Austenite Stability and Deformation Behavior in Medium Mn Steel Processed by Cyclic Quenching ART Heat Treatment

This study investigated the austenite stability and deformation behavior of cyclic quenching-austenite reverse transformation processed Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb medium Mn steel. A number of findings were obtained. Most importantly, the extent of the TRIP effect was mainly determined by an appropriately retained austenite stability rather than its content. Simultaneously, chemical elements were the key factors affecting austenite stability, of which Mn had the greatest impact, while the difference of retained austenite grain size and Mn content resulted in different degrees of retained austenite stability. Additionally, there were still large amounts of strip and granular-retained austenite shown in the microstructure of the CQ3-ART sample after tensile fracture, revealing that the excessively stable, retained austenite inhibited the generation of an extensive TRIP effect.     

Effect of Molybdenum on the Impact Toughness of Heat-Affected Zone in High-Strength Low-Alloy Steel

The microstructure, precipitates, and austenite grain in high-strength low-alloy steel were characterized by optical microscope, transmission electron microscope, and laser scanning confocal microscopy to investigate the effect of Mo on the toughness of steel. The microstructure was refined and the toughness was enhanced after the addition of 0.07% Mo in steel. The addition of Mo can suppress the Widmanstätten ferrite (WF) formation and promote the transformation of acicular ferrite (AF), leading to the fine transformed products in the heat-affected zone (HAZ). The chemical composition of precipitates changed from Nb(C, N) to (Nb, Mo)(C, N) because of the addition of Mo. The calculated lattice misfit between Nb(C, N) and ferrite was approximately 11.39%, while it was reduced to 5.40% for (Nb, Mo)(C, N), which significantly affected the size and number density of precipitates. A detailed analysis of the precipitates focusing on the chemical composition, size, and number density has been undertaken to understand the contribution of Mo on the improvement of steel toughness.     

Phase Transition Kinetics in Austempered Ductile Iron (ADI) with Regard to Mo Content

The phase transformation to ausferrite during austempered ductile iron (ADI) heat treatment can be significantly influenced by the alloying element Mo. Utilizing neutron diffraction, the phase transformation from austenite to ausferrite was monitored in-situ during the heat treatment. In addition to the phase volume fractions, the carbon enrichment of retained austenite was investigated. The results from neutron diffraction were compared to the macroscopic length change from dilatometer measurements. They show that the dilatometer data are only of limited use for the investigation of ausferrite formation. However, they allow deriving the time of maximum carbon accumulation in the retained austenite. In addition, the transformation of austenite during ausferritization was investigated using metallographic methods. Finally, the distribution of the alloying elements in the vicinity of the austenite/ferrite interface zone was shown by atom probe tomography (APT) measurements. C and Mn were enriched within the interface, while Si concentration was reduced. The Mo concentration in ferrite, interface and austentite stayed at the same level. The delay of austenite decay during Stage II reaction caused by Mo was studied in detail at 400 °C for the initial material as well as for 0.25 mass % and 0.50 mass % Mo additions.     

Austenite Decomposition of a Lean Medium Mn Steel Suitable for Quenching and Partitioning Process: Comparison of CCT and DCCT Diagram and Their Microstructural Changes

The present work deals with the dilatometric study of a hot-rolled 0.2C3Mn1.5Si lean medium Mn steel, mainly suitable for the quenching and partitioning (Q&P) heat treatment in both hot-rolled or cold-rolled condition, subjected to a variation of austenitization temperature. These investigations were performed in a temperature range of 800–1200 °C. In this context, the martensite transformation start temperature (Ms) was determined as a function of austenitization temperature and in turn obtained prior austenite grain size (PAGS). The results show rise in prior austenite grain size due to increasing austenitization temperature, resulting in elevated Ms temperatures. Measured dilatation curves were confronted with the metallographic analysis by means of scanning electron microscopy (SEM). The present paper also focuses on the construction of a continuous cooling transformation (CCT) and deformation continuous cooling transformation (DCCT) diagram of the investigated lean medium Mn steel in a range of cooling rates from 100 to 0.01 °C/s and their subsequent comparison. By comparing these two diagrams, we observed an overall shift of the DCCT diagram to shorter times compared to the CCT diagram, which represents an earlier formation of phase transformations with respect to the individual cooling rates. Moreover, the determination of individual phase fractions in the CCT and DCCT mode revealed that the growth stage of ferrite and bainite is decelerated by deformation, especially for intermediate cooling rates. Microstructural changes corresponding to cooling were also observed using SEM to provide more detailed investigation of the structure and present phases identification as a function of cooling rate. Moreover, the volume fractions obtained from the saturation magnetization method (SMM) are compared with data from X-ray diffraction (XRD) measurements. The discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements. In that regard, the volume fraction of retained austenite increased with a decrease of cooling rate as a result of larger volume fraction of ferrite and bainite. The hardness of the samples subjected to the deformation was slightly higher compared to non-deformed samples. The reason for this was an evident grain refinement after deformation.     

Effect of Hot Deformation on Phase Transformation Kinetics in Isothermally Annealed 3Mn-1.6Al Steel

The kinetics of ferritic transformation and the corresponding microstructural evolution in 0.17C-3.1Mn-1.6Al-0.04Nb-0.22Mo-0.22Si medium-Mn steel during isothermal annealing was investigated in dilatometric studies. The material was subjected to thermal and thermo-mechanical treatments aimed at obtaining, by the austenite → ferrite transformation, a sufficient fraction of ferrite to stabilize the retained austenite by C and eventual Mn partitioning. The samples were isothermally held for 5 h in a temperature range from 600 to 750 °C to simulate simplified temperature conditions of an industrial coiling process following hot rolling. Some of the samples were plastically deformed at a temperature of 900 °C before isothermal holding in order to study the effect of hot deformation on the kinetics of phase transformations. After the dilatometric investigations the material was subjected to light and scanning electron microscopy to reveal relationships between the holding temperature, deformation and microstructure evolution. Hardness tests were performed to assess the mechanical behavior. A significant effect of manganese in slowing down diffusional transformations during the cooling of steel was found. The influence of austenite deformation on the kinetics of austenite to ferrite transformation was noted. The plastically deformed samples showed an accelerated start of ferritic transformation and the extension of its range. During dilatometric tests, low-range dynamic ferritic transformation was recorded, which was also confirmed by the microscopic tests.     

The Influence of Annealing Temperature on the Morphology of Structures and the Mechanical Properties of Prequenching—Quenching and Partitioning Steel

In this study, we comparatively study the microstructures and mechanical properties of prequenching—quenching and partitioning (QQ&P) and traditional Q&P samples at different annealing temperatures (intercritical annealing temperatures). When the annealing temperature is 780 °C, the ferrite and retained austenite in QQ&P samples with lath and blocky morphologies. The lath retained austenite is mainly distributed along the lath ferrite. As the annealing temperature increases, the lath ferrite recrystallizes and gradually grows into the blocky (equiaxed) shape, leading to a decrease in the lath retained austenite content. When the annealing temperature increases to 870 °C, the ferrite content decreases significantly, and the retained austenite is mainly blocky and thin film, distributed at the boundaries of prior austenite grains and between martensite laths, respectively. Different from QQ&P samples, the ferrite and retained austenite in Q&P samples are mainly blocky when the annealing temperature is 780 °C or 810 °C. When the annealing temperature is increased to 870 °C, the microstructures of the Q&P sample are basically the same as that of the QQ&P sample. The 780 °C-QQ&P sample and the 810 °C-QQ&P sample have higher total elongation and product of strength and elongations (PSEs) than their counterpart Q&P samples due to the fact that lath ferrite and retained austenite are conducive to carbon diffusion and carbon homogenization in austenite grains, thereby improving the thermal stability and volume fraction of the retained austenite. In addition, the lath structures can release local stress concentration and delay the formation of voids and microcracks. The difference of mechanical properties between QQ&P samples and Q&P samples decreases with the increase in the annealing temperature. The results show that the low annealing temperature combined with prequenching—Q&P heat treatments can significantly improve the elongation and PSE of Q&P steel.     

Study on Kinetics of Transformation in Medium Carbon Steel Bainite at Different Isothermal Temperatures

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.     

Abrasive Wear of High-Carbon Low-Alloyed Austenite Steel: Microhardness,Microstructure and X-ray Characteristics of Worn Surface

A high-carbon, high-silicon steel (1.21 wt% C, 2.56 wt% Mn, 1.59 wt% Si) was subjected to quenching from 900 and 1000 °C, resulting in microstructures containing 60 and 94% of retained austenite, respectively. Subsequent abrasive wear tests of quenched samples were performed using two-body abrasion and three-body abrasion testing machines. Investigations on worn surface and subsurface were carried out using SEM, XRD, and microhardness measurement. It was found that the highest microhardness of worn surface (about 1400 HV0.05) was achieved on samples quenched from 900 °C after three-body abrasion. Microhardness of samples after two-body abrasion was noticeably smaller. with a maximum of about 1200 HV0.05. This difference correlates with microstructure investigations along with XRD results. Three-body abrasion has produced a significantly deeper deformed layer; corresponding diffractograms show bigger values of the full width at half maximum parameter (FWHM) for both α and γ alone standing peaks. The obtained results are discussed in the light of possible differences in abrasive wear conditions and differing stability of retained austenite after quenching from different temperatures. It is shown that a structure of metastable austenite may be used as a detector for wear conditions, as the sensitivity of such austenite to phase transformation strongly depends on wear conditions, and even small changes in the latter lead to significant differences in the properties of the worn surface.     

Effects of Intercritical Annealing Temperature on Mechanical Properties of Fe-7.9Mn-0.14Si-0.05Al-0.07C Steel

A medium Mn steel has been designed to achieve an excellent combination of strength and ductility based on the TRIP (Transformation Induced Plasticity) concept for automotive applications. Following six passes of hot rolling at 850 °C, the Fe-7.9Mn-0.14Si-0.05Al-0.07C (wt.%) steel was warm-rolled at 630 °C for seven passes and subsequently air cooled to room temperature. The sample was subsequently intercritically annealed at various temperatures for 30 min to promote the reverse transformation of martensite into austenite. The obtained results show that the highest volume fraction of austenite is 39% for the sample annealed at 600 °C. This specimen exhibits a yield stress of 910 MPa and a high ultimate tensile stress of 1600 MPa, with an elongation-to-failure of 0.29 at a strain rate of 1 × 10⁻³/s. The enhanced work-hardening ability of the investigated steel is closely related to martensitic transformation and the interaction of dislocations. Especially, the alternate arrangement of acicular ferrite (soft phase) and ultrafine austenite lamellae (50–200 nm, strong and ductile phase) is the key factor contributing to the excellent combination of strength and ductility. On the other hand, the as-warm-rolled sample also exhibits the excellent combination of strength and ductility, with elongation-to-failure much higher than those annealed at temperatures above 630 °C.     

Kinetics of Austenite Phase Transformations in Newly-Developed 0.17C-2Mn-1Si-0.2Mo Forging Steel with Ti and V Microadditions

This work presents the results of phase transformation kinetics during continuous cooling in newly developed high strength low-alloy steel (HSLA). Initial theoretical calculations for the determination of heat treatment parameters were conducted. To determine the structural constituents formed due to the austenite decomposition the dilatometry approach was used. The material was cooled down from the austenitization temperature of 1000 °C with cooling rates between 0.1 °C/s to 60 °C/s. Then, light and scanning electron microscopy investigations were carried out. The microstructure after cooling at rates between 0.1 °C/s up to 1 °C/s is mainly ferritic with some fraction of granular bainite. Increasing the cooling rate led to formation of a higher fraction of bainitic ferrite. At 60 °C/s the microstructure was mainly bainite with some fraction of ferrite. To determine the presence of retained austenite, color etching using Klemm solution was used. The results show that the increase of cooling rate decreases the amount of retained austenite in the microstructure of the steel. Hardness measurements were made to determine the changes in the mechanical properties as a function of the cooling rate.     

Effect of Annealing Temperature on the Mechanical and Corrosion Behavior of a Newly Developed Novel Lean Duplex Stainless Steel

The effect of annealing temperature (1000–1150 °C) on the microstructure evolution, mechanical properties, and pitting corrosion behavior of a newly developed novel lean duplex stainless steel with 20.53Cr-3.45Mn-2.08Ni-0.17N-0.31Mo was studied by means of optical metallographic microscopy (OMM), scanning electron microscopy (SEM), magnetic force microscopy (MFM), scanning Kelvin probe force microscopy (SKPFM), energy dispersive X-ray spectroscopy (EDS), uniaxial tensile tests (UTT), and potentiostatic critical pitting temperature (CPT). The results showed that tensile and yield strength, as well as the pitting corrosion resistance, could be degraded with annealing temperature increasing from 1000 up to 1150 °C. Meanwhile, the elongation at break reached the maximum of 52.7% after annealing at 1050 °C due to the effect of martensite transformation induced plasticity (TRIP). The localized pitting attack preferentially occurred at ferrite phase, indicating that the ferrite phase had inferior pitting corrosion resistance as compared to the austenite phase. With increasing annealing temperature, the pitting resistance equivalent number (PREN) of ferrite phase dropped, while that of the austenite phase rose. Additionally, it was found that ferrite possessed a lower Volta potential than austenite phase. Moreover, the Volta potential difference between ferrite and austenite increased with the annealing temperature, which was well consistent with the difference of PREN.     

Effect of Initial Microstructure on the Toughness of Coarse-Grained Heat-Affected Zone in a Microalloyed Steel

Toughness of the coarse-grained-heat-affected-zone (CGHAZ) strongly depends on the prior austenite grain size. The prior austenite grain size is affected not only by chemical composition, thermal cycle, and dissolution of second-phase particles, but also by the initial microstructure. The effect of base metal microstructure (ferrite/pearlite obtained by air cooling and martensite obtained by water-quenching) on Charpy impact toughness of the CGHAZ has been investigated for different heat inputs for high-heat input welding of a microalloyed steel. A welding thermal cycle with a heat input of 100 kJ/cm and 400 kJ/cm were simulated on the MMS-300 system. Despite a similar microstructure in the CGHAZ of both the base metals, the average Charpy impact energy for the air-cooled base metal was found to be higher than the water-quenched base metal. Through thermo-kinetic simulations, it was found that a higher enrichment of Mn/C at the ferrite/austenite transformation interface of the CGHAZ of water-quenched base metal resulted in stabilizing austenite at a lower A1 temperature, which resulted in a coarser austenite grain size and eventually lowering the toughness of the CGHAZ.     

The Effect of Inclusions on a Medium Temperature Transformation Microstructure and Toughness of High-Strength HSLA Steel

In this study, the evolution of high-strength HSLA steel microstructure was studied using high-temperature laser confocal microscopy and SEM, TEM, and EPMA techniques. The effect of precipitates on grain boundary migration of austenite during high-temperature heating and the effect of inclusions in undercooled austenite on AF phase transformation were studied. The effect of multiphase microstructure on impact toughness was studied by Gleeble thermal simulation at 550, 600, and 650 °C. The results show that the austenite grain is refined by TiN pinning at high temperatures, and a large number of NbC and VCN are precipitated in ferrite for precipitation strengthening. The (Ti-Mn-O) + (Al + Si + Mn-O) + MnS composite inclusions with smaller sizes have a greater promoting effect on the nucleation of acicular ferrite than single-phase MnS. With a decrease in isothermal temperature, the content of acicular ferrite increases. When the isothermal temperature is 550 °C, an increase in the maximum impact toughness of acicular ferrite with large-angle grain boundary is clearly observable.     

Optimisation of Thermal Processes with Plasma Nitriding on Vanadis 4 High Speed Steel

Vanadis 4 steel is a tool steel processed by powder metallurgy (PM). Its main alloying elements are Cr, V and Mo. Through the application of a design of experiments with six factors and eight experiments, the parameters of the process related to the thermal treatment of these steels are deliberately varied. Those thermal parameters related to the destabilisation of austenite were analysed: the cooling method in quenching, tempering and the application of an ionic nitriding treatment. Through XRD, the percentage and types of precipitated crystalline phases were determined, and, through SEM-EDX, the microstructure was revealed. At the same time, through a pin-on-disc test, those factors with a significant influence on resistance to wear were determined. It has been concluded that, in order to increase resistance to wear, treatments of destabilisation of the austenite at 900 °C with oil quenching, tempering at 550 °C for 4 h and a subsequent treatment of plasma nitriding would be very favourable. This tempering favours a second destabilisation of the austenite and its transformation into martensite, as well as the joint precipitation of type M₇C₃ and MC carbides. The thickness of the nitrided layer exceeds 100 microns and generates a fully adhesive wear mechanism.     

Microstructure Evolution and Mechanical Properties of X6CrNiMoVNb11-2 Stainless Steel after Heat Treatment

X6CrNiMoVNb11-2 supermartensitic stainless steel, a special type of stainless steel, is commonly used in the production of gas turbine discs in liquid rocket engines and compressor disks in aero engines. By optimizing the parameters of the heat-treatment process, its mechanical properties are specially adjusted to meet the performance requirement in that particular practical application during the advanced composite casting-rolling forming process. The relationship between the microstructure and mechanical properties after quenching from 1040 °C and tempering at 300–670 °C was studied, where the yield strength, tensile strength, elongation and impact toughness under different cooling conditions are obtained by means of mechanical property tests. A certain amount of high-density nanophase precipitation is found in the martensite phase transformation through the heat treatment involved in the quenching and tempering processes, where M₂₃C₆ carbides are dispersed in lamellar martensite, with the close-packed Ni₃Mo and Ni₃Nb phases of high-density co-lattice nanocrystalline precipitation created during the tempering process. The ideal process parameters are to quench at 1040 °C in an oil-cooling medium and to temper at 650 °C by air-cooling; final hardness is averaged about 313 HV, with an elongation of 17.9%, the cross-area reduction ratio is 52%, and the impact toughness is about 65 J, respectively. Moreover, the tempered hardness equation, considering various tempering temperatures, is precisely fitted. This investigation helps us to better understand the strengthening mechanism and performance controlling scheme of martensite stainless steel during the cast-rolling forming process in future applications.     

Wear-Resistance Improvement of 65Mn Low-Alloy Steel through Adjusting Grain Refinement by Cyclic Heat Treatment

Refined microstructures achieved by cyclic heat treatment significantly contribute to improving the wear resistance of steels. To acquire the refined microstructures of 65Mn low-alloy steel, first, the specimens were solid solution-treated; then, they were subjected to cyclic heat treatment at cyclic quenching temperatures of 790–870 °C and quenching times of 1–4 with a fixed holding time of 5 min. The mechanical properties of 65Mn low-alloy steel in terms of hardness, tensile strength, elongation and wear resistance were characterized. Afterwards, the effect of cyclic heat treatment on microstructure evolution and the relationships between grain refinement and mechanical properties’ improvement were discussed. The results show that the average grain size firstly decreased and then increased with the increase in the quenching temperature. Hardness increased with grain refinement when the temperature was lower than 830 °C. Once the temperature exceeded 830 °C, hardness increased with the temperature increase owing to the enrichment of carbon content in the martensite. With the increase in cyclic quenching times, hardness continuously increased with grain refinement strengthening. In addition, both tensile strength and elongation could be significantly improved through grain refinement. The relationships among wear loss, hardness and average grain size showed that wear resistance was affected by the synthesis reaction of grain refinement and hardness. Higher hardness and refined grain size contributed to improving the wear resistance of 65Mn low-alloy steel.     

Effect of Organizational Evolution on the Stress Corrosion Cracking of the Cr-Co-Ni-Mo Series of Ultra-High Strength Stainless Steel

Different microstructures were obtained under various thermal conditions by adjusting the heat treatment parameters of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel. The effect of organizational evolution on the stress corrosion cracking (SCC) of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel was investigated using potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and other test methods in combination with slow strain rate tensile tests (SSRTs). The results show that the Mo- and Cr-rich clusters and precipitation of the Laves phase reduce the corrosion resistance, while increasing the austenite content can improve the corrosion resistance. The Cr-Co-Ni-Mo series of ultra-high strength stainless steel has a high SCC resistance after quenching at 1080 °C and undergoing deep cooling (DC) treatment at −73 °C. With increasing holding time, the strength of the underaged and peak-aged specimens increases, but the passivation and SCC resistance decreases. At the overaged temperature, the specimen has good SCC resistance after a short holding time, which is attributed to its higher austenite content and lower dislocation density. As a stable hydrogen trap in steel, austenite effectively improves the SCC resistance of steel. However, under the coupled action of hydrogen and stress, martensitic transformation occurs due to the decrease in the lamination energy of austenite, and the weak martensitic interface becomes the preferred location for crack initiation and propagation.     

Effect of Yttrium on the Microstructure and Mechanical Properties of PH13-8Mo Stainless Steels Produced by Selective Laser Melting

In the present work, PH13-8Mo stainless steel parts without yttrium and with yttrium (Y) were manufactured by selective laser melting (SLM). The microstructure, phase composition and grain orientation of the stainless steels parts with Y and without Y were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The characterization results revealed that the addition of Y clearly refined the grain size of the PH13-8Mo steel formed part, resulting in more equiaxed massive grains and in a less anisotropic microstructure. PH13-8Mo stainless steel formed parts were mainly composed of martensite and retained austenite. The addition of Y could significantly increase the content of retained austenite and also generate nano-sized precipitates containing Y. The mechanical test results showed that both strength and toughness of the shaped parts containing Y were improved synergistically. The yield strength reached 1443 MPa, the elongation was 12.2%, and the room temperature impact energy reached 124.25 J/cm². The strengthening and toughening by Y of the formed parts were mainly attributed to grain refinement, higher volume fraction of the retained austenite and the formation of nano-sized precipitates containing Y.     

Mechanical Behavior of Multi-Phase Steels Comprising Retained Austenite

The retained austenite (RA) in advanced high-strength steel (AHSS) grades, such as dual-phase (DP) steels, plays an important role on their formability. Thanks to the transformation-induced plasticity (TRIP) effect that occurs during the mechanically induced transformation of RA into martensite, additional ductility is obtained. Martensite has a higher flow stress than austenite; hence, the transformation results in an apparent hardening, which is beneficial for the stability of deformation. The stability of RA at a given temperature strongly depends on its carbon content, which, in AHSS, is not uniform but distributed. The aim of this study is to build a model that predicts the transformation as well as TRIP in a DP steel grade with RA. A physics-based kinetic model is presented that captures the transformation of retained austenite based on the thermodynamic driving force of the applied stress. A direct analytical estimate of transformation plasticity is provided, which is consistent with the kinetic model. Transformation kinetics is incorporated in a self-consistent, mean-field homogenization-based constitutive model. Finally, an indication of the effect of transformation of retained austenite on formability is given.     

Bainitic Ferrite Plate Thickness Evolution in Two Nanostructured Steels

Bainitic ferrite plate thickness evolution during isothermal transformation was followed at the same holding temperatures in two nanostructured steels containing (in wt.%) 1C-2Si and 0.4C-3Si. A dynamic picture of how the bainitic transformation evolves was obtained from the characterization of the microstructure present at room temperature after full and partial transformation at 300 and 350 °C. The continuous change during transformation of relevant parameters influencing the final scale of the microstructure, YS of austenite, driving force of the transformation and evolution of the transformation rate has been tracked, and these variations have been correlated to the evolution of the bainitic ferrite plate. Instead of the expected refinement of the plate predicted by existing theory and models, this study revealed a thickening of the bainitic ferrite plate thickness as the transformation progresses, which is partially explained by changes in the transformation rate through the whole decomposition of austenite into bainitic ferrite.