Showing papers on "Austenite published in 2004"

Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe–Mn–C alloys

Abstract: A model is proposed for the evaluation of the stacking fault energy (SFE) in Fe–Mn–C austenitic alloys, at different temperatures. It accounts for the variation of the Gibbs energy of each element during the austenite to e martensite transformation, plus their interactions. The Gibbs energy due to the antiferromagnetic to paramagnetic transition is also taken into account. The required data have been obtained from the literature. The result shows a decrease of the SFE with temperature, with a saturation below the austenite Neel temperature. The result agrees with the mechanical and thermal martensitic transformation limits proposed by Schumann. The plasticity mechanisms depend on the SFE. The mechanical martensitic transformation occurs below 18 mJ/m 2 , and twinning between 12 and 35 mJ/m 2 , in agreement with the tensile tests and the deformation microstructures observed in an Fe–22 wt.% Mn–0.6 wt.% C alloy at 77, 293 and 693 K.

Magnetic and martensitic transformations of NiMnX(X=In,Sn,Sb) ferromagnetic shape memory alloys

Abstract: Martensitic and magnetic transformations of the Heusler Ni50Mn50−yXy (X=In, Sn and Sb) alloys were investigated by differential scanning calorimetry measurement and the vibrating sample magnetometry technique. In all these alloy systems, the austenite phase with the ferromagnetic state was transformed into the martensite phase, which means that these Heusler alloys have potential as Ga-free ferromagnetic shape memory alloys (FSMAs). Furthermore, multiple martensitic transformations, such as two- or three-step martensitic transformations, occur in all these alloy systems. It was confirmed by transmission electron microscopy observation that the crystal structure of the martensite phase is an orthorhombic four-layered structure which has not been reported in other FSMAs. Therefore, the present Ga-free FSMAs have the great possibility of the appearance of a large magnetic-field-induced strain.

Partitioning of carbon from supersaturated plates of ferrite, with application to steel processing and fundamentals of the bainite transformation

Abstract: A model is reviewed, that describes the endpoint of carbon partitioning between supersaturated ferrite and retained austenite. A new process, quenching and partitioning (Q&P), has been developed recently to intentionally employ such partitioning in creating useful ferrous microstructures containing retained austenite. The process involves quenching austenite below the martensite-start temperature, followed by a partitioning treatment to enrich the remaining austenite with carbon, thereby stabilizing it to room temperature. Recent experimental studies have confirmed that Q&P provides a viable means to create microstructures containing carbon-enriched retained austenite, and attractive property combinations have been achieved in a variety of materials, while opportunities remain for further optimization. Furthermore, some implications of the partitioning model with respect to fundamentals of the bainite transformation are discussed, including the possibility of displacive growth under carbon diffusion control, with an austenite composition at the α/γ interface represented by the (adjusted) T0 composition. It is suggested that individual movements of iron atoms are likely during growth of Widmanstatten ferrite, and that there may be a need for further consideration of thermally activated iron-related processes in general.

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Abstract: Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.

Transformation-induced plasticity for high strength formable steels

Abstract: Recent advances in the development of high performance steels presenting improved properties of strength and ductility rely on the TRIP effect, i.e. on the mechanically-induced martensitic transformation of the retained austenite dispersed in a soft ferrite-based matrix. As a consequence, the stabilisation and retention of austenite at room temperature have become of primary importance, leading to specifically designed steel grades and thermal or thermomechanical treatments. Particularly, carbon enrichment of the austenite during intercritical annealing and bainite transformation was found to be very effective in retaining austenite. This metastable austenite then progressively transforms during straining, bringing about a large increase of the work hardening rate. This increase results from the stress and strain partitioning continuously evolving with the appearance of the hard martensite. (c) 2004 Elsevier Ltd. All rights reserved.

The mechanism of acicular ferrite in weld deposits

Abstract: Research has shown that the acicular ferrite microstructure in steel weld metal, which provides an optimum combination of strength and toughness, is indeed intragranularly nucleated bainite. It is possible to maximize the content of acicular ferrite by increasing the intragranular nucleation sites while maintaining a critical weld metal cooling rate and the steel hardenability. This paper highlights recent research related to nucleation and growth of acicular ferrite during decomposition of austenite.

Austenite formation during intercritical annealing

Abstract: A systematic experimental study has been conducted on ferrite recrystallization and intercritical austenite formation for two low-carbon steels with chemical compositions typically used for dual-phase and transformation-induced plasticity (TRIP) steels. Different initial heating rates, holding temperatures, and times were applied to the materials to examine the ferrite recrystallization and austenite formation kinetics. An Avrami model was developed to describe the isothermal ferrite recrystallization behavior and was applied successfully to the nonisothermal conditions. It was found that the initial heating rate affects the isothermal austenite formation kinetics for both the hot-rolled and cold-rolled materials albeit the effect is more pronounced for the cold-rolled material. This can be attributed to the interaction between the ferrite recrystallization and austenite formation processes. Furthermore, it was found that the distribution of austenite phase is also affected by the ferrite recrystallization process. When ferrite recrystallization is completed before the austenite formation (i.e., under sufficiently slow heating rate conditions), austenite is to a large extent randomly distributed in the ferrite matrix. On the other hand, incomplete recrystallization of ferrite due to higher heating rates leads to the formation of banded austenite grains. It is proposed that this observation is characteristic of simultaneous recrystallization and austenite formation where moving ferrite grain boundaries do not provide suitable sites for austenite nucleation.

Strengthening via the formation of strain-induced martensite in stainless steels

Abstract: The strengthening that results from the low-temperature formation of strain-induced martensite in austenitic stainless steel was studied. Specifically, the work hardening behaviour was characterized, as well as the spatial distribution of the martensite as a function of prior strain. Neutron diffraction measurements revealed the degree of elastic strain partitioning between the austenite and martensite. It was found that a sufficiently high initial dislocation density leads to a localization of the martensite transformation in the form of a Luders front. The martensite acts as an elastic reinforcing phase as it supports a higher stress than the austenite tensile loading, even though the martensite co-deforms plastically with the austenite. A model was developed that predicts the volume fraction of martensite formed as a function of plastic strain. © 2004 Elsevier B.V. All rights reserved.

Nanoscale-twinning-induced strengthening in austenitic stainless steel thin films

Abstract: Magnetron-sputter-deposited austenitic 330 stainless steel (330 SS) films, several microns thick, were found to have a hardness ∼6.5 GPa, about an order of magnitude higher than bulk 330 SS. High-resolution transmission electron microscopy revealed that sputtered 330 SS coatings are heavily twinned on {111} with nanometer scale twin spacing. Molecular dynamics simulations show that, in the nanometer regime where plasticity is controlled by the motion of single rather than pile-ups of dislocations, twin boundaries are very strong obstacles to slip. These observations provide a new perspective to producing ultrahigh strength monolithic metals by utilizing growth twins with nanometer-scale spacing.

Stabilization of martensitic microstructure in advanced 9Cr steel during creep at high temperature

Abstract: In order to improve the long-term creep strength of 9%Cr steel, the stabilization of martensitic microstructure in the vicinity of prior austenite grain boundaries during creep has been investigated by the addition of boron and by a dispersion of nano-size MX nitrides. Creep tests were carried out at 923 K for up to about 3×10 4 h. Boron is enriched in the M 23 C 6 carbides during aging and creep, especially in the vicinity of prior austenite grain boundaries. This reduces the coarsening rate of M 23 C 6 carbides, which effectively stabilizes the martensitic microstructure in the vicinity of prior austenite grain boundaries. A dispersion of nano-sized MX nitrides but no M 23 C 6 along boundaries also gives rise to excellent pinning force for migrating boundaries during creep, as shown by approximately two orders of magnitude longer time to rupture than ASME-P92. The stabilization of martensitic microstructure retards the onset of tertiary or acceleration creep, which results in lower minimum creep rate and longer time to rupture.

Effect of Grain Refinement on Thermal Stability of Metastable Austenitic Steel

Abstract: In martensitic steels, it is well known that a certain chemical driving force (about 180 MJ/m 3 ) is required to start martensitic transformation (Ms), and additional driving force has to be charged further to complete the transformation (Mf). In the case of metastable austenitic steels with Ms temperature at around room temperature, however, only the chemical driving force needed to start martensitic transformation has been stored at room temperature. Hence, the state of austenite is very unstable thermally. It has already been known that such a metastable austenite undergoes a partial martensitic transformation during isothermal holding at room temperature or cooling to a low temperature. It is very convenient to investigate the behavior of martensitic transformation of austenite. In this study, the effect of austenite grain size on martensitic transformation is introduced from the viewpoint of microstructural analysis and thermo-dynamics. The steel used in this investigation is an Fe-16 mass%Cr-10 mass%Ni ternary alloy, which has Ms temperature at around room temperature. The grain size of this steel can be controlled from 0.8 mm to 80 mm using the technique of reversion of deformation induced martensite. In the material with coarse grain size (80 mm), about 18% of martensite was detected at room temperature and the amount of martensite was increased to 50% by the following subzero treatment to 77 K. However, martensite was hardly detected in the material with ultra fine grains (0.8 mm) even after the subzero treatment. It was found that such a stabilization occurs in the materials with the grain size below 10 mm and the stabilization was reasonably explained by considering the relation between austenite grain size and elastic strain energy which is required on the single variant martensitic transformation.

Tempering of hard mixture of bainitic ferrite and austenite

Abstract: Recent work has shown that bainitic ferrite plates produced by transformation at low temperatures can be as thin as 20 nm with a hardness in excess of 650 HV30, tensile strength ~2.3 GPa and toughness ~30 MPa m1/2. Because these properties rely on the fine scale of the microstructure, a study has been carried out in relation to the tempering resistance of steel over the temperature range 350 – 750°C. It is found that significant softening occurs only after the plates of ferrite begin to coarsen. The coarsening process is hindered by the intense precipitation of carbides resulting from decomposition of the carbon enriched retained austenite. The carbides themselves lead to some precipitation strengthening during the early stages of tempering. The ferrite is found to contain excess carbon, beyond its solubility limit, and X-ray analysis indicates that the carbon is associated with heterogeneous strains in the microstructure. It does not readily precipitate until the onset of substantial recovery dur...

Microstructure of Steels and Cast Irons

Abstract: First Part The history of iron and steel - of swords and ploughshares.- 1 From iron to steel.- 2 Of swords and swordmaking.- 2 The Genesis of Microstructures.- 3 The principal phases in steels.- 4 The basic phase diagrams.- 5 The formation of solidification structures.- 6 Liquid/solid structural transformations.- 7 Grains, grain boundaries and interfaces.- 8 Diffusion.- 9 The decomposition of austenite.- 10 The pearlite transformation.- 11 The martensite transformation.- 12 The bainite transformation.- 13 Precipitation.- 3 Steels and cast irons.- 14 Steel Design.- 15 Solidification macrostructures.- 16 Macro- and microstructures of sintered powder products.- 17 Plain carbon and low alloy steels.- 18 Quench hardening steels.- 19 Stainless steels.- 20 Heat resisting steels and iron-containing superalloys.- 21 Cast irons.- 22 Appendices.- 23 References.- 24 Index.

Deformation induced martensitic transformation in stainless steels

Abstract: Deformation induced martensitic transformation was investigated in metastable austenitic stainless steel. This steel can present a microstructure of austenite (γ), α′ martensite and non magnetic e martensite. Uni-axial tensile test was used for loading at different temperatures below room temperature (from −120 to 20 °C). During the deformation the transformation takes place at certain places in an anisotropic way and texture also develops. Quantitative phase analysis was done by X-ray diffraction (XRD) and magnetic methods while the texture was described by X-ray diffraction using a special inverse pole figure. The quantitative phase analysis has shown that the formation of α′ and e martensite from austenite is the function of deformation rate, and deformation temperature. The transformation of the textured austenite takes place in an anisotropic way and a well defined crystallographic relationship between the parent and α′ martensite phase has been measured.

The low-temperature aging embrittlement in a 2205 duplex stainless steel

Abstract: The effect of isothermal treatment (at temperatures ranging between 400 and 500 °C) on the embrittlement of a 2205 duplex stainless steel (with 45 ferrite–55 austenite, vol.%) has been investigated. The impact toughness and hardness of the aged specimens were measured, while the corresponding fractography was studied. The results show that the steel is susceptible to severe embrittlement when exposed at 475 °C; this aging embrittlement is analogous with that of the ferritic stainless steels, which is ascribed to the degenerated ferrite phase. High-resolution transmission electron microscopy reveals that an isotropic spinodal decomposition occurred during aging at 475 °C in the steel studied; the original δ-ferrite decomposed into a nanometer-scaled modulated structure with a complex interconnected network, which contained an iron-rich BCC phase (α) and a chromium-enriched BCC phase (α′). It is suggested that the locking of dislocations in the modulated structure leads to the severe embrittlement.

Stability of retained austenite in TRIP-assisted steels

Abstract: A theory is developed for the quantitative representation of the strain induced transformation of retained austenite in low alloy, TRIP-assisted steels of the type developed for the automobile industry. It is possible, therefore, to calculate the fraction of austenite as a function of the plastic strain, chemical composition, deformation temperature and the starting amount of austenite. The effect of composition and temperature is expressed through the free energy available for transformation. Good agreement has been obtained with published experimental data. The model can be used to investigate the stability of the austenite during plastic deformation.