Low-temperature Bainite Transformation Research Articles

Kinetics of Carbon Enrichment in Austenite during Partitioning Stage Studied via In-Situ Synchrotron XRD

The present study reveals the microstructural evolution and corresponding mechanisms occurring during different stages of quenching and partitioning (Q&P) conducted on 0.6C-1.5Si steel using in-situ High Energy X-Ray Diffraction (HEXRD) and high-resolution dilatometry methods. The results support that the symmetry of ferrite is not cubic when first formed since it is fully supersaturated with carbon at the early stages of partitioning. Moreover, by increasing partitioning temperature, the dominant carbon source for austenite enrichment changes from ongoing bainitic ferrite transformation during the partitioning stage to initial martensite formed in the quenching stage. At low partitioning temperatures, a bimodal distribution of low- and high-carbon austenite, 0.6 and 1.9 wt.% carbon, is detected. At higher temperatures, a better distribution of carbon occurs, approaching full homogenization. An initial martensite content of around 11.5 wt.% after partitioning at 280 °C via bainitic ferrite transformation results in higher carbon enrichment of austenite and increased retained austenite amount by approximately 4% in comparison with partitioning at 500 °C. In comparison with austempering heat treatment with no prior martensite, the presence of initial martensite in the Q&P microstructure accelerates the subsequent low-temperature bainitic transformation.

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.

A Comparative Study on the Dynamic Tensile Behavior of Nanostructured Bainitic and Quenched-Tempered Martensitic Steels

In order to assess the dynamic tensile behavior of a newly developed three-step low-temperature-transformed bainitic microstructure, and a conventional one-step isothermal bainitic microstructure, and a quenched and tempered martensitic microstructure, a comparative research was conducted by using the dynamic tensile test. The results showed that more film-like retained austenite was obtained in the three-step low-temperature-transformed sample. The carbon content in the film-like retained austenite was found to be higher as compared to the blocky retained austenite. The dynamic tensile properties were all improved with an increase in strain rates (from 0.1 to 500 s−1). However, the three-step bainitic low-temperature-transformed sample showed the most remarkable dynamic tensile properties, including ultimate tensile strength and total elongation. This is attributed to a better transformation-induced plasticity effect that is exhibited by the higher stability of the film-like retained austenite. Therefore, the present study suggests that performing a three-step low-temperature bainitic transformation promotes grain refinement and the formation of film-like retained austenite and improves the dynamic tensile properties.

Effect of ausforming on the anisotropy of low temperature bainitic transformation

Ausforming is the process by which austenite is plastically deformed prior to bainitic or martensitic transformation. The advantages of such a process on the kinetics of the bainitic transformations have already been reported, but little is found about the effect on the morphology and crystallography of the final microstructure, and in most cases the studies deal with high C steels. Since ausforming has been suggested to be an alternative to transfer the concept of nanostructured bainite to medium carbon contents, the necessity to study the effect of prior plastic deformation on the subsequent bainitic transformation in those steels arises. By means of techniques such as high-resolution dilatometry, scanning electron microscopy, electron backscatter diffraction and X-Ray diffraction, it has been proven that, in a medium carbon steel (0.4 wt%), the macroscopic isotropy that characterizes the bainitic transformation can be altered, under certain conditions, by plastically deforming austenite prior to transformation. The alteration is such that, at low temperatures, the interpretation of the dilatometric response or the plate thickness measurements can be hindered. In addition, the final microstructure is also affected, presenting highly ordered bainitic ferrite plates, most of them correspondent to specific crystallographic variants.

Features of Isothermal Formation of Carbide-Free Bainite in High-Carbon Manganese-Silicon Steel

Change in the stability of overcooled austenite of high-carbon manganese-silicon steel relative to decomposition via the intermediate stage upon isothermal holding and martensitic γ → α transformation upon subsequent cooling has been studied using dilatometry and X-ray diffraction. The kinetics of low-temperature bainite transformation has been studied. The tetragonality of the crystal lattice of bainite α-phase caused by the superequilibrium content of carbon was found.

A comparative study on the three-body abrasive wear performance of Q&P processing and low-temperature bainitic transformation for a medium-carbon dual-phase steel

A comparative study to assess the three-body abrasive wear performance of medium-carbon dual-phase steels, obtained by the two heat treatments: quenching and partitioning (Q&P) process and low-temperature bainitic transformation, was conducted by employing a designed three-body abrasive wear method. The microstructures were examined using the optical, scanning and transmission electron microscopes. The carbon content of the retained austenite in the samples was measured by electron probe microanalysis. The results showed that the ultrafine dual-phases, consisting of martensite/bainite and retained austenite, were obtained in both the heat treatments. Both the samples were observed to be extremely hard (458–471 HV1). The results of the three-body abrasive wear tests indicated that the abrasion weight loss decreased as the surface hardness increased. Further, compared to the low temperature transformed bainitic sample, the Q&P treated martensitic sample had a higher retained austenite stability, superior transformation-induced plasticity effect and thicker deformed layer which showed a better three-body abrasive wear performance.

Improved uniformity of hardness by continuous low temperature bainitic transformation in prehardened mold steel with large section

Expanding automobile industry energized the demand for large section plastic mold steel, large (465mm × 1325mm × length) pre-hardened mold (PHM) steel was trial-produced by quenching the forged slab at temperature between start (Ms) and finish (Mf) temperature of martensite, followed by tempering, according to the thermal expansion experiments and finite element modeling (FEM) of cooling rate from core to surface. Continuous low temperature bainitic transformation in PHM steel created high volume fractions of refined bainitic-ferrite laths, filmy retained austenite and dispersed carbides. Uniformity of microstructure and hardness, and the impact toughness of the PHM steel were remarkably superior to the one produced by conventional quenching & tempering (QT) process.

Effect of Pre-Existing Martensite on Bainitic Transformation in Low-Temperature Bainite Steel

The effect of different volume fractions of pre-existing martensite on the low-temperature bainitic transformation and microstructures was quantitatively analyzed by dilatometer, optical microscope and scanning electron microscope. The results showed that pre-existing martensitic transformation accelerated the subsequent low-temperature bainitic transformation, and the incubation period and completion time of bainitic reaction were significantly shortened. This phenomenon was attributed to the increasing nucleation sites caused by the introduced dislocations in austenite due to the formation of pre-existing martensite. However, it was noteworthy that, because of the increased bainitic plates adjacent to the pre-existing martensitic plates, the probability of the impingement of bainitic plates during growth was increased, which resulted in a decrease in the maximum attainable volume fraction of bainite.

Transformation behavior and microstructure feature of large strain ausformed low-temperature bainite in a medium C - Si rich alloy steel

Large-strain ausforming and low-temperature bainite transformation were carried out in a medium C - Si rich alloy steel on a thermomechanical simulator. Then, temperature–time–transformation curves of the bainite transformation after ausforming at different temperatures were obtained by dilatometry. The effects of ausforming on transformation kinetics, microstructure, and hardness of samples were studied. Results show that the entire process of bainite transformation can be accelerated by ausforming. With decreasing ausforming temperature, the transformation rates greatly increased at the initial transformation stage but slightly decreased at the final stage. Compared with non-ausformed sample, not only was the maximum growth rate in the ausformed samples larger, but the maximum growth rate also appeared earlier. In addition, the maximum rate of the ausformed bainite transformation was increased and the time of reaching the maximum rate was shortened with decreasing the ausforming temperature. The effect of ausforming on bainite transformation kinetics depended not only on the ausforming temperature and strain but also on the isothermal transformation temperature. The reduction in transformation time was increased under decreased isothermal transformation temperature. Ausforming refined the bainite plates and increased the fraction of retained austenite formed in the subsequent low-temperature isothermal transformation. Consequently, nanostructured bainite with enhanced hardness was prepared in the medium C - Si rich alloy steel.

Effect of retained austenite on wear resistance of nanostructured dual phase steels

Nanostructured super bainitic and quenching–partitioning (Q&P) martensitic steels with a significant amount of retained austenite obtained by low temperature bainitic transformation and Q&P respectively were studied to explore the effect of retained austenite on stirring wear resistance. The results suggest that the Q&P martensitic steel significantly enhanced the hardness of the worn surface (from 674 to 762 HV1) and increased the thickness of the deformed layer (∼3.3 μm), compared to the nanostructured bainitic steel. The underlying reason is that the Q&P martensitic steel has a higher stability of retained austenite thereby providing a superior transformation induced plasticity effect to increase surface hardness and reduce wear rate during the wear process.

The effects of external compressive stress on the kinetics of low temperature bainitic transformation and microstructure in a superbainite steel

Abstract The effects of external compressive stress below the yield strength of austenite on the kinetics of low temperature bainitic transformation and microstructure in a superbainite steel were studied through dilatometry, scanning electron microscopy and X-ray diffraction analysis simultaneously. The results show that even under a relatively low isothermal temperature of 300°C, compressive stress less than yield stress can clearly accelerate bainitic transformation because of the increase in total driving force for bainitic transformation and anisotropic transformation strain. Bainite ferrite becomes longer, the orientations of bainite sheaves become more uniform and the transformation strain becomes anisotropic under external compressive stress because of variants selection. The amount of bainite increases while the amount of martensite decreases with the application of external stress.

Effect of Ausforming Temperature on Bainite Transformation of High Carbon Low Alloy Steel

The effect of ausforming temperature on bainite transformation of high carbon low alloy steel was studied by in situ experiments using a Gleeble 3500 thermal and mechanical testing system. Morphology and crystallography of ausforming bainite were examined by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). It has been found that deformation at all temperatures range from 230°C to 600°C can accelerate low temperature bainite transformation, and transformation rate increased with deformation temperature reduced. Quantitative X-ray analysis shows that the volume fraction of retained austenite was about 35.84% after deformation and isothermal transformation for 20 hours, it was approximately the same amount with austempering bainite transformation process (no strain) which austenite volume fraction was about 32.01%. Low temperature bainite formation can be accelerated with a smaller increase amount of retained austenite by deformation at a low temperature range of 230~600 oC.

Acceleration of Bainite Transformation at Low Temperature by Warm Rolling Process

In this paper, the acceleration effect of warm rolling process on bainite transformation at 230oC for a high-carbon, silicon-rich steel with nanostructured bainite was studied. The warm rolling temperature was determined by simulation experiments using a Gleeble 3500 thermal and mechanical testing system. It is found that the bainite transformation will take more than 50hours to complete at the same isothermal temperature without warm rolling, while the incubation time can be shortened from 6hours to 0.5hours when warm rolling was conducted within a temperature range 300-600oC. In addition, the quantitative X-ray analysis indicated that the volume fraction of retained austenite after warm rolling (total strain 0.20) and isothermal transformation for 20hours was about 33.8%, keeping a same level compared with the austempering bainite transformation process (no strain) which austenite volume fraction was about 32%. Low temperature bainite transformation can be accelerated by warm rolling effectively without compromising strength. And the ultimate tensile strength of warm rolling nanostructured bainite steel is 2127MPa.

Multi-step isothermal bainitic transformation in medium-carbon steel

The effect of multi-step low-temperature bainitic transformation on the microstructure and mechanical properties of a medium-carbon steel was investigated. Compared with the microstructure obtained by conventional bainitic transformation, the blocky microstructure was almost eliminated due to the formation of higher volumes of nanoscale bainitic–ferrite plates and film-like austenite, which lead to a refinement in the average thickness of the bainite plates. The samples with refined microstructure showed superior mechanical properties.

On low temperature bainite transformation characteristics using in-situ neutron diffraction and atom probe tomography

In-situ neutron diffraction was employed to monitor the evolution of nano-bainitic ferrite during low temperature isothermal heat treatment of austenite. The first 10 peaks (austenite, γ and ferrite, α) were monitored during austenization, homogenisation, rapid cooling and isothermal holding at 573K. Changes in the α-110 and γ-111 peaks were analysed to determine the volume fraction changes and hence the kinetics of the phase transformation. Asymmetry and broadening in the α-200 and γ-200 peaks were quantified to lattice parameter changes due to carbon redistribution as well as the effects of size and dislocation density. Atom Probe Tomography was then used to confirm that, despite the presence of 1.5 mass% Si, carbide formation was evident. This carbide formation is the cause of poor ductility, which is lower than expected in such steels.

Isothermal Transformation of Low Temperature Super Bainite

Fine-scale bainitic microstructure with excellent mechanical properties has been achieved by transforming austenite to bainite at low temperature ranging from 200oC to 300oC. Microstructural observations and hardness measurements show that transformed microstructures consist of bainitic ferrite and carbon-enriched retained austenite. The thickness of bainitic ferrite plates is less than 50 nm. The hardness reaches approximately 640 HV1. Strong austenite and/or large driving force at the low transformation temperature leads to ultra fine bainitic ferrite plates. X-ray diffraction analysis indicates that low-temperature bainite transformation is an incomplete reaction. The carbon content in carbon-enriched retained austenite is below the para-equilibrium (Ae3′) phase boundary predicted. The carbon content in bainitic ferrite is less than that T0′ phase boundary predicted.

401 鋼の低温ベイナイト変態挙動と変態組織の特徴(OS4-(1)オーガナイズドセッション《材料と組織》)

401 鋼の低温ベイナイト変態挙動と変態組織の特徴(OS4-(1)オーガナイズドセッション《材料と組織》)

High performance low cost steels with ultrafine grained and multi-phased microstructure

Ultrafine grained ferrite was obtained through tempering cold rolled martensite with an average grain size of 200–400 nm in a low carbon and a microalloyed steel. Thermal and mechanical stability of the two steels was studied. Due to the pinning effect of microalloyed precipitates on the movement of dislocations and grain boundaries, the recrystallization and grain growth rate were retarded, and the thermal stability of ultrafine grained microstructure was improved. The ultrafine grained ferritic steel was strengthened, but its strain hardening rate was reduced. It seems that the tiny carbide precipitates have no significant effect on work hardening rate. The ultrafine grained ferrite+martensite dual phase microstructure was obtained in the microalloyed steel through intercritically annealing cold rolled martensite. The resulting multiphase microstructure has a tensile strength higher than 1.0 GPa with a yield ratio lower than 0.7. Another type of multiphase microstructure with nanoscaled lath bainite+retained austenite was obtained through an isothermal heat treatment in low temperature bainite transformation region in high carbon steel. The tensile strength was as high as 1.64 GPa with a yield ratio of 0.84.

611 鋼の低温ベイナイト変態と強靭化機構(材料と組織)

611 鋼の低温ベイナイト変態と強靭化機構(材料と組織)