Prior Deformation Of Austenite Research Articles

A critical review on deformation-induced transformation kinetics of austenitic stainless steels

Deformation-induced austenite to martensite transformation is an important phenomenon during the deformation of metastable fully austenitic transformation induced plasticity (TRIP) steels. The kinetics of austenite to martensite transformation influence the deformation behaviour of austenitic stainless steels but is often neglected by the materials community. In this paper, after an initial discussion on thermodynamics and mechanism, the importance of deformation-induced transformation kinetics is briefly outlined and the influence of various parameters like grain size, deformation temperature, strain rate, stress state and prior austenite deformation on the transformation kinetics is critically assessed. Variation of transformation kinetics with various parameters has been identified and justified. These variations could act as a ready reference for designing austenitic stainless steels with the desired set of mechanical properties and deformation behaviour. Further in this paper, selected models that can predict the transformation kinetics are reviewed and compared. In the end, research fronts related to transformation kinetics that can be further explored have been identified.

Phase transformation testing and modeling for hot stamping of boron steel considering the effect of the prior austenite deformation

Hot stamping of boron steels is a thermal-mechanical and phase transformation coupled process. The phase transformation of boron steels during continuous cooling is a complex process since the phase transformation is tightly coupled with plastic deformation during the hot stamping process. In the present work, the effects of hot deformation amount, deformation temperature, and strain rate on phase transformation are investigated by using a comprehensive set of methods including in-situ isothermal tensile tests combined with dilatometry and ex-situ microstructure characterization. The dilatation curves, phase fraction evolution curves, continuous cooling transformation (CCT), and deformed continuous cooling transformation (DCCT) diagrams are constructed. Finally, based on the testing results, a new non-isothermal kinetic model of phase transformation are proposed to introduce the effect of the austenite deformation on transformation during the continuous cooling. The proposed model is validated by testing Vickers hardness of DCCT specimens and hot stamping W-channel parts. The results demonstrate that the deformation accelerates the diffusive phase transformation, causing the CCT curves to move in the direction of shorter incubation time. But the acceleration of diffusion transformation is rapidly saturated as the plastic strain increases. Lower deformation temperature and higher strain rate are more favorable to the diffusion phase transformation, and the higher the critical cooling rate of the martensite transformation is needed. The proposed kinetic model not only accurately predicts the final phase fraction and hardness of the DCCT specimen, but also the distribution of mechanical properties of the hot stamping W-channel parts.

In Situ Investigation of the Bainitic Transformation from Deformed Austenite During Continuous Cooling in a Low Carbon Mn-Si-Cr-Mo Steel

The effects of hot deformation on the bainitic transformation of a low carbon steel during continuous cooling were comprehensively studied through in situ high-energy synchrotron X-ray diffraction, dilatometry, and ex situ microstructural characterizations. The obtained results indicated that the prior deformation of austenite at 950 °C accelerates the bainite formation at the early stages. During the ongoing of the transformation, both the overall kinetics of bainite and carbon enrichment of austenite are lower in deformed austenite. The bainitic microstructure developed from deformed austenite is more refined and presents the same retained austenite content at room temperature with slightly lower carbon content when compared with the undeformed sample. Besides, a significant higher dilatation strain was measured during the bainitic transformation in the deformed sample, which can be explained by the crystallographic texture in hot deformed austenite. The evolution of the peak broadening of the {220}γ and {211}α reflections during bainitic transformation are discussed in detail.

Modelling of the Influence of Prior Deformation of Austenite on the Martensite Formation in a Low-Alloyed Carbon Steel

The current work aims at developing models supporting design of the rolling and quenching processes. This requires a martensite formation model that can account for effect of previous plastic deformation as well as evolution of stress and temperature during the quenching step. The effect of deformation prior to the cooling on the transformation is evaluated. The experimental result shows that prior deformation impedes the martensite transformation due to the mechanical stabilisation of the austenite phase. Larger deformation above 30 % reduces the effect of the mechanical stabilisation due to increase in martensite nucleation sites. The computed transformation curves, based on an extended version of the Koistinen-Marburger equation, agree well with experimental results for pre-straining less than 30 %.

Prediction of part shape and associated material properties in hot-press forming using Unite element analysis

The hot-press forming of a U-channel was conducted on a boron-steel blank. The die consisted of two separate parts in order to perform the partial quenching process. The cold die was initially at 25 °C while the heated die was set to five different temperatures, namely, 25, 120, 220, 320 and 400 °C. The cooling temperature history, Vickers hardness and springback of the channel were measured. A thermo-mechanical-metallurgical model, which accounts for the prior austenite deformation effect, was successfully implemented in the LS-DYNA explicit solver to simulate the hot-press forming process under partial quenching conditions. The predicted and experimental results were compared and found in reasonable agreement.

Thermo-mechanical-metallurgical modeling for hot-press forming in consideration of the prior austenite deformation effect

In this study, a prior austenite grain refinement model was incorporated into semi-empirical diffusive transformation kinetics for application to hot-press forming. In particular, the kinetics equations were modified to include the effects of boron addition and austenite deformation on transformation behaviors during forming. To simulate the hot-press forming process, a thermo-mechanical-metallurgical model was formulated implicitly and implemented into the finite element program ABAQUS using the user subroutines UMAT and UMATHT. This nonconventional finite element modeling is appropriate to consider thermal- and transformation-associated strains. The proposed model was validated through simple finite element simulation examples, i.e., dilatometry simulation with and without external loading, and hot torsion and quenching of a rod. Finally, the hot-press forming of a U-channel-type part was simulated to study the effect of austenite deformation on the phase kinetics, hardness and residual stress. The simulation results showed that the austenite deformation had considerable influence on the final strength and residual stress distribution in the hot-press formed sheet, which resulted from an increase in ferritic phases due to the modified kinetics. In particular, the austenite deformation effect was more noticeable in the side-wall region of the U-channel where plastic deformation was the most severe.

Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Ferrite Formation in Low-Carbon Steels

Effects of prior austenite deformation and non-metallic inclusions on the ferrite nucleation and grain refinement of two kinds of low-carbon steels have been studied. The ferrite nucleation on MnS and V(C,N) is observed. The combination of thermomechanical processes with adequate amounts of non-metallic inclusions formed in low-carbon steels could effectively refine the grain size and the microstructure. Ferrite nucleated on the single MnS or V(C,N) inclusions and complex MnS+V(C,N) inclusion. The proper addition of elements S and V could effectively promote the formation of ferrite and further refinement of ferrite grains.

Effiect of silicon and prior deformation of austenite on isothermal transformation in low carbon steels

Isothermal transformation (TTT) behavior of the low carbon steels with two Si contents (0.50 wt pct and 1.35 wt pct) was investigated with and without the prior deformation. The results show that Si and the prior deformation of the austenite have significant effiects on the transformation of the ferrite and bainite. The addition of Si refines the ferrite grains, accelerates the polygonal ferrite transformation and the formation of M/A constituents, leading to the improvement of the strength. The ferrite grains formed under the prior deformation of the austenite become more homogeneous and refined. However, the influence of deformation on the tensile strength of both steels is dependent on the isothermal temperatures. Thermodynamic calculation indicates that Si and prior deformation reduce the incubation time of both ferrite and bainite transformation, but the effiect is weakened by the decrease of the isothermal temperatures.

Strong and Ductile Martensitic Steels for Automotive Applications

The limits of strength and ductility of a medium‐carbon silicon chromium spring steel are investigated for the case of conventional heat treatment including austenitization, quenching and tempering. The effect of phosphorus and austenite deformation prior to quenching was studied by measuring mechanical properties after quenching and tempering and by microstructural investigation. Strong influence of phosphorus on the ductility is observed for the quenched and tempered martensite without prior austenite deformation. The minimum in ductility found after tempering at 350°C is explained by the formation of cementite and grain boundary segregation of phosphorus. Two thermomechanical treatments were tested involving different austenite conditions produced by variation of the deformation temperature. The deformed conditions, recrystallized or work‐hardened, exhibit higher ductility at all tempering temperatures tested. A combined thermomechanical treatment is proposed that provides the highest ductility after tempering at 300°C independent of the phosphorus content. All thermomechanical treatments described in this study refine or eliminate carbide films at prior austenite grain boundaries. It was found possible to increase the tensile strength and the fatigue limit by deformation of austenite prior to quenching while maintaining or increasing the ductility level.

The use of strain memory alloys in structural health monitoring systems

There is an increasing drive toward the monitoring of the structural health of costly and safety critical infrastructure. Quite often, however, this does not require the cost and nuisance of active monitoring, and a simple passive system will suffice. One of the most robust, cost effective, and neat methods of passive peak monitoring involves the use of strain memory alloys; which are ferrous alloys that display paramagnetism in the unstrained state, but transform proportionally to display varying degrees of ferromagnetism depending on the level of peak strain induced in the material. This effect is achieved using a transformation in crystal structure from a metastable austenitic structure to a stable strain-induced martensitic structure. Because of the physical properties and the relatively low cost of strain memory alloys it is actually possible to manufacture complete components from this family of materials, and the materials can be tailored in terms of physical as well as transformation properties using alloying chemistry and prior deformation of austenite in the ausforming temperature range. Several dual purpose components have been developed for a variety of different applications, using strain memory alloys: a smart composite laminate, a smart rock anchor, and a smart aircraft bolt among others. This paper presents the general principles involved in using strain memory alloys for structural health monitoring, as well as specific application designs and results.

Grain refinement and microstructural evolution of grain boundary allotriomorphic ferrite/granular bainite steel after prior austenite deformation

Abstract The prior compressive deformation of austenite followed by air cooling was simulated by Gleeble-1500 machine on the grain boundary allotriomorphic ferrite/granular bainite (F GBA /Bg) dual phase steel. F GBA grains of less than 5 μm and intragranular ferrite grains less than 3 μm were obtained in the specimen with 30% compressive deformation at 780 °C. The prior compressive deformation of austenite promotes the formation of granular structure instead of granular bainite, reduces the length of bainitic plates and results in a random distribution of ultrafine martensite–austenite (MA) islands. The results of Vickers hardness and Charpy V-notch (CVN) impact experiments show that the prior austenite deformation with air cooling is an effective way to improve both the strength and toughness of this dual phase steel.

Simulation of the Hot Rolling of Trip-Assisted Multiphase Steels

The influence of intercritical rolling on the microstructural development during the processing of hot rolled multiphase steels is characterised. Intercritical rolling leads to dynamic recovery of ferrite and to strain-induced transformation of austenite into ferrite. The prior deformation of austenite considerably influences the bainite transformation. It is shown that a smaller amount of austenite transforms into bainite when bainite forms from deformed austenite. This is explained by the physics of the bainite transformation.

The effect of compressive deformation of austenite on the bainitic ferrite transformation in FeMnSiC steels

The influence of different amounts (10, 20, and 40%) of compressive deformation of austenite on the subsequent isothermal transformation of bainite has been investigated in FeMnSiC alloy steels. The overall transformation kinetics became slower and the final attained amount of bainite decreased in deformed austenite after the isothermal transformation was completed. The prior deformation of austenite had a strong impact on the bainitic transformation, because it created defects which hinder the growth of bainite and resulted in finer microstructure. If the recovery of deformed austenite occurred, the effect of mechanical stabilization of austenite would be mitigated. These results emphasized that the accumulated strain in austenite matrix retarded significantly the growth of bainite.

The effect of compressive deformation of austenite on the Widmanstätten ferrite transformation in Fe–Mn–Si–C steel

The effect of different amounts (10, 20, and 40%) of compressive deformation of austenite on the isothermal transformation of Widmanstätten ferrite has been investigated in Fe–Mn–Si–C alloy steels. The prior deformation of austenite has a strong impact on the Widmanstätten ferrite formation. The results showed that the austenite with higher amount of deformation transformed initially at a higher rate. However, they also indicated that as a higher amount of prior deformation was applied, a smaller quantity of Widmanstätten ferrite was obtained after the isothermal transformation was completed. These results emphasized that the accumulated strain in austenite matrix retarded significantly the growth of Widmanstätten ferrite.

Effect of Composition and Prior Austenite Deformation on the Transformation Characteristics of Ultralow-Carbon Microalloyed Steels

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of ultralow-carbon microalloyed steels containing additions of B and/or Mo to enhance hardenability. Each alloy was subjected to simulated recrystallisation and non-recrystallisation rolling schedules, followed by controlled cooling at rates from 0.1°C/s to about 100°C/s. The resulting transformation products were analysed using optical microscopy in conjunction with hardness measurements which, together with the dilatometry data, facilitated the construction of CCT diagrams. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element content, through to lath bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. The combined addition of B and Mo was found to be most effective in increasing steel hardenability, while B was significantly more effective than Mo as a single addition. Large plastic deformation of the prior austenite markedly enhanced PF formation, both in the base and Mo-containing steels, thus decreasing their hardenability significantly. In contrast, the steels containing B displayed only very small decreases in hardenability, resulting from the lack of sensitivity to strain in the austenite of the non-equilibrium microstructure constituents forming in the absence of PF.

Mechanical Stabilization of Austenite against Bainitic Reaction in Fe–Mn–Si–C Bainitic Steel

The influence of different amounts (5, 10, 20, 40%) of compressive deformation of austenite on the subsequent isothermal transformation of bainite has been investigated in a Fe–Mn–Si–C alloy steel. The results show that the prior deformation of austenite retards significantly the bainitic transformation. At the same isothermal transformation temperature, as the amount of prior deformation is higher, the quantity of bainite (which can be obtained after the isothermal transformation is completed) becomes less and it is also more difficult to develop the classical sheaf-like structure. The experiments reported here manifest that severe deformation causes mechanical stabilization of austenite against bainitic transformation.

Acicular ferrite transformation in deformed austenite of an alloy-steel weld metal

The effect of different amounts (5, 10 and 15%) of compressive deformation of austenite on the isothermal transformation of acicular ferrite in an alloy-steel weld metal has been investigated. It was found that prior deformation of austenite significantly enhanced acicular ferrite transformation. At the same isothermal transformation temperature, as a higher amount of prior deformation was applied, a greater quantity of acicular ferrite could be obtained and the size of acicular ferrite plates became much finer. These results implied that the effective nucleation sites of acicular ferrite increased with increasing amount of prior deformation. The other results also emphasized that the accumulated strain (due to prior deformation of austenite) could trigger acicular ferrite to nucleate on inclusions at high temperatures, where undeformed austenite remained stable. The acicular ferrite start temperature was found to be raised continuously by increasing the amount of prior deformation of austenite. Further evidence suggests that the application of deformation can boost the driving force for acicular ferrite formation. This phenomenon is similar to the case in which martensite forms under the influence of deformation.

Mechanical Stabilisation for Bainitic Reaction in a Fe-Mn-Si-C Bainitic Steel

In this work, the effects of different amounts (5, 10, 20, 40%) of compressive deformation of austenite on the isothermal transformation of bainite in a Fe-Mn-Si-C alloy steel have been investigated. It is found that the prior deformation of austenite retards significantly the bainitic transformation. At the same isothermal transformation temperature, as the amount of prior deformation is higher, the quantity of bainite (which can be obtained after the isothermal transformation is completed) becomes less and it is also more difficult to develop the classical sheaf-like structure. The evidence implies that severe deformation causes mechanical stabilisation of austenite for bainitic transformation.

Effect of Compressive Deformation on the Transformation Behavior of an Ultra-Low-Carbon Bainitic Steel

In this work, an experimental ultra-low-carbon-bainitic (ULCB) steel was prepared to study the effect of a prior compressive deformation on the transformation behavior during continuous cooling. It is found that prior deformation of austenite significantly enhances the bainitic transformation. At the same cooling rate the specimen with the prior deformation has a higher bainite start temperature (Bs) than the specimen without the prior deformation; in addition, the difference of Bs temperature becomes much larger at the higher cooling rate. This result is not consistent with that reported by Tamehiro et al. On the other hand, an isothermal experiment has been utilized to investigate the stability of the deformed austenite. It is suggested that the kinetics of precipitation of Nb(C, N) in deformed austenite influences the hardenability of bainite in local regions of the austenite matrix. This reason can explain why the deformed austenite grains produce finer allotriomorphic ferrite grains in a relatively slow cooling condition.

Effect of Prior Deformation of Austenite on the γ→ε Martensitic Transformation in Fe–Mn Alloys

The influence of prior deformation of austenite (γ) on the formation of hcp (∈) martensite in Fe-Mn binary alloys during subsequent cooling was investigated. The formation of bcc (α') lath martensite in deformed austenite was also examined for comparison. In both of the martensitic transformations to α'in the Fe-9%Mn alloy and to ∈ in the Fe-16%Mn and Fe-24%Mn alloys, the transformation start temperature (M s ) monotonously decreases with increasing the reduction of austenite at 773 K