Transition from upper to lower bainite in Fe–C–Cr steel

Abstract

An analytical evaluation of transition temperature from upper to lower bainite in Fe – 0·38C – 0·93Cr (wt-%) steel was carried out. Calculations were based on the model constructed by Takahashi and Bhadeshia, which involves a comparison between the time t_θ needed to precipitate cementite within the bainitic ferrite plates with the time t_θ required to decarburise supersaturated ferrite plates. It was found that the distribution of lath widths, shown by histograms, of the bainitic ferrite varies with isothermal transformation temperatures and holding times. The transition between upper and lower bainite is found to occur over a narrow range of temperatures (350 – 410°C) and depends on the thickness of bainitic ferrite laths and the volume fraction of precipitated cementite. On comparing t _d and t_θ it was found that a transition temperature from upper to lower bainite reaction L _S of about 350°C could be predicted if the thickness of bainitic ferrite laths is set as w _o = 0·1 μm and the volume fraction of cementite set as ξ = 0·01. Calculated differences in the relative behaviour of t _d and t_θ revealed the occurrence of upper and lower bainite in steel Fe – 0·38C – 0·93Cr consistent with the results of transmission electron microscopy investigation.     

Continuous cooling transformation behaviour and bainite formation kinetics of new bainitic steel

In order to avoid the appearance of soft particles composed of ferrite or pearlite in the actual production of new bainitic steel, the phase transformation behaviour and bainite formation kinetics were investigated by DIL805A dilatometer, optical microscopy, scanning electron microscopy and Vickers-durometer. The results show that the soft particles cannot appear when the cooling rate exceeds 0.025?K?s^?1, and this condition can be ensured by direct spray cooling in production. The local activation energy decreases with increasing transformed bainite volume fraction (f_b), and the average energy is about 136.7?kJ?mol^?1. The local Avrami exponent mainly lies between 0.5 and 3 in a wide f_b range, indicating that the dominating mechanism of bainite formation is two-dimensional and one-dimensional growth.     

Application of Koistinen and Marburger’s athermal equation for volume fraction of martensite to diffusional transformations obtained on continuous cooling 0·13%C high strength low alloy steel

Abstract

Koistinen and Marburger’s (KM) equation for the variation of volume fraction of athermal martensite y with temperature Tbelow the M _s has been applied to continuous cooling diffusional data. The data consisted of dilatometer curves obtained on continuous cooling of a 0·13%C high strength low alloy steel. The KM equation takes the form, ln(1 - y) = α(M _s - T ). Plots of -ln (1 - y) against temperature for what are thought to be grain boundary ferrite, intragranular ferrite, proeutectoid ferrite, and pearlite give a series of straight lines of increasing slope α. Intersections of these lines give the temperature of transformation points in good agreement with those on the dilatometry curves. Values of α obtained for each transformation are compared with those previously obtained for martensite in plain carbon and alloy steels and ferrite in Fe–9%Ni.     

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Abstract

Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.     

Laser transformation hardening of low alloy hypoeutectoid steel

Abstract

The principles of laser transformation hardening were investigated using a low alloy special steel having a microstructure of pearlite and proeutectoid ferrite. Temperature fields and phase transformations were modelled. Particular attention was paid to increases of the Ac₁ and Ac₃ transformation temperatures owing to the rapid thermal cycles produced by laser heating. Dissolution of proeutectoid ferrite is shown to control the formation of a homogeneous hardened case. Experimental data are in good agreement with the predictions of the model. A diagram was constructed which describes the case geometry and microstructure in terms of the process variables and is an aid to optimising practical processing parameters. The models are flexible and may be used for laser transformation hardening of other ferrous alloys having inhomogeneous microstructures.

MST/1606     

Factors influencing ferrite/pearlite banding and origin of large pearlite nodules in a hypoeutectoid plate steel

Abstract

The microstructure and distribution of alloying elements in a hot rolled, low alloy plate steel containing (wt-%) 0·15%C, 0·26%Si, l·49%Mn, and 0·03%Al were examined using light microscopy and electron probe microanalysis. Microstructural banding was caused by microchemical banding of manganese, where alternate bands of proeutectoid ferrite and pearlite were located in solute lean and solute rich regions, respectively. Bands were well defined for a cooling rate of 0·1 K s^?1, but banding was much less intense after cooling at 1 K s^?1. At a cooling rate of 0·1 K s^?1 and for austenite grains smaller than the microchemical band spacing, austenite decomposition occurred via the formation of ‘slabs’ of proeutectoid ferrite in manganese lean regions resulting in the growth of ferrite grains across austenite grain boundaries. Abnormally large austenite grains result in the formation of large, irregularly etching pearlite nodules which traversed several bands. In specimens cooled at 1 K s^?1, ferrite/pearlite banding did not exist in regions where austenite grains were two or more times larger than the microchemical band spacing.

MST/1397     

The bainite transformation behavior,the influencing factors and control strategy in ferritic‐pearlitic forged crankshafts with coarse grains

Microstructural characterization of the bainite in a ferritic–pearlitic forged crankshaft was carefully investigated. A Gleeble thermo‐mechanical simulator as well as a high resolution dilatometer were also used to analyze the effect of cooling rate on the bainite formation and the bainite transformation mechanism in steels with different austenite grain sizes. Results show that the fine structure of the bainite mainly consists of bainitic ferrite and martensite. No segregations are found where bainite forms. Bainite tends to form in the slower cooled inner part of the crankshaft with an austenite grain size exceeding 100 μm. The formation of bainite is mainly affected by the austenite grain size as well as the cooling rate in the crankshaft studied. As the austenite grain size increases, ferrite start, pearlite finish and bainite finish temperatures are decreased. More bainite forms when bainite finish temperature decreases. The critical cooling rate of bainite transformation is increased from 0.34 °C?s^‐1 to 0.44 °C?s^‐1, if the maximum austenite grain size is refined from 216 μm to 100 μm. For ferritic–pearlitic crankshafts, or other bulky products, the elimination of bainite can be achieved through austenite grain refinement.     

Effects of cooling process on microstructure and hardness for 1·0C–1·5Cr bearing steel

Effects of cooling rate (V_cr) and final cooling temperature (T_ft), after hot deformation, on microstructure and hardness for 1·0C–1·5Cr bearing steel were investigated. The results show that if V_cr increases from 2 to 25°C s^?1 and T_ft remains at 650°C, pearlite colony size and grain size both decrease, hardness increases. When V_cr exceeds 8°C s^?1, carbide network can be restrained effectively. TEM micrographs indicate that there exist branches in the local region of lamellar cementite and ferrite, and a ferrite thin film is also found around the proeutectoid carbide. Under the cooling rate of 10°C s^?1, with the increase in T_ft, the microstructure changes from martensite into pearlite, carbide network becomes more serious and hardness decreases.     

Effect of laser incident energy on microstructures and mechanical properties of 12CrNi2Y alloy steel by direct laser deposition

This work aims to establish the effect of laser energy area density(EAD) as the laser incident energy on density, microstructures and mechanical properties of direct laser deposition(DLD) 12CrNi2 Y alloy steel.The results show that the density of DLD 12CrNi2 Y alloy steel increases at initial stage and then decreases with an increase of EAD, the highest density of alloy steel sample is 98.95%. The microstructures of DLD12CrNi2 Y alloy steel samples are composed of bainite, ferrite and carbide. With increase of EAD, the microstructures transform from polygonal ferrite(PF) to granular bainite(GB). The martensite-austenite constituent(M-A) in GB transforms from flake-like paralleling to the bainite ferrite laths to granular morphology. It is also found that the average width of laths in finer GB can be refined from 532 nm to 302 nm, which improves the comprehensive properties of DLD 12 CrNi2 Y alloy steel such as high hardness of 342 ± 9 HV_(0.2), yield strength of 702 ± 16 MPa, tensile strength of 901 ± 14 MPa and large elongation of15.2%±0.6%. The DLD 12CrNi2 Y material with good strength and toughness could meet the demand of alloy steel components manufacturing.     

Microstructure,phase evolution and corrosion behaviour of the Zn–Al–Mg–Sb alloy coating on steel

ABSTRACT

In the present work, the influence of antimony (Sb) addition in Zn–Al–Mg alloy on the microstructure, phase characteristic, solidification behaviour and corrosion resistance of hot dipped Zn–0.5Al–0.5Mg–xSb (x?=?0, 0.1, 0.3 and 0.5?wt-%) coated steel wires were evaluated. Thermal analysis revealed that cooling rate of the liquid metal using the steel mould (5.3°C?s^–1) was higher than using ceramic mould (0.3°C?s^–1). Based on the phase analysis and verified by thermodynamic calculations, it was revealed that Zn₁₁Mg₂ and Zn₂Mg phases appeared for Zn–Al–Mg alloy at slow and fast cooling rates while, the Mg₃Sb₂ phase was observed after addition of Sb at both cooling rates. Corrosion behaviour of the alloys determined through electrochemical measurements shows that Zn–Al–Mg alloy with 0.3?wt-%Sb has the lowest corrosion rate indicating an excellent corrosion resistance.     

3Cr2Mo塑料模具钢连续冷却相变行为

为了调节塑料模具钢3Cr2Mo的组织,以实现在线预硬化,使用Gleeble1500热模拟试验机、光学显微镜以及透射电子显微镜等研究3Cr2Mo钢变形及未变形奥氏体的连续冷却相变行为及相变组织.实验结果表明,3Cr2Mo钢奥氏体稳定性较高,在所研究的实验条件下,连续冷却过程中没有出现先共析铁素体和珠光体,而是发生贝氏体和马氏体相变.热变形使奥氏体发生了机械稳定化,贝氏体相变推迟到较低温度下才完成.随着冷却速度的降低,贝氏体的形态由常规板条状变成粒状,最终可获得粒状贝氏体组织.     

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract

Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains d_α formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size d_γ. In the cooling rate range studied (q= 0·05–5 K s^?1), d_α was found to be proportional to q^?0·26d_γ^0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q^?0·17d_γ^0·33. This was in close agreement with the dependence obtained in the present experiment.

MST/466     

Effect of austenite deformation on crystallographic texture during transformations in microalloyed bainitic steel

Abstract

The evolution of the texture of ferrite as a function of the coiling temperature has been studied in a hot rolled Nb alloyed CMnMoCrB complex phase steel by means of electron backscatter diffraction. Coiling that steel at 720 ° C led to ferrite and pearlite, and coiling at 550 ° C produced a bainite-martensite microstructure. The presence of residual austenite in the steels coiled at 680 and 550 ° C allowed for texture measurements in γ. Analyses of texture gave fundamental information on the decomposition of γ in both the recrystallised state and the deformed state. It was found that austenite, initially deformed below the non-recrystallisation temperature T_nr, recrystallised statically d partially during the γ α and the γ ^d α _b transformations. In the specimen coiled at 680 ° C, primary ferrite and bainite could be distinguished based on the confidence indexof the diffraction pattern. A clear variant selection was observed for the γ ^d α _b transformation, as arotation of ? ₁ = 30 ° occurred inthe austenite between the ferrite and the bainite formations. The bainite was found to result mainly from the decomposition of the brass {110} 〈 112 〉 and Goss {110} 〈 001 〉 orientations of deformed austenite. The residual austenite was found to be recrystallised γ γ austenite with the cube{001} 〈 100 〉 orientation. Coiling simulations were performed in a dilatometer starting from different austenite grains sizes and deformation states. In the most deformed specimens, the deformation state of the austenite and the combined effects between the different alloying elements presentin the steel were responsible for a solute drag like effect.     

Transformation of austenite remaining after intercritical rolling to ferrite

Abstract

The impact of austenite deformation in the intercritical range on the rate of transformation in continuous cooling to ferrite, pearlite, bainite or martensite has been studied. The austenite associated with the rolled ferrite is much higher in carbon content, which does not influence the pearlite transformation but retards bainite and martensite. Furthermore, in comparison with rolling of stable austenite the increased strain hardening of the intercritically cooled austenite accelerates the formation of ferrite and pearlite (+ 10–30°C) and refines them but retards the bainite and martensite transformations (?20–40°C). At the intermediate cooling rate near 16 K s^?1, these several influences combined with near doubling of the ferrite production give rise to the suppression of bainite formation and to maximum increased delay of martensite start.     

A new empirical formula for the bainite upper temperature limit of steel

The definition of the practical upper temperature limit of the bainite reaction in steels is discussed. Because the theoretical upper temperature limit of bainite reaction, B ₀, can neither be obtained directly from experimental measurements, nor from calculations, then, different models related to the practical upper temperature limit of bainite reaction, B _S, are reviewed and analyzed first in order to define the B ₀ temperature. A new physical significance of the B _S and B ₀ temperatures in steels is proposed and analyzed. It is found that the B ₀ temperature of the bainite reaction in steels can be defined by the point of intersection between the thermodynamic equilibrium curve for the austeniteferrite transformation by coherent growth (curve Z ) and the extrapolated thermodynamic equilibrium curve for the austenitecementite transformation (curve ES in the Fe-C phase diagram). The B _S temperature for the bainite reaction is about 50–55 °C lower than the B ₀ temperature in steels. Using this method, the B ₀ and B _S temperatures for plain carbon steels were found to be 680 °C and 630 °C, respectively. The bainite reaction can only be observed below 500 °C because it is obscured by the pearlite reaction which occurs prior to the bainite reaction in plain carbon steels. A new formula, B _S(°C) =, 630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W, is proposed to predict the B _S temperature of steel. The effect of steel composition on the B _S temperature is discussed. It is shown that B _S is mainly affected by alloying elements other than carbon, which had been found in previous investigations. The new formula gives a better agreement with experimental results than for 3 other empirical formulae when data from 82 low alloy steels from were examined. For more than 70% of these low alloy steels, the B _S temperatures can be predicted by this new formula within ±25°C. It is believed that the new equation will have more extensive applicability than existing equations since it is based on data for a wide range of steel compositions (7 alloying elements).     

Formation of ultrafine grained ferrite during thermomechanical treatment in microalloyed steel

Abstract

In the present work, the formation of ultrafine grained ferrite has been studied by applying suitable thermomechanical treatment. A high amount of deformation (～80%) at varying strain rates (0·01–10 s^?1) was applied in the temperature range of Ar₃ to Ac₃ followed by water quenching. This treatment resulted in a two-phase ferrite–martensite microstructure as compared to fully martensite structure after quenching without deformation. The formation of ultrafine ferrite (?3 μm) during deformation was favourable at a lower temperature and a slower strain rate. A maximum ～50% ferrite formed during deformation at 780°C with a strain rate of 0·01 s^?1. Experimental rolling with a high strain (～1·3) with finish rolling temperature just above Ar₃ (～750°C) resulted in fine ferrite–pearlite of ?3 μm, and the properties showed a high value of strength as compared to steels rolled in a conventional way. Dual phase microstructure (ferrite and martensite) was produced after partial austenisation to 780°C followed by quenching in water, and this resulted in an excellent combination of properties (high ultimate tensile strength, low yield strength/ultimate tensile strength, high elongation and high n values).     

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Abstract

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s^-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.     

Additivity hypothesis and effects of stress on phase transformations in steel

Theoretical analysis shows that the fraction of pearlite formed from nucleation is additive and that from growth is not. A modified additivity model is established with two continuous cooling experiments. Calculations of Ae₃ temperature for proeutectoid ferrite formation under stress and nucleation rate as well as incubation period of ferrite or pearlite transformation under stress are successfully made. Kinetics equations for ferrite and pearlite transformations under stress are expressed from modification of J–M–A equation with addition of a stress item. The acceleration effect of stress on bainite formation is mainly attributed to the increase of diffusivity of solute atoms and even iron. By consideration of the grain size effect, Patel and Cohen’s equation expressing the effect of stress on M_s is modified. Calculations of M_s for fcc → bcc(bct) and fcc → hcp under stress are introduced. An equation showing the relationship between strain and nucleation of martensite which can well explain the morphology of martensite formed under stress is mentioned. Appearance and mechanism of mechanical stabilization of austenite in martensitic transformation, i.e., the lowering of M_s, resulted from the work hardening of austenite, are different from retardation of bainite formation under stress, i.e., after B_s raising, occurring the retardation of bainite growth resulted from hindrance by defects.     

Effects of hot deformation on austenite transformation in 10w carbon Mo-Nb and C-Mn steels

Abstract

The 0.15C-1.5Mn and 0.07C-0.3Mo-0.055Nb steels were subjected to hot rolling to determine the effects on transformation to ferrite and to hot torsion to measure strength. After 25 or 50% reduction at 830°C, the acceleration during isothermal transformation in the range 700–600°C is much greater for C-Mn than for Mo-Nb; delays of 5–10 s before cooling reduce the acceleration markedly. In continuous cooling, three passes of 20% reduction raise the temperature of austenite decomposition, most noticeably at the higher rate 9 K S^-l. At this rate, for the C-Mn the pearlite is refined and for Mo-Nb the promoted ferrite is refined more and enhanced over bainite than at lower rates. Intercritical rolling below Ar₃ enhanced ferrite formation more in C-Mn in the range 780–720°C than in Mo-Nb over 750–690°C. The stronger Mo-Nb in torsion (900–1100°C, 0.1-5 S^-l) exhibits an activation energy of 353 kJ mol^-l compared with 316 kJ mol^-l for C-Mn.     

Transformation kinetics of a superhardenability treated steel

Abstract

The influence of high-temperature austenitization on the isothermal transformation kinetics of a superhardenability treated steel has been studied by comparing the isothermal transformation diagrams. It is shown that the superhardenability effect is more pronounced in bainite than in ferrite–pearlite transformations. The critical transformation temperatures M_s, M_f, Ac₁ and Ac₃ are unchanged. The superhardenability effect is completely suppressed by austenitization above 1200°C and recovered by reaustenitization at typical (880°C) temperatures. It is also shown that this effect is related to a low oxygen content. From comparison of the transformation kinetics of a superhardenability treated steel and boron containing steels after high-temperature and grain refining treatments it is suggested that the superhardenability effect is possibly related to the boron effect by which boron increases the hardenability of low- and medium-carbon alloy steels.

MST/397