and allotriomorphic ferrite formed in the interlayer regions in addition to polygonal ferrite within the melt pools’ center. Moreover, the uniaxial tensile and Charpy impact results exhibited isotropic tensile, yield, elongation, and impact properties for both Horizontal and 45-degree samples; however, the Vertical sample showed a lower mechanical performance. The improved mechanical properties of the Horizontal and 45-degree samples were correlated to their uniform ferritic microstructure.

Influence of Multiphase High Silicon Steel (Retained Austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) and Carbon Content of RA-Cγ on Rolling/Sliding Wear

Abstract: In this research work, heat-treatment processes have been utilized to obtain multiphase microstructure in the silicon rich steel samples, silicon in the steel helps in the development of multiphase microstructure and to keep away from carbide precipitation development through the austempering. The desired multiphase microstructure (Retained austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) consisting of continuous cooling (CC) for 0, 20 and 40 s respectively after austenization followed by austempering at (300, 350 and 400 °C) to form a high wear resistance multiphase steels with microstructure varies amount of F,B, P and RA during continuous cooling. Steels with varies retained austenite up to (5±1.1 to 18±1.9%) along with excellent specific wear rate (2.038 × 10−9-1.061 × 10−8 m3/N-m) were obtained. Further, the rolling/sliding wear rate has been obtained through the disc-on-disc experimental setup. The effect of continuous cooling on retained austenite, carbon content of retained austenite on specific rolling/sliding SWR (specific wear rate) has been studied; phase fraction (P, B and F) and the materials characterization with the help of XRD, AFM and FE-SEM.

The Mechanical Properties of Ductile Iron at Intermediate Temperatures: The Effect of Silicon Content and Pearlite Fraction

Abstract: The use of ductile irons in thermo-mechanically loaded components is increasing, necessitating more knowledge of material properties at intermediate temperatures. A study of the mechanical properties of ductile irons at intermediate temperatures was conducted, investigating the effect of different pearlite fractions along with silicon content tests in fully ferritic microstructures. The effect of the pearlite fraction and silicon content on tensile and yield strength was measured at different temperatures, from room temperature to 450 °C. Models of tensile and yield strengths were developed based on those measurements. These resulting regression models were tested with data from the literature. Such models can be applied in various design tools, such as FEM calculations and in the optimisation of thermally and cyclic loaded ductile iron components.

Assessment of Operational Degradation of Pipeline Steels

Abstract: This paper summarizes a series of the authors’ research in the field of assessing the operational degradation of oil and gas transit pipeline steels. Both mechanical and electrochemical properties of steels are deteriorated after operation, as is their resistance to environmentally-assisted cracking. The characteristics of resistance to brittle fracture and stress corrosion cracking decrease most intensively, which is associated with a development of in-bulk dissipated microdamages of the material. The most sensitive indicators of changes in the material’s state caused by degradation are impact toughness and fracture toughness by the J-integral method. The degradation degree of pipeline steels can also be evaluated nondestructively based on in-service changes in their polarization resistance and potential of the fracture surface. Attention is drawn to hydrogenation of a pipe wall from inside as a result of the electrochemical interaction of pipe metal with condensed moisture, which facilitates operational degradation of steel due to the combined action of operating stresses and hydrogen. The development of microdamages along steel texture was evidenced metallographically as a trend to the selective etching of boundaries between adjacent bands of ferrite and pearlite and fractographically by revealing brittle fracture elements on the fracture surfaces, namely delamination and cleavage, indicating the sites of cohesion weakening between ferrite and pearlite bands. The state of the X52 steel in its initial state and after use for 30 years was assessed based on the numerical simulation method.

Ductility and plasticity of ferritic-pearlitic steel after severe plastic deformation

Abstract: A ferritic-pearlitic steel was subjected to severe plastic deformation (SPD) through Equal Channel Angular Pressing (ECAP) at room temperature, obtaining strengths greater than 1 GPa. Steel constituents were identified by light microscopy displaying similar grain sizes around 14 μm for the pearlite, and 13 μm and 18 μm for the ferrite before and after heat treatment, respectively. Texture changed from a rolling type to a simple shear texture with higher intensity after 4 ECAP passes. After different severe plastic deformation magnitudes, an ultra-fine grain structure was obtained with grain sizes between 0.9 μm-0.36 μm. The substantial grain size reduction was related to heterogeneous Geometrically Necessary Dislocations (GNDs) distribution in the as-received material with pearlite showing higher GNDs densities than ferrite. The remarkable strength increase after ECAP processing was found to be dependent on both small grain sizes and high dislocation densities. On the other hand, the low ductility of the ultrafine-grained (UFG) material was associated with a high annihilation rate of mobile dislocations at deformations greater than 0.39. Additionally, excellent plasticity properties were associated with a high density of immobile dislocations as well as high GND densities inside the deformed grains.

Structure and Properties of Carbon Steel Cast by the Ablation Process

Abstract: Ablation casting is a relatively new casting technique which can improve the mechanical properties of castings. In this paper, the microstructure and mechanical properties of plain carbon steel castings produced by conventional casting and ablation casting techniques were compared. It was found that ablation increased the ultimate strength from 638 to 1094 MPa and tensile elongation increased from approximately 13 to 16% as a result of severe microstructural changes from ferrite and pearlite to acicular ferrite, bainite, and martensite. Ablation increased the core hardness of the casting from 182 to 467 HB and the surface hardness from 205 to 508 HB.

Comparison on the microstructure, bending properties and tribological behaviors of rail materials treated by laser dispersed quenching and induction assisted laser dispersed quenching

Abstract: Laser dispersed quenching technology (LDQ), which can get composite structure with martensite hardening arrays embedded in the untreated matrix on rail surface, can improve the rail’ s wear resistance while not threaten its service safety, thus has attracted a lot of notices recently. However, the severe spalling appearing at quenched zones due to the low toughness of martensite structure therein may reduce the rail's ride-ability and rolling contact fatigue (RCF) life. In view of above problems, an induction assisted laser dispersed quenching technology (IA-LDQ) was proposed to adjust the microstructure and mechanical properties of the quenched zones in this paper. The microstructure and hardness of the quenched zones treated by IA-LDQ with different induction heating temperature (T), and their effects on the bending properties and tribological behaviors of rail materials were studied systemically. Results indicated that the microstructure of the quenched zones changed significantly with the increase of T in IA-LDQ, mixed structure of pearlite + bainite with HV399 and approximate pure pearlite structure with HV377 could be obtained at T = 450 °C and T = 500 °C, respectively. Compared with the rail material treated by LDQ, the bending resistances of rail materials treated by IA-LDQ were enhanced significantly, and that treated by IA-LDQ with T = 450 °C showed the best bending performance. On the other hand, the surface damages such as the vertically extended cracks and severe spalling appearing at the LDQ treated zones could be eliminated by IA-LDQ significantly. By IA-LDQ with T = 400 °C and 450 °C, the rail rollers' resistance to wear and fatigue could be balanced best, in which the wear rates were decreased by 59.7% and 41.3% than the untreated rail substrate, respectively, but the surface damage characteristics were kept almost unchanged. Thus, the IA-LDQ technology shows the potential to hardface the rail.

Effects of Microstructural Morphology on Formability, Strain Localization, and Damage of Ferrite-Pearlite Steels: Experimental and Micromechanical Approaches

Abstract: This paper attempts to predict how the microstructural features and mechanical properties of the individual constituents affect the deformation behavior and formability of ferrite-pearlite steels under quasi-static loading at room temperature. For this purpose, finite element simulations using representative volume elements (RVEs) based on the real microstructures were implemented to model the flow behavior of the ferrite-pearlite steels with various microstructural morphologies (non-banded and banded). The homogenized flow curves obtained from the RVEs subjected to periodic boundary conditions together with displacement boundary conditions were validated with the experimental results of the uniaxial tensile tests. Then, the initial microstructural inhomogeneity and Johnson–Cook damage criteria were employed for both non-banded and banded RVEs to estimate the onset of plastic instability under different loading paths ranging from uniaxial tension to equi-biaxial tension. Finally, the forming limit diagrams of both ferritic-pearlitic microstructures were predicted, which show a good agreement with the experimental results of the Nakazima stretch-forming tests (less than 13 pct error). It implies that the initial microstructural inhomogeneity criterion adequately enables to predict the plastic instability in the ferritic-pearlitic steel sheets without using any damage or failure criterion. The most commonly observed damage mechanism is the severe plastic deformation of the ferrite grains near the pearlite colonies due to the strength contrast between ferrite and pearlite. Another significant finding is that the microstructural morphology has a crucial influence on the strain partitioning, strain localization, and formability of the ferritic-pearlitic steels.

Influence of Cooling Rate on Microstructure Formation of Si–Mo Ductile Iron Castings

Abstract: The present study highlights the effect of the cooling rate on the microstructure formation of Si–Mo ductile iron. In this study, experiments were carried out for castings with different wall thicknesses (i.e., 3, 5, 13, and 25 mm) to achieve various cooling rates. The simulation of the cooling and solidification was performed through MAGMASOFT to correlate the cooling conditions with the microstructure. The phase diagram of the investigated alloy was calculated using Thermo-Calc, whereas the quantitative metallography analyses using scanning electron microscopy and optical microscopy were performed to describe the graphite nodules and metallic matrix morphologies. The present study provides insights into the effect of the cooling rate on the graphite nodule count, nodularity, and volumetric fractions of graphite and ferrite as well as the average ferritic grain size of thin-walled and reference Si–Mo ductile iron castings. The study shows that the cooling rates of castings vary within a wide range (27 °C–1.5 °C/s) when considering wall thicknesses of 3 to 25 mm. The results also suggest that the occurrence of pearlite and carbides are related to segregations during solidification rather than to cooling rates at the eutectoid temperature. Finally, the present study shows that the longitudinal ultrasonic wave velocity is in linear dependence with the number of graphite nodules of EN-GJS-SiMo45-6 ductile iron.

Parametric study and characterization of wire arc additive manufactured steel structures

Abstract: This paper focuses on selecting optimal process parameters for uniform single-layer weld bead deposition and the characterization of structures manufactured by GMAW-based wire arc additive manufacturing (WAAM) process. The results reveal that the obtained grain structures vary due to the local thermal cycle. Near to the base of the fabricated part, ferrite with pearlite structures is observed. At the same time, the observed grains become coarser along the deposition direction. Again adjacent to the final layer, finer grains are observed with ferrite and thin strips of bainite, which has been confirmed through XRD analysis. Also, the formation of different chemical compounds such as ferrite, cementite, bornite, and martensite has been identified at different layers. Investigations of surface defects using dye penetration test and corrosion behavior of the component using the weight loss method have also been conducted. The observed surface defects like cracks and porosity are primarily present in the interface of the upper layers. The measured Vickers micro-hardness is found different in different wall structures. The micro-hardness values in the flat and circular wall are recorded as 162.806 HV and 172.191 HV, respectively.

Effect of Cr/Mn segregation on pearlite-martensite banded structure of high carbon bearing steel

Abstract: The effect of Cr/Mn segregation on the abnormal banded structure of high carbon bearing steel was studied by reheating and hot rolling. With the use of an optical microscope, scanning electron microscope, transmission electron microscope, and electron probe microanalyzer, the segregation characteristics of alloying elements in cast billet and their relationship with hot-rolled plate banded structure were revealed. The formation causes of an abnormal banded structure and the elimination methods were analyzed. Results indicate the serious positive segregation of C, Cr, and Mn alloy elements in the billet. Even distribution of Cr/Mn elements could not be achieved after 10 h of heat preservation at 1200°C, and the spacing of the element aggregation area increased, but the segregation index of alloy elements decreased. Obvious alloying element segregation characteristics are present in the banded structure of the hot-rolled plate. This distinct white band is composed of martensitic phases. The formation of this abnormal pearlite—martensite banded structure is due to the interaction between the undercooled austenite transformation behavior of hot-rolled metal and the segregation of its alloying elements. Under the air cooling after rolling, controlling the segregation index of alloy elements can reduce or eliminate the abnormal banded structure.