Ferrite Austenite Research Articles

Advances in the Optimization of Thin Strip Cast Austenitic 304 Stainless Steel

This study is about the latest advances in the optimization of the microstructure and properties of thin strip cast austenitic stainless steel (AISI 304, 1.4301). Concerning the processing steps the relevance of different thin strip casting parameters, in‐line forming operations, and heat treatments for optimizing microstructure and properties have been studied. The microstructures obtained from the different processing strategies were analysed with respect to phase and grain structures including the grain boundary character distributions via EBSD microtexture measurements, the evolution of deformation‐induced martensite, the relationship between delta ferrite and martensite formation in austenite, and the texture evolution during in‐line deformation. It is observed that different process parameters lead to markedly different microstructures and profound differences in strip homogeneity. It is demonstrated that the properties of strip cast and in‐line hot rolled austenitic stainless steels are competitive to those obtained by conventional continuous casting and hot rolling. This means that the thin strip casting technique is not only competitive to conventional routes with respect to the properties of the material but also represents the most environmentally friendly, flexible, energy‐saving, and modern industrial technique to produce stainless steel strips.

Al as solid solution hardener in steels

Aluminium in solution in the ferrite in ferrite/pearlite steels has been found to have little influence on strength of the ferrite probably because up-till recently it has been added in very small amounts (0˙02–0˙08 wt-%Al) as a grain refining addition. There is indeed some doubt in the literature as to whether Al can solid solution harden. This lack of knowledge has not been important while Al additions are small but recently Al has been added in much greater amounts 0˙5–2% in some new grades of transformation induced plasticity (TRIP) steels. These steels have often an essentially ferritic structure with retained austenite and bainitic ferrite. The main thrust of the paper has therefore been to determine the solid solution hardening effect of Al in ferritic structures. Steels with varying Al levels from 0˙02 to ∼2% at two C levels, 0˙02 and 0˙1% were tensile tested after hot rolling so as to determine the solid solution hardening effect of Al. It was found in the 0˙1%C steel that some of the pearlite transformed to martensite on raising the Al level to 1% and this resulted in preyielding and lower strength levels than would have been obtained from a ferrite/pearlite structure. However, when the Al level was kept to the maximum allowable before martensite formed a marked solid solution hardening by Al was observed. Hall-Petch plots were also obtained from steels in which martensite was not present and it was found that a 1%Al addition increased the yield strength by 70–80 MPa and this increase was not dependent on the grain size as has been found for Si. It can be concluded that Al is a substantial solid solution hardener of steel but the strengthening effect of Al is often obscured at low additions by its ability to remove nitrogen from solution and at high additions by its tendency to favour martensite formation and cause preyielding of the ferrite.

Transformation characteristics, microstructure and mechanical properties of austempered ductile iron welds

The isothermal transformation of austempered ductile iron (ADI) weld at 370°C for a time period ranging from 0·5 to 1440 min involves three stages. The welding process has the effect of accelerating the subsequent transformation during austempering to bainitic ferrite and high carbon austenite (stage 1) and delaying the transformation to carbide (Fe3C) and ferrite (stage 3). Within the austempering time range 15–240 min, the ADI welds consisting of bainitic ferrite and retained austenite had superior properties (namely tensile strengths between 1030 and 1060 MPa and elongations between 8·0 and 8·5%). As the austempering temperature increased from 310 to 400°C, the tensile strength of ADI weld decreased from 1141 to 1040 MPa and its elongation increased from 4·0 to 9·1%. Under tensile loading a large number of slip bands appeared in the retained austenite and the austenite/ferrite interfaces behaved as obstacles limiting the movement of dislocations. The superior properties of ADI weld are mainly associated with the presence of large amount of retained austenite (30–32%), grain boundary strengthening effect and solid solution strengthening effect.

Study on Characteristics of ADI Coated DLC/ TiN /TiAlN Coatings by Cathodic Arc Evaporation

In this study, austempered ductile iron (ADI) substrate containing acicular ferrite and high-carbon austenite in the microstructure is made by austempering treatment at 360. Cathodic arc evaporation (CAE) coating technique was used to coat DLC, TiN and TiAlN films on ADI for surface modification. The results showed that the three coatings could be successfully coated onto ADI through CAE process without altering the unique microstructure of ADI. The structures of the coatings were identified by XRD, Raman and TEM, respectively. After HRC indentation testing, it was found that nodular graphite was the initial site of cracking for the coatings and then peeled. Surface roughness of all coated specimens was increased because the droplets generated on the substrate surface during the CAE process. Coated ADI had better wearability performance than uncoated ADI due to higher surface hardness. Coated specimens exhibited higher corrosion resistance than uncoated ones when they were immersed in separate solutions of both 3.5 wt. % NaCl and 10 vol. % HCl. In particular, TiAlN had the best corrosion resistance among the coated specimens.

Atomic scale observations of bainite transformation in a high carbon high silicon steel

A fine-scale bainitic microstructure with high strength and high toughness has been achieved by transforming austenite at 200 °C. X-ray diffraction analysis showed the carbon concentration of these bainitic ferrite plates to be higher than expected from para-equilibrium. Atom-probe tomography revealed that a substantial quantity of carbon was trapped at dislocations in the vicinity of the ferrite–austenite interface. These results suggest that the carbon trapping at dislocations prevents the decarburization of super-saturated bainitic ferrite and therefore alters the carbide precipitation sequence during low-temperature bainite formation.

Bainite transformation and TRIP effect in 20Mn2SiVB steel

Transformation-induced plasticity (TRIP) steel is a relatively new type of automotive steel known for its combination of high-strength and high ductility which was developed in the 1990s. 20Mn2SiVB steel is a kind of TRIP steel with low-carbon and low-alloying contents and high-strength. Specimens of a tested 20Mn2SiVB steel austenitized at 920 °C and austempered at 420 °C in a salt bath at different time are investigated. The microstructure obtained is studied by means of optical microscopy, scanning electron microscopy and X-ray diffraction. The results show that bainitic ferrite precipitates at the boundary of the austenite first, and with the prolongation of the isothermal time, the amount of bainitic ferrite increase. Then the ferrite decollates the austenite grain and lath-shaped bainitic ferrite with little island-shaped austenite forms during the holding time. The microstructure contains carbide-free bainite, granular bainite, retained austenite and martensite in the process of bainite transformation. Tensile test of the different treated specimens indicates that a better comprehensive property can be gained after austenized at 920 °C following austempered at 420 °C for 5 min, a certain TRIP effect can be also obtained under this condition.

Simulations of Transformation Kinetics in a Multi-Pass Weld

This paper discusses the thermal welding cycle's influence on microstructural changes in the weld metal. We examined the kinetics of austenite formation by the application of simulated microstructures. Special attention was dedicated to the thermal welding cycle's influence on the formation of microstructures with very low toughness, which could be potential triggers of brittle fracture. The simulated microstructures were prepared by the application of several simulated thermal cycles with different peak temperatures, on samples of real single-pass weld metal. The results of this research show that martensite-austenite constituents form in the overcritical temperature regions (over 950°C), too. The dissolution of carbides (particularly cementite) occurs at temperatures much higher than their expected solubility temperature, particularly over a short heating time. The nucleation rate of austenite in ferrite is not very rapid, in spite of the great amount of interfacial area on which nucleation can occur. The surface energies associated with austenite, ferrite, and carbide are not favorable for nucleation. Nucleation occurred at high angle boundaries where the surface energies are more favorable. The rate of austenite formation is controlled by the rate of growth of the austenite grains. The kinetics of austenite transformation in the weld metal are slower compared to the kinetics of austenite transformation in the heat affected zone. The weld metal is very stable and less sensitive to the thermal influence of the subsequent weld passes. The simulation of kinetics of austenite transformation in the weld metal enables the determination optimal of welding parameters, particularly the cooling rate from 800°C to 500°C.

Orientation Relationship during Partial α-γ-Phase Transformation in Microalloyed Steels

The ferrite to austenite phase transformation in microalloyed steel was studied, with a special focus on the orientation relationship between prior ferrite and subsequent austenite. Also the role of growth selection and preferred nucleation was investigated in this context. Their effects were examined at partial phase transformation.

An assessment of interfacial dissipation effects at reconstructive ferrite–austenite interfaces

In support of accurate prediction of carbon content in austenite for control of austenite mechanical stability in “triple-phase” steels, paraequilibrium growth simulations with the DICTRA software were used to predict the carbon concentration profile at the ferrite–austenite interface during rapid cooling from an intercritical annealing temperature. A mobility model developed from previous literature data was used to estimate a temperature dependent interfacial dissipation energy function. Addition of the interfacial dissipation energy to the ferrite free energy lowers the austenite carbon content near the interface to levels consistent with experimental measurements of retained austenite average carbon content.

Modeling austenite–ferrite transformation in low carbon steel using the cellular automaton method

Austenite–ferrite transformation at different isothermal temperatures in low carbon steel was investigated by a two-dimensional cellular automaton approach, which provides a simple solution for the difficult moving boundary problem that governs the ferrite grain growth. In this paper, a classical model for ferrite nucleation at austenite grain boundaries is adopted, and the kinetics of ferrite grain growth is numerically resolved by coupling carbon diffusion process in austenite and austenite–ferrite (γ–α) interface dynamics. The simulated morphology of ferrite grains shows that the γ–α interface is stable. In this cellular automaton model, the γ–α interface mobility and carbon diffusion rate at austenite grain boundaries are assumed to be higher than those in austenite grain interiors. This has influence on the morphology of ferrite grains. Finally, the modeled ferrite transformation kinetics at different isothermal temperatures is compared with the experiments in the literature and the grid size effects of simulated results are investigated by changing the cell length of cellular automaton model in a set of calculations.

Laser-Ultrasonics as a Technique to Study Recrystallisation and Grain Growth

Laser-Ultrasonics (LUS) provides a means of obtaining microstructure information continuously and non-destructively both in the laboratory and for quality control on-line in industry. Ultrasound is both generated and recorded using lasers which permits remote, non-contact operation with fast sampling and also the capability of working at high temperatures or at moving surfaces, for example during industrial continuous annealing. Examples of dynamic heating trials will be presented for samples of cold rolled steel sheets where primary recrystallisation and ferrite austenite transformation are monitored in-situ as a function of temperature. Examples are also presented where the grain size of low carbon steels have been quantitatively analysed and show very good agreement with microscopy methods.

A cellular automaton model for austenite to ferrite transformation in carbon steel under non-equilibrium interface conditions

The isothermal decomposition of austenite into ferrite is investigated using a two-dimensional cellular automaton (CA) algorithm. This CA model provides a simple solution for the difficult moving boundary problem that governs the ferrite grain growth. In this model, the growth of ferrite grains is controlled by both the austenite–ferrite (γ–α) interface mobility and the carbon diffusion in the retained austenite. The competition between the γ–α interface dynamics and the carbon diffusion in the austenite results in a non-equilibrium γ–α interface condition. In order to predict the growth kinetics of ferrite grains, the carbon diffusion coupled with the γ–α interface dynamics is resolved numerically. The driving force for the γ–α interface mobility is calculated using a regular solute sub-lattice model. The kinetics of ferrite transformation predicted by this CA method is compared with that of a JMA model and experiments in the literature. The simulation provides an insight into the carbon diffusion in austenite and the microstructure evolution during transformation. Meanwhile, the results also show that the γ–α interface under the present non-equilibrium condition is numerically stable in two dimensions and the simulated morphology of the ferrite grains is an almost equiaxed polygon.

Corrosion Fatigue Crack Growth Inhibition OfDuplex Stainless Steel

The main objective of this study is to examine the inhibition effect of chromate on corrosion fatigue crack growth in a super duplex stainless steel alloy. Corrosion fatigue crack propagation tests were performed for Zeron 100 in air, 3.5% NaCl, and in 3.5% NaCl inhibited with sodium chromate (Na2CrO4) using two chromate concentrations. Specimens were tested in the TL orientation. Results show that crack growth rates are enhanced in 3.5% NaCl solution relative to air over the entire range of ∆K tested. Addition of 0.02 M Na2CrO4 to 3.5% NaCl solution results in notable decreases in crack growth rates up to a specific value of ∆K above which crack growth rates approach values measured in pure 3.5% NaCl solution. When the chromate concentration is increased by ten fold, corrosion fatigue crack growth rates are markedly reduced and approach measurements obtained in air for the entire range of ∆K. Examination of fracture surfaces by the scanning electron microscope reveals the presence of ductile striations in austenite and brittle striations in ferrite. The basic striations characteristics in the austenitic phase are apparently the same in all the environments. However, ferrite striations appear to be ∆K and environment dependent. Scratch tests were carried out to examine the effect of chromate concentration on repassivation kinetics of the duplex alloy. Chromate inhibition effectiveness during corrosion fatigue of duplex stainless steel is proposed to correlate with chromate repassivation kinetics and the crack tip strain rate.

The Role of Inclusions and Austenite Grain Size on Intragranular Nucleation of Ferrite in Medium Carbon Microalloyed Steels

The beneficial aspects of intragranular ferrite formation on mechanical properties of welds have been reported in the literature for decades. In recent years, this concept has been successfully extended to medium carbon forging steel to refine the microstructure and optimise ductility and toughness. The aim of this work is to demonstrate that intragranular formation of ferrite could be enhanced by increasing the austenite grain size and/or optimising the nature of the inclusions. In this sense, the isothermal decomposition of austenite in allotriomorphic and idiomorphic ferrite for two medium carbon steels microalloyed with vanadium and titanium have been studied. The experimental results reported in this work allows to conclude that austenite grain size and the nucleation potency of inclusions are two parameters that should be considered linked to promote the full decomposition of austenite into intragranularly nucleated ferrite.

Avrami theory for transformations from non-uniform austenite grain structures

Classical overall transformation kinetics theory for grain boundary nucleated transformations relies on the assumption that the parent phase grain size is uniform. In the present work the theory is adapted to deal with the case where there is a distribution in the size of the parent grains, as is often found in practice. It is demonstrated that the overall transformation rate is fastest when the starting grain structure is uniform. The method has been applied to the formation of allotriomorphic ferrite in austenite and the results have been compared against published experimental data.

The influence of morphology on grain-boundary and twin-boundary bainite growth kinetics at the bay in Fe–C–Mo

The growth kinetics morphology and crystallography of grain-boundary bainite (GBB) and twin-boundary bainite (TBB) formed in Fe–0.24 wt% C–4 wt% Mo were reassessed. Optical microscopy and transmission electron microscopy observations of these coalesced allotriomorphic slabs showed that the bainite–austenite growth fronts were rough, even spiky in place, characteristic of Widmanstätten growth. Furthermore, a significant substructure of regions having different ferrite orientations (subunits) was revealed, the GBB subunit arrangement was quite complex, suggestive of competitive growth between subunit, whereas TBB had a simpler subunit arrangement that yielded less subunit competition. These crystallographic and interfacial structure effects are instrumental for explaining the more rapid thickening of GBB relative to TBB. The differing subunit arrangements in GBB versus TBB originate in the differing ferrite–austenite orientation relationships established during nucleation at austenite grain boundaries versus twin boundaries. The complexities inherent in a multicrystalline allotriomorphic slab migrating with a rough growth front prevents direct interpretation of thickening kinetics data in terms of a fundamental growth process.

Modelling of ausferrite growth in ADI

A model of ausferrite growth in ADI was developed together with a simulation program. Their application enables tracing of the process of ausferrite growth, i.e. of the high-carbon ferrite in austenite. The model was based on a transient diffusion growth of ferrite lamellae in austenite under the boundary conditions proper for the austempering temperature and allowing for the function of ferrite nucleation. The results of the simulation were compared with the data obtained in experiments.

Microstructural evolution in austempered ductile iron during non-isothermal annealing

This work investigates the thermal stability of ausferritic microstructures such as those found in NiCuMo austempered ductile iron (ADI). Typical ADI microstructures consisting of acicular ferrite and high carbon austenite were produced by heat treating NiCuMo ductile iron for 2 h at 315 °C and 370 °C. It was found that low temperature austempering led to the precipitation of a hexagonal epsilon-carbide phase. Hence, differential thermal analysis was used to closely follow the active phase transformations found in the experimental ADI during non-isothermal annealing. From these tests, it was apparent that the ausferrite stability is strongly influenced by the austempering conditions and heating rates. In particular, the experimental outcome shows that the ADI microstructures decompose at temperatures of 428°C and 459°C for irons austempered at 315°C and 370°C, respectively. Further evidence for the decomposition reactions involving acicular ferrite and high carbon austenite was provided by TEM observations. The TEM results indicate that ausferrite decomposes into ferrite-cementite during non-isothermal annealing via the precipitation of transition carbides. In the iron austempered at high temperature, orthorhombic η-Fe2C was identified as the transition carbide, whereas tricilinic silicon-carbide precipitates became dominant in ADI treated at 315°C.

Austenite Grain Size Effects on Isothermal Allotriomorphic Ferrite Formation in 0.37C-1.45Mn-0.11V Microalloyed Steel

Allotriomorphic ferrite is the morphology of ferrite formed at relatively small undercooling below the Ae 3 temperature. Because it is the first transformation in austenite decomposition during cooling, allotriomorphic ferrite affects indirectly the subsequent austenite phase transformations, and then its study is of vital importance. The present paper is concerned to the theoretical and experimental study of the isothermal decomposition of austenite in allotriomorphic ferrite in a modern medium carbon vanadium-titanium microalloyed forging steel. This paper deals with the isothermal austenite -toallotriomorphic ferrite as a whole, considering the specific role of different features such as prior austenite grain size (PAGS) and isothermal temperature, in nucleation and growth processes independently.

Modelización de la formación isotérmica de ferrita idiomórfica en aceros de medio carbono microaleados con vanadio-titanio

A theoretical model is presented in this work to calculate the evolution of isothermal austenite-to-idiomorphic ferrite transformation in a medium carbon vanadium-titanium microalloyed steel. This model has been developed on the basis of the study of the nucleation and growth kinetics of idiomorphic ferrite, considering the influence of the nature, size and distribution of the inclusions, which are responsible of the intragranular nucleation of idiomorphic ferrite. Moreover, the influence of the austenite grain size on the isothermal decomposition of austenite in idiomorphic ferrite has been thoroughly analysed. An excellent agreement (85 % in R) has been obtained between experimental and predicted values of volume fraction of idiomorphic ferrite.