Microstructural examination of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

A detailed microstructural characterization of two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~ 400–250°C), has been carried out using principally electron microscopy, X-ray diffraction, and dilatometry. Upper bainite in these silicon-containing steels consists of bainitic ferrite laths and interwoven thin films of retained austenite instead of cementite. Coarser granular regions of retained austenite may also be obtained. The bainitic ferrite laths (or plates) in lower bainitic structures contain intralath carbides, but the interlath morphology of retained austenite still occurs. The variations in these microstructures with isothermal transformation temperature, and the thermal stability of the retained austenite phase is described and discussed.

MST/526     

Effect of boron on bainitic transformation kinetics after ausforming in low carbon steels

The addition of boron(B) is frequently adopted to increase the hardenability of bainitic steels. Although it is well known that B can retard the bainitic transformation kinetics, it is still not clear how the B affects the bainitic transformation kinetics after ausforming. By systematic high-resolution dilatometry tests, the present work reveals that the bainitic transformation kinetics is accelerated in a low C steel with B addition after ausforming from all aspects including incubation time, transformation velocity and transformed volume fraction. In contrast, for the same steel without B addition, both transformation velocity and transformed volume fraction are retarded after ausforming. It is proposed that ausforming can reduce B segregation at prior austenite grain boundaries as some boron can interact with dislocations and therefore enhance bainite nucleation rate. Furthermore, auforming can refine the average volume of bainitic sheaf. Based on the competing mechanisms between increase of nucleation rate and refinement of bainitic sheaf, the effects of B and ausforming on the bainitic transformation kinetics are discussed.     

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.     

Retained austenite stabilisation in low carbon high silicon steel during isothermal holding

We elucidate here the role of isothermal hold temperature of 300–500°C after intercritical annealing at 760°C on bainitic transformation and in governing the stabilisation of retained austenite in a 0.23C-1.35Si-1.82Mn steel. A critical analysis was attempted to explain the observations using displacive mechanism of bainite formation in the attempt to endeavour to understand the kinetics of bainitic transformation during isothermal holding. The model predicted that carbon enrichment in austenite was of particular significance in governing the stability of retained austenite. Thus, through the contribution of transformation induced plasticity effect of retained austenite, high tensile strength (964?MPa) and excellent ductility (uniform elongation of 24.5% and total elongation of 32%) was obtained on isothermal holding at 400°C.     

Carbide-free bainite in medium carbon steel

Heat-treatment processes to obtain carbide-free upper bainite, low bainite and low-temperature bainite in the 34MnSiCrAlNiMo medium-carbon steel were explored. Results show that in the steel bainite transformation mainly goes through three stages: short incubation, explosive nucleation and slow growth. When transformation temperature, T > Ms + 75 °C, upper bainite consisted of catenary bainitic ferrite and blocky retained austenite is obtained in the steel. When Ms + 10 °C < T < Ms + 75 °C, lower bainite is the main morphology composed of lath-like bainitic ferrite and flake-like retained austenite. When T < Ms + 10 °C, the lower bainite, also known as low-temperature bainite, is obtained, which contains much thinner lath-like bainitic ferrite and film-like retained austenite. Mechanical testing results show that the lower the transformation temperature is, the better comprehensive performance is. The low-temperature bainite has the very high tensile strength and impact toughness simultaneously. The lower bainite has lower tensile strength and higher impact toughness. The upper bainite has higher tensile strength and lower impact toughness. The big difference of the mechanical performance between these kinds of bainite is mainly caused by interface morphology, size, and phase interface structure of the bainitic ferrite and the retained austenite. Additionally, when the bainite transformation temperature is decreased, the high-angle misorientation fraction in packets of bainite ferrite plates is increased. High-angle misorientation between phase interfaces can prevent crack propagation, and thus improves impact toughness.     

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.     

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

Abstract

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

In situ synchrotron X-ray study of bainite transformation kinetics in a low-carbon Si-containing steel

The bainite transformation in a low-carbon Si-containing steel has been studied in situ by synchrotron X-rays. While the austenite is homogeneous prior to transformation, the carbon distribution becomes nonuniform as bainite plates form. This is because of the different degrees of physical isolation of films and blocks of residual austenite. The method for converting dilatational strain into bainite volume fraction, using lattice strain as a reference, during isothermal transformation was found to overestimate it. The bainitic and martensitic ferrite did not exhibit a tetragonal unit cell due to the low-carbon content of the steel and the high transformation temperature.     

Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding

A Fe-based coating with nano-scale bainitic microstructure was fabricated using laser cladding and subsequent isothermal heat treatment. The microstructure of the coating was observed and analyzed using optical microscope (OM), field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results showed that nanostructured bainitic ferrite and carbon-enriched retained austenite distributed uniformly in the coating. Blocky retained austenite was confined to the prior austenite grain boundaries resulting from the elements segregation. The bainitic microstructure obtained at 250 °C had a finer scale compared with that obtained at 300 °C. The volume fraction of austenite increased with increasing transformation temperature for the fully transformed bainitic coating. The bainitic transformation was accelerated as a result of the fine prior austenite generated during the laser cladding. The evolution of the carbon contents in bainitic ferrite and retained austenite revealed the diffusionless mechanism of the bainitic transformation.     

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 austenisation temperature on bainite transformation below martensite starting temperature

Effects of austenisation temperature on martensite and bainite transformation behaviour, microstructure, and mechanical properties of a bainitic steel austempered below martensite starting temperature were investigated in this study. Results show that the amount of athermal martensite gradually increased with the increase of austenisation temperature, whereas the amounts of bainite and retained austenite initially increased and then decreased, resulting in the trend of the first increase and then decrease in the product of tensile strength and elongation. In addition, the transformation rate of isothermal bainite after athermal martensite formation revealed a trend of deceleration and then acceleration with austenisation temperature at the beginning period. Moreover, the size of bainite plates decreased first and then increased with austenisation temperature.     

Acoustic emission monitoring of bainitic and martensitic transformation in medium carbon steel during continuous cooling

Abstract

This paper concerns acoustic emission (AE) measurements during continuous cooling of steel C45 using a Gleeble 1500 thermomechanical simulator. After austenising at a certain temperature, the studied specimen was cooled down and the root mean square (RMS) value of the continuous AE signal was measured. During cooling two distinct peaks in the RMS data were observed at temperatures of 200-300°C and 500-600°C, which have been attributed to martensite and bainite formation respectively. The observed bainite peak strongly indicated that the mechanism of bainite growth is displacive. The AE monitoring of bainite and martensite formation was supported by dilatation measurements, which were performed simultaneously. The effect of the austenite grain size on the evolution of the bainitic and martensitic transformation was studied by varying the austenising temperature T _a. It was found that upon lowering T _a, i.e. with decreasing austenite grain size, the bainite peak increases while the martensite peak decreases.     

Tensile deformation of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

Two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~400–250°C) have been deformed in tension. The bainitic microstructures in these steels contain appreciable amounts of retained austenite (instead of interlath cementite), and the behaviour of this phase during tensile deformation, and its apparent influence on the mechanical properties, has been examined. In particular, it is shown that provided the retained austenite exists in an interlath, thin-film morphology it has appreciable mechanical stability. Larger volumes of retained austenite have less mechanical and thermal stability, forming plate martensite structures and also undergoing deformation twinning. The effects of these variations on tensile strength and ductility are discussed.

MST/527     

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.     

Mechanical stabilisation of bainite

Abstract

Compression experiments in which plastically deformed austenite is allowed to transform to bainite have revealed that bainite, like martensite, is susceptible to mechanical stabilisation. The overall transformation kinetics becomes slower and the maximum attainable fraction of bainite decreases in deformed austenite. This is because the motion of the transformation interface is hindered by the accumulated debris of dislocations in the austenite. The number density of nucleation sites is increased in deformed austenite, resulting in a more refined microstructure. Severe deformation eventually leads to a recovery in the maximum attainable fraction of bainite because of the corresponding increase in nucleation site density.

MST/3148     

Modification of microstructure to increase impact toughness of nanostructured bainite–austenite steel

Abstract

To improve impact toughness of the nanostructured bainite–austenite steel, a heat treatment operation was developed to divide prior austenite grains by plates of martensite directly before isothermal transformation. In the investigation, nanostructured steel containing 0·55%C, 1·95%Mn, 1·82%Si, 1·29%Cr and 0·72%Mo was used. It was found that a partial transformation to martensite achieved by cooling to 160°C followed by direct isothermal transformation to bainite at 225°C was the most promising treatment to improve Charpy impact energy of the investigated steel. For each testing temperature: ambient, 0, ?20, ?40 and ?60°C, the specimens subjected to the developed treatment showed a higher averaged impact energy than the specimens subjected to the standard treatment.     

Transition from bainite to acicular ferrite in reheated Fe–Cr–C weld deposits

Abstract

Factors controlling the transition from acicular ferrite to bainite in Fe–Cr–C weld metals have been investigated. It appears that the presence of allotriomorphs of ferrite at austenite grain boundaries has the effect of suppressing the formation of bainitic sheaves. This in turn allows the acicular ferrite plates to develop on intragranular nucleation sites. A theoretical analysis indicates that bainitic transformation is prevented from developing at the allotriomorphic ferrite/austenite boundaries by the carbon concentration field present in the austenite at the allotriomorphic ferrite/austenite interface. This field does not homogenise within the residual austenite during the time scale of the experiments.

MST/1217     

The effect of alloying elements on the structure and mechanical properties of ultra low carbon bainitic steels

Microalloying with Nb and B leads to a granular bainite microstructure which is composed of a bainitic ferrite matrix and a uniformly distributed martensite/austenite-constituent in the as-rolled condition. Due to this transformation strengthening mechanism, high strength and toughness could be achieved even though the C content was extremely low. It was found that both dissolved Nb in austenite and free B are prerequisites for granular bainite formation. Furthermore, there is a critical B content to achieve the complete bainitic transformation strengthening effect. The critical B content increases with C content. C thus diminishes the effect of B in promoting bainite transformation, due to the formation of boron carbides or the depletion of dissolved Nb in austenite. The effect of Mn, Mo and Ni on the decomposition of austenite is similar. A parameterMneq which relates the effect of these alloying elements on the Bs temperature was derived. It was confirmed that the strength of bainitic steels is inversely proportional to theBs temperature.     

High temperature mechanical properties of triple phase steels

A 0.15% C–1.2% Si–1.7% Mn steel was intercritically annealed at 780 °C for 5 min and then isothermally held at 400 °C for 4 min followed by oil quenching to room temperature and the annealed microstructure consist of 75% ferrite , 15% bainite and 10% retained austenite was produced. Samples of this steel with triple phase structure were tensile tested at temperature range of 25–450 °C. Stress–strain curves showed serration flow at temperature range of 120–400 °C and smooth flow at the other temperatures. All of the stress–strain curves showed discontinuous yielding at all testing temperatures. Both yield and ultimate tensile strength decreased with increasing temperature, but there exists a temperature region (120–400 °C) where a reduction of strength with increasing temperature is retarded or even slightly increased. The variation in the mechanical properties with temperature was related to the effects of dynamic strain aging, high temperature softening, bainite tempering and austenite to martensite transformation during deformation.