Bainite Reaction Research Articles

Effect of Heat Treatment on Microstructure and Mechanical Properties of High-Carbon and High-Manganese Cast Steel Subjected to Bainitic Reaction

Effect of Heat Treatment on Microstructure and Mechanical Properties of High-Carbon and High-Manganese Cast Steel Subjected to Bainitic Reaction

Development of advanced cementite-free bainitic steel using Ni, Al and Cu additions – Design concept and phase transformations study

A novel concept, consisting of the stabilization of retained austenite with Ni, Al, and Cu additions instead of using Si as the major alloying element to prevent carbide formation, is proposed to design nanocrystalline, cementite-free bainitic steels. In this work, the novel steel (wt.%) Fe-0.40C-0.50Mn-8.0Ni-2.65Al-0.8Cu was designed. Dilatometry investigations were performed to obtain the experimental CCT and TTT diagrams for this material. It was concluded that the bainitic reaction occurs between 400 and 225 °C during a transformation time ranging from 2 h to 66.5 h, respectively. An in-depth analysis performed on the resulting materials confirmed the presence of an ultrafine structure free of cementite consisting of carbon supersaturated laths of tetragonal bainitic ferrite and retained austenite with thin film and block morphologies. The use of Ni, Al and Cu as alloying elements leads to high carbon content in both populations of retained austenite, which prevents, the formation of fresh martensite during cooling to room temperature. The results of this research constitute a novel insight into the design of ultra-fine, cementite-free bainitic steels.

Non-isothermal kinetics and the effects of alloying elements on bainite precipitation in the Cu74.5Al15.0Mn10.5 alloy

In as-quenched CuAlMn-based alloys, the bainitic precipitation can occur between 450 and 625 K on heating. This reaction can promote diffusion into the martensitic phase and can damage the shape memory effect. In this study, the effect of the fourth alloying element (Ag, Ga, and Gd) on bainitic precipitation kinetics in the Cu74.5Al15.0Mn10.5 alloy was evaluated using differential scanning calorimetry, optical microscopy, X-ray diffraction, and the non-isothermal isoconversional method. The study of the Gd addition to the Cu74.5Al15.0Mn10.5 alloy indicated that the Cu5Gd phase increased the number of nucleation sites, thereby reducing the average isoconversional activation energy for the bainitic reaction. Nonetheless, the Ag and Ga additions promoted a more effective change in the kinetic mechanism of the bainitic precipitation. The Ga addition reduced the isoconversional activation energy value and induced the bainitic phase formation at lower temperatures, but it also introduced parallel reactions that turned their lateral growing slower. On another hand, Ag elevated the initial isoconversional activation energy value and shifted up the bainitic precipitation starting, but the presence of Ag precipitates induced the bainite formation in a narrower temperature range. Therefore, the additions of Ag, Ga, and Gd to the Cu74.2Al15.0Mn10.5 alloy changed bainitic precipitation kinetics, which can be associated with modifications in the Cu diffusion energy and the number of nucleation points.

Effect of coiling temperature on the structure and properties of thermo-mechanically rolled S700MC steel

The boron-free S700MC steel is usually produced by exploiting the properties of a ferrite-bainite mixed microstructure formed by coiling the strips at a temperature of about 450?C, i.e.below the bainite starting temperature. With the aim of further enhancing the mechanical properties of 6 to 10 mm thick strips, industrial tests were carried out at a coiling temperature of 600?C to promote the formation of a structure of ferrite and carbides, which is also acceptable for this type of steel. Unexpectedly, a microstructure composed of ferrite and martensite was obtained. Compared to the ferritic-bainitic grade, the new structure is characterized by a slight decrease of the yield point but by an increase of the ultimate tensile strength by no less than 80 MPa, with a transition from a quasi-discontinuous to a clearly continuous yielding behaviour. Accordingly, the ratio of yield strength to tensile strength decreases from 0.90 to 0.75 and the impact energy decreases by 35 J and 60 J for the two gauge levels, respectively. The mechanical behaviour of the strips coiled at high temperature is explained as a direct consequence of the dual phase structure with a hard phase interspersed in a soft ferrite matrix. The presence of martensite is explained by the so-called incomplete bainite reaction. The partial transformation into ferrite after coiling and the long time required for the coil to cool down stabilize the untransformed austenite due to the carbon enrichment making bainite formation at lower temperatures impossible.

Effect of the Austenitization Route on the Bainitic Reaction Kinetics and Tensile Properties of an Alloyed Austempered Ductile Iron

The effect of the austenitization route on the bainitic reaction kinetics of an alloyed (3.2C–2.8Si–1.8Ni–1.4Cu–0.4Mn–0.2Mo–0.1Cr) austempered ductile iron was studied. Two-step, conventional and rapid austenitization heat treatments were employed to produce different austenite grains sizes (94, 39, and 15 μm, respectively) and, in one case, secondary graphite precipitation. The overall bainitic transformation kinetic at 350 °C was described using the Johnson-Mehl-Avrami-Kolmogorov equation, and the values of the Avrami's adjustable parameters were discussed. The austempering reaction of the coarser austenite microstructure with secondary graphite precipitation featured the fastest kinetics, while the one derived from the medium austenite grain showed the slowest reaction rate. The increase in the graphite/austenite interfacial area reduced the half-transformation time (t50-value) by one magnitude compared to the austenite grain boundary area. The austempered samples from the rapid austenitization route comparatively featured the best tensile properties and the highest ISO and ASTM standards grades despite the considerable proportion of grain boundary allotriomorphic ferrite.

Effect of Microsegregation and Bainitic Reaction Temperature on the Microstructure and Mechanical Properties of a High-Carbon and High-Silicon Cast Steel

Bainitic microstructures obtained in high-carbon (HC) and high-silicon (HSi) steels are currently of great interest. Microstructural evolution and the bainitic transformation kinetics of a high-carbon and high-silicon cast steel held at 280, 330, and 380 °C was analyzed using dilatometry, X-ray diffraction, optical and scanning electron microscopy, and electron backscatter diffraction (EBSD). It is shown that the heterogeneous distribution of silicon (Si), manganese (Mn), and chromium (Cr) associated to microsegregation during casting has a great impact on the final microstructure. The transformation starts in the dendritic zones where there is a lower Mn concentration and then expands to the interdendritic ones. As Mn reduces the carbon activity, the interdendritic areas with a higher Mn concentration are enriched with carbon (C), and thus, these zones contain a greater amount of retained austenite plus martensite, resulting in a heterogeneous microstructure. Higher transformation temperatures promote higher amounts of residual austenite with poor thermal/mechanical stability and the presence of martensite in the final microstructure, which has a detrimental effect on the mechanical properties. Tensile tests revealed that the ultra-fine microstructure developed by the transformation at 280 °C promotes very high values of both tensile and yield stress (≈1.8 GPa and 1.6 GPa, respectively), but limited ductility (≈2%).

Influence of the Austempering Time on the Mechanical Properties of Carbide-Free Bainitic Cast Steels

Three medium-carbon, high-silicon cast steels with different alloy contents were austempered at 330 °C for different holding times in order to obtain carbide-free bainitic microstructures. Aiming at evaluating the influence of holding time and microstructural features on strength and ductility, tensile properties were measured for each steel at selected austempering times. The results obtained indicate that it is possible to adjust holding time in order to obtain the best strength/ductility combination at determined austempering temperature. Moreover, it has been shown that the mechanical stability of retained austenite is the key factor in controlling tensile performance. Short austempering times result in low carbon enrichment of the austenite (low stability) and promote higher ultimate tensile strength and lower ductility. For longer austempering times, steels present a slight decrease in ultimate tensile strength but a marked increase in ductility. This work shows that it is possible to obtain cast steels with ultimate tensile strength of 1682 MPa, yield strength of 1493 MPa and total elongation of 12.5% by means of bainitic reaction. This strength/ductility combination and others reported in this study are remarkable for cast steels.

Isothermal Treatment at Low Temperatures Applied to AISI 4340 Steel: Effects on Hardness, Toughness and Microstructure

The effect of the parameters of isothermal treatment at low temperatures, also called Austempering, on the impact toughness, hardness and microstructure of AISI 4340 steel was investigated. For this purpose, specimens for impact toughness tests were machined according to ASTM E 23-93a. For hardness tests, prismatic bars of 10mmx10mmx30 mm were manufactured. Austempering was performed in a salt bath furnace using 55% sodium nitrate and 45% potassium nitrate. Three temperatures: 300, 325 and 350 °C, and four soaking times: 1 min, 10 min, 1 hour and 10 hours, were used. Hardness tests were performed on Rockwell durometer and three indentations per sample on HRc scale were made. The impact toughness test was done with the Charpy method. The specimens, previously machined, were subjected to austempering, and then subjected to the respective tests. The maximum hardness value 56.75 HRC was found at 300 °C with 10 min of soaking time, and the maximum impact toughness value was 27.67 J, found under the same conditions. It was observed that the hardness and impact resistance are not always inversely related during Austempering. In all microstructure tests, a biphasic structure composed of lower bainite with lath martensite in different proportions was observed, showing that the bainitic reaction is never complete. it is concluded that when alloyed steels are subjected to the Austempering treatment, a decrease in hardness with the soaking time or variation in temperatura, is not always accompanied by an increase in impact toughness, since undissolved alloyed carbides can produce embrittlement at any time and temperature interval.

Effect of the Microsegregation on Martensitic and Bainitic Reactions in a High Carbon-High Silicon Cast Steel

Casting processes show some weaknesses. A particular problem is presented when the workpiece needs to be subjected to heat treatments to achieve a desired microstructure. This problem arises from the microsegregation phenomena typically present in cast parts. The effect of the microsegregation on the martensitic and bainitic transformations has been investigated in a high carbon-high silicon cast steel, with the approximate composition Fe-0.8C-2Si-1Mn-1Cr (in wt. %), which was poured into 25 mm keel block-shaped sand molds. The microsegregation maps of Cr, Si, and Mn characterized by electron probe microanalysis (EPMA) show that interdendritic regions are enriched while dendrites are impoverished in these elements, implying that their partition coefficients are lower that the unity (k < 1). As-quenched martensitic and austempered bainitic microstructures (at 230 °C) were obtained and analyzed after applying an austenitization heat treatment at 920 °C (holding for 60 min). The thermal etching method used to reveal the prior austenite grain size showed a bimodal grain size distribution, with larger grains in the dendritic regions (≈22.4 µm) than in the interdendritic ones (≈6.4 µm). This is likely due to both the microsegregation and the presence of small undissolved cementite precipitates. Electron Backscatter Diffraction (EBSD) analysis carried out on the martensitic microstructure do not unveil any differences in misorientation distribution frequency and block size between the dendritic and interdendritic zones related to the microsegregation and bimodality of the austenite grain size. On the contrary, the bainitic transformation starts earlier (incubation time of 80 min), proceeds faster and bainitic ferrite plates are longer in the dendritic zones, were prior austenite grains are larger and impoverish in solute. The presence of these microsegregation pattern leads to the non-uniform development of the bainitic reaction in cast parts, modifying its kinetics and the resulting microstructures, which would probably have a major impact on the mechanical properties.

Effect of microsegregation on carbide-free bainitic transformation in a high-silicon cast steel

ABSTRACTA high silicon cast steel was studied in the as-cast condition in order to characterise its solidification macrostructure and microsegregation. The steel, poured into 32 mm-keel-block-shaped moulds, has a coarse solidification structure and marked microsegregation, containing low-alloyed areas with a total alloy content (Cr + Mn + Si) of 2.3 wt-% and high-alloyed zones of 5.3 wt-%. The bainitic transformation behaviour at 300°C was studied at different austempering times. The bainitic reaction occurs at different rates within the specimen volume, because of its chemical heterogeneity. An austempering heat treatment leads to an inhomogeneous carbide-free bainitic microstructure with different phase amounts, morphologies and sizes. The heterogeneous distribution of sizes and chemical compositions of retained austenite is speculated to benefit mechanical properties.

Effect of austempering time on microstructure and properties of a low-carbon bainite steel

The effect of austempering time after the bainitic transformation on the microstructure and property in a low-carbon bainite steel was investigated by metallography and dilatometry. The results showed that by prolonging the austempering time after the bainite transformation, the amount of large-size martensite/austenite islands decreased, but no significant change of the amount and morphology of bainite were observed. In addition, more austenite with a high carbon content was retained by prolonging the holding time at the bainite transformation temperature. Moreover, with a longer holding time, the elongation was improved at the expense of a small decrease in tensile strength. Finally, the Avrami equation BRF = 1− exp(−0.0499 × t0.7616) for bainite reaction at 350°C was obtained for the tested steel. The work provided a reference for tailoring the properties of low-carbon steels.

Effect of nickel aluminide on the bainite transformation in a Fe-0.45C–13Ni–3Al–4Co steel, and associated properties

It is known that precipitates that are present in austenite can influence its subsequent transformation behaviour. This investigation deals specifically with the role of nickel aluminide precipitates on the rate of the bainite reaction, in recent alloys intended for applications at about 400∘C. It is demonstrated that the aluminides lead to an increase in the nucleation kinetics of the bainite. Furthermore, their presence increases the total quantity of bainite that forms, thus helping to reduce the scale of the austenite retained in the final microstructure. There is therefore, an increase in the fracture and impact toughnesses without sacrificing the strength. Diffraction experiments show that the retained austenite is optimally stable during the tensile test of the alloy containing NiAl.

The Effect of Stress on Bainite Transformation, Microstructure, and Properties of a Low‐Carbon Bainitic Steel

The effects of applying external stress on bainite transformation, microstructure, and mechanical properties are investigated by metallography, dilatometry, etc. Results show that compressive stress during bainite transformation accelerates the bainite transformation and increases the final bainite amount due to the additional mechanical driving force. The lath‐like bainite becomes shorter, and smaller blocky martensite/austenite islands form under stress during the bainite reaction. Different from the results with stress during bainite transformation, applying stress after bainite transformation during isothermal holding cannot increase the volume fraction of bainite, and no obvious change in bainite morphology is observed. In addition, the subsequent martensite transformation is promoted during the cooling process. Moreover, the less amount of retained austenite (RA) but more carbon content in RA is obtained with applying stress after the bainite reaction. Finally, the tensile strength increases, while the elongation decreases when the external stress is imposed after bainite transformation.

Effect of Carbon Content on Bainite Transformation Kinetics and Microstructure of 4140/4150 Steels

4140 steel and 4150 steel are different only in their carbon content. A comparison between the kinetics and microstructure of the bainite transformation of both steels has been carried out under a wide range of isothermal temperatures, in which four bainite phase matrices were generated. The four phases are upper bainite, mixed upper and lower bainite, lower bainite and mixed martensite and lower bainite (lower bainite transformed under Ms temperature). The bainite transformation kinetics-morphology characteristics of the four phases were studied and compared. Kinetics behavior patterns are shown with the developed TTT diagrams and kinetics plots- (bainite volume fraction vs isothermal holding time). The upper bainite and lower bainite transition temperatures are different and the bainite reaction time span is shorter when carbon content is higher. The n values in the Johnson-Mehl- Kolmogorov-Avrami (JMKA) equation have a wider deviation when carbon content is higher, indicating more bainite reaction variation. The activation energy required for the bainite phase transformations were compared and it was found that higher activation energy is needed when carbon content is higher. The subtle microstructure morphology differences are illustrated. This work will aid future development of 4140/4150 steels with bainitic microstructures having improved mechanical properties.

Effect of Nickel Aluminide on the Bainite Transformation

It is known that the presence of precipitates in the austenite can influence its subsequent transformation kinetics. This investigation deals specifically with the role of nickel aluminide precipitates on the rate of the bainite reaction, in recent alloys intended for applications at about 400°C. It is demonstrated that the aluminides lead to an increase in the nucleation kinetics of the bainite. Furthermore, their presence increases the total quantity of bainite that forms, thus helping to reduce the amount of blocky retained-austenite in the final microstructure.

Influence of Prior Martensite on Bainite Transformation, Microstructures, and Mechanical Properties in Ultra-Fine Bainitic Steel.

A multiphase microstructure comprising of different volume fractions of prior martensite and ultra-fine bainite (bainitic ferrite and retained austenite) was obtained by quenching to certain temperatures, followed by isothermal bainitic transformation. The effect of the prior martensite transformation on the bainitic transformation behavior, microstructures, and mechanical properties were discussed. The results showed that the prior martensite accelerated the subsequent low-temperature bainite transformation, and the incubation period and completion time of the bainite reaction were significantly shortened. This phenomenon was attributed to the enhanced nucleation ratio caused by the introduced strain in austenite, due to the formation of prior martensite and a carbon partitioning between the prior martensite and retained austenite. Moreover, the prior martensite could influence the crystal growth direction of bainite ferrite, refine bainitic ferrite plates, and reduce the dimension of blocky retained austenite, all of which were responsible for improving the mechanical properties of the ultra-fine bainitic steel. When the content of the prior martensite reached 15%, the investigated steels had the best performance, which were 1800 MPa and 21% for the tensile strength and elongation, respectively. Unfortunately, the increased content of the prior martensite could lead to a worsening of the impact toughness.

Elastically induced pattern formation in the initial and frustrated growth regime of bainitic subunits

We present analytical and numerical results for the dominant mechanisms of pattern selection in two growth regimes which are crucial in elastically influenced solid-solid transformations like the bainitic one. The first growth regime comprises the very early regime, when in a nucleation scenario the size of the nucleus is so small that the bulk crystal structure is typically not yet fully developed and the phase is elastically softened. Here we see a dominant effect of curvature effects in analogy to the theory on growth of lenticular melt inclusions. The second growth regime is of specific interest to bainitic steels. During the bainitic reaction subunits form, grow up to a point where the thermodynamic driving force is kinetically overcome by a deformation-induced growth barrier, stop growth and then nucleate new subunits. Thus, the regime prior to the new subunit nucleation corresponds to the limiting case of vanishing growth velocity. For both, analytical and numerical approach, we use sharp interface descriptions of the problem, for the numerical approach we invoke a representation of the problem in terms of boundary integral equations.

Low-Temperature Bainite: A Thermal Stability Study

The thermal stability of nanobainitic structures obtained by heat treating two different high-carbon high-silicon steels at temperatures between 200 °C and 600 °C has been investigated by means of three complementary techniques, i.e., field emission gun-scanning electron microscopy, X-ray diffraction, and high-resolution dilatometry. Three main stages have been established, each of them characterized by a distinctive microstructure. Furthermore, the nanocrystalline structure generated by the bainite reaction confers the steel with an extraordinary tempering resistance.

Bainitic reaction and microstructure evolution in two normalized and tempered steels designed for service at elevated temperatures

Abstract In the present work conventional heat treatment like normalizing (bainitic microstructure) and tempering of the alloys has been performed. The materials used in this study were two steels, one the laboratory prepared experimental low alloy Cr-Mo steel in comparison to typical commercial 10CrMo9-10 steel. The determined carbon concentrations of the residual austenite at the different temperatures of bainite transformation supports the hypothesis that the growth of bainitic ferrite occurs without any diffusion with carbon being partitioned subsequently into the residual austenite. It was found that bainitic reaction has stopped when average carbon concentration of the untransformed austenite is close to the T0 line and supports formation of bainitic ferrite by a shear mechanism, since diffusionless transformation is not possible beyond the T0 curve. Normalized samples were air cooled down to room temperature before tempering at various temperatures in the range of 500-750°C. Samples have been austenitized at 980°C for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750°C for 1 hour. After heat treatment, the assessment in the microstructure and phase precipitation was made using the samples prepared for metallographic and transmission electron microscope (TEM) on thin foils analysis. Quantitative X-ray analysis was used to determine the retained austenite content after heat treatment like normalizing and tempering and the total volume fraction of the retained austenite was measured from the integral intensity of the (111)γ and (011)α peaks. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure, the growth of the size of M23C6 carbides, and precipitation of the secondary M2C precipitates. The magnitude of these changes depended on the temperature of tempering.

Influence of bainite reaction on the kinetics of carbon redistribution during the Quenching and Partitioning process

In the present study the microstructural evolution and kinetics of carbon redistribution during the partitioning step of the Quenching and Partitioning process are investigated by means of a modeling approach that simultaneously considers the martensite-austenite carbon partitioning and the decomposition of austenite into bainitic ferrite. The development of the phase fractions, interface position, and carbon compositions are analyzed for two different binary Fe-C alloys with distinct initial carbon compositions and simulation geometries. The composition dependence of carbon diffusivity in austenite is taken into account for solving the diffusion field equations. Simulations indicate that kinetics of carbon partitioning from martensite to austenite is controlled by carbon diffusion in martensite and it is little affected by simultaneous occurrence of bainite reaction. On the other hand martensite-austenite carbon partitioning strongly influences the bainite reaction by inhibiting the growth of bainitic ferrite.