Ferritic-pearlitic Steel Research Articles

The effect of specimen size and surface conditions on the local mechanical properties of 14MoV6 ferritic–pearlitic steel

The paper describes multiscale experimental techniques required for determining the mechanical response of a ferritic–pearlitic steel (14MoV6). Accordingly, five different sets of specimens ranging from macro to micro size scales are utilized and whose experimental results have been compared. Digital image correlation is employed to measure deformations on the surface of the test specimens under tensile loading conditions. The influence of the surface conditions on the residual stress distribution was investigated by means of X-ray synchrotron diffraction. In addition, the effects of specimen size, surface conditions and the number of grains in the cross-section of the specimens on mechanical properties are examined. It is observed that key material parameters including the yield stress, tensile strength and elongation to failure are dependent on specimen size. In addition, the results demonstrate that the number of grains in the cross-section of the specimens significantly influence the material response during uniaxial tensile testing. On the other hand, the surface treatment of the micro tensile test specimens bring about reducing differences in mechanical properties between standard and miniature specimens.

Propagation of Long Fatigue Cracks under Remote Modes II and III in Ferritic-Pearlitic Steel

Propagation of Long Fatigue Cracks under Remote Modes II and III in Ferritic-Pearlitic Steel

Effects of microstructure and crystallography on crack path and intrinsic resistance to shear-mode fatigue crack growth

ABSTRACT. The paper focuses on the effective resistance and the near-threshold growth mechanisms in the ferritic-pearlitic and the pure pearlitic steel. The influence of microstructure on the shear-mode fatigue crack growth is divided here into two factors: the crystal lattice type and the presence of different phases. Experiments were done on ferritic-pearlitic steel and pearlitic steel using three different specimens, for which the effective mode II and mode III threshold values were measured and fracture surfaces were reconstructed in three dimensions using stereophotogrammetry in scanning electron microscope. The ferritic-pearlitic and pearlitic steels showed a much different behaviour of modes II and III cracks than that of the ARMCO iron. Both the deflection angle and the mode II threshold were much higher and comparable to the austenitic steel. Mechanism of shear-mode crack behaviour in the ARMCO iron, titanium and nickel were described by the model of emission of dislocations from the crack tip under a dominant mode II loading. In other tested materials the cracks propagated under a dominance of the local mode I. In the ferritic-pearlitic and pearlitic steels, the reason for such behaviour was the presence of the secondary-phase particles (cementite lamellas), unlike in the previously austenitic steel, where the fcc structure and the low stacking fault energy were the main factors. A criterion for mode I deflection from the mode II crack-tip loading, which uses values of the effective mode I and mode II thresholds, was in agreement with fractographical observations.

Effect of electric current pulses on the microstructure and niobium carbide precipitates in a ferritic-pearlitic steel at an elevated temperature

Abstract

Application of the Master Curve to ferritic steels in notched conditions

This paper evaluates the application of the Master Curve methodology for the prediction of the apparent fracture toughness of ferritic-pearlitic steels in notched conditions. With this purpose, a new parameter is defined named the notch reference temperature (T0N), which is different from the reference temperature (T0) obtained in cracked specimens and varies with the notch radius. In order to validate this application, the fracture resistance results obtained in 240 CT specimens have been used. The tests cover two different structural steels (S275JR and S355J2) and five different notch radii, from 0.15mm up to 2.0mm. The predictions provided by the Master Curve have also been compared to those provided by the Notch-Master Curve, revealing the temperature ranges where each prediction tool provides the best results.

Multiscale modeling of failure initiation in a ferritic–pearlitic steel

A representative volume element (RVE)-based strategy for modeling the hardening and failure behavior of a ferritic–pearlitic steel at different length scales – mesoscale and microscale – is presented. At first, pearlite properties were considered to be isotropic and homogeneous. Micrographs taken on the undeformed material were transformed to a finite element mesh by using the software OOF2, a public domain FE-analysis software created at the National Institute of Standards and Technology (NIST) for the investigation of microstructures. Boundary conditions of the RVE were defined based on the macroscopic deformation history of a region of interest of an axisymmetric impact extrusion part. Crack initiation in pearlite is modeled within the extended finite element method (XFEM) framework. Pearlite cracking modeled at the mesoscale corresponds well to the observed cracks on SEM-micrographs. In a further approach, the crystallographic orientation of ferrite as well as various distributions of cementite lamellae were considered to take the inhomogeneous structure of the pearlite into account. For this purpose, in the framework of RVE computation, a spectral solver of the code DAMASK (Dusseldorf Advanced Material Simulation Kit, an open source crystal plasticity general purpose solver) was applied to model strain localizations at grain level. X-ray measurements were carried out to determine the orientation of ferrite grains and to determine the parameters of the applied crystal plasticity material model (critical resolved shear stress and slip hardening parameters). Investigations showed that the orientations of cementite lamellae have a significant influence on strain localization. The concept of coupling the FE-method to simulate the macroscopic behavior of the material and the spectral solver to achieve a high resolution of the microstructure in the framework of RVE computations leads to an efficient strategy regarding computational time and modeling of the microstructure.

Some observations on short fatigue cracks under biaxial fatigue

This work presents a new methodology for evaluating crack initiation under biaxial conditions. The methodology consists of evaluating a number of crack parameters automatically with digital processing of high-magnification images of the crack. Five different strain conditions were evaluated on a low carbon ferritic–pearlitic steel specimen with tubular shape. A hole of 150μm diameter was drilled to enforce the crack to initiate at a particular spot. Different combinations of axial and torsional strains were analysed during the initiation stage of the crack. Fatigue crack propagation curves clearly showed oscillations due to microstructure. It was also observed that these oscillations decreased as the torsional component of the strain was increased. Driving force was also evaluated in crack opening and sliding direction through crack opening displacement and crack sliding displacement. The results demonstrate that crack opening displacement is sensitive to microstructural barriers and crack sliding displacement is sensitive to crack deviations caused by the microstructure.

X-Ray Studies of Welded Joints of Ferrite-Pearlite Steels Working in Conditions of Low-Frequency Temperature and Force Loading

The article is aimed to solve one of the important scientific and technical problems of improving operational reliability and safety of critical constructions exploited in Siberia and the Far North by developing new technological approaches to welding. In the article the results of X-ray diffraction studies of welded joints of steel 09G2S, obtained by different conditions of welding. Research results prove that pulsed arc welding is more preferred than direct current welding.

Enhanced true stress–true strain relationships due to grain refinement of a low-carbon ferrite–pearlite steel

The fine-grained low-carbon ferrite–pearlite (FP) steel with the ferrite grain size of 3.6μm showed enhanced true stress (σ)–true strain (ε) relationships up to fracture, i.e., the plastic deformation limit. Both σ and ε increased with decreasing ferrite grain size in the FP steels. The effect of ferrite grain size on the σ–ε relationships up to the plastic deformation limit was dependent on the microstructure, in spite of belonging to the same low-carbon steel. In the FP steels, the smaller the pearlite size became with decreasing ferrite grain size, the more the pearlite could deform. The decrease in not only the ferrite grain size but also the secondary microstructure is effective in improving the σ–ε relationship up to the plastic deformation limit.

Tool Wear Prediction in Machining Process Using a Microstructure-Based Finite Element Model

The microstructure of materials has a significant influence on tool life, however most of the research in modelling to date considers the material as homogeneous. This research aims to develop a microstructure-based Finite Element Model in order to qualitatively analyze the influence of the scale of the microstructure on the generated tool wear. In particular, this paper is focused on the orthogonal cutting process of a ferrite-pearlite dual-phase steel using uncoated carbide tools. Based on individual mechanical properties of these phases, a 3D coupled Eulerian Lagrangian heterogeneous model was developed. An empirical tool wear rate prediction model was implemented by a user subroutine in both models (heterogeneous and homogeneous) to predict wear and wear rate values. A comparison between the microstructure-base model (heterogeneous) and the homogeneous model considering wear and wear rate values was made. The results demonstrate the validity of microstructure-based Finite Element Model for an improved prediction of the wear phenomena.

Changes in physical-mechanical properties and structure of ferritic-pearlitic steel 15Kh2NMFA caused by severe low-temperature deformation and exposure to alternating magnetic field

It is shown for the low-plasticity ferritic-pearlitic steel 15Kh2NMFA (2Cr, 1Ni, 0.5Mn, 0.5Mo), subjected to severe rolling deformation at 90 K, that the treatment by the alternating magnetic field results in a substantial decrease of the yield strength and an increase of fracture stress, total elongation, necking and dynamic shear modulus. This is accompanied by reduction of the internal friction background and coercive force. The changes in the physical-mechanical properties and the structure are related to magnetic- and electrical-nature processes giving rise to stress relaxation in the microvolumes with a high density of deformation defects.

Development of the multi-scale model of cold rolling based on physical and numerical investigation of ferritic–pearlitic steels

With the aim of developing dual phase (DP) microstructure using a continuous annealing process, four steels with different chemical compositions were investigated in the axisymmetrical plastometric tests and during the cold rolling process. The rheological model for these steels was developed using the results of the plastometric tests and application of the inverse analysis. Load–displacement curves were used to identify parameters of the rheological model, which was incorporated into the finite element code for rolling. First, the model was validated with experiments realized on the laboratory cold rolling mill. Calculated loads were compared with experimental data and good agreement was obtained. Beyond this, metallographic analysis was performed and deformation of the ferritic–pearlitic microstructure during rolling was investigated. Finally, the generated results were combined in the form of the multi-scale numerical model based on the digital material representation approach capable of investigating local microstructural inhomogeneities.

Extension of Fatigue Cracks in a Low-Alloy Steel After Massive Plastic Deformation

Abstract Fatigue crack extension was studied in a ferritic–pearlitic steel in two different material conditions. The first one was the virgin state with a typical pearlitic microstructure containing comparatively large stacks of cementite lamellae. The second material contained predominantly globular cementite and stacks of very fine cementite lamellae and was found after massive plastic deformation during a thermo-mechanical forming process. These differences in the microstructure lead to significantly different mechanisms in the damage accumulation process under fatigue loading. Stacks of cementite lamellae pearlite were both crack initiation sites and microstructural barriers for small cracks in the virgin material, whereas the fine lamellae in the plastically deformed (cold-worked) material could cause crack deviation, but no crack initiation and no significant crack arrest or crack retardation.

Residual stresses in multi-pass butt-welded ferritic-pearlitic steel pipes

This study is dedicated to the investigation of the residual stress development in multi-pass butt-welded joints of ferritic-pearlitic steel pipes. Therefore, temperature profiles are taken during welding, and the residual stresses are determined by X-ray diffraction measurements on the outer and inner surfaces of the pipes. The distortion of the specimens is measured by laser triangulation. In order to identify the influence of the tubular geometry in contrast to a plane one, plates are welded with the same parameters, and the results are compared. The residual stress distributions in both pipes and plates can be explained by phase transformations and thermal contraction. However, it is shown that the latter is the governing mechanism in the residual stress development in the pipes considered in this study, which is due to their distinct geometry.

The effect of electropulsing on the interlamellar spacing and mechanical properties of a hot-rolled 0.14% carbon steel

The application of electropulsing treatment to a low carbon ferritic–pearlitic steel is studied. The effect of electric current pulses on the interlamellar spacing of pearlite is investigated. It is found that the interlamellar spacing increases as the number of pulses increases. The mechanism of electropulse-effect was discussed by analysing the change in the free energy and the reduction of electrical resistance due to electropulsing. Mechanical properties are also examined for the samples with and without electropulsing treatment. It is shown that softening occurs during the treatment which is attributed to the formation of precipitation free zone (PFZ), increase in the value of interlamellar spacing and the spheroidization of the lamellar structure.

フェライト・パーライト鋼のへき開破壊靱性予測モデルの構築

A numerical model to quantitatively predict cleavage fracture initiation in ferrite-pearlite steel is proposed. The model is based on microscopic fracture process of three stages; Stage-I: formation of fracture origin in pearlite colony, Stage-II: propagation of the pearlite crack into ferrite matrix, and Stage-III: propagation of the crack across ferrite grain boundary. In the proposed model, fracture conditions are formulated by the probability of pearlite cracking based on the experimental results on Stage-I and the concept of fracture stress of ferrite matrix on Stage-II and Stage-III. Ferrite grains and cementite particles are assigned based on their distributions into each volume element. Applied plastic strain and stress of each volume element are calculated by finite element analysis. Cleavage fracture is assumed to initiate at the time when the fracture conditions of the all stages are satisfied in any one of the volume elements. Cleavage fracture toughness of three point bend test is simulated by the proposed model. The numerical predicted results of fracture toughness show good agreement with experimental ones. The bottleneck process of cleavage fracture is then evaluated by the number of arrested micro-cracks until all of the fracture process is satisfied. Influence of ferrite and pearlite size on cleavage fracture toughness is evaluated. It is shown that steel with finer pearlite colony is tougher, and then the developed model can reproduce the size effect of cleavage fracture toughness. Based on the aforementioned results, the validation and the effectiveness of the proposed model are found out.

訂正：フェライト－パーライト複合組織鋼の組織形態と力学特性に関する実験と数値解析の比較研究［鉄と鋼 Vol.98（2012），No.6，pp.290-295］

訂正：フェライト－パーライト複合組織鋼の組織形態と力学特性に関する実験と数値解析の比較研究［鉄と鋼 Vol.98（2012），No.6，pp.290-295］

Increasing the properties of structural ferrite-pearlite steels

We have studied the ways of increasing the durability of low-alloy structural ferrite-pearlite steels and researched their possible effective use in a grade composition. The statistic processing of an array of industrial smelts has proved possible increasing of the durability owing to both solid-solution strengthening of ferrite and, to a greater extent, dispersive and grain-boundary strengthening. The suggested way of increasing the durability of low-alloy steels is based on the mechanism of carbonitride hardening through a complex steel microalloying with nitrogen alongside nitride-forming titanium and aluminum, instead of vanadium. Owing to nanodispersive carbonitride phases, such microalloying provides a fine grain microstructure of rolling and heat-treated cast and secures a high level of durability. The research has proved that the forming of nanodispersive carbonitride phases requires optimal correlation of nitrogen (0.012-0.015 percent by mass for cast steels and 0.014-0.020 percent by mass for heat-deformed steels) and the suggested microalloying elements (titanium—0.015-0.025 percent by mass and aluminum—0.015-0.025 percent by mass).

Wear Behavior of Newly Developed Bainitic Wheel Steels

The present work concentrates on the analysis of wear behavior of bainitic steels made by austempering from a microalloyed steel MAS2, meant for making railway wheel, and comparison with that of a conventional railway wheel steel, wheel-R19. Austempering of the MAS2 steel samples has been performed at different times and temperatures to obtain different morphologies of bainite. Linearly reciprocating dry sliding wear tests of these samples have been carried out at laboratory scale using five different loads. The wear behavior of the bainitic steels has been compared with that of the ferritic-pearlitic steel, wheel-R19. Mechanical properties of the bainitic MAS2 steels are found to be more than that of the wheel-R19 steel. Considerable enhancement in wear resistance of the bainitic steels is attributed to high hardness and strength of the steels. The wear mechanism has been critically analyzed by examining wear track morphology. The wear data gathered have been graphically presented in the form of wear mechanism map to understand the material behavior under different sliding conditions and subsequent morphological variations.

The Influence of the Ferrite and Pearlite Grain Size on the S-N Fatigue Characteristics of Steel

In this work an investigation of internal structure influence on mechanical and fatigue properties of ferritic-pearlitic steels is shown. Ferrite grain size and phase volume fraction of three grades of structural steel with similar chemical composition, but different mechanical properties, were examined. Afterwards, samples of the materials were subjected to cyclic bending tests. The results and conclusions are presented in this paper