Isothermal Pearlitic Transformation Research Articles

Microstructure and mechanical properties of Nb microalloyed high‑carbon pearlitic steels subjected to isothermal transformation

Niobium (Nb) is mainly added to low-carbon steels, in which the main microstructure is ferrite and/or bainite, to refine grains and improve their properties. However, the studies on Nb in high-carbon pearlitic steels are insufficient, and there are some debates regarding the existing studies. High-carbon pearlitic steel is widely used in bars, wires, and steel rails. The role of Nb in high-carbon steels may differ for different steel grades and under different heat treatments. Hence, further detailed studies are required to improve the properties of high-carbon pearlitic steels. In this study, two series of high-carbon pearlitic steels (68C and 75C series) were designed and subjected to isothermal pearlite transformation at different austenitization and isothermal transformation temperatures. The microstructures and mechanical properties of the steels were investigated by optical microscopy, field-emission scanning electron microscopy, electron backscatter diffraction, scanning transmission electron microscopy, and hardness tests. The results showed that the pearlite lamellae were refined by the addition of Nb to high-carbon pearlitic steels, which was attributed to the solute drag effect of Nb. The pearlite colony and nodule sizes also became finer with the addition of Nb because of the finer prior austenite grains and smaller pearlite growth rates. However, the refinement of the pearlite lamellae, nodules, and colonies by the addition of Nb was not significantly enhanced by increasing the Nb content from 0.014 wt% to 0.027 wt% in 75C series samples. In addition, the pearlite interlamellar spacing and colony and nodule sizes of the samples decreased with decreasing isothermal and austenitization temperatures. Moreover, the addition of Nb to high-carbon steels increased their hardness and yield strength. The main strengthening mechanisms of Nb in high-carbon steel are grain refinement strengthening and precipitation strengthening in the samples austenitized at 900 °C, whereas the improvement of strength is mainly attributed to grain refinement strengthening in the samples austenitized at 1180 °C. This study systematically characterized the pearlite morphology of Nb microalloyed high-carbon steels and analyzed the strengthening mechanism, providing a theoretical reference for the development of Nb-containing high-carbon pearlitic steels.

Enhancement of pearlite transformation by Warm Rolling in 1.0C-1.5Cr Steel

Isothermal and non-isothermal pearlite transformations under 840°C warm-rolled and 950°C hot-rolled were researched. It was found that the pearlite nuclei at grain boundary and defects were increased by warm deformation. The results showed that the incubation time was shortened in the isothermal pearlite transformation and the volume fraction of pearlite was increased in continuous cooling condition by warm deformation. The pearlite transformation behaviour under continuous cooling condition was predicted by the formula of Umemoto. The critical cooling rates of the fully pearlite structures formed under warm deformation condition and hot deformation condition were calculated.

Effects of high magnetic field on isothermal pearlite transformation and microstructure in a hypereutectoid steel

A high magnetic field was applied during the isothermal pearlite transformation of a hypereutectoid carbon steel at three different temperatures. It was found that the magnetic field applied can increase the transformation rate and the pearlite nodule growth rate, decrease the interlamellar spacing of pearlite and the amount of low angle boundary in ferrite lamella. The magnetic field effects are more obvious at high temperature. The mechanism of field-effect was discussed by analyzing the influences of high magnetic field on the eutectoid temperature, diffusion activation energy in austenite, Gibbs free energy of ferrite and dislocation mobility.

Electron microscopic study of austenite microtwins and their influence on the crystallographic features of the pearlitic transformation

The crystallographic relationship between the structural components in high-carbon 120G4 steel after a partial isothermal pearlitic transformation has been studied by TEM. It has been found that a high density of microtwins and stacking faults was observed in the retained austenite. A new type of crystallographic relationship has been detected—parallelism of close-packed directions and planes of the structural components of fine-plate pearlite to one of the variants of twinned austenite.

Influence of magnetic field on isothermal pearlite transformation in Fe–C–Ni hypoeutectoid alloy

The influence of an external magnetic field of 8 T on isothermal pearlite transformation was studied in an Fe–0·4 mass%C–2 mass%Ni hypoeutectoid alloy, in which both proeutectoid ferrite and pearlite transformations were accelerated by the field. Whereas the pearlite volume fraction increased with decreasing reaction temperature, the magnetic field increased the proportion of pearlite relative to proeutectoid ferrite only at low undercoolings. The interlamellar spacing of pearlite was reduced 5–15% by the magnetic field, which may have led to a substantial increase in growth rate of pearlite nodules. Whereas the pearlite carbon content fell near the paraequilibrium γ/(γ+cem) phase boundary, other features, e.g. interlamellar spacing, seem to be in accord with the condition of non-partition local equilibrium at the pearlite/austenite boundary as reported in Fe–C base hypoeutectoid alloys.

Crystal geometric analysis of the orientational relationships between austenite and fine pearlite in steel 120G4

Electron-microscopic examination of the crystallographic relationships between the structural constituents in the high-carbon steel 120G4 has been carried out after a partial isothermal pearlitic transformation. On a large number of experimentally investigated fragments of the microstructure, the parallellism has been established between the close-packed directions and planes of structural constituents of fine pearlite and one of the variants of twinned austenite.

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.

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.

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.

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.

Transformation plasticity at different phase transformations of bearing steel

Simulation of heat treatment processes presupposes that material data exist for the description of the phase transformation. During the heat treatment transformation and thermally caused stresses develop, which affect the behaviour of the phase transformation. The subject of this paper is the interaction between phase transformation and internal stress. The investigations show the influence of constant tensile loading on transformation strains occurring during the isothermal pearlitic and bainitic transformation and also the martensitic transformation of the bearing steel 100Cr6. The transformation plasticity parameter K increases during pearlitic transformation, with increasing transformation temperature linear, but decreasing for bainitic transformation with increasing temperature. Umwandlungsplastizität eines Wälzlagerstahls bei unterschiedlichen Phasenumwandlungen Die FE-Simulation von Wärmebehandlungsvorgängen erfordert Stoffwerte zur Beschreibung der Phasenumwandlung. Während der Wärmebehandlung treten thermisch und umwandlungsbedingte Spannungen auf, die das Umwandlungsverhalten beeinflussen. Der Einfluss derartiger Spannungen auf die Umwandlung waren Gegenstand der hier vorgestellten Arbeit. Die durchgeführten Untersuchungen zeigen den Einfluss einer während der Umwandlung angelegten konstanten Spannung auf die Umwandlungsplastizität. Der Parameter K, der das Ausmaß der Umwandlungsplastizität beschreibt, nimmt während der perlitischen Umwandlung mit steigender Temperatur zu, bei bainitischer Umwandlung dagegen ab.

Effects of Rare Earth Element on Isothermal and Martensitic Transformations in Low Carbon Steels.

Rare earth elements (RE) may segregate at the grain boundaries of austenite, lead to form carbide and refine the austenite grain. In case of no change of grain size and carbon content of austenite, an addition of RE is beneficial to the hardenability of steels. In case of a marked refinement of austenite grain, addition of RE will deteriorate the hardenability. The incubation period of the proeutectoid ferrite can be expressed as a function of grain boundary energy, grain size, activation energy for growth and the driving force for transformation and the calculated results are in good agreement with the experimental data. RE may retard the isothermal pearlitic transformation, because RE diminishes the diffusion coefficient of carbon as well as tends to segregate at Fe3C/α interface, showing a pinning effect on the transformation. RE reduces the lamella spacing of the pearlite owning to lowering the interfacial energy, e.g., from 0.7 to 0.53 J/m2 in 0.27C–1Cr–RE steel. RE tends to segregate at ferrite/island interface in the granular bainite. In grain refined steel, at the earlier stage of bainite formation, the transformation rate is high while at later stage it becomes sluggish. The activation energies of pearlitic and bainitic transformations increase by the addition of RE. The segregation of RE at ferrite/island interface may act as a drag effect. A drag factor α is expressed as a function of transformation fraction and calculated in a 0.27–1Cr–RE steel. RE segregates at the grain boundary of austenite and this kind of distribution will not be changed during the martensitic transformation. It is reasonable to predict that RE will lower the martensite/austenite interface energy, resulting in the formation of a finer lath structure. RE lowers Ms, decreases the amount of the retained austenite and retards the autotempering process. It is emphasized that the amount of the retained austenite, γ, in quenched low carbon steel depends on not only the Ms and the temperature of quenching medium, Tq, but also the influence of alloying elements on the carbon diffusion during quenching. A general equation modified from the Magee's equation is derived as γ=exp{α(C1–C1)-β(Ms–Tq)} where C0 and C1 are carbon concentrations in austenite before and after quenching respectively, α and β are constants. RE decreases Ms but also lowers C1 so as to reduce the amount of the retained austenite.

Micromechanical analysis of the finite element calculation of a diffusional transformation

A micromechanical model of a diffusional transformation has been developed, which describes the progress of the transformation within a three-dimensional “unit cell” submitted to an external stress state. The example chosen is that of an isothermal pearlitic transformation of a steel. The transformation plastic strain is due to interactions occurring between the local stresses and the transformation process, resulting in an oriented plastic flow in or in the vicinity of the material layers swept by the transformation front. The analysis of the local mechanical states of the simulation provides a good interpretation of the evolution of transformation plastic strain, when considering the effect of applied stress level and the way mechanical properties are imposed on the newly formed phase. In particular, the normality law for transformation plasticity is related to the shape of the local plastic zones. The discrepancy observed between simulation and experience is then discussed, following two main points: the influence of the behaviour law of the phases and the way interactions between neighbouring cells are prescribed. The difficulty and importance of obtaining realistic mechanical properties of the forming pearlite is pointed out.

A microscopic examination of the precipitation phenomena occuring during the isothermal pearlitic transformation in high carbon-copper nickel steels

During isothermal pearlitic transformation of high carbon austenite, the formation of proeutectoid phases precedes that of pearlite. Widmanstatten cementite is the major proeutectoid phase that forms at all temperatures. Within this constituent the precipitation of ε-Cu always occurs, together with the formation of stabilized coarse austenite entrapments. Within pearlitic cementite interphase precipitation of ε-Cu occurs at all transformation temperatures, while within pearlitic ferrite a banded interphase precipitation of ε-Cu occurs only at temperatures close to the eutectoid point. As the temperature of the isothermal pearlitic transformation is lowered the formation of non-partitioned pearlitic ferrite is possible and ε-Cu precipitation only forms within this ferrite as a result of ageing occurring during prolonged isothermal holding. It was concluded that interphase precipitation of ε-Cu always occurred on the migrating austenite/cementite interphase boundaries, but ε-Cu interphase precipitation occurred on the migrating austenite/ferrite boundaries only at temperatures close to the eutectoid point.

A microscopic investigation of the precipitation phenomena observed during the pearlite reaction in vanadium alloyed carbon steels

During the isothermal pearlitic transformation in vanadium alloyed low and medium carbon steels the formation of proeutectoid ferrite precedes that of pearlitic formation. Within this proeutectoid ferrite interphase precipitation of vanadium carbide (VC) occurs in a form of linear uniformly sized and shaped arrays of particles. When pearlite subsequently forms, two distinctive pearlitic morphologies exist. Lamellar and non-lamellar spheroidised types of pearlite. Within the pearlitic constituent of both of these pearlitic microstructures VC interphase precipitation occurs. However, in the lamellar type of pearlite curved arrays of uniformly sized and shaped VC particles whose direction is perpendicular to the cementite long axis are observed while in the non-lamellar type of pearlite apparently random distributions of VC particles exist. It was consistently found that VC interphase precipitation always occurred on the moving austenite/ferrite interphase boundaries at all temperature of isothermal pearlitic transformation.

Microscopic characterisation of ε-Cu interphase precipitation in hypereutectoid Fe-C-Cu alloys

During isothermal pearlitic transformation in medium or high carbon copper steels the decomposition of austenite leads, apart from the formation of proeutectoid phases and pearlite, to the precipitation of the ε-Cu phase. At temperatures close to that of the eutectoid of the system interphase precipitation of ε-Cu occurs within proeutectoid ferrite (medium carbon steels), within grain boundary proeutectoid cementite (high carbon steels) and within both pearlitic ferrite and cementite. As the temperature of the isothermal pearlitic transformation is lowered the formation of copper supersaturated pearlitic ferrite occurs while within proeutectoid cementite (grain boundary or Widmanstatten) and pearlitic cementite interphase precipitation of ε-Cu continues. This study of the isothermal pearlitic transformation in the Fe-Cu-C system revealed that interphase precipitation of ε-Cu always occurs on moving cementite/austenite interphase boundaries but ε-Cu interphase precipitation only occurs on moving ferrite/austenite boundaries at temperatures close to the eutectoid temperature range of the system.

Plasticite de transformation durant la transformation perlitique d'un acier eutectoïde

The transformation plasticity of an eutectoid steel has been studied during the isothermal pearlitic transformation. The variations of the deformation due to transformation plasticity have been measured in function of the applied tensile stresses and of the progress of the transformation. We take into account the modification of the transformation kinetics by the applied tensile stress. In the range of transformation temperature and applied stresses studied, we observe that the deformation due to transformation plasticity increases linearly with the applied stress and the progress of the transformation. These results are compared to the models of Greenwood-Johnson and Poirier. The comparison to these models and a further analysis lead us to propose that the transformation plasticity does not only depend on mechanical properties of the phases or on the volume variation during transformation. The relative velocity of the growth interface in regard of the mean velocity of the propagation of defects due to transformation strain accommodation, must intervene and conditions the magnitude of the deformation due to transformation plasticity.

Prediction of hardenability effects from isothermal transformation kinetics

The relationship between the size of material, fraction of martensite and austenite grain size is considered for a certain eutectoid steel under a certain cooling condition. It is assumed that the isothermal pearlite transformation can be expressed as X =1 -exp {-k(T) (tn/dm )} for the whole transformation temperature range and that the transformation is additive over the whole transformation range. The cooling rate is determined as a function of section size (which is commonly found in actual quenching). With the application of the obtained relation, the Jominy curve, DI vs DI,X (the diameter of the bar with X pct martensite at the center after an ideal quenching) and the effect of austenite grain size on the DI and Jominy distance are shown to be predictable.

等温パーライト変態におけるMnの効果について

等温パーライト変態におけるMnの効果について

共析炭素鋼の上部ベイナイトおよび恒温パーライト変態におよぼすオ一ステナイト化処理条件の影響

As has previously been reported on the lower bainite transformation, the effects of austenitizing temperature and time (longer than 60 min) on the upper bainite transformation (at 380°C) and the isothermal pearlite transformation (at 675°C) are investigated for a commercial eutectoid carbon steel heat treated at 1200°C for 20 min like those on the lower bainite transformation which were reported earlier. The principal findings are as follows.(1) The amount of bainite formed during a certain transformation period increases to a maximum with increasing time and then starts to decrease. On the other hand, the amount of pearlite formed decreases to a minimum as the time increases, after which it increases to a maximum and again decreases.(2) The higher the austenitizing temperature the shorter is the time to reach the maximum, and the lower the temperature the higher is the maximum in both bainite and pearite transformations. Besides, the higher the temperature the shorter is the time to reach the minimum pearlite formation, and the lower the temperature the higher is the minimum.(3) Bainite or pearlite starts to nucleate at grain boundaries in the case of a short austenitizing time. However, this tendency of nucleation becomes a little weaker as the time increases and the nucleation also starts inside the grains. Furthermore, in the case of a long austenitizing time, bainite plates and pearlite nodules at grain boundaries or inside the grains are fine compared to those at grain boundaries in the case of a short austenitizing time.(4) Nucleation of bainite or pearlite inside the grains is caused by the nuclear action by AlN. The nuclear action by AlN seems to be stronger in the bainite tranformation than that in the isothermal pearlite transformation.(5) The value of activation energy obtained by assuming that a process to show the maximum amount of transformation is associated with that for thermal activation can be taken as 33.8 kcal/mol in all the upper bainite, lower bainite and pearlite transformations.