Ferritic-pearlitic Steel Research Articles

Yield-Point Phenomenon and Plastic Bands in Ferrite–Pearlite Steels

Lüders deformation is one type of discontinuous yielding in ferrite-pearlite steel. The yield-point phenomenon and localized plastic bands are two features of the Lüders phenomenon. It is believed that the yield-point phenomenon is related to the formation of plastic bands, but the correlation between them is unclear. In this study, this correlation was investigated by examining the global and local deformation behaviors in the tension processes of four ferrite-pearlite steels (carbon content, 0.05-0.3%; pearlite fraction, 1.2-32%) via an extensometer and digital image correlation (DIC) technique. The main obtained results are as follows: (1) the degree of yield drop decreased with an increase in the pearlite fraction (the magnitude of the yield stress drop was 8.6-0 MPa), and (2) a plastic band was formed at a certain stress level smaller than the upper yield stress; when the stress level was larger than 92% of the upper yield stress, the upper yield point disappeared.

Comparative evaluation of the yield strength of ferrite-pearlite steels by structure parameters

The influence of different hardening mechanisms on the yield strength of carbon and low-alloy steels is studied. It is established that for Ст.5пс hot-rolled steel, the greatest influence on the yield strength is exerted by solid-solution and grain-boundary hardening, and for 16Г2АФ low-alloyed steel, in addition to these types of hardening, it is also dispersion type. It is shown that thermomechanical treatment of Ст.5пс steel increases dislocation hardening as a result of a decrease in the increase in the density of dislocations and their preservation during accelerated cooling of hot-worked austenite in rolled products. Keywords: hardening, yield strength, thermomechanical treatment, accelerated cooling, phase components, grain size. aman-kanaev2012@yandex.ru, remshev@mail.ru, mash-kalugin@yande.ru, aliya-981@mail.ru

EBSD analysis of strain distribution and evolution in ferritic-Pearlitic steel under cyclic deformation at intermediate temperature

Strain accumulation as a result of cyclic loading affects the microstructure and mechanical performance of low-alloy steels under cyclic operation conditition. This manuscript presents a comprehensive analysis of a ferritic-pearlitic SA204 Grade C steel under strain-controlled low-cycle fatigue tests to understand the microstructural deformation and strain localization in the ferritic-pearlitic steel. Low-cycle fatigue tests were performed in a Gleeble® thermo-mechanical simulator at 0.008, 0.01, 0.015, and 0.02 strain amplitudes under an isothermal condition at 250 °C. The microstructure evolution during cyclic loading was evaluated using the samples from interrupted 0.01 strain amplitude tests. The LCF analysis shows strain localization at the ferrite-cementite interfaces and ferrite-pearlite interfaces. The characterization results using OM, SEM, EBSD, and nanoindentation analysis reveals the grain misorientation in the ferritic-pearlitic microstructure due to strain accumulation increases with the strain amplitudes and the loading cycles.

Effect of nitrogen content on the mechanical properties and deformation behaviors of ferritic-pearlitic steels

The effect of nitrogen content (0.001 wt%, 0.009 wt%, 0.017 wt%) on the microstructure, mechanical properties and deformation behaviors of a medium-carbon ferritic-pearlitic steel is investigated in this study. The addition of nitrogen is shown to have little effect on the ferrite-pearlite microstructure and only marginally coarsens it because austenite grain growth is promoted. Although nitrogen is a strong element for solid solution strengthening, nitrogen addition only marginally improves the strength of the steel and has little effect on the ductility because fine Fe3N phases are precipitated during the tensile process. The toughness of the steel is markedly decreased by the addition of nitrogen because the dislocation locking effect caused by interstitial nitrogen atoms is enhanced. When the nitrogen content increases from 0.001 wt% to 0.017 wt%, the yield-point phenomenon in the tensile process is first strengthened and then weakened. The yield-point phenomenon is closely related to the dislocation behaviors in proeutectoid ferrite, and the different yield-point phenomena under different nitrogen contents are related to the dislocation locking effect caused by interstitial nitrogen atoms and to the dislocation pinning effect caused by Fe3N phases.

Abrasive wear behavior in dry condition of a plasticity-induced transformation steel

The microstructural, mechanical, and tribological evaluation was carried out through an abrasion test of low alloy steel: 0,21%C, 1,22%Mn, and 2,16%Si with/without heat treatment. The steel was manufactured in an electric induction furnace from AISI-1018 steel with additions of FeMn and FeSi. Later it was hot rolled and heat treated. The microstructure of steel without heat treatment is ferritic-pearlitic with σ u = 714 MPa, S = 8%, and a hardness of 21 HRC. The microstructure of the heat-treated steel corresponds to a Plasticity Induced Transformation (TRIP) steel with σ u = 890 MPa, S = 30%, and hardness of 22 HRC. Both types of steel were subjected to abrasive wear (ASTM G-65) with silica sand particles and loads of 45, 66,5, and 130 N. The TRIP effect (transformation of retained austenite into martensite during plastic deformation) was evaluated before and after the tensile test by x-ray diffraction analysis. The results of the abrasion tests showed that the TRIP steel did not present high resistance characteristics, however, it shows better performance than steels with high carbon and/or alloy contents. The results show that with 45 N the TRIP steel is less resistant than the ferritic-pearlitic steel, however, with 66,5 and 130 N, its abrasive wear performance improved by 17% concerning the ferritic-pearlitic steel, which is attributed to the deformation capacity of TRIP steel. The main wear mechanisms present in TRIP steel are: ploughing, fractures, plastic deformation, pits, cracks, cutting, and micro-cutting. On the other hand, the surface hardness in the abrasion test footprint did not show any change that could be related to the TRIP effect. The microstructure analysis reveals that the material’s detachment occurs at the grain boundaries of the ferrite, in which the cracks that cause the detachment of the material were generated and propagated.

Regularities of the creep deformation and failure of 15Kh2NMFA-A steel within the temperature range of 900–1200 °C

This paper presents and summarizes main results of the research to define the high-temperature creep parameters and strength characteristics of the unirradiated steel 15Kh2NMFA-А tested in the temperature range of 900–1200 °C. This ferritic-pearlitic steel is one of the main types of steel used for the production of the VVER RPVs of the Russian design. The results of research of the strength properties and creep parameters for 15Kh2NMFA-А steel are meant to support, first and utmost, the estimation of the integrity and predicting the behavior of the VVER pressure vessel during the SA, when it may deform due to high-temperature creep and the overpressure in the RPV. One of the main goals of the investigation was to determine such strength properties and creep parameters of the investigated steel as, for instance, steady-state creep rate, creep tensile strength under the condition of isothermal creep at different temperatures and test stress. The creep tensile tests were conducted in vacuum and in the protective argon medium. A series of creep tests was conducted in air. The analysis and comparison of the test results demonstrated significant impact of the surficial oxidation process on the nature of their damage and the time to failure of the test samples. The absence of a protective medium during testing, as observed in the case of the test in air, causes decarburization and the damage of the surficial layer of the steel samples, so that the specimen fractures prematurely. Based on the statistic estimation the (B) and (n) parameters for the secondary creep rate equations were defined for the researched steel within the temperature range of 900–1200 °C. The agreement between the values of the steady-state creep rate of 15Kh2NMFA-A steel from the conducted creep tensile tests and similar calculation values from the obtained parameters (B) and (n) for the temperature range from 900 to 1200 °C was satisfactory. The paper also presents the definition of the LMP parametric dependence coefficients, completed on the basis of the creep test results for 15Kh2NMFA-А steel with a time span up to 100 h in the given temperature range. The comparative analysis of the defined LMP dependence demonstrated good agreement of the predicted values for the failure time with similar values defined during the experiment.The research of the steel specimens’ microstructure following the tests demonstrated a difference in the mechanisms of damage accumulation and fracture at the temperatures over 1000 °C in comparison to similar mechanisms at lower temperatures. It is important to apply more precise research methods to understand this phenomenon to a better extent; such research may take place later.

Hardness measurements as a technique for measuring accumulated fatigue damage

PurposeBrinell, Vickers and low-force Vickers hardness measurements are herein adopted to investigate and quantify the fatigue damage evolution in specimens made of S355J2+AR ferritic pearlitic steel. Though nano and microhardness measurements have been well adopted, they require a strict preparation routine, whereas macroscopic hardness measurements are not as stringent.Design/methodology/approachThe feasibility of adopting macroscopic hardness measurements as a means of measuring fatigue damage is investigated through a combination of experimental tests and finite element analyses with both Brinell and Vickers hardness indenter.FindingsIt is found that the Brinell hardness measurements method seems more feasible, regarding finding a continuous and significant change during the fatigue life, in comparison to both Vickers and low-force Vickers. Thereafter, the question regarding the feasibility of the hardness measurements as a method of assessing accumulated fatigue damage in situ is discussed.Originality/valueMuch work has previously been performed towards correlation of the micro and nano hardness indentations, which generally has stringent preparation requirements before testing. Herein, the adoption of macroscopic hardness measurements as a means of assessing accumulated fatigue damage is considered both experimentally and theoretically.

A Comparison of Two Methods Modeling High-Temperature Fatigue Crack Initiation in Ferrite–Pearlite Steel

High-temperature fatigue tests are carried out to investigate the fatigue crack initiation behavior of ferrite–pearlite steel. Two approaches to modeling fatigue crack initiation under high temperatures in ferrite–pearlite steel are developed and compared in this study. In the first approach, basal energy is introduced as the criterion for crack initiation. Various initial basal energy values are assigned to each potential location. The increments in basal energy after each cycle can be calculated. In the second approach, a crystal plasticity model is developed, combined with the cyclic hardening/softening effect and three fatigue indicator parameters (two of them are adopted from previous works and one is developed in this work). Results indicate that both approaches are capable of predicting the fatigue crack initiation location and the corresponding life. The first approach requires a lower computational cost but has many limitations, and the second approach is suitable for calculating mesoscale stress and strain distributions but with a higher computational cost.

Structure formation during sheet steel production in a unit of continuous casting and deformation

The paper considers the main problem of improving quality of steel sheets for welded pipes obtained at plate rolling mills and the main reasons that reduce it. There is a possibility of improving quality of steel sheets using technological capabilities of the unit of combined continuous casting and deformation. Problem of determining stress state of metal in cyclic deformation zone is posed and solved. The results of obtaining 09G2S sheet steel in pilot unit of continuous casting and deformation are presented. Magnitude of stresses in the center of cyclic deformation during production of steel sheets was determined for welded pipes at the unit. The authors present results of experimental study of the process of steel sheets production at pilot unit of continuous casting and deformation. The article considers the results of microstructure analysis, microhardness determination and microanalysis of chemical composition of the studied steel in various zones of the strip cross section. Structural inhomogeneity development in ferritic-pearlitic steel is interpreted along the thickness of sheet under conditions of combined thermomechanical processing of metal solidified in the mold. Predominantly ferritic microstructure is generated in the central light interlayer of the strip compared to base metal, characterized by a sharp decrease in carbon concentration in it. There is a possibility of such structure genesis at the first stages of crystallization during forging of low-carbon steel, when the excess phase of high-temperature ferrite that solidifies first is located mainly in the strip center. The size of pearlite grains in the light interlayer covariates with the grain structure of pearlite in the strip base metal and does not form precipitates of lath morphology, which increase tendency of steel to brittle fracture.

Post-radiation annealing influence on the evolution of the materials properties of the supporting structures of WWER-440 reactor vessels. Part 1: Problem statement and test results

Experimental studies have been carried out regarding the effect of annealing temperature on the degree of properties recovery of supporting structures materials for WWER-440 reactor vessels (low-strength ferritic-pearlitic steel and its weld metal) irradiated at low temperatures (50–90°C). Properties connected with material embrittlement have been determined on the basis of test results of tensile specimens, impact bending specimens and fracture toughness specimens. Dependence of properties recovery coefficient on annealing temperature have been determined. It is shown that for material with high content of copper dependence of properties recovery coefficient on annealing temperature may be nonmonotonic.

Post-radiation annealing influence on the evolution of the materials properties of the supporting structures of WWER-440 reactor vessels. Part 2: Analysis of the influence of material annealing after low temperature irradiation

The results of studying the influence of annealing temperature on the restoration degree of the materials properties of WWER-440 reactor vessels supporting structures (low-strength ferritic-pearlitic steel and its weld metal) irradiated at low temperatures (50–90°C) are analyzed. The main processes that occur during the annealing of the supporting structures materials after low-temperature irradiation and lead to an ambiguous effect of the annealing temperature on the recovery degree of the properties of supporting structures materials are revealed. The influence of impurities (phosphorus and copper) on the embrittlement of the material during irradiation and on the recovery of its properties after annealing is considered.

The influence of pearlite fraction on the shock properties of ferrite–pearlite steel microstructures: Insight into the effect of second-phase particles

The goal of this work is to investigate the effect of varying phase fractions on the overall spall strength and damage behavior of a material. Specifically, two plain carbon, ferrite–pearlite steels (1045 and A283) were subjected to spall recovery experiments to investigate the effect of pearlite fraction on spall strength and total damage. The A283 (20% pearlite) alloy exhibited a higher Hugoniot elastic limit and spall strength compared with 1045 (60% pearlite). Discontinuous and continuous yielding behaviors were observed at quasi-static and dynamic rates for A283 and 1045, respectively. The yielding behavior was connected to pearlite fraction and the prevalence of dislocation-emitting, ferrite/cementite interfaces. Postmortem characterization revealed cementite lamellae cracking within pearlite of 1045, suggesting that pearlite reduces spall strength by providing low-energy damage nucleation sites. The rate of damage growth and coalescence was similar between the two alloys; however, 1045 exhibited more continuous cracks than A283, which exhibited a greater prevalence of discrete voids.

Multiresolution investigations of thermally aged steels using spherical indentation stress-strain protocols and image analysis

In recent work, our research group has developed and demonstrated novel multi-resolution protocols capable of extracting indentation stress-strain (ISS) curves from tests on individual microscale constituents (e.g., phases, grains) as well as bulk properties of material microstructures. In addition, we recently developed protocols for design of consistent segmentation of micrographs. This work combines these recent advances in multi-resolution spherical indentation and image segmentation protocols to address the current challenges in the critical evaluation and advancement of physics-based composite models. These new research avenues are identified and demonstrated through a case study on thermally aged ferrite-pearlite steel samples, where the respective indentation yield strengths of the microscale constituents (i.e., ferrite, and pearlite) and the bulk yield strength of the samples were estimated from ISS tests measurements. The constituent volume fractions were extracted from segmented optical microscopy images. It is shown that the multi-resolution indentation yield strength and volume fraction measurements are highly consistent with the homogenization estimates from simple composite theories.

True Stress–True Strain Relationship up to the Plastic Deformation Limit in Ferrite–Pearlite Steel at Various Temperatures

True Stress–True Strain Relationship up to the Plastic Deformation Limit in Ferrite–Pearlite Steel at Various Temperatures

Influence of microstructural parameters on nanohardness of various dual-phase steels: Experiment, Finite Element simulation and Statistical analysis

This study aims to achieve insight related to the role of various microstructural parameters on nanohardness of different phases in ferrite-martensite (FM), ferrite-bainite (FB) and ferrite-pearlite (FP) steels. Low-carbon microalloyed steel has been subjected to suitable heat treatment schedules to develop FM, FB and FP dual-phase (DP) microstructures with nearly the same amount of ferrite. Macro, micro, and nanohardness values and tensile properties of the developed DP steels have been measured while their microstructures have been characterized. It has been found that the characteristics of hard phase strongly affect the nanohardness of the ferrite phase. Load-depth profiles of diverse phases obtained from the nanoindentation tests have been simulated by the finite element (FE) technique. The dislocation-based model has been modified to estimate the flow behavior of constituent phases which have been provided as input to the FE simulations. The predicted nanoindentation load-depth profiles are found to be in good agreement with the experiment ones. Subsequently, FE simulations have been performed by incorporating the microstructural parameters to predict the nanohardness of individual phases in the developed DP microstructures. Finally, statistical analyses following the Taguchi method and ANOVA have been conducted to demonstrate the influence of different microstructural parameters on nanohardness in various DP steels. It has been revealed that the nanohardness values of constituting phases are significantly influenced by the percentage of carbon and the hardness ratio of two phases in FM, FB and FP DP steels except the pearlite for which the interlamellar spacing is the most contributing factor.

Investigation of the rate dependences of the stress of plastic flow and fracture of Mn2-Si steel at normal and elevated temperatures

The Hugoniot elastic limit and spall strength of reactor Mn2-Si steel under shock compression were measured by recording and subsequent analysis of the wave profiles. The temperature-rate dependences of the resistance to high-strain rate and fracture of steel at normal and elevated temperatures are determined. The results of measurements of the strength characteristics of steel under spall are supplemented by a metallographic analysis of the fracture zone and compared with data for 15Kh2NMFA reactor steel and Armco iron. Keywords: ferrite-pearlite steel, shock waves, deformation, temperature, Hugoniot elastic limit, spall strength.

Microstructure and strength in ultrastrong cold-drawn medium carbon steel

Via traditional wire drawing, the medium carbon ferrite-pearlite (MCFP) steel wires can achieve the ultrahigh strength beyond 4 GPa normally for high-carbon pearlitic steel wires, but have a 30–60% lower production cost. The microstructural evolution and mechanical properties of medium carbon ferrite-pearlite steel wires have been investigated by means of scanning electron microscopy, transmission electron microscopy and tensile testing. The tensile strength of medium carbon ferrite-pearlite steel wires increases from 750 MPa up to 4120 MPa when the drawing strain increases up to ε = 6.4, which represents the highest strength reported so far – to our knowledge for a carbon steel with such low carbon content. At low and medium strains (ε ≤ 1.95), the proeutectoid ferrite forms dense dislocation walls (DDWs) via dislocation activities, including sliding, accumulation, interaction, and tangling. With the drawing strain increase, the reorientation of DDWs to the drawing direction forms the coarse proeutectoid ferrite lamellae. Finally, the proeutectoid ferrite deformed to high strains is characterized by a lamellar morphology and the average lamellar spacing of proeutectoid ferrite is about 55 nm at ε = 6.4. The interlamellar spacing of pearlite and thickness of cementite decreases with the drawing strain increases. The dislocation density in ferrite lamellae increases with the drawing strain increases, and the dislocation density in ferrite lamellae is 7.8 × 1015 m−2 at ε = 4.19. A higher dislocation density of 3.1 × 1016 m−2 can be obtained at ε = 6.4 by means of extrapolation and TEM investigations. The stress contributions of proeutectoid ferrite and pearlite to the flow stress are estimated based on quantified structural parameters. Based on the assumption that the stress contributions from different strengthening mechanisms are linearly additive and the general rule of mixtures, a good agreement between the measured and estimated flow stresses has been found in a large range of flow stress. The good application of the general rule of mixture to the medium carbon ferrite-pearlite steel wires indicates the importance of quantitative characterization of microstructural evolution and parameters with the strain.

Observation and Quantification of Crack Initiation from TiN Particle in a Ferrite-pearlite Steel

Observation and Quantification of Crack Initiation from TiN Particle in a Ferrite-pearlite Steel

A modified Johnson-Cook constitutive model for improved thermal softening prediction of machining simulations in C45 steel

A new modified Johnson-Cook model is developed to capture the non-linear thermal softening behaviour below the austenite transformation temperature (723°C) in C45 ferritic-pearlitic steel, which is not captured by the original Johnson-Cook (JC) model. The physical mechanisms responsible for this behaviour are also studied. The model is developed by experimental quasi-static and high strain rate axisymmetric compression testing from 20°C to 720°C and implemented into 3D finite element drilling simulations. The improvement over the original JC model is demonstrated by comparison to experimentally measured torque and thrust force.

Hydrogen Stress Cracking Resistance and Hydrogen Transport Properties of ASTM A508 Grade 4N

Low alloy steels combine relatively low cost with exceptional mechanical properties, making them commonplace in oil and gas equipment. However, their strength and hardness are restricted for sour environments to prevent different forms of hydrogen embrittlement. Materials used in sour services are regulated by the ISO 15156-2 standard, which imposes a maximum hardness of 250 HV (22 HRC) and allows up to 1.0 wt% Ni additions due to hydrogen embrittlement concerns. Low alloy steels that exceed the ISO 15156-2 limit have to be qualified for service, lowering their commercial appeal. As a result, high-performing, usually high-nickel, low alloy steels used successfully in other industries are rarely considered for sour service. In this work, the hydrogen stress cracking resistance of the high-nickel (3.41 wt%), quenched and tempered, nuclear-grade ASTM A508 Gr.4N low alloy steel was investigated using slow strain rate testing as a function of applied cathodic potential. Results showed that the yield strength and ultimate tensile strength were unaffected by hydrogen, even at a high negative potential of −2.00 VAg/AgCl. Hydrogen embrittlement effects were observed once the material started necking, manifested by a loss in ductility with increasing applied cathodic potentials. Indeed, A508 Gr.4N was less affected by hydrogen at high cathodic potentials than a low-strength (yield strength = 340 MPa) ferritic-pearlitic low alloy steel of similar nickel content. Additionally, hydrogen diffusivity was measured using the hydrogen permeation test. The calculated hydrogen diffusion coefficient of the ASTM A508 Gr.4N was two orders of magnitude smaller when compared to that of ferritic-pearlitic steels. Hydrogen embrittlement and diffusion results were linked to the microstructure features. The microstructure consisted of a bainitic/martensitic matrix with the presence of Cr23C6 carbides as well as Mo- and V-rich precipitates, which might have played a role in retarding hydrogen diffusion, kept responsible for the improved HE resistance.