Showing papers on "Bainite published in 2012"

Evolution of microstructure and mechanical properties of medium Mn steels during double annealing

Abstract: A double annealing process was applied to cold rolled medium Mn steel. The evolution of both microstructure and mechanical properties during the second annealing were analysed. Austenite reverted transformation (ART) was observed during intercritical annealing. It was shown that a complex ultra-fine microstructure composed of three phases (retained austenite/martensite/ferrite) was formed and two types of morphologies were detected (lath-like and polygonal). Furthermore, a high volume fraction of retained austenite (22%), which was stabilized at room temperature, was the origin of a TRIP effect. A good balance between strength and ductility can be achieved by optimizing the heat treatment. The various results are discussed and some mechanisms are proposed to explain the observations.

Bainite and martensite start temperature calculated with exponential carbon dependence

Abstract: Analysis of published data demonstrates that the start temperatures of bainite (Bs) and martensite (Ms) formation exhibit an exponential carbon dependence. Empirical models are proposed to describe this specific carbon dependence. The models are relatively simple and sufficiently accurate for conventional steels with 0·1–1·9 wt-% carbon and less than 7 wt-% in total of other alloying elements. Predictions of the Bs and Ms temperatures show a better accuracy than those obtained with equations from literature. An improved prediction of the Ms temperature is important to accurately determine of the amount of martensite at a certain arrest temperature using the Koistinen and Marburger (KM) equation. Predictions of the volume fraction martensite are also influenced by the rate parameter αm controlling the kinetics of martensite formation. Based on the improved models for the composition dependence of Ms and αm, the volume fraction of retained austenite at room temperature has been calculated for Fe–C a...

Tensile behaviour of a nanocrystalline bainitic steel containing 3 wt% silicon

Abstract: Much recent work has been devoted to characterize the microstructure and mechanical properties bainitic nanostructured steels. The microstructure is developed by isothermal heat treatment at temperatures as low as 125–350 °C and adapted steel grades typically contain high carbon contents to achieve sufficient depletion of the B S – M S temperature range, and above 1.5 Si wt.% to suppress carbide formation during isothermal holding. On the latter, most of the published literature agrees on a limit of around 1.2–1.5 wt.% to suppress cementite in high carbon steels. For this reason perhaps, additions of Si significantly above this limit have not been investigated systematically in the context of nanostructured bainitic steels. The present work is concerned with the effect of up to ∼3 Si wt.% in a steel grade adapted to low temperature bainitizing. Tensile properties as compared to similar grades, though with lower Si contents, exhibited unrivalled combinations of strength and ductility, with above 21% total elongation for a UTS above 2 GPa. An attempt is made to explain the mechanical properties of this microstructure in terms of some of its most relevant and unique morphological and microstructural features.

Influence of hot plastic deformation and cooling rate on martensite and bainite start temperatures in 22MnB5 steel

Abstract: During hot stamping process, hot forming, cooling and phase transformations are performed in a single step As a matter of fact, multifunctional phenomena happen and affect each other Among these phenomena, martensitic and bainitic transformations have the greatest importance In the current research, the start temperatures of martensite and bainite of 22MnB5 boron steel have been measured in undeformed and 40% deformed conditions, and in various cooling rates from 04 °C/s to 100 °C/s by means of deformation dilatometer It is concluded that, reduction of cooling rate, could bring about an increase or decrease in M s and M f , depending on other phases formation before martensite Also, hot plastic deformation, hindered the martensitic transformation and decreased M f and M s especially at lower cooling rates, while B s increased Furthermore, the critical cooling rate, increased about 40 °C/s by applying 40% hot plastic deformation

Influence of bainite morphology on impact toughness of continuously cooled cementite free bainitic steels

Abstract: The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of martensite/austenite constituent. Results suggest that the mechanism of brittle fracture of cementite free bainitic steels involves the nucleation of microcracks in martensite/austenite islands but is controlled by the bainite packet size.

On the Effect of Manganese on Grain Size Stability and Hardenability in Ultrafine-Grained Ferrite/Martensite Dual-Phase Steels

Abstract: Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material.

High-strength steel sheet and method for manufacturing same

Abstract: A high strength pressed member has excellent ductility and stretch flangeability and tensile strength of 780-1400 MPa, with a predetermined steel composition and steel microstructure relative to the entire microstructure of steel sheet, where area ratio of martensite 5-70%, area ratio of retained austenite 5-40%, area ratio of bainitic ferrite in upper bainite 5% or more, and total thereof is 40% or more, 25% or more of martensite is tempered martensite, polygonal ferrite area ratio is above 10% and below 50% to the entire microstructure of steel sheet, and average grain size is 8 μm or less, average diameter of a group of polygonal ferrite grains is 15 μm or less, the group of polygonal ferrite grains represented by a group of ferrite grains of adjacent polygonal ferrite grains, and average carbon content in retained austenite is 0.70 mass % or more and tensile strength is 780 MPa or more.

Investigation on the microstructure and toughness of coarse grained heat affected zone in X-100 multi-phase pipeline steel with high Nb content

Abstract: Effect of increasing heat input on microstructure evolution and impact toughness of coarse grained heat affected zone (CGHAZ) in high Nb X-100 multi-phase pipeline steel was investigated by means of Gleeble simulator, optical microscope (OM), scanning electron microscope (SEM) and electron backscattering diffraction (EBSD). Charpy impact test confirmed the optimum toughness of CGHAZ was achieved at heat input of 20 kJ/cm, equivalent to the excellent toughness of the base plate. Observations performed by OM, SEM and EBSD show that the microstructure of CGHAZ varies dramatically with heat input without a noticeable changing in prior austenite grain size, and the optimum toughness achieved at the heat input of 20 kJ/cm is related to the cumulative contribution of its well-refined martensite/austenite (M/A) constituent and the highest density of high angle boundaries. Analysis on crystallography shows that high angle boundaries are mainly the boundaries between the products from different Bain groups produced from the fcc to bcc coherent transformation within prior austenite grain, and the density of high angle boundary is controlled by the configuration of Bain groups within the crystallographic packet in each austenite grain. With the ideal configuration, the density of high angle boundary can be optimized to be beneficial to keep high toughness in CGHAZ, together with well-refined M/A constituent. This indicates that in addition to M/A refinement, the characteristic in crystallography of the crystallographic packet (the configuration of Bain groups within it) is related to the mechanical properties of CGHAZ and should be controlled to be optimum.

Iron and Steel

Abstract: Steels Steels are iron-base alloys usually containing carbon. Figure 7.1 shows the iron-carbon phase diagram. Below 911°C and between 1410°C and the melting point, iron has a bcc crystal structure called ferrite . Between 1410°C and 911°C it has an fcc crystal structure called austenite . Austenite dissolves much more carbon interstitially than ferrite. On slow cooling below 727°C, the austenite transforms by a eutectoid reaction into ferrite and iron carbide or cementite (which contains 6.7%C). The ferrite and cementite form alternating platelets called the eutectoid temperature. The resulting microstucture is called pearlite (see Figure 7.2). Pearlite Formation When a steel containing less than 0.77%C ( hypo-eutectoid steel ) is slowly cooled, some ferrite forms before any pearlite. A steel containing more than 0.77%C ( hyper-eutectoid steel ) will form some cementite before any pearlite. The formation of pearlite from austenite on cooling requires diffusion of carbon from ahead of the advancing ferrite platelets to the advancing carbide platelets as indicated in Figure 7.3. Because diffusion takes time, pearlite formation is not instantaneous. The rate at which pearlite forms depends on how much the temperature is below 717°C. The rate of diffusion increases with temperature, but the driving force for the transformation increases as the temperature is lowered. The result is that the rate of transformation is fastest between 500 and 600°C, as indicated schematically in Figure 7.4.

On measurement of carbon content in retained austenite in a nanostructured bainitic steel

Abstract: In this study, the carbon content of retained austenite in a nanostructured bainitic steel was measured by atom probe tomography and compared with data derived from the austenite lattice parameter determined by X-ray diffraction. The results provide new evidence about the heterogeneous distribution of carbon in austenite, a fundamental issue controlling ductility in this type of microstructure.

A new effect of retained austenite on ductility enhancement in high strength bainitic steel

Abstract: A designed high strength bainitic steel with considerable amount of retained austenite is presented in order to study the effect of retained austenite on the ductility enhancement in bainitic steels. Transformation induced plasticity (TRIP) effect is verified by both X-ray diffraction (XRD) measurement of retained austenite fraction in various deformation stages and transmission electron microscopy observation of the deformed twin-type martensite. Results from XRD line profile analysis reveal that the average dislocation density in bainite during the deformation is lower than that before deformation, and such a phenomenon can be explained by a new effect, dislocations absorption by retained austenite (DARA) effect, based on our previous investigation of martensitic steels. DARA effect availably enhances the compatibility of deformation ability of bainite with retained austenite. In view of microstructure similarity of bainitic steels with martensitic steels, the conditions of DARA effect are proposed. The effects of retained austenite on the ductility enhancement in bainitic steels are clarified.

Effect of hardness of martensite and ferrite on void formation in dual phase steel

Abstract: The influence of the hardness of martensite and ferrite phases in dual phase steel on void formation has been investigated by in situ tensile loading in a scanning electron microscope. Microstructural observations have shown that most voids form in martensite by evolving four steps: plastic deformation of martensite, crack initiation at the martensite/ferrite interface, crack propagation leading to fracture of martensite particles and void formation by separation of particle fragments. It has been identified that the hardness effect is associated with the following aspects: strain partitioning between martensite and ferrite, strain localisation and critical strain required for void formation. Reducing the hardness difference between martensite and ferrite phases by tempering has been shown to be an effective approach to retard the void formation in martensite and thereby is expected to improve the formability.

TRIP aided deformation of a near-Ni-free, Mn–N bearing duplex stainless steel

Abstract: A near-Ni-free, Mn–N bearing duplex stainless steel (D-SS) that shows transformation induced plasticity was developed. The present D-SS exhibited an excellent strength–ductility combination over 1000 MPa tensile strength and 50% elongation. An analysis of the element partitioning during annealing revealed that the stacking fault energy of austenite was low enough for a strain induced martensite (SIM) transformation to occur. The strain hardening rate began to increase at ∼10% strain with the same manner of SIM fraction. The TEM and EBSD analyses showed that not only the ɛ martensite band intersections but the austenite grain boundaries acted as the SIM nucleation sites. The SIM transformation was saturated because of the austenite grain refinement and the corresponding austenite stabilization. The austenite grain refinement was caused by the mutual impingement of growing SIM and as a result by the engulfment of remaining austenite by SIM. The deformation behavior of the present D-SS was characterized by analyzing the kernel average misorientation (KAM) of the constituent phases with strain. The KAM distribution of austenite, ferrite and SIM exhibited different characteristics. The average KAM of austenite and ferrite increased as the strain increased, but its increasing rate of austenite was higher than that of ferrite. These KAM characteristics were discussed along with the dislocation glide modes of austenite and ferrite. By contrast, the average KAM of SIM was insensitive to strain and higher than that of the other two phases.

Dual phase structure formed by partial reversion of cold-deformed martensite

Abstract: Effect of prior deformation and heating rate on the dual phase (DP) structure formed by partial reversion of cold-rolled martensite was investigated in a low carbon steel (0.15%C–1.0%Mn). The steel plate was quenched after austenitization to obtain full martensitic structure and then cold-rolled with varying reductions. The cold-rolled specimens were continuously heated at a slow (0.083 K/s) or fast (100 K/s) heating rate up to a temperature above A1 point to partially form reversed austenite. Increasing rolling reduction rate or lowering heating rate enhanced recrystallization on heating before the onset of reversion, while the undeformed martensite never caused recrystallization irrespective of heating rate. The matrix of DP structure was changed from tempered martensite to equiaxed ferrite through the recrystallization, which resulted in a large difference in the distribution of fresh martensite (reversed austenite). Tensile testing revealed that the excellent strength-elongation balance was obtained in the DP steel produced from undeformed martensite, while higher strength was realized in the steel with prior deformation. With increasing the rolling reduction and the heating rates, the grain size of recrystallized ferrite becomes finer and the tensile strength is more increased. It was also suggested that the competition between recrystallization and reversion during continuous heating could be predicted by the modified tempering parameter.

Role of ɛ martensite in tensile properties and hydrogen degradation of high-Mn steels

Abstract: Effects of ɛ martensite on tensile properties and hydrogen degradation behaviors of a high Mn steel were investigated. For this purpose, a Fe–15Mn–2Cr–0.6C steel containing various amount of ɛ martensite was prepared and tensile tested at room temperature. Microstructures were examined by electron back scattered diffraction and transmission electron microscopy. Then, a series of electrochemical hydrogen pre-charging, slow strain rate tests, and thermal desorption spectrometry (TDS) analyses was conducted to examine the hydrogen degradation behaviors. Deformation of the steel without ɛ martensite (i.e. fully austenitic) was dominated by slip and mechanical twinning, but that of the steel containing ɛ martensite was mainly attributed to transformation induced plasticity in association with strain induced martensitic transformation during deformation, resulting in higher work hardening rate. However, tensile strength and elongation on the steel containing ɛ martensite were lower than those of the fully austenitic steel, since cracks were prone to be initiated and propagated at the region of ɛ martensite which is harder than austenite. Furthermore, it was found that ɛ martensite provided many diffusible hydrogen trapping sites. Consequently, the notch fracture stress of the steel containing ɛ martensite decreased significantly as the diffusible hydrogen content increased. The activation energy for hydrogen detrapping from its trapping sites was also calculated by means of the TDS analyses, ∼22 kJ/mol for the γ/ɛ interfaces, and ∼37 kJ/mol for dislocations/γ grain boundaries.

Microstructure and mechanical properties of weld-bonded and resistance spot welded magnesium-to-steel dissimilar joints

Abstract: The aim of this study was to evaluate microstructures, tensile and fatigue properties of weld-bonded (WB) magnesium-to-magnesium (Mg/Mg) similar joints and magnesium-to-steel (Mg/steel) dissimilar joints, in comparison with resistance spot welded (RSW) Mg/steel dissimilar joints. In the WB Mg/Mg joints, equiaxed dendritic and divorced eutectic structures formed in the fusion zone (FZ). In the dissimilar joints of RSW and WB Mg/steel, FZ appeared only at Mg side with equiaxed and columnar dendrites. At steel side no microstructure changed in the WB Mg/steel joints, while the microstructure in the RSW Mg/steel joints consisted of lath martensite, bainite, pearlite and retained austenite leading to an increased microhardness. The relatively low cooling rate suppressed the formation of shrinkage porosity but promoted the formation of MgZn 2 and Mg 7 Zn 3 in the WB Mg/steel joints. The added adhesive layer diminished stress concentration around the weld nugget. Both WB Mg/Mg and Mg/steel joints were significantly stronger than RSW Mg/steel joints in terms of the maximum tensile shear load and energy absorption, which also increased with increasing strain rate. Fatigue strength was three-fold higher for WB Mg/Mg and Mg/steel joints than for RSW Mg/steel joints. Fatigue failure in the RSW Mg/steel joints occurred from the heat-affected zone near the notch root at lower load levels, and in the mode of interfacial fracture at higher load levels, while it occurred in the Mg base metal at a maximum cyclic load up to ∼10 kN in both WB Mg/Mg and Mg/steel joints.

Morphology and substructure of martensite in steels

Abstract: Martensite in ferrous alloys exhibits various morphologies, chiefly lath, lenticular and thin plate, depending on chemical compositions and M s temperature. This chapter reviews crystallographic features and substructures for each of these specific forms. Furthermore, the chapter discusses crystallographic features of packet and block in lath martensite, the origin of dislocation structure in lath and lenticular martensites, and the origin of the midrib in lenticular martensite.