Ferritic-pearlitic Steel Research Articles

Artificial Neural Network: Some Applications in Physical Metallurgy of Steels

Artificial neural networks are parameterized nonlinear models used for empirical regression and classification modelling. Their flexibility enables them to discover very complex relationships among large number of variables with complex interdependencies. Hence it is a very appropriate technique for developing predictive models in the short term. This article discusses the development of neural network models based on a Bayesian framework and some applications of the same. These examples include: (i) prediction of yield and tensile strengths of hot-rolled low-carbon ferrite-pearlite steel plates as a function of composition and rolling parameters, (ii) estimation of bainite plate thickness, (iii) estimation of retained austenite as a function of process parameters in Transformation Induced Plasticity aided (TRIP-aided) steels, and (iv) analysis of strain-induced transformation behavior of retained austenite during uniaxial tensile testing of TRIP-aided steels. While examples (i), (ii), and (iv) provide an overview of the work reported earlier, example (iii) is reported here in open literature for the first time. In all these four cases, it has been shown that the results are consistent with the established physical metallurgy principles.

Micromachining of coarse-grained multi-phase material

The high demand of miniaturized components, coupled with geometric and material range limitations of traditional lithographic techniques has generated a strong interest in micromechanical machining. In micromachining the so-called size effect is a dominant factor. This is attributed to the fact that the unit or physical size of the material to be removed can be of the same order of magnitude as the tool edge radius or grain size. This paper explores the micro-machinability of multi-phase ferrite—pearlite steel that has a relatively large average grain size (10 μm). The investigation and cutting tests examined the effect of undeformed chip thickness, tool edge radius, and workpiece grain size on the specific cutting force, burr size, surface finish, and tool wear. The work clearly shows that micro tool edge radius and workpiece material grain size are valuable inputs in determining micromilling conditions that ensure the best surface finish and reduced burr size. Cutting conditions recommendations are also put forwards for roughing and finishing passes in micromilling of AISI 1045 tool steel.

Fatigue Properties of Steels with Ultrasonic Attrition Treated Surface Layers

Severely deformed surface layers have been created by ultrasonic attrition technique on four steel sheets to investigate their influence on fatigue behaviour. A low-carbon (0.05%) ferritic steel and a medium-carbon (0.47%) normalized ferritic-pearlitic steel were selected to study the effect of carbon content on fatigue properties of carbon steels. Two stainless steels, Type 316L and Type 301LN, were also tested to study the influence of stability of the austenitic structure. Microstructural features were characterized by hardness measurements, X-ray diffraction and optical and electron microscopy. Fatigue properties were determined in flexural bending in the range 104 to 107 cycles. Crack nucleation and propagation stages were followed. In the attrition treatment thin severely deformed surface layers were found to form. Highly increased hardness was measured in these layers, especially for stainless steels, where also strain-induced martensite was formed. Drastic improvement in fatigue resistance was observed for all steels due to the surface nanocrystallization treatment.

Microstructure based modeling for the mechanical behavior of ferrite–pearlite steels suitable to capture isotropic and kinematic hardening

This study proposes a new microstructure based model for the mechanical behavior of non-microalloyed ferrite–pearlite steels. Its main originality consists in estimating the internal stresses evolving with strain. It takes into account two contributions corresponding to two different scales of the microstructure: the huge internal stresses generated in pearlite all along the deformation and the mesoscopic strain incompatibility between ferrite and pearlite. This estimation allows the distinction to be made between the isotropic and kinematical components of work-hardening. The parameters of the model have been adjusted on numerous results from the literature concerning fully ferritic or pearlitic steels. The performance of the model is then demonstrated on datasets from the literature concerning steels with different fractions of pearlite. The predictions of the model are excellent concerning both tensile behavior and Bauschinger effects. The size effects of lamellar pearlite have been reviewed in this paper, i.e., the impact of interlamellar spacing on mechanical properties. In pearlite, the yield strength scales with s −1, the internal stress with s −1/2 and the macroscopic strain-hardening (i.e., slope of the tensile curve) do not depend on interlamellar spacing. This review gives new perspectives for the understanding of the mechanical behavior of lamellar structures, such as pearlite or bainite.

Formation of Bimodal-Sized Structure and Its Tensile Properties in a Warm-Rolled and Annealed Ultrafine-Grained Ferrite/Cementite Steel

An ultrafine-grained ferrite/cementite (UGF/C) steel with a local high density of cementite particles was fabricated through caliber-warm-rolling followed by annealing and resulted in a bimodal-sized microstructure. The characteristic bimodal-sized microstructure was attributed to the original ferrite-pearlite structure and cementite spacing, and reflected the original ferrite-pearlite structure. The smaller-sized clusters corresponded to the former pearlite regions and the larger-sized clusters to the proeutectoid ferrite regions. The cementite particles naturally localized within the former pearlite region. Most of the ferrite coarsening did not occur until the cementite particle spacing reached a critical value. The UGF/C microstructure with a bimodal grain size showed a yield strength elongation combination that was better than conventional ferrite pearlite steels, and is a significant improvement as a result of the processing and resultant microstructure. The Hall–Petch relationship was best demonstrated when the yield strength was correlated to the average size (determined from the two inverse grain sizes that were normalized by the area fractions) rather than the larger or smaller size of the bimodal distribution, which reflected the strengthening effect of the subgrained structure. The grain refinement contributed to the athermal component of the yield strength but hardly affected its thermal component.

Correlation between residual stress and plastic strain amplitude during low cycle fatigue of mechanically surface treated austenitic stainless steel AISI 304 and ferritic–pearlitic steel SAE 1045

Mechanical surface treatments such as deep rolling are known to affect the near-surface microstructure and induce, e.g. residual stresses and/or increase the surface hardness. It is well known that, e.g. compressive residual stress states usually increase the lifetime under fatigue loading. The stress relaxation behaviour and the stability of the residual stress during fatigue loading depend on the mechanical surface treatment method. In this paper three different surface treatments are used and their effects on the low cycle fatigue behaviour of austenitic stainless steel (AISI 304) and ferritic–pearlitic steel (SAE 1045) are investigated. X-ray diffraction is applied for the non-destructive evaluation of the stress state and the microstructure. It is found that consecutive deep rolling & annealing as well as high temperature deep rolling produce more stable near-surface stress states than conventional deep rolling at room temperature. The plastic strain amplitudes during fatigue loading are measured and it is shown that they correlate well with the induced residual stress and its relaxation, respectively. Furthermore, Coffin–Manson plots are presented which clearly show the correlation between the plastic strain amplitude and the fatigue lifetime.

Kinetics of polymorphic transformation in ferrite-pearlite steel on heating in the intercritical temperature range

However, the processes in structural steel on heating in the intercritical range and their influence on the structure and properties of the metal have not been adequately studied. Taking account of the scientific and practical interest in polymorphic transformation within the intercritical temperature range, it is appropriate to study the transformations in ferrite‐pearlite steel (considered in the present work) and in high-strength steel and to determine the physicomechanical properties of the rolled steel. In the first stage of this work, attention focuses on carbon and low-alloy sheet steel: eU 3 OO , 09 E 2 e , 09 E 2 aiA , and 16 E 2 Ai steel (Tables 1 and 2). We know that the position of the intercritical range and the transformations within it depend not only on the chemical composition of the steel but also on its initial structure and heating conditions [5‐12]. For steel subjected to mechanical or phase hardening, at high heating rates, the critical points A c 1 and A c 3 lie lower than for steel with equilibrium structure. With slower heating, facilitating recovery and recrystallization, the critical points are higher. Therefore, to obtain more general results, the steel is investigated in two structural states: after annealing (heating to 920 ° C, holding for 20 min, cooling in the furnace to 300 ° C and then in air); and after phase hardening (quenching in water from 920 ° C after 20-min holding at 920 ° C). The thickness of the plate samples is equal to the sheet thickness, with a width of 200 mm and a length of 300 mm. The position of the critical points A c 1 and A c 3 and the relative volume of α γ transformation in heat-treatment conditions or in the thermal cycles of welding may be determined by means of a high-speed vacuum dilatometer, for example [13]; however, this method is slow. Therefore, a different dilatometer design is proposed: the rod samples are electrically heated in air, and oxidation and scale formation are eliminated by applying a metallicchrome layer of thickness 4‐8 µ m. To equalize the temperature over the sample length, a working section of stepped form is employed (Fig. 1). Thermal changes in

Effect of various dual-phase heat treatments on the corrosion behavior of reinforcing steel used in the reinforced concrete structures

In this study, corrosion behavior of six different dual-phase (DP) steels with varying morphologies and martensite content has been examined in comparison to ferrite–pearlite steel in concrete. Intercritical annealing and intermediate quenching heat treatments have been applied to the reinforcing steel in order to obtain DP steels with different morphologies and content of martensite. Corrosion experiments were conducted in two stages. In the first stage, the corrosion potential of steels embedded in concrete was measured every day for a period of 30 days in accordance with ASTM C 876 standard. In the second stage, anodic and cathodic polarization values of these steels were obtained and then the corrosion currents were determined with the aid of cathodic polarization curves. It has been observed that both the amount of martensite and the morphology of the phase constituents have definite effect on the corrosion behavior of DP steel embedded in concrete. As a result of this study, it is found that corrosion rate of dual-phase steel has increased with increase amount of martensite.

Correlation of the microdislocation parameters with the hardness of plastically deformed heat-resistant steels

By the methods of electron-transmission microscopy and nondestructive testing, we study basic regularities of the influence of high-temperature plastic deformation on the kinetics of hardening of 15Kh13MF and 25Kh1M1F heat-resistant steels. The difference between the intensities of exhaustion of the plasticity of steels is discovered. Thus, the process of hardening of 15Kh13MF ferritic-martensitic steel is more intense, which is explained by the increase in nonparallelism and the decrease in the distance between subboundaries of the dislocation microstructure, reduction of sizes, and fragmentation of dislocation martensite. The 25Kh1M1F ferritic-pearlitic steel is characterized by less intense hardening explained by a different type of evolution of the dislocation structure connected with the transformation of dislocation nets and cells into globular and cellular disoriented structures and then into fragmented structures. It is shown that the relationship between the characteristics of substructural hardening, the density of dislocations within the low-angle boundaries, and the hardness (microhardness) of the material in the process of plastic deformation of heat-resistant steels can be described by a linear dependence independently of the level of strains.

Kinetic Measurements of Hydrogen Distribution between Different States in Exploited Steels

The Hydrogen Analyzer AV-1 equipped with the high resolution registration system has been used to measure the kinetics of hydrogen extraction and thus to distinguish the different states of hydrogen in metals. The hydrogen distribution between the various energetic states as measured in the pipe line ferrite-pearlite steels being in service for different time has been shown to depend on conditions and on time of exploitation. Results of such measurements may be used for assessing the life time of industrial installations.

Fatigue Strength of Ultrafine Ferrite-Cementite Steels and Effects of Strengthening Mechanisms

To investigate the effects of precipitation, solid-solution, and dislocation strengthening on fatigue strength, fatigue tests were conducted on a series of ultrafine ferrite-cementite steels, the ferrite grains of which were smaller than 1 μm. To investigate the effect of precipitation strengthening by carbon addition, which increased the number of cementite particles in the ultrafine ferrite-cementite steel, several steels containing 0.15 pct (in mass pct) carbon and others containing 0.45 pct were studied. The effect of solid-solution strengthening was investigated by adding 0.1 pct phosphorus to some steels. Fatigue tests were also conducted on annealed versions of the ultrafine ferrite-cementite steels, to investigate the effect of dislocation strengthening, since the as-rolled versions were work hardened, due to warm-caliber rolling to refine the ferrite grains. The resultant tensile strength of the lab-prepared ultrafine ferrite-cementite steels ranged from 721 to 1048 MPa. All the lab-prepared ultrafine ferrite-cementite steels showed high fatigue-limit ratios (fatigue-limit/tensile strength) exceeding 0.5. The fatigue strength of the ultrafine ferrite-cementite steel was higher than that of ferrite-pearlite steel, and matched that of tempered martensite steel. In conclusion, all of these strengthening mechanisms satisfactorily improved the fatigue strength of the ultrafine ferrite-cementite steels. The ultrafine ferrite-cementite steel showed stable fatigue strength, due to its microstructure being both ultrafine and uniform.

Coupled simulation of microstructural formation and deformation behavior of ferrite–pearlite steel by phase-field method and homogenization method

A coupled simulation by the phase-field (PF) method and the finite element method based on the homogenization theory (FEH) is developed to predict the microstructure formations and mechanical properties of ferrite–pearlite steels. The formation of the α phase during the isothermal γ → α transformation is simulated by the PF method. Furthermore, the FEH analysis is performed to clarify the effects of the predicted microstructure on the deformation behavior of the steels. In order to link to the FEH analysis, the microstructure in the steel is described by the representative volume element (RVE) based on the results of the PF simulation. The results reveal that although the macroscopic stress–strain relationship is mainly characterized by the volume fraction of the constituent phase, the localization of plastic strain is reduced due to the fine-grained α phase. This numerical model provides a systematic way of predicting the mechanical properties of steel depending on the microstructure.

A material and environment-dependent criterion for the prediction of fatigue crack paths in metallic structures

Crack growth under mode II cyclic loading was investigated in maraging steel, ferritic–pearlitic steel and TA6V. When Δ K II exceeds a threshold value, cracks do not bifurcate but grow in mode II over a distance which increases with Δ K II. Shear mode crack growth was much more extensive in maraging steel than in TA6V and ferritic–pearlitic steel. This result is discussed in relation with the cyclic behaviour of the materials and the importance of friction along the crack faces. The maximum growth rate criterion is shown to be suitable for the prediction of crack paths when shear mode crack growth is likely to occur.

Genetic Algorithms in Optimization of Strength and Ductility of Low-Carbon Steels

A comparative study between the conventional goal attainment strategy and an evolutionary approach using a genetic algorithm has been conducted for the multiobjective optimization of the strength and ductility of low-carbon ferrite-pearlite steels. The optimization is based upon the composition and microstructural relations of the mechanical properties suggested earlier through regression analyses. After finding that a genetic algorithm is more suitable for such a problem, Pareto fronts have been developed which give a range of strength and ductility useful in alloy design. An effort has been made to optimize the strength ductility balance of thermomechanically-processed high-strength multiphase steels. The objective functions are developed from empirical relations using regression and neural network modeling, which have the capacity to correlate high number of compositional and process variables, and works better than the conventional regression analyses.

Competition between mesoplasticity and damage under HCF – Elasticity/damage shakedown concept

The aim of this paper is to present new modelling dedicated to multiaxial high cycle fatigue (HCF), and applied to polycrystalline metals. The model presented is based on the experimental characterization of damage during HCF tests, under pure tension and torsion modes. The origin of this approach is a mesoscopic model considering three plastic behaviour stages (hardening, saturation and softening) suggested by Papadopoulos [Papadopoulos I.V. Fatigue limit of metals under multiaxial stress conditions: the microscopic approach. Technical Note No. I.93.101, Commission of the European Communities, Joint Research Centre; 1993. [ISEI/IE 2495/93].], and used by Morel [Morel F. A critical plane approach for life prediction of high cycle fatigue under multiaxial variable loading. Int J Fatigue 2000;22:101–119.]. The principal evolution brought in by this study is a competition description during all the sample lifetime of the plasticity and damage effects. The plasticity mechanisms induce a hardening saturating effects (resulting from movement and accumulation of dislocations), especially significant at the beginning of the crystal life. Damage, present at the end of crystal lifetime, is considered as a degradation process inducing a strong reduction of the crystal ductility, leading to its failure (decohesion). The coexistence and the competition between these two effect (hardening and damage-induced softening) describe cyclic crystal behavior, including shakedown phase. The model is formulated in the framework of the continuum damage mechanics, according to the identified physical mechanisms during the tests. The second purpose is to compare the model predictions with experimental data, after identification of the parameter for a ferritic-pearlitic steel. The case of in-phase loading is merely studied here. In particular, the evolution of a few internal variables is discussed and correlated with the available physical features. It is shown that this model provides a complementary insight into a crystal with respect to the endurance criterion of Dang Van. The model predicts, in a particular stress amplitude range, the damage growth arrest.

Stress shielding and fatigue crack growth resistance in ferritic–pearlitic steel

The effect of pearlite morphology on stage IIb (Paris regime) fatigue crack growth behavior in ferritic–pearlitic steel was investigated. Networked and distributed pearlite structures were prepared. Constant-Δ K fatigue crack growth tests were performed in situ in a scanning electron microscope. The results revealed that a distributed pearlite structure had better fatigue crack growth resistance than a networked pearlite structure. From the in situ observations, the distributed pearlite structure indicated a tortuous crack path, which induced crack interlocking as well as crack closure. For the networked pearlite structure, some crack branching was found on the crack path. The crack growth curves for the two microstructures, plotted using the effective stress intensity factor range Δ K eff, where crack closure behavior is taken into consideration, did not coincide. The crack growth curves plotted using the crack tip effective stress intensity factor range Δ K eff,tip, where crack tip shielding behavior as well as crack closure are taken into consideration, successfully coincided on one line.

Elastic constants and internal friction of martensitic steel, ferritic-pearlitic steel, and α-iron

The elastic constants and internal friction of induction hardened and unhardened SAE 1050 plain-carbon steel at ambient temperatures were determined by resonant ultrasonic spectroscopy. The hardened specimen contained only martensite and the unhardened specimen was ferrite-pearlite. Using an inverse Ritz algorithm with assumed orthorhombic symmetry, all nine independent elastic-stiffness coefficients were determined, and, from the resonance peak widths, all nine components of the internal-friction tensor were determined. Similar measurements and analysis on monocrystalline α-iron were performed. The steel has slight elastic anisotropy, and the isotropically approximated elastic moduli were lower in the martensite than in ferrite-pearlite: shear modulus by 3.6%, bulk modulus by 1.2%, Young modulus by 3.2%, and Poisson ratio by 1.5%. Isotropically approximated elastic moduli of α-iron were 0.6–1.3% higher than ferrite-pearlite. All components of the internal-friction in martensite were higher than those of ferrite-pearlite, but lower than those of α-iron.

Plastic anisotropy of ferritic-pearlitic steel on the microstructural scale

Carbon steel is tested for monotonic and cyclic torsion up to high plastic strains. The analyzed steel is regarded as a composite formed by different cementite lamellae embedded in the ferrite matrix. This leads to the formation of internal stresses in the process of deformation and anisotropic hardening, especially under alternating loading. Plastic deformation leads to the initiation of lattice defects in the ordered structure. The characteristics of strain hardening are studied under the conditions of multiaxial deformation. It is shown that anisotropic hardening depends on the path and direction of deformation. These conclusions are confirmed by the formation of microstrains and the distribution of dislocations. Under loading, we observe the phenomenon of dynamic reversible recovery, which manifests itself in the formation of dislocation cells and is connected with significant kinematic hardening. The experimental data reveal the necessity of creation of new mathematical models capable of the adequate description of the phenomenon of anisotropic hardening with regard for the microstructure of the material.

Fatigue crack growth behavior in ferritic–pearlitic steels with networked and distributed pearlite structures

Effect of pearlite morphology on fatigue crack growth behavior in ferritic–pearlitic steels was investigated. Networked-like and distributed pearlite structures were prepared. Δ K decreasing/increasing and constant-Δ K fatigue crack growth tests have been performed inside an SEM chamber. The results revealed that distributed pearlite structure had better fatigue crack growth resistance than networked-like pearlite structure. Grain size and yield strength are the main factors controlling threshold value of the steels. In-situ observation revealed that the distributed pearlite structure indicated tortuous crack path. This tortuous path contributed to crack interlocking as well as crack closure. It was observed that ferritic–pearlitic steel with distributed pearlite structure inhibited the extent of plastic deformation around the crack compared to that in the networked-like pearlite structure. Plastic zone size estimation by using their cyclic stress–strain curves showed that the plastic zone size in distributed pearlite structure was smaller than that in the networked-like pearlite structure.

Evolution of Microstrains at Cyclic Incremental Deformation of Steel 42CrMo4

X-ray diffraction line profiles of a ferritic-pearlitic steel at different stages of monotonic and cyclic incremental strain paths were recorded and peak broadening was regarded. Evaluation of integral peak widths shows, that the distortion of structure is strongly dependent on the deformation path. The results for peak broadening at different deformation states correlate with flow stress behaviour and can be explained with certain dislocation arrangements.