Multiscale prediction of mechanical behavior of ferrite–pearlite steel with numerical material testing

Multiscale mechanical behaviors of ferrite–pearlite steel were predicted using numerical material testing (NMT) based on the finite element method. The microstructure of ferrite–pearlite steel is regarded as a two‐component aggregate of ferrite crystal grains and pearlite colonies. In NMT, the macroscopic stress–strain curve and the deformation state of the microstructure were examined by means of a two‐scale finite element analysis method based on the framework of the mathematical homogenization theory. The microstructure of ferrite–pearlite steel was modeled with finite elements, and constitutive models for ferrite crystal grains and pearlite colonies were prepared to describe their anisotropic mechanical behavior at the microscale level. While the anisotropic linear elasticity and the single crystal plasticity based on representative characteristic length have been employed for the ferrite crystal grains, the constitutive model of a pearlite colony was newly developed in this study. For that reason, the constitutive behavior of the pearlite colony was investigated using NMT on a smaller scale than the scale of the ferrite–pearlite microstructure, with the microstructure of the pearlite colony modeled as a lamellar structure of ferrite and cementite phases with finite elements. On the basis of the numerical results, the anisotropic constitutive model of the pearlite colony was formulated based on the normal vector of the lamella. The components of the anisotropic elasticity were estimated with NMT based on the finite element method, where the elasticity of the cementite phase was numerically evaluated with a first‐principles calculation. Also, an anisotropic plastic constitutive model for the pearlite colony was formulated with two‐surface plasticity consisting of yield functions for the interlamellar shear mode and yielding of the overall lamellar structure. After addressing the microscopic modeling of ferrite–pearlite steel, NMT was performed with the finite element models of the ferrite–pearlite microstructure and with the microscopic constitutive models for each of the components. Finally, the results were compared with the corresponding experimental results on both the macroscopic response and the microscopic deformation state to ascertain the validity of the numerical modeling. Copyright © 2011 John Wiley & Sons, Ltd.     

Microstructure based flow stress modeling for quenched and tempered low alloy steel

Quenching and tempering (Q&T) process is commonly applied in part making industries for improving mechanical properties of carbon low alloy steels. After Q&T, microstructure of the steel consists of temper martensite and carbide precipitations. In this work, material modeling for describing flow stress behavior of the SNCM439 alloy steel under different tempering conditions was introduced. Microstructure based models were developed on both macro- and micro-scale. The models were afterwards applied in FE simulations for predicting stress–strain responses of the tempered steels. For the macroscopic model, the Ludwik equation was used, in which precipitation strengthening depending on particle size was incorporated by the Ashby–Orowan relationship. For the microscopic model, representative volume elements (RVEs) were generated considering microstructure characteristics of the examined steels. Flow curves of the individual constituents were described based on dislocation theory and chemical compositions. The FE simulations of tensile tests and RVE simulations under uniaxial tension were performed using the introduced models. The influences of the carbide precipitations on mechanical behavior of the tempered steels were investigated. The resulted effective stress–strain curves were determined and compared with the experimental ones. Both macroscopic and microscopic approaches accurately predicted mechanical properties and strain hardening behaviors of the tempered steels.     

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

Abstract

A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrite. The volume fraction of ferrite and its contiguity ratio in the rolled microstructure were greater than in the forged grade. The rolled microstructure exhibited a better combination of strength and toughness than that of the forged material. The rolled steel work hardened more than the forged variety owing to its fine, uniform (bainite–martensite and ferrite) microstructure. Retained austenite present in these steels underwent a strain induced transformation to martensite during tensile deformation. The J–C analysis of the work hardening rates revealed typical three stage behaviour in both varieties during tensile deformation.     

Microstructure and mechanical properties of UFG medium carbon steel processed by HPT at increased temperature

Using the high pressure torsion (HPT) deformation method the medium carbon steel (AISI 1045) was the experimental material used to conduct the deformation process. The torsion deformation experiment was performed at increased temperature of 400 °C. The influence of deformation processing parameters, resolved shear strain γ (number of turns N = 1–6) and applied pressure p (constant pressure of 7 GPa), was evaluated by microstructure analysis and mechanical properties. The strength behaviour was assessed by microhardness measurements across the disc to detect the positional hardening, by tensile tests and in situ measured torque. In situ measurement of torque during deformation allows characterizing the changes in mechanical properties due to the large shear deformation developed across the disc. To obtain absolute values of strength the ultimate tensile strength was measured in radial direction with respect to the deformed sample. From each deformed disc two sub-sized tensile test specimens with gauge length of 2.5 mm were machined. The tensile strength in samples increased markedly with the number of turns. The hardness measured at disc edge gradually increases as straining increases until it saturates after 2–3 turns. However, the hardness values at edge were different from those measured in disc centre and for applied straining no saturation was reached across the disc. The SEM and TEM investigations were carried out to analyze the fine microstructure evolution regarding the strain introduced. To follow the difference in strain distribution across the deformed disc the microstructure analysis was performed at edge and central site of the disc in order to evaluate the effect of the strain distribution. TEM investigation confirmed the increasing misorientation even in very small grains, the fragmentation and dissolution of the cementite lamellae, (diffuse cementite/ferrite boundaries), the alignment of the fragments to the shear plane with increasing deformation. Indistinct deformation of ferrite and preserved cementite lamellae morphology were found at the centre of the disc.     

Behaviour of low and medium carbon free cutting steels during deformation to large strains

Abstract

Low and medium carbon free cutting steels were deformed by cold rolling to reductions of up to 98%. The resultant microstructures were observed and characterised using optical microscopy, SEM, and TEM. Deformation of the pearlite grains and manganese sulphide inclusions was quantified in terms of their relative plasticity (compared to that of the steel). The evolution of the ferrite microstructure in the steels was seen to be dependent on the volume fraction of pearlite present. The ferrite grains in the low carbon steels underwent structural subdivision characterised by the formation of dense dislocation walls and microbands. At intermediate rolling deformations much of the strain was accommodated inhomogeneously in narrow bands of shear (S bands). Strain in the pro-eutectoid ferrite of the medium carbon steel occurred in a more homogeneous manner owing to the constraints imposed by the pearlite. The manganese sulphides and pearlite in the steels also acted as fiducial markers of the surrounding ferrite flow. Plasticity of the sulphides was generally found to be less than the overall rolling strain. However, within certain narrow strain ranges, sulphide plasticities greater than that of the steel were measured.     

Deformation characteristics of low carbon steel subjected to dynamic impact loading

The effects of impact loading on changes in microstructure have been studied in low carbon steel. Low to moderate shock loading tests have been carried out on steel specimens using a single stage gas gun with projectile velocities ranging from 200 to 500 m/s. Stress history at the back face of the target specimen and projectile velocity prior to impact were recorded via manganin stress gauges and velocity lasers, respectively. A Johnson-Cook constitutive material model was employed to numerically simulate the material behavior of low carbon steel during impact and obtain the particle velocity at the impact surface as well the pressure distribution across the specimens as a function of impact duration. An analytical approach was used to determine the twin volume fraction as a function of blast loading. The amount of twinning within the α-ferrite phase was measured in post-impact specimens. A comparison between experimental and numerical stress histories, and analytical and experimental twin volume fraction were used to optimize the material and deformation models and establish a correlation between impact pressure and deformation response of the steel under examination. Strain rate controlled tensile tests were carried out on post-impact specimens. Results of these tests are discussed in relation to the effects of impact loading on the yield and ultimate tensile strength as well as the hardening and strain energy characteristics.     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Microstructure in adiabatic shear bands in a pearlitic ultrahigh carbon steel

Abstract

Adiabatic shear bands, obtained in compression deformation at a strain rate of 4000 s^?1, in a pearlitic 1·3%C steel, were investigated. Shear bands initiated at 55% compression deformation with the width of the band equal to 14 μm. Nano-indentor hardness of the shear band was 11·5 GPa in contrast to the initial matrix hardness of 3·5 GPa. The high strength of the shear band is attributed to its creation from two sequential events. First, large strain deformation, at a high strain rate, accompanied by adiabatic heating, led to phase transformation to austenite. Second, retransformation upon rapid cooling occurred by a divorced eutectoid transformation (DET). The result is a predicted microstructure consisting of nano size carbide particles within a matrix of fine ferrite grains. It is proposed that the DET occurs in iron–carbon steels during high rate deformation in ball milling, ball drop tests and in commercial wire drawing.     

Effects of carbon and niobium on microstructure and properties for Ti bearing steels

The objective of the study described here is to elucidate the effect of carbon and niobium on the microstructure, precipitation behaviour, and mechanical properties of 0·09C–0·11Ti (%) steel and 0·05C–0·025Nb–0·11Ti (%) steel under ultra fast cooling condition. The strengthening mechanisms are also discussed. The ferrite grains size and the size of precipitates in Ti and Nb–Ti steels were measured respectively. The mechanical properties obtained in Ti steel were similar to Nb–Ti steel with yield stress >700 MPa, elongation >20%, and good low temperature impact toughness. The study underscores that addition of higher carbon content by 0·04% under controlled rolling and ultra fast cooling conditions, we can achieve similar strength in the absence of micro-alloying element, niobium.     

The empirical estimation of tensile strength of dual‐phase steels depending on carbon content and austenitizing temperature

Surface and thin film analysis – indispensable tool for coating development and ‐production A low‐carbon (0.097 wt%) steel was annealed at 820°C for 60 min. then quenched in water to develop the desired dual phase steel (DPS) structure of martensite and ferrite. The intercritically annealed steel exhibited a microstructure of equiaxed ferrite‐martensite. Continuous yielding was observed at the stress‐strain curves. New equation was developed to predict the empirical tensile strengths of DPS. Empirical results were compared with experimental data in this study and the literature. A difference changing between ± 10% was observed for experimental and empirical results.     

Prediction of ferrite grain size and tensile properties of a low carbon steel

Abstract

A relationship between ferrite grain size, cooling rate from austenitising temperature, austenitising time, and austenitising temperature is developed to predict the ferrite grain size of a low carbon steel. The coefficients of that relationship are determined experimentally. A Hall - Petch relationship is used to predict the yield stress and fracture stress from the predicted ferrite grain size. Considering the experimental results, maximum errors of 12.5% and 6.5% were found in the prediction of ferrite grain size and strengths, respectively.     

Fatigue damage evaluation of a power plant component from analysis of the dislocation structures

The paper presents a method for estimating the degree of fatigue damage in an engineering ferritic–bainitic steel. A dislocation microstructure chart is first established from laboratory fatigue tests. Misorientation measurements are carried out after fatigue failure between dislocation cells produced by cyclic loading in ferrite grains and between dislocation cells in bainite originated by the heat treatment. There is shown to be a clear dependence of these values on the strain amplitude. On this basis, the analysis of the microstructure of a steel taken from a component which failed in service allowed the determination of the strain amplitude value to which the component had been subjected.     

Study on silicon carbide nanowires produced from carbon blacks and structure of carbon blacks

Silicon carbide nanowires were produced from carbon blacks at 1473 K and their microstructure was characterized by TEM, X-ray diffraction, FTIR and Raman spectroscopy. Nanowires of uniform diameters, the smallest averaging 10 nm, and narrow size distribution were obtained from graphitized carbon blacks, and their morphology depends on the properties of carbon black pecursors. High concentration of stacking faults and twins was detected. In addition to silicon carbide nanowires, a silicon carbide layer, about 20 nm thick, was formed on the surface of carbon black aggregates. The interior of the aggregates did not react and analysis of the data showed that it is composed of a mixture of amorphous carbon and small graphitic crystallites. The small lateral sizes of these crystallites remain unchanged during the graphitization process which is limited to the outer layer of the aggregates.     

Microstructure of X52 and X65 pipeline steels

The microstructure of two commercial pipeline steels X52 and X65 was examined to provide a foundation for the understanding of the IGSCC mechanism of pipeline steels. Observation of the microstructure was carried out using scanning electron microscopy (SEM) and an analytical electron microscope. The microstructure of X52 and X65 pipeline steels shows banding of pearlite rich and ferrite rich areas. The ferrite grains were about 10 μm in size with curved grain boundaries. There was carbide at the ferrite grain boundaries for X52 steel, and there was circumstantial evidence to suggest carbon segregation at the boundaries. The pearlite colonies were consistent with nucleation by a number of different mechanisms. This revised version was published online in September 2006 with corrections to the Cover Date.     

Fatigue life studies in carbon dual-phase steels

An investigation has been conducted to examine the morphological influence on fatigue life of low carbon steel with dual phase microstructure. The results showed that dual-phase microstructure, composed by ferrite and martensite had superior symmetrical bending fatigue strength when compared with ferrite-pearlite steel. Through those tests, evidences of different mechanisms were verified (such as ferrite cyclic hardening, slip band formation and beginning of crack nucleation and propagation). Based on the fatigue tests results, various mechanisms stages were discussed associated with different microstructure morphology.     

Microstructural control by secondary processing of strip cast low carbon steel

Abstract

The secondary processing of low carbon steel strip produced by twin roll casting was investigated to examine its effect on microstructural development and mechanical properties. The as cast microstructure is predominantly acicular ferrite with regions of bainitepearlite and polygonal ferrite. Deformation at temperatures below Ar1 produces a heterogeneous microstructure with regions of moderately deformed acicular ferrite adjacent to highly deformed regions containing shear bands. Cold rolled and warm rolled steels show similar behaviour to conventional hot band in that dynamic recovery during warm rolling results in sluggish recrystallisation and produces a coarse final grain size. However, the initial as cast microstructure recrystallises at a slower rate than conventional hot band and produces a weaker recrystallisation texture. This can be attributed to the heterogeneous microstructure of the as cast strip such that, after rolling, nucleation occurs within shear bands and more ill defined sites, which results in nucleation of randomly oriented grains thereby producing a weak final texture. It was found that austenitising the as cast strip followed by rolling in the vicinity of Ar3 produces a uniform distribution of equiaxed, ultrafine ferrite UFF grains throughout the thickness of the strip. The production of UFF by twin roll casting and subsequent rolling represents a simple processing route for the production of fine grained low carbon sheet steel products.     

On cavitation and macroscopic behaviour of amorphous polymer-rubber blends

Abstract

The macroscopic behaviour of rubber-modified polymethyl methacrylate (PMMA) was investigated by taking into account the microdeformation mechanisms of rubber cavitation. The dependence of the macroscopic stress–strain behaviour of matrix deformation on the cavitation of rubber particles was discussed. A phenomenological elastic-viscoplastic model was used to model the behaviour of the matrix material, while the rubber particles were modelled with the hyperelasticity theory. A two-phase composite material with a periodic arrangement of reinforcing particles of a circular unit cell section was considered. Finite-element analysis was used to determine the local stresses and strains in the two-phase composite. In order to describe the cavitation of the rubber particles, a criterion of void nucleation is implemented in the finite-element (FE) code. A comparison of the numerically predicted response with experimental result indicates that the numerical homogenisation analysis gives satisfactory prediction results.     

Low temperature synthesis of high quality carbon nanospheres through the chemical reactions between calcium carbide and oxalic acid

Carbon nanospheres (CNSs) were synthesized through the chemical reactions of calcium carbide and oxalic acid without using catalysts. The chemical reactions were carried out in a sealed stainless steel pressure vessel with various molar ratios at temperatures of 65–250 °C. The synthesized CNSs have been characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) attached to the SEM, transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The total yield of carbonaceous materials relative to the starting material is about 4% (w/w). SEM and TEM results reveal that the percentage of CNSs is high (>95%). The CNSs that have been synthesized are roe-like spheres of relatively uniform size with diameters of 60–120 nm. The attached EDS result shows that the carbon content of CNSs reaches up to 98%.     

A study of local deformation and damage of dual phase steel

Deformation and fracture behavior of Dual Phase (DP) high strength steel were investigated by means of a microstructure based Finite Element (FE) modeling. Representative Volume Elements (RVEs) were applied to consider effects of various microstructure constituents and characteristics. Individual stress–strain curves were provided for ferrite, martensite as well as transformation induced Geometrically Necessary Dislocations (GNDs) taking into account in the RVEs. Principally, the GNDs occurred around phase boundaries during quenching process due to the austenite–martensite transformation. Flow behaviors of individual phases were defined on the basis of dislocation theory and partitioning of local chemical composition. Then, flow curves of the examined DP steel were predicted. Furthermore, the Gurson–Tvergaard–Needleman (GTN) model was used to represent ductile damage evolution in the microstructure. Occurrences of void initiation were characterized and damage parameters for RVE simulations were hence identified. Finally, influences of the GNDs, local stress and strain distributions and interactions between phases on predicted crack initiation in the DP microstructure were discussed and correlated with experimental results.     

"针状"先共析铁素体形成及对钢丝性能的影响

运用膨胀法及显微组织观察分析了35#钢连续加热和冷却过程中相变特点,讨论了经正火处理的中碳钢组织中出现一定量的"针状"先共析铁素体形成原因.通过对正火处理的35#钢丝室温拉伸变形行为测试与分析,发现其呈现两个阶段屈服变形行为,两次屈服分别对应于先共析铁素体与珠光体塑性屈服.