Pearlite Transformation Research Articles

Microstructure of warm rolling and pearlitic transformation of ultrafine-grained GCr15 steel

Pearlitic transformation mechanisms have been investigated in ultra-fine grained GCr15 steel. The ultrafine-grained steel, whose grain size was less than 1μm, was prepared by thermo-mechanical treatment at 873K and then annealing at 923K for 2h. Pearlitic transformation was conducted by reheating the ultra-fine grained samples at 1073K and 1123K for different periods of time and then cooling in air. Scanning electron microscope observation shows that normal lamellar pearlite, instead of granular cementite and ferrite, cannot be formed when the grain size is approximately less than 4(±0.6)μm, which yields a critical grain size for normal lamellar pearlitic transformations in this chromium alloyed steel. The result confirms that grain size has a great influence on pearlitic transformation by increasing the diffusion rate of carbon atoms in the ultra-fine grained steel, and the addition of chromium element doesn't change this pearlitic phase transformation rule. Meanwhile, the grain growth rate is reduced by chromium alloying, which is beneficial to form fine grains during austenitizing, thus it facilitating pearlitic transformation by divorced eutectoid transformation. Moreover, chromium element can form a relatively high gradient in the frontier of the undissolved carbide, which promotes carbide formation in the frontier of the undissolved carbide, i.e., chromium promotes divorced eutectoid transformation.

Numerical prediction on microstructure evolution for multi-stage hot forging of micro-alloyed piston

Purpose – The purpose of this paper is to build a flow stress model and microstructure evolution models which can be used to fulfill the multi-physics prediction of hot forging process, in this way the process design can be virtually verified and optimized. This is especially crucial for micro-alloyed steel forging which microstructure determines the component properties, since the downstream quenching is usually not needed. Design/methodology/approach – First, hot compression tests have been completed; second, experimental data are used to build the flow stress model and models for microstructure evolution; third, programming has been finished to integrate the proposed models into the commercial finite element method (FEM) code; fourth, case study is conducted to simulate multi-stage hot forging process of micro-alloyed steel F38MnV piston; and fifth, simulation results are validated by experiment. Findings – First, simulation results in grain size and phase volume fraction are in well agreement with experimental ones; second, the austenite grain is dramatically refined by the dynamic recrystallization in pre-forging process and static recrystallization in the two intervals has no obvious change during the following final forging and cooling above the Ae3 temperature; third, during the cooling process below the Ae3 temperature, ferrite and pearlite transformation begin from the thin skirt to the thick skirt and piston bottom because of different cooling speeds at different areas. Originality/value – First, flow stress model, dynamic recrystallization model, static recrystallization model, austenite grain growth model and phase transformation models are established for a micro-alloyed steel; second, the multi-physics FEM simulation of multi-stage hot forging of industrial piston has been conducted and verified by experiment, which show good agreement.

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.

Inner Connection of Bainite and Pearlite Transformation in Steels

Decomposition of austenite in steels from high to low temperatures is pearlite, bainite and martensite phase transformation. According to studies, magnetic field promotes the three transformations. The phenomenons indicate the three transformations have coherence in magnetics. There is a transformation from paramagnetism (γ) to ferromagnetism (α) among all these three. Magnetic drive ΔGm plays an important role in the phase transition. The bainite transformation was carried out at 300°C during different isothermal time. Combining the results, it studied the relationship of bainite and pearlite transformation from characteristics of the nucleation of BF, microstructures etc. Even without magnetic field, spontaneous magnetization makes contributions to phase transformation driving force. Small deflections cause by spontaneous magnetization may promote the lattice reorganization from FCC to BCC in a certain extent, while promote the formation of the new phase with BCC cube.

Development of ferrite/bainite bands and study of bainite transformation retardation in HSLA steel during continuous cooling

The development of banded structure in HSLA steel during continuous cooling has been systematically studied using dilatometry and microstructural observations. At low cooling rates ( 10 °C/min), the formation of pearlite is suppressed, and the corresponding microstructure consists of banded ferrite and bainite. Dilatometric analysis indicates that as ferrite and pearlite transformation at low rates (or ferrite transformation at higher rates) completes, Manganese (Mn) and Carbon (C) concentration in austenite can retard the bainitic transformation and result in the transformation stagnancy phenomenon. The magnitude of retardation and the bainite morphology are affected by Mn and C concentration. The increased cooling rate decreases Mn and C content in bainite, and then the length of the stagnant stage decreases and bainite morphology changes from lower bainite to acicular ferrite.

Enhanced superplasticity in an Al-alloyed multicomponent Mn–Si–Cr–C steel

Excellent superplasticity (elongation ∼720%) is observed in a novel multi-component (Mn–S–Cr–Al alloyed) ultrahigh carbon steel during tensile testing at a strain rate of 2×10−3s−1 and a temperature of 1053K (just above the equilibrium austenite–pearlite transformation temperature). In order to understand superplasticity in this material and its strong Al dependence, the deformation-induced microstructure evolution is characterized at various length scales down to atomic resolution, using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, energy-dispersive X-ray spectroscopy and atom probe tomography. The results reveal that 1wt.% Al addition influences various microprocesses during deformation, e.g. it impedes Ostwald ripening of carbides, carbide dissolution, austenite nucleation and growth and void growth. As a result, the size of the austenite grains and voids remains relatively fine (<10μm) during superplastic deformation, and fine-grained superplasticity is enabled without premature failure.

Effect of Prior Austenite Grain Size on Pearlite Transformation in a Hypoeuctectoid Fe-C-Mn Steel

The aim of this work is to evaluate the influence of the prior austenite grain size (AGS) on the austenite-to-pearlite isothermal decomposition in a Fe-C-Mn hypoeutectoid steel. Due to the strong influence, grain boundaries have on pearlite transformation kinetics, morphological aspects of pearlite from two conditions with very different AGS were studied and characterized. Results allow us to conclude that the formation of pearlite and ferrite are favored for small AGS values, whereas a larger AGS led to an increase in the total amount of pearlite volume fraction. Furthermore, the average size of pearlitic colonies increased with increasing AGS, and it appears that the interlamellar spacing of the pearlite does not depend on AGS, but instead, is controlled by the isothermal decomposition temperature. Finally, it was observed that the ratio between lamellar thickness of ferrite and cementite depended on AGS.

High-temperature mechanical properties of near-eutectoid steel

The high-temperature mechanical properties of near-eutectoid steel were studied with a Gleeble-1500 simulation machine. Zero strength temperature (ZST), zero ductility temperature (ZDT), hot ductility curves, and strength curves were measured. Two brittle zones and one plastic zone were found in the temperature range from the melting point to 600°C. Embrittlement in zone I is caused by the existence of liquid film along dendritic interfaces. Ductility loss in zone III mainly results from precipitates and inclusions as well as S segregation along grain boundaries. Pearlite transformation also accounts for ductility deterioration in the temperature range of 700–600°C. Moreover, the straightening temperature of the test steel should be higher than 925°C for avoiding the initiation and propagation of surface cracks in billets.

A study of self‐hardening bainitic cast steel

AbstractBainitic cast steel is a kind of wear resistant material which has high strength and toughness, and can usually be obtained by isothermal quenching or molybdenum alloying. However, isothermal quenching has lower production efficiency and molybdenum alloying has higher production cost. In this paper, according to the characteristics that manganese and boron elements delayed the pearlitic transformation, the authors developed a new type of self‐hardening bainitic cast steel in which manganese and boron were main alloy elements and a small amount of titanium, nitrogen, calcium, barium and yttrium elements were also added in the steel that could refine and purify the solidification structure of steel. On this basis, the author studied the effect of tempering treatment on microstructures, mechanical properties and wear resistance of bainitic cast steel. The results showed that impact toughness of bainitic cast steel increased ceaselessly with the increase of tempering temperature, and there was tempering brittleness while tempering from 450°C–500°C. Moreover, the hardness of bainitic cast steel decreased with the increase of tempering temperature, and hardness decreased slowly and maintained at 55HRC or above when tempering temperature was lower than 300°C. Under the condition of two‐body pin‐on‐disc wear, the wear resistance of bainitic cast steel decreased with the increase of tempering temperature, but bainitic cast steel tempering at 300°C had excellent wear resistance in the condition of impact wear. In the practical use, the bucket teeth of excavator and the hammer of crusher making from self‐hardening bainitic cast steel were safe and reliable, and their service life were increased by 120–150% than Hadfield manganese steel.

Effects of Ce or Nb Addition on Nucleation of Pearlite in High Carbon Steel

The high-carbon steel was micro-alloying by Ce and Nb respectively, and the effect of micro-alloying on the nucleation of pearlite was reported in the present work. The results show that the pearlite could nucleate in rare earth sulfide inclusions and Nb-containing precipitates. The formation of NbC caused the C-depleted zone, promote the formation of proeutectoid ferrite of the pearlite. And the strain energy caused by rare earth sulfide inclusions will provide activation energies for pearlitic transformation. The inclusion and precipitates will increase the rate of nucleation of pearlite and refined the microstructures of pearlitic steel.

Effect of Destabilizing Heat Treatment on Solid-State Phase Transformation in High-Chromium Cast Irons

This work describes the influence of secondary carbide precipitation at destabilizing heat treatment on kinetics of austenite phase transformation at a subcritical range of temperatures in high-Cr cast irons, alloyed with 4 to 6 wt pct of Mn or by complex Mn-Ni-Mo (Mn-Cu-Mo). The samples were soaked at 1073 K to 1373 K (800 °C to 1100 °C) (destabilization) or at 573 K to 973 K (300 °C to 700 °C) (subcritical treatment); the combination of destabilization and subcritical treatment was also used. The investigation was carried out with application of optical and electron microscopy and bulk hardness measurement. Time-temperature-transformation (TTT) curves of secondary carbide precipitation and pearlite transformation for as-cast austenite and destabilized austenite were built in this work. It was determined that the secondary carbide precipitation significantly inhibited the pearlite transformation rate at 823 K to 973 K (550 °C to 700 °C). The inhibition effect is more evident in cast irons alloyed with complex Mn-Ni-Mo or Mn-Cu-Mo. The possible reasons for transformation decelerating could be austenite chemical composition change (enriching by Ni, Si, and Cu, and depleting by Cr) and stresses induced by secondary carbide precipitation.

Volume Fractions of Proeutectoid Ferrite/Pearlite and Their Dependence on Prior Austenite Grain Size in Hypoeutectoid Fe-Mn-C Alloys

It has been generally believed that pearlite transformation in hypoeutectoid steels starts when the average carbon concentration in untransformed austenite reaches the Acm line after the formation of proeutectoid ferrite. To test this concept experimentally, volume fractions of proeutectoid ferrite/pearlite and carbon contents in the austenite being transformed into pearlite were measured for the Fe-2Mn-0.3C alloy isothermally transformed in the temperature range 848 K to 898 K (575 °C to 625 °C). It was found that lamellar pearlite can form even when the average carbon content in untransformed austenite is much lower than the Acm line. This peculiar observation is probably due to the two-dimensional diffusion of carbon, i.e., parallel to and normal to the austenite/pearlite interface, which enables lamellar cementite to grow continuously by supplying carbon atoms to its growth front. This results in proeutectoid ferrite fractions with respect to pearlite being much lower than those predicted by the lever rule. With decreasing prior austenite grain size, proeutectoid ferrite fractions with respect to pearlite were found to increase, but the thickness of proeutectoid ferrite was constant within the range of grain size investigated. This is due to the existence of the critical α/γ interface velocity only below which pearlite (actually cementite) can be nucleated at the migrating α/γ interface. Furthermore, the upper limit temperatures for pearlite formation in the Fe-1Mn-0.33C and Fe-2Mn-0.3C alloys were found to be well between the PLE/NPLE and PE Ae1 temperatures.

Reasonable Temperature Schedules for Cold or Hot Charging of Continuously Cast Steel Slabs

Some continuously cast steel slabs are sensitive to transverse fracture problems during transportation or handling away from their storage state, while some steel slabs are sensitive to surface transverse cracks during the following rolling process in a certain hot charging temperature range. It is revealed that the investigated steel slabs with high fracture tendency under room cooling condition always contain pearlite transformation delayed elements, which lead to the internal brittle bainitic structure formation, while some microalloyed steels exhibit high surface crack susceptibility to hot charging temperatures due to carbonitride precipitation. According to the calculated internal cooling rates and CCT diagrams, the slabs with high fracture tendency during cold charging should be slowly cooled after cutting to length from hot strand or charged to the reheating furnace directly above their bainite formation temperatures. Based on a thermodynamic calculation for carbonitride precipitation in austenite, the sensitive hot charging temperature range of related steels was revealed for the determination of reasonable temperature schedules.

Effect of recalescence on microstructure and phase transformation in high carbon steel

This paper deals with the effect of recalescence on the microstructure of high carbon steels. Controlled experiments are conducted with varying cooling rates and different specimen geometries in a thermomechanical simulator to completely or partially suppress the transformation enthalpy. Different amounts of recalescence obtained from various experiments are then correlated with the final microstructures. The results clearly indicate the importance of recalescence and also show the limitations of using thermomechanical simulators in predicting the characteristics of γ→pearlite transformation in high carbon steels. A mathematical model has been developed to predict the phase transformation in high carbon steels under continuous cooling conditions incorporating the influence of recalescence. The newly developed model gives better prediction of microstructure than currently available models in high carbon steels under higher cooling rates.

Theoretical and numerical study of lamellar eutectoid growth influenced by volume diffusion

We investigate the lamellar growth of pearlite at the expense of austenite during the eutectoid transformation in steel. To begin with, we extend the Jackson–Hunt-type calculation (previously used to analyze eutectic transformation) to eutectoid transformation by accounting for diffusion in all the phases. Our principal finding is that the growth rates in the presence of diffusion in all the phases are different compared to the case when diffusion in growing phases is absent. The difference in the dynamics is described by a factor ’ρ’ which comprises the ratio of the diffusivities of the bulk and the growing phases, along with the ratios of the slopes of the phase coexistence lines. Thereafter, we perform phase-field simulations, the results of which are in agreement with analytical predictions. The phase-field simulations also reveal that diffusion in austenite as well as ferrite leads to the formation of tapered cementite along with an overall increase in the transformation kinetics as compared to diffusion in austenite (only). Finally, it is worth noting that the aim of present work is not to consider the pearlitic transformation in totality; rather it is to isolate and thereby investigate the influence of diffusivity in the growing phases on the front velocity.

Kinetics and crystal geometry of precipitation of vanadium carbides at the interphase boundary upon pearlitic transformation of steel

In high-carbon vanadium steel, the temperature range (600°C and higher) and time (∼0.1 s) of the formation of vanadium carbide particles 5–10 nm in size at the interphase boundary upon pearlitic transformation have been established. Their distribution in ferrite has been subjected to crystallographic analysis with allowance for the habit of cementite plates. Based on the results of this analysis, a model has been suggested for the volume distribution of vanadium carbide nanoparticles in ferrite interlayers between cementite plates of the pearlitic colony in the form of successive (periodic) corrugated spherical surfaces with a period of ∼20 nm. This particle distribution is determined by the shape of the moving pearlite-austenite interphase boundary and by the direction of growth of the pearlitic colony.

Composition Design, Microstructure and Properties of Ultra-High-Strength Steel Using for Energy Storage Flywheels

According to the properties request of energy storage flywheels running in high speed, chemical composition of a new ultra-high-strength steel has been designed. The designed steel specimen was prepared using intermediate frequency induction furnace and its transformation point, which had been simulated and calculated through J-Mat software in advance together with cooling curves, was investigated using by thermal dilatometer. Then the microstructure and mechanical properties of the designed steel have been evaluated by means of OM, SEM, durometer and universal material tensile tester. Simulation results showed that the pearlitic transformation of designed steel occurred at 687-453°C and bainite transformation at 453-340°C. Martensitic transformation started at 340°C and terminated at 220°C. The experimental results indicated that the casting microstructure of the designed steel was a duplex structure consisting of martensite and acicular bainite with a small amount of retained austenite. The austenization temperature ranged from 698°C to 790°C. The superior comprehensive mechanical properties of tensile strength of 1900MPa and elongation of 6.65% as well as the microstructure of tempered martensite was obtained after heat treatment.

Transformation Behavior of Austenite Continuous Cooling of Nb-Ti Micro-Alloyed Low Carbon Steel

The transformation behavior of Nb-Ti micro-alloyed low carbon steel during continuous cooling was studied trough thermomechanical simulator and metallographic analysis. The results show that the dynamic CCT diagrams shift to the left and upper compared with the static ones, the begin temperature of γ→α transformation is gradually lower with the increase of cooling rate. The high temperature deformation improves Ferrite and Pearlite transformation, also improves Bainite transformation and decreases ferrite transformation zone. The rapid cooling can obtain better obdurability mixed microstructure of Martensite and Bainite within a certain cooling rate after deformation.

Study of New On‐Line Pre‐Hardening Technology of 3Cr2NiMo Plastic Die Steel

AbstractThe thermal property parameters of 3Cr2NiMo plastic die steel were tested and the two‐dimensional finite element model was established in the paper. The temperature fields of 3Cr2NiMo plastic die steel plate with the thickness of 130 mm were calculated and analyzed under three quenching processes. The results showed that for all of the three processes the cooling rates could avoid the pearlite transformation zone when the temperature of the steel plate was more than 500°C. When the temperature was less than 500°C, the cooling rate of the third process was slower, and the temperature difference was effectively alleviated, which avoided large transformation stress appearance. For 3Cr2NiMo plastic die steel with the thickness of 130 mm, the third process was the best process. In the paper, the hardness and the microstructures were tested after the third process and tempering. After tempering process, the structures were all tempered sorbite, and the tempering hardness difference in the whole steel plate was less than 3HRC, and there were no cracks. The study provided references for on‐line pre‐hardening process formulation and optimization of large plastic die steel.

Ultrafine grain effect on pearlitic transformation in hypereutectoid steel

Abstract