Pearlite Transformation Research Articles

Microstructure and Mechanical Property of Copper Bearing Eutectoid Steel

Isothermal heat treatment was performed for austenitized Cu bearing eutectoid steels (Fe–0.8%C–Cu alloys). Microstructural observations revealed that the pearlite transformation is significantly retarded by the addition of Cu, but the morphology of pearlite, such as lamellar spacing and block size, are hardly influenced by the Cu addition. From the result of chemical analysis for cementite phase, it was found that the redistribution of Cu had occurred between ferrite and cementite through the interfacial diffusion during pearlite transformation and then the precipitation of Cu takes place simultaneously within ferrite phase. Furthermore, in the 2% Cu steel, age hardening behavior was found at the same temperature applied to the isothermal pearlite transformation. This means that the precipitation of Cu does not complete on pearlite transformation and the ferrite in pearlite structure is in the supersaturated state. Yield strength and tensile strength of the pearlite steels depend on not only lamellar spacing but also the dispersion of Cu particles within the ferrite phase. For example, 2% Cu addition increases yield strength and tensile strength by about 250 MPa without reducing ductility so much.

Identification of Transformation Plasticity Coefficient for Some Steels during Pearlite Transformation and the Dependence on Chemical Composition

A newly developed testing system available to control temperature and the rate as well as applied stress or strain with high quality of extensometer is used to measure the transformation plastic strain occurring in the course of diffusion type transformation (including pearlite reaction). Specimens of six kinds of steels widely used for quenching operation are heated to austenite temperature of 900°C and kept at the steady temperature followed by cooling to lower temperature below transformation finish point subjected to several applied loads. Since the difference in the strain between zero and nonzero stress corresponds to the transformation plastic strain, the value of transformation plasticity coefficient is identified by the ratio of the strain to applied stress. Dependence of the coefficient on the chemical composition is discussed by defining an equivalent carbon content and compared with the data for plastic hardening coefficient.

On Phenomenological Mechanism of Transformation Plasticity and Inelastic Behavior of a Steel Subjected to Varying Temperature and Stress –Application of Unified Transformation-Thermoplasticity Theory

Based on the well known fact for the transformation plasticity that the plastic deformation takes placee in mother phase due to the progressing new phase when applying small stress during transformation, an simple phenomenological mechanism on pearlite and martensite transformation from austenite is discussed by considering the difference in thermal expansion coefficients : Numerical analysis of both phases connected each other shows that stress in mother phase increases to the yield level under small applied stress even lower than the yield stress, which gives the background of theory of unified transformation-thermoplastic constitutive law proposed by the present author. As an application of the theory, thermoplastic behavior for a fire resistant steel is simulated for complicated variation of stress and temperature, which shows fairly good agreement with experimental data. This may motivate to provide key knowledge for the decrease in strength of steel structure due to fire sometimes occurring after earthquake.

Transformation plasticity at different phase transformation of a through hardening bearing steel

A simulation of the heat treatment processes requires material data for the description of phase transformation. Those data refer not only to the transformation without external loads, since during the heat treatment transformation and thermally caused stresses develop, which affect the behaviour of the phase transformation. Interaction between phase transformation and external stress is the subject of these investigations. The investigations show the influence of constant tension and compression stresses on the transformation kinetics during the isothermal pearlitic and bainitic transformation and also the martensitic transformation of the bearing steel 100Cr6 (SAE 52100). This paper shows the transformation plasticity parameter K for different phases. Also, we introduce some models to describe the evolution of the transformation plasticity.

Grain Refinement in Steels and the Application Trials in China

In order to control microstructure and performances of steel products, the investigation has been taken to the ways to refine grains in steels and the mechanisms, the availability in industrial production of ultrafine grain steel products, and the applications of ultrafine grain steel products. It is shown that strength of steels can be increased without detriment to ductility when ferrite grains are refined to less than 4 μm in low carbon steels. Based on the study of deformation induced transformation, a new rolling technology has been developed to refine ferrite grains in plain low carbon steel rebar and strip. Plain low carbon steel rebars of 400 MPa grade, with ferrite grains in the range of 4-8 μm, have been developed as the candidate to replace microalloyed steel rebars. High strength plain low carbon steel strips in the yield strength range from 345 to 420 MPa have been used for components conventionally made of microalloyed steels. Ultrafine grained microalloyed steel strip has also been developed. A new on-line cementite spheroidization softening technology has been studied for the wire rod production of medium carbon bolt steel. It is obvious that deformation could also induce pearlite transformation in eutectoid steel.

Phase Transformation from Fine-grained Austenite

Microstructure formed by diffusional or martensitic transformation from fine-grained austenite of which grain size is smaller than 5 μm was studied. Grain refinement of austenite was established through two kinds of reversion processes; (1) cyclic transformation between martensite and austenite and (2) reverse transformation from tempered and cold-rolled lath martensite (or pearlite). In the process of (1), the fine austenite structures whose grain sizes of 5-10 μm are obtained. Refinement of austenite grain size results in the increase of hardness. In the process of (2), austenite grain size can be refined down to about 2 μm in low-carbon Mn steels by microalloying through pinning of austenite grain growth by alloy carbides. The ferrite grain size after continuous cooling transformation becomes finer as austenite grain size is refined. However, the grain size ratio of austenite and ferrite, d α /d γ , increases by refining austenite grain size. For the austenite of grain size smaller than 5 μm, the ferrite grain size becomes coarser than that of austenite for slow cooling. A similar trend in the change of ferrite grain size by refinement of austenite was recognized for isothermal pearlite transformation in eutectoid alloys. Thus, it is suggested that extensive accelerated cooling is important to obtain fine-grained ferrite by diffusional transformations from the fine-grained austenite. Packet and block sizes of lath martensite in low carbon steels are also refined by decreasing the austenite grain size. Several packets and blocks are formed even from the austenite matrix of 2 μm in grain size.

New insights into crystallographic correlations between ferrite and cementite in lamellar eutectoid structures, obtained by SEM–FEG/EBSD and an indirect two-trace method

Four different ferrite/cementite orientation relationships (ORs) in near-eutectoid steel are derived using SEM–FEG/EBSD (scanning electron microscopy–field emission gun/electron back-scatter diffraction) and an indirect two-trace method. They show a common feature of close-packed plane parallelism between ferrite and cementite. Their crystallographic compatibility with habit planes shows a variety of possible habit planes and excludes the existence of the exact conventional Bagaryatsky and Pitsch–Petch ORs. Each of these new ferrite/cementite ORs is correlated with a different edge-to-edge matching condition between austenite and pearlitic ferrite, and between austenite and pearlitic cementite, and possesses specific morphological features. The present results may give deep insight into the crystallography of pearlitic transformation and provide useful information for materials design through interface tailoring in steels.

Continuous Cooling Transformation Behaviour of C-Si-Mn TRIP Steel

Abstract Thermomechanical simulations were performed by a Gleeble 1500 thermomechanical simulator in C-Si-Mn TRIP steel. The continuous cooling transformation (CCT) behaviors of this steel were investigated in the undeformed and deformed conditions, respectively, and the corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed. The microstructures were observed by scanning electron microscopy (SEM). The microstructural evolution obtained from the thermal simulation specimens revealed that the fastest cooling rate produced fully martensitic microstructure. As the cooling rate decreased, granular bainite, polygonal ferrite and pearlite were formed. A rapid cooling rate depressed the formation of ferrite and pearlite, which resulted in higher hardness. Alloying elements of Si and Mn made the pearlite and bainite curves of CCT diagrams move to the right. Hot deformation shifted the ferrite, pearlite and bainite transformation left to high temperature region in dynamic CCT diagram, because deformation increased nucleation sites and energy and accelerated the transformation. Polygonal ferrite, granular bainite and retained austenite could be obtained at cooling rates of 3.3–16.7°C/s in the deformed condition, and this might contribute to a satisfactory TRIP effect due to the presence of retained austenite in the microstructure.

A Mathematical Model to Predict Microstructure of Heat-Treated Steel

The dual effects of chemical composition and cooling rate on microstructural evolution were theoretically simulated extending the Johnson-Mehl equation for non-isothermal pro-eutectoid ferrite transformation kinetics incorporating the effect of supercooling (ΔT) as well as for isothermal pearlite transformation kinetics assuming the exponent on time ‘n’ to vary inversely with time as, n = p/tm, for case carburized steel quenched in oil. Carbon concentration profile of steel carburized at 930 °C for 10 h was theoretically computed by solving the Fick’s diffusion equation. The cooling curves from surface to core were generated for typical 20.32 and 15.6 mm diameter steels using FEM package (ANSYS) for oil quenching, water quenching, and air cooling. The effect of varying carbon concentration from surface to core was incorporated in the heat transfer equations while generating cooling curves, at different case depths (surface, 0.31, 0.558, 1.239, and 3.469 mm). The cooling curves for oil quenching were superimposed on the published TTT diagrams of the steels of corresponding carbon content and using the empirical equations the evolution of different microconstituents, e.g., ferrite, pearlite, and martensite from the parent austenite phase were computed for the carburized 20.32 and 15.6 mm diameter steel samples. These steel samples were also case carburized experimentally at 930 °C for 10 h followed by oil quenching. The theoretically predicted case depth of 3.469 mm matched closely with the experimentally observed value. Microstructural studies were done on inverted microscope and quantitative image analyzer at different case depths/nodal points. Microhardnesses were also measured at case depths from 0.1 mm to center of the samples at selected areas to identify the different phases. The experimentally observed microstructures matched well with the theoretically predicted evolution of microconstituents.

Phase Transformation Modeling of Medium-Carbon Forging Steel

The kinetics of phase transformations in medium-carbon forging steels (MCFS) have been modeled based on CALPHAD multicomponent thermodynamics and the classical nucleation-growth theory. New treatments include the time dependency of parabolic growth rate of proeutectoid ferrite (α) , which account for the soft impingement effect by carbon enrichment in austenite (γ). And a potential transition of γ/α interface equilibrium has also been considered depending on temperatures and velocity of the moving interface. To make a realistic prediction of the onset of pearlite (P) transformation, a normal distribution of γ grain size has been assumed and successive α→P transformation kinetics in each grain size have been summated. The developed program coupled with thermodynamic solver, 'ThermoCalc', calculated the isothermal kinetics of MCFS and has been found to predict well the effect of minor difference of chemical composition / holding temperatures.

Substructure and Crystallography of Degenerate Pearlite in an Fe-C Binary Alloy

Microstructures formed by degenerate pearlite transformation in an Fe-0.38mass%C alloy were studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Degenerate pearlite which contains fine cementite particles even at the growth front was observed with other structures such as proeutectoid ferrite, lamellar pearlite and bainite in a temperature range between 773K and 923K. As the isothermal transformation temperature is lowered, a fraction of the degenerate pearlite increases. The degenerate pearlite consists of ‘block’ (a region in which ferrite orientations are nearly the same) and ‘colony’ (a region containing cementite particles of nearly the same orientation), both of which are similar to those in lamellar pearlite. Block boundaries within an austenite grain are generally of high-angle type and their misorientations deviate largely from intervariant relationships for the K-S orientation relationship. In contrast, colony boundaries are of low-angle type. Cementite films are formed along those ferrite boundaries in the degenerate pearlite, presumably formed by encounter of the blocks or colonies.

Phase transformation behaviour in two C–Mn–Si based steels under different cooling rates

The continuous cooling transformation (CCT) behaviour of two C–Mn–Si based steels was investigated. The effects of chemical composition and cooling rate on γ→α transformation were studied using dilatometric measurements. Quantitative phase analysis was carried out in order to determine the effect of cooling rate on the precise phase distribution after transformation. Presence of Cr and Mo in the experimental steels appears to retard pearlitic transformation and promotes formation of acicular products (combination of acicular ferrite, Widmanstatten ferrite and bainite). Martensitic transformation also starts at a perceptibly lower cooling rate in the Mo containing alloy as compared with the one without any Mo.

大型品対応新熱間ダイス鋼ＤＨ３１‐ＳＳ１の開発

The strongly commercial interests have been focused on hot working die steels with improved hardenability to attain higher impact value and ensure adequate dies life even in the larger mass blocks. Hardenability, avoiding pearlite transformation and attain a finer bainitic microstructure, is quite important on quenching of large die blocks. In addition strength of die is essential to improve sensitivity for ware at elevated temperature. In this paper, several properties such as hardnenability, inpact value, fracture toughness and high temperature strength of newly developed hot working die steel 'DH31-SS1' will be represented. The given results by comparing with conventional hot working die steel DHA1-ES (corresponding to AISI H13) are as follows. (1) Retarded quenched material has finer bainitic microstructure and superior impact value of 30 J/cm2. (2) Stress of resistance to deform at 973 K after short time holding is higher from 100 MPa upto 150 MPa. (3) Fracture toughness on high hardness is same but on low hardness, inferior.

Conversional model of transformation strain to phase fraction in low alloy steels

Computational models of austenite decomposition are widely used to predict external shape and internal microstructural changes as functions of time and temperature. One modeling approach involves converting the measured transformation strain to phase fractions using the lattice parameters of parent and product phases, because the transformation strain is caused by crystal structural change and an enrichment/depletion of the solute atoms in solid solution. A conversional model of transformation strains occurring during austenite decomposition to phase fractions was developed using an optimal set of lattice parameters and thermal expansion coefficients of each phase associated with low alloy steels. The model is composed of four different transformation strain formulae for ferrite, pearlite, bainite and martensite transformations, respectively, and can be applied to continuous cooling transformations as well as isothermal ones. The conversional model demonstrated a good match between calculated and measured phase fractions of industrial low alloy steels cooled at the different rates after austenitization.

Reverse transformation of ferrite and pearlite to austenite in an ultrafine-grained low-carbon steel fabricated by severe plastic deformation

Reverse transformation characteristics of a low-carbon steel consisting of ultrafine-grained (UFG) ferrite and severely deformed pearlite by severe plastic deformation were investigated and compared to those of the steel having coarse-grained (CG) ferrite and undeformed pearlite by austenitization and subsequent air cooling. Coarse-grained steel exhibited two serial transformation stages, i.e., pear-lite → austenite followed by ferrite → austenite. Contrarily, UFG steel transformed with the three serial stages, i.e., probably carbon-supersaturated ferrite → austenite, not-fully-dissolved pearlite → austenite, and ferrite → austenite transformations.

Validation of a TP model with backstress for the pearlitic transformation of the steel 100Cr6 under step-wise loads

Transformation plasticity (TP) may cause distortion of steel work-pieces during production processes like heat treatment, for example. Recent results show, that there are effects in the material behaviour which cannot be described by models widely used today. For instance, a varying load during the transformation may lead to a backstress effect which is not predicted by the well-known Franitza–Mitter–Leblond TP model. The extended TP model presented here consists of a back stress due to TP itself, i.e., generated by the TP strain. The authors continue their investigations (e.g., at the IWCMM14 in Goa [M. Wolff, M. Böhm, M. Dalgic, G. Löwisch, N. Lysenko, J. Rath, Comput. Mater. Sci. 37 (1–2) (2006) 37–41], at the IDE2005 in Bremen [M. Wolff, M. Böhm, M. Dalgic, G. Löwisch, J. Rath, in: H.-W. Zoch, T. Lübben (Eds.), Proceedings of the 1st International Conference on Distortion Engineering, September 2005, Verlag Universität Bremen, Bremen, Germany, 2005, pp. 14–16]) of the pearlitic transformation of the steel 100Cr6 under step-wise loads and validate some proposals for TP.

The role of carbon diffusion in ferrite on the kinetics of cooperative growth of pearlite: A multi-phase field study

Cooperative growth of pearlite is simulated for eutectoid steel using the multi-phase field method. The model considers diffusion of carbon not only in γ phase, but also in α phase. The lamellar spacing and growth velocity are estimated for different undercoolings and compared with experimental results from the literature and theoretical results from analytical models. The important finding of this work is that carbon diffusion in ferrite and growth of cementite from the ferrite increase the kinetics of the pearlitic transformation by a factor of four as compared to growth from austenite only, which is assumed by the classic Zener–Hillert model. This growth mode therefore must be considered to be the dominating growth mode and it explains at least some of the differences between experiment and theory, where diffusion in ferrite is excluded.

Effects of a high magnetic field on bainitic transformation in Fe-based alloys

It has been found that magnetic fields affect solid/solid phase transformation behavior and structures since helium-free superconducting magnet has been developed, and magnetic fields are new effective tools for the structural and functional control of materials. Structures of materials are controlled by magnetic field not only due to the difference of magnetic moment between parent and product phases but also magnetocrystalline anisotropy, shape magnetic anisotropy, induced magnetic anisotropy and magnetostriction. However, studies on the effects of magnetic field on bainitic transformation are very few. So the effects of magnetic field of 10 T on transformation temperature, kinetics and transformed structure for bainitic transformation have been investigated for an Fe–3.6Ni–1.5Cr–0.5C (mass%) alloy. Transformation temperature is increased by more than 40 K by applying a magnetic field of 10 T. This increment is larger than that for ferrite transformation in pure iron (9 K), pearlite transformation in Fe–0.8C (15 K) and lath martensite transformation in 18Ni maraging steel (20 K). The isothermal transformation behavior at 360 °C is accelerated by applying 10 T. The elongation or alignment of transformed structure is observed under some conditions for ferrite transformation, but has not been observed for bainitic transformation so far.

Transient thermal stress analysis on rapid post-weld heat treatments applied to flash butt welded rails

A finite element (FE) model has been established to estimate the transient thermal stresses developed during rapid post-weld heat treatments (RPWHT) and post-weld quenching (PWQ) of rail flash butt welds. Rapid post-weld heat treatment parameters, such as heat flux intensity, location and heating duration, were varied. Altering the PWQ initiation times was also studied. The sequentially coupled thermomechanical FE model incorporated rate independent plasticity with a constitutive linear kinematic hardening rule. This described the inelastic behaviour of the rail material caused by post-weld thermal cyclic loadings. Representative temperature dependent thermal and mechanical properties including the pearlitic transformation behaviour of rail steel were also used in the FE model. Significant reduction of local tensile residual stresses in the web region of the weld was obtained by RPWHT applied to the entire foot region. RPWHT applied to both the web and the foot regions of the weld resulted in further reductions in vertical tensile residual stresses but a lesser effect on the longitudinal tensile residual stresses. A series of PWQ conditions, which were initiated after completion of the austenite–pearlite phase transformation, had significantly affected residual stresses. Numerical predictions revealed that PWQ initiated after transformation induced higher tensile residual stresses in the web region of the weld with respect to normal cooled welds, without affecting the microstructure.

Effect of hot deformation on phase transformation kinetics of 86CrMoV7 steel

The time–temperature–transformation (TTT) and continuous cooling transformation (CCT) diagrams of 86CrMoV7 steel with and without hot deformation were constructed by means of dilatometry, metallography and transmission electron microscopy (TEM). The results show that the pearlite and bainite transformations of 86CrMoV7 steel can be promoted and the microstructure can be refined by hot deformation. The undissolved carbides associated with hot deformation increase the inhomogeneity of carbon distribution in deformed austenite. The inhomogeneities of the austenite increase the number of nucleation sites for pearlite and bainite, and promote pearlite and bainite formation, which result in refinement of both the pearlite and bainite microstructures. In contrast, the undissolved carbides do not play a direct role on the pearlite and bainite transformation of 86CrMoV7 steel in the absence of hot deformation.