Effect Of Austenite Deformation Research Articles

Continuous cooling transformation behaviour of 30CrMo steel simulated by CSP process

ABSTRACT The continuous cooling transformation (CCT) diagram of 30CrMo steel simulated by compact strip production process was constructed. And the effect of austenite deformation on the CCT behaviour was investigated. The results showed that the microstructure gradually transformed from proeutectoid ferrite and pearlite to bainite and martensite with the increase of cooling rate, accompanying a refined microstructure and an increasing hardness. When the cooling rate was lower than 0.1°C s−1, the desired microstructure of ferrite and pearlite could be obtained. And the austenite deformation strongly promoted the formation of proeutectoid ferrite, which led to a decreasing hardness of deformed steel. When the cooling rate was larger than 10°C s−1, the fully martensite microstructure was obtained. And the austenite deformation inhibited martensite transformation, resulting in a decrease of the martensite transformation starting (M s) temperature from 391°C to 368°C.

Effect of deformation and grain size on austenite decomposition during quenching and partitioning of (high) silicon‑aluminum steels

The effect of austenite deformation on carbon partitioning and transformation to athermal and isothermal martensite, and bainite during quenching and partitioning (QP) is described for three steel compositions: Fe-0.3C-0.6Si-1.1Al, Fe-0.3C-1.0Si and Fe-0.3C-0.5Si-0.5Al. Microstructures were characterized using SEM-EBSD, TEM and XRD. Austenite decomposition kinetics was investigated using dilatometry. Deformation close to the no-recrystallization temperature refines the grain size, lowers the martensite start temperature and affects the stability of austenite during QP processing. The results in respect of dilatation measurements and retained austenite contents at room temperature corroborate the QP microstructures and their hardness and can help in designing optimal QP treatments for these novel steels.

Effects of austenite deformation on continuous cooling transformation of the pearlite heat-resistant steel

ABSTRACT Continuous cooling transformation (CCT) diagrams of undeformed and 40% compressively deformed pearlite heat-resistant steels at 950°C were obtained by a combined method of dilatometry and metallography. The microstructure and hardness of the steel under these two experimental conditions were compared and analysed. The effects of austenite deformation on the CCT of pearlite heat-resistant steel are discussed. The CCT diagrams show that the austenite deformation extends the ferrite and pearlite transformation fields and increases the critical cooling rates to achieve the ferrite and pearlite microstructure, which causes the CCT curve to shift to the left. However, austenite deformation suppresses the transformation of bainite and martensite. Moreover, it reduces the hardness of the tested steel at the same cooling rate and grain size of the ferrite at low cooling rates.

Erratum to: Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Erratum to: Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Although there has been much research regarding the effect of austenite deformation on accelerated cooled microstructures in microalloyed steels, there is still a lack of accurate data on boundary densities and effective grain sizes. Previous results observed from optical micrographs are not accurate enough, because, for displacive transformation products, a substantial part of the boundaries have disorientation angles below 15 deg. Therefore, in this research, a niobium microalloyed steel was used and electron backscattering diffraction mappings were performed on all of the transformed microstructures to obtain accurate results on boundary densities and grain refinement. It was found that with strain rising from 0 to 0.5, a transition from bainitic ferrite to acicular ferrite occurs and the effective grain size reduces from 5.7 to 3.1 μm. When further increasing strain from 0.5 to 0.7, dynamic recrystallization was triggered and postdynamic softening occurred during the accelerated cooling, leading to an inhomogeneous and coarse transformed microstructure. In the entire strain range, the density changes of boundaries with different disorientation angles are distinct, due to different boundary formation mechanisms. Finally, the controversial influence of austenite deformation on effective grain size of low-temperature transformation products was argued to be related to the differences in transformation conditions and final microstructures.

Effect of austenite deformation in non-recrystallization region on microstructure development in low-silicon content TRIP-assisted steels

Abstract The influence of austenite deformation in non-recrystallization region on microstructural development in low-silicon content TRIP-assisted steels was investigated. Laboratory simulation of a typical thermomechanical control processing was carried out in an automated hot-compression testing machine. Specimens subjected to a typical multi-stage isothermal deformation/cooling program were deformed to true strains of 0, −0.15, −0.25 and −0.35 at various temperatures in austenite non-recrystallization region. Mossbauer spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and a novel tint-etching method were used to investigate the microstructure of deformed specimens. The results indicated that the maximum volume fraction ( V RA ) and carbon content ( C RA % ) of retained austenite can be obtained by deforming samples to some intermediate strains ( e =−0.15 for V RA and e =−0.25 for C RA % ). However, further straining of samples to e =−0.35 resulted in a drastic reduction of both parameters due to formation of pearlite. It was found that a decrease in deformation temperature resulted in increasing V RA and C RA % . Moreover, deformation of austenite was associated with morphology changes in retained austenite particles from interlath film-like type in undeformed specimens to blocky and encapsulated types in the deformed specimens.

On the ferrite and bainite transformation in isothermally deformed 22MnB5 steels

Abstract The effect of austenite deformation on ferrite transformation and bainite transformation is reviewed in this paper. In the ferrite zone on continuous cooling transformation (CCT) diagram of 22MnB5 steel, isothermal tensile tests were carried out in various deformation conditions on Gleeble 3800 thermo-mechanical machine, and microstructures were observed. The influence of hot deformation on the driving force of ferrite transformation was theoretically analyzed based on the 22MnB5 steel. The effects of deformation temperature, strain amount, strain rate and cooling rate on the ferrite transformation of the 22MnB5 steel were also investigated. The ferrite transformation of the 22MnB5 is induced by isothermal deformation and the incubation period is significantly shortened. Below the start temperature of bainite transformation ( B s ), isothermal tensile tests and microstructure observation were performed. The effects of deformation temperature and strain amount on the bainite transformation of the 22MnB5 steel were investigated. The isothermal deformation below B s promotes the bainite transformation obviously, while the hot deformation above the B s promoting the bainite transformation is not observed in the investigated testing conditions.

Effect of Austenite Deformation on Recrystallisation Behaviour in an X-70 Microalloyed Steel

In the present study, the effect of austenite deformation on the recrystallisation behaviour in terms of recrystallisation-stop and recrystallisation-limit temperatures (T5% and T95%) of an X70 niobium microalloyed pipeline steel have been investigated by interrupted plane strain compression tests. The extents of recrystallisation are calculated using a modified fractional softening parameter. And the 20% and 60% of fractional softening were correlated to T5% and T95%. Quantitative optical metallography indicates that this method provides for a convenient and reliable experimental measurement of the critical temperatures associated with the recrystallisation of austenite. The recrystallisation kinetics and the precipitation kinetics of Nb(CN) were calculated using two widely applied models. The experimental results from this study suggest that the current model of precipitation kinetics might overestimate the precipitation start time.

Effect of deformation of austenite and cooling rates on transformation microstructures in a Mn–Cr gear steel

The effect of austenite deformation and cooling rates on continuous cooling transformation microstructures for a Mn–Cr gear steel were investigated using a Gleeble 1500 thermomechanical test system. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, leading to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The deformation enhances the stability of austenite against bainite transformation, which results in an increase in amount of martensite/austenite (M/A) constituent with deformation at some cooling rates studied. Moreover, cooling rate also affects amount of M/A constituent. With decrease of cooling rate, amount of M/A constituent increases at first, but decreases subsequently till disappears eventually.

Effect of Austenite Deformation on Continuous Cooling Transformation Microstructures for 22CrSH Gear Steel

The effect of compressive deformation of austenite on continuous cooling transformation microstructures for 22CrSH gear steel has been investigated using a Gleeble 1500 thermal simulator. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, and leads to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The grain boundary allotriomorphic ferrite occupies the austenite grain surfaces when the prior deformation takes place or the cooling rate is decreased, which causes a transition from bainite to acicular ferrite. The deformation enhances the stability of transformation from austenite to acicular ferrite, which results in an increase of M/A constituent.

The effect of austenite deformation on bainite formation in an alloyed eutectoid steel

The isothermal bainite formation of austenite with and without pre-strain has been investigated in an alloyed eutectoid steel of Fe–0.80C–0.27Si–0.61Mn–0.15V (wt.%). It is found that the isothermal decomposition of deformed austenite within the regimes of bainite and finer pearlite is significantly promoted as the incubation period is remarkably shortened for small strain (e.g. 2%) and almost vanishes for strain larger than 5%. According to the TTT curves obtained for deformed (2%) and non-deformed austenite, B s (B s for isothermal bainite formation) for deformed austenite is about 10 °C higher than that for the latter. On the basis of the diffusion mechanism of bainite formation, the experimental results are explained by taking account of defects brought about by plastic deformation.

Effect of austenite deformation and chemical composition on the microstructure and hardness of low-carbon and ultralow-carbon bainitic steels

The effect of hot deformation and chemical composition on the microstructure and hardness of experimental grades of low-carbon and ultralow-carbon C-Mn-Mo-Nb-B steels is studied by the method of physical simulation. It is shown that deformation of austenite at a temperature below that of recrystallization leads to formation of polygonal ferrite and high-temperature ferrite with bainitic morphology. The effect is the most pronounced in boron-bearing steels. Simultaneous alloying with Mo, Nb, and B seems to be the most effective for ensuring high hardness.

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

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 Tnr, 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 ϖ 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.

Bainite transformation during continuous cooling of low carbon microalloyed steel

The evolution of the final microstructure for a low carbon Nb–Ti microalloyed plate steel was studied during a simulation of thermomechanical processing for hot rolling following by accelerated cooling. The effects of austenite deformation below the non-recrystallisation temperature TNR, cooling rate, and interrupt temperature on the formation of conventional (intergranular) bainite (CB), acicular ferrite (intragranular) (AF), and martensite–austenite (MA) constituents were determined. With increases in austenite deformation and cooling rate, and decrease in the interrupt temperature, the final microstructure changed from a mixture of CB+MA through CB+AF+MA to a dual phase AF+MA.

Microstructural development and austempering kinetics of ductile iron during thermomechanical processing

A new method of refining the microstructure of austempered ductile iron (ADI) by thermome chanical processing is investigated. Refinement of microstructure is effected by grain refinement of parent austenite by hot deformation in the austenitizing temperature range, before the austempering treatment. The effects of austenite deformation on the kinetics of austempering reaction and the microstructure development were studied using metallography and X-ray diffraction (XRD), at different austempering temperatures and deformations. The process window for optimum microstructure was determined in terms of the parameters involved. Deformation of 40 to 60 pct could be imparted in the temperature range 900 °C to 1025 °C, resulting in a reduction in the prior austenite grain size by 35 to 50 pct and ferrite size in ausferrite by 70 to 75 pct. The effects of austenitization temperature on the austempered microstructure were also studied.

Changes in transformation behaviour and microstructure of a CrV-spring steel by thermomechanical treatment

The effect of austenite deformation on the transformation behaviour was investigated on a CrV-spring steel with the major attention put on the martensitic transformation. In the first part, a small review is given on the relation between the state of austenite after hot deformation and its influence on the formation of martensite. In the laboratory tests, the second part of the paper, a conventional heat treatment (CHT) was compared with two types of austenite conditioning by thermomechanical treatment (TMT) : TMTR - with deformation above the recrystallization temperature vR leading to a fully recrystallized austenite and TMT N - with deformation below vR with a not recrystallized but possibly polygonized austenite. For the laboratory tests, the hot deformation simulator Wumsi was employed. After quenching in oil, the martensite after TMT consisted of associations of many fine fragments with a smaller number of large acicular martensitic units than observed after CHT. In both TMT-variants small ferritic areas (< 1 pm) could be revealed. Different behaviour of martensite during tempering at low temperatures was observed after CHT and TMT. It can be explained by reduced inherent stresses generated during martensitic transformation after TMT, presumably as a result of a better ability of deformed austenite to withstand the accommodation strain during martensitic transformation. This may have considerable consequences for the toughness properties of tempered martensite.

Effect of deformation on ferrite nucleation and growth in a plain carbon and two microalloyed steels

Isothermal compression tests were carried out on plain C, Mo, and Mo−Nb−V microalloyed steels in order to study the effect of austenite deformation on the ferrite nucleation and growth rates. The nucleation rate increases with deformation and the degree of supersaturation, Ae3−T; it appears to be reduced by the substitutional elements Mo, Nb, and V through reduction of the austenite grain boundary energy. The growth rate increases with the degree of supersaturation and is also reduced by these elements, apparently through the solute drag-like effect. Under static conditions, increasing the prestraining strain rate increases the nucleation rate, but this increase is small compared to the effect of concurrent deformation. The growth rate under static conditions decreases as the deformation or the strain rate is increased.

Ferrite Formation from Deformed Austenite in a Ti Modified HSLA Steel

Abstract The effect of austenite deformation in the recrystallization, partial recrystallization and nonrecrystallization temperature range on the subsequent ferrite transformation was investigated in a commercial HSLA steel with 0.076% Ti and a reference C-Mn steel of same base chemistry. The steel samples were given reductions in the range 20–60% at 850, 950 and 1050°C by single pass rolling followed by isothermal transformation to ferrite at 690°C for up to 1800 s. It was found that the austenite to ferrite transformation rate was increased both with increase in the amount of austenite deformation and lowering the finish rolling temperature. The addition of titanium resulted in decrease in the rate of ferrite formation as well as led to the formation of polygonal ferrite grains. Non-recrystallized elongated austenite grains appear to give fine ferrite structure whilst mixed ferrite grains resulted from partially recrystallized austenite. It was observed that not only the austenite grain boundaries, but...

The influence of austenite strength upon the austenite-martensite transformation in alloy steels

The resistance of austenite to plastic deformation (austenite flow stress) was measured using a high temperature tensile apparatus. The flow stress was then correlated with the Ms temperature as determined magnetically during subsequent cooling. In one part of the study, the flow stress of the austenite was varied only by work hardening the austenite, allowing the austenite composition, which is known to affect Ms, to be held constant. A decrease in Ms temperature with increasing austenite flow stress was observed. This observation was supported by the observation of a decrease in the amount of austenite transformed at 25°C. In the other part of the study, a series of alloy steels of different chemical compositions was tested. A decrease in Ms temperature with increasing austenite flow stress was again observed. Strengthening of austenite by plastic deformation was shown not to change the chemical driving force for transformation. The effect of deformation on Ms temperature thus results from its influence on either the nucleation or the growth process. While the effect of austenite deformation on martensite nucleation is uncertain, specific nucleation models can account for only approximately one-third of the nonchemical free energy change which accompanies transformation. A proposal, consistent with the observations, was made that the energy expended for the deformation of austenite during martensite plate growth could reasonably account for a substantial part of the nonchemical free energy change.