Showing papers on "Austenite published in 2002"

Decomposition of Austenite in Austenitic Stainless Steels

Abstract: Austenitic stainless steels are probably the most important class of corrosion resistant metallic materials. In order to attain their good corrosion properties they rely essentially on two factors: a high chromium content that is responsible for the protective oxide film layer and a high nickel content that is responsible for the steel to remain austenitic. Thus the base composition is normally a Fe-Cr-Ni alloy. In practice the situation is much more complex with several other elements being present, such as, Mo, Mn, C, N among others. In such a complex situation one almost never has a single austenite phase but other phases invariably form. Those phases are, with few exceptions, undesirable and they can be detrimental to the corrosion and mechanical properties. It is therefore of considerable importance to study the formation of such phases. In this work the decomposition of austenite in austenitic stainless steels is reviewed in detail. First the binary equilibrium diagrams relevant to the system Fe-Cr-Ni are briefly presented as well as other diagrams, such as the Schaeffler diagram, that traditionally have been used to predict the phases present in these steels as a function of composition. Secondly the precipitation of carbides and intermetallic phases is presented in detail including nucleation sites and orientation relationships and the influence of several factors such as composition, previous deformation and solution annealing temperature. Next, the occurrence of other constituents such as nitrides, sulfides and borides is discussed. TTT diagrams are also briefly presented. Finally the formation of martensite in these steels is discussed.

Phase Transformation and Mechanical Properties of Si-Free CMnAl Transformation-Induced Plasticity-Aided Steel

Abstract: Conventional CMnSi transformation-induced plasticity (TRIP)-aided steels are a promising solution for producing lighter, crash-resistant car bodies, due to their high-strength and large uniform elongation. The CMnSi TRIP-aided steels, with more than 1 mass pct Si, have the drawback of poor galvanizability due to the presence of complex Si-Mn oxides on the surface. The full substitution of the Si by Al in cold-rolled and intercritically annealed TRIP-aided steels, therefore, was evaluated in detail. The phase-transformation kinetics during the intercritical annealing and the isothermal bainitic transformation were investigated by means of dilatometry. The allotropic phase-boundary was determined both by thermodynamic calculations and the experimental determination of the C content in the retained austenite. The results imply that short isothermal bainitic transformation times are sufficient to obtain the TRIP microstructure and that the processing of CMnAl TRIP-aided steels in a continuous annealing line not equipped for overaging is possible. The mechanical properties were evaluated for CMnAl TRIP-aided steels obtained using an industrial thermal cycle: the properties matched those of the conventional CMnSi TRIP-aided steels, where it was found that the high-Al CMnAl TRIP-aided steel had a high strain-hardening coefficient of 0.25, which was stable up to a true strain of 0.25.

Three-dimensional Landau theory for multivariant stress-induced martensitic phase transformations. I. Austenite↔martensite

Abstract: A three-dimensional Landau theory of stress-induced martensitic phase transformations is presented. It describes transformations between austenite and martensitic variants and transformations between martensitic variants. The Landau free energy incorporates all temperature-dependent thermomechanical properties of both phases. The theory accounts for the principal features of martensitic transformations in shape memory alloys and steels, namely, stress-strain curves with constant transformation strain and constant, or weakly temperature dependent, stress hysteresis, as well as nonzero tangent elastic moduli at the phase transformation point. In part I, the austenite$\ensuremath{\leftrightarrow}$martensite phase transformation is treated, while transformations between martensitic variants are considered in part II.

Development of 9Cr-ODS Martensitic Steel Claddings for Fuel Pins by means of Ferrite to Austenite Phase Transformation

Abstract: For use as fuel cladding of liquid metal fast reactors, Fe-0.12C-9Cr-2W ODS martensitic steel claddings were developed by cold-rolling under the softened ferrite phase induced by slow cooling from austenite phase, subsequently by ferrite to austenite phase transformation to break up substantially elongated grains produced by cold-rolling at the final heat-treatment. The produced claddings showed noticeable improvement in tensile and creep rupture strength that are considerably superior to PNC-FMS and even austenitic PNC316 at higher temperature and extended time to rupture. The strength improvement is mainly attributed to titanium addition in ODS martensitic steels through its reduction of Y2O3 particle size and shortening inter-particles spacing. The behavior of oxide particle size reduction is associated with stoichiometry between Y2O3 and TiO2.

Surface engineering of austenitic stainless steel

Abstract: The inherent poor tribological behaviour of austenitic stainless steels has been a barrier to their wider application under corrosion wear conditions. Therefore, researchers have long dreamt of developing new technologies to enhance the wear resistance of stainless steel surfaces without loss of their attractive ‘stainless’ characteristics. For example, the discovery of low temperature plasma nitriding in the mid 1980s allowed a new phase with a high hardness and good corrosion resistance to be produced on austenitic stainless steel surfaces. Since then much research and development has aimed to combine improvements in wear, corrosion and fatigue properties. This has been demonstrated by various low temperature surface alloying processes (including plasma, ion beam, gaseous and salt bath methods) and increasing numbers of academic publications, reflecting rapidly expanding niche markets in the food, chemical, nuclear and medical sectors. In this paper, the historical evolution and development of l...

Properties of thermomechanically processed dual-phase steels containing fibrous martensite

Abstract: Manganese partitioning and long fibrous martensite were obtained in several dual-phase steels by annealing, followed by hot rolling in their respective intercritical annealing temperature regions. The volume fraction of martensite was varied by changing the intercritical annealing temperature. Long annealing times were required to obtain complete manganese partitioning in order to increase the strength of the austenite phase and subsequently the martensite phase. Post-roll annealing improves the ductility through ferrite recrystallisation and also by changes to the martensite morphology, however, this results in a decrease in strength. The as-rolled steel shows two distinct work-hardening processes with different exponents and shows a laminated fracture appearance due to the ribbon-like morphology of the martensite.

TEM study on the surface white layer in two turned hardened steels

Abstract: The structure of surface white layers was examined using transmission electron microscopy. Surface specimens were machined from a BS 817M40 steel (0.4C 1.2Cr 1.4Ni 0.2Mo) of 52 HRC and a low alloy tool steel (0.8C 1.7Cr 0.4Mo) of 58 HRC, with unworn and worn alumina/TiC composite cutting tools. Thin foil specimens were prepared such that the direction of observation was normal to the machined surface. The as-tempered microstructure of both steels was lath martensite. The structure of the machined surfaces of both steels was characterised by very fine, mis-orientated cells, less than 100 nm in size. The accompanying selected area electron diffraction patterns indicated the presence of retained austenite, the volume fraction of which increased with cutting tool wear. A refinement in the size of cementite particles was also evident. In the surface of the BS 817M40 steel machined with a worn cutting tool, there was evidence to suggest a degree of recrystallization. This may be accounted for by a transition from dynamic recovery to dynamic recrystallization during surface generation; a phenomenon which is favoured by the decrease in the work materials stacking fault energy as a result of the reverse martensite transformation.

On the measurement of the nanohardness of the constitutive phases of TRIP-assisted multiphase steels

Abstract: The nanohardness of the phases present in the microstructure of two TRIP (for TRansformation Induced Plasticity)-assisted multiphase steels differing by their silicon content was measured by nanoindentation in an atomic force microscope. It is observed that the softest phase in both steels is the ferritic matrix, followed by bainite, austenite and martensite. It is also shown that the silicon content of the steel grades is responsible for an increase of the hardness of the ferritic matrix due to solid solution strengthening. Finally, the influence of the preparation mode of the surface prior to the nanoindentation measurements has been investigated. An electropolishing stage after mechanical polishing is acceptable to allow valuable nanohardness measurements. (C) 2002 Elsevier Science B.V. All rights reserved.

Effects of martensite formation and austenite reversion on grain refining of AISI 304 stainless steel

Abstract: The austenite–martensite transformation followed by annealing for austenite reversion in AISI 304 stainless steel has been investigated in order to study the effect of this thermo-mechanical process on grain refinement. In particular the effect of cold reduction, annealing temperature and annealing times have been analysed. After getting ultrafine grains the effect of the grain size on the hardness and on the tensile properties has been evaluated, showing a Petch-Hall dependency in the fully analysed range (down to 0.8 μm grain size).

Dilution and microsegregation in dissimilar metal welds between super austenitic stainless steel and nickel base alloys

Abstract: Super austenitic stainless steels are often welded using high Mo, Ni base filler metals to maintain the corrosion resistance of the weld. An important aspect of this processing is the weld metal dilution level, which will control the composition and resultant corrosion resistance of the weld. In addition, the distribution of alloying elements within the weld will also significantly affect the corrosion resistance. Dissimilar metal welds between a super austenitic stainless steel (AL-6XN) and two Ni base alloys (IN625 and IN622) were characterised with respect to their dilution levels and microsegregation patterns. Single pass welds were produced over the entire dilution range using the gas tungsten arc welding process. Microstructural characterisation of the welds was conducted using light optical microscopy, scanning electron microscopy, and quantitative image analysis. Bulk and local chemical compositions were obtained through electron probe microanalysis. The quantitative chemical information w...

The effect of ageing upon the microstructure and mechanical properties of type 15-5 PH stainless steel

Abstract: Microstructures developed in commercial 15-5 PH precipitation-hardened stainless steel after different heat treatments have been studied. In the as received condition, two types of carbides, NbC and M 7 C 3 , were present. Age hardening involves initial formation of fine precipitates rich in copper. Conventional transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) studies have revealed the formation of a 9R structure copper precipitates (4 nm) in the samples aged at temperatures below 500 °C. However, at higher temperatures, in addition to the formation of these precipitates, the austenite phase was formed. After ageing at 500 °C for 128 h, M 23 C 6 carbides were observed. The majority of the M 23 C 6 carbides were in the interface of martensitic matrix and retained austenite. A second type of copper precipitate being, spherical in shape, were observed on ageing at 650 °C. TEM and X-ray microanalysis on thin foils and on carbon extraction replicas used for analysing the structure and metallic compounds of these precipitates. The mechanical properties following strain deformation has been carried out using scanning electron microscope (SEM), TEM and HREM. At peak aged, the 15-5 PH alloy exhibit brittle failure, the major fracture mode was cleavage and/or quasicleavage.

Development of microstructural banding in low-alloy steel with simulated Mn segregation

Abstract: The development of microstructural banding in low-alloy steel with Mn segregation has been investigated through the use of artificially segregated steel, interrupted cooling techniques, and optical microscopy. Mn segregation was simulated by hot roll bonding thin sheets of 5140 steel with 0.82 wt pct Mn and modified 5140M with 1.83 wt pct Mn into a plate with 20- and 160-µm-thick segregated layers. Samples were austenitized at 850 °C, continuously cooled at 1 °C/s and 0.1 °C/s, and quenched from progressively lower temperatures to observe the evolution of the microstructure. The segregated band thickness had a striking effect on microstructural development. Samples with 160 µm bands cooled at 1 °C/s had martensite and bainite in high-Mn bands. In contrast, samples with 20 µm bands cooled at the same rate had pearlite in high-Mn bands. The dramatic effect of band thickness on microstructural development was due to growth of a fully pearlitic band at the interface between segregated layers. The formation of interfacial pearlite is discussed relative to redistribution of carbon between adjacent high- and low-Mn bands during cooling.

Effects of the grain size on the corrosion behavior of refined AISI 304 austenitic stainless steels

Abstract: Although there have been many studies on fine grained ferritic steels, only a few research reports are available on refined austenitic stainless steels and, in particular, on the influence of the grain size on the corrosion resistance of this class of material [1, 2]. The grain size of ferritic steels can be easily induced by phase transformation, but in austenitic alloys, following the absence of a phase transformation, the grain diameter is usually controlled by recrystallization after cold working [3]. This method is mainly affected by the working temperature, amount of deformation and recrystallization temperature. Recrystallization after hot rolling is reported to have the effect of grain refining [4] but this method seems to be limited. In a previous paper [5] we examined the effect of subzero working on the grain refining of austenitic stainless steels. In particular, ultrafine grained AISI 304 stainless steel of ca. 1 μm average grain size was obtained by applying the reverse transformation of martensite to austenite on subzeroworked steel annealed at low temperatures. Up to now, the corrosion behavior of such ultrafinegrained austenitic stainless steels has not been reported. This paper deals with the corrosion behavior, especially general corrosion (GC), intergranular corrosion (IGC) and pitting corrosion (PC) of ultrafine-grained AISI 304 stainless steel. Results are compared with those of similar measurements on standard AISI 304 steel. The chemical composition of the AISI 304 stainless steel, obtained from a commercial batch, is shown in Table I. After subzero working down to 90% thickness reduction, the material was subjected to the following four heat treatments in order to obtain different microstructures: annealing at 800 ◦C for 160 s and 900 s (specimens A and B respectively) and at 1000 ◦C for 10 s and 600 s (specimens C and D respectively). The grain sizes corresponding to the above specimens, as measured by automatic image analyzer, are shown in Table II. The typical microstructures of the 1 μm and 50μm specimens are shown in Fig. 1. Tensile properties of the specimens are shown in Fig. 2. Ultimate tensile stress and 0.2% yield stress increase with decreasing grain size, according to the Hall Petch relation [6]. Steel materials were machined into corrosion test specimens of 15 × 15 × 1 mm. The specimen surface was polished by using increasingly finer abrasive papers, starting with a 300 grit paper and finishing up with

Microstructural characterization of simulated heat affected zone in a nitrogen-containing 2205 duplex stainless steel

Abstract: In order to investigate the microstructural evolution in a nitrogen-bearing 2205 duplex stainless steels (DSS) during welding, a simulated weld thermal cycle with 5 kJ cm−1 heat input followed by exposure at 700 °C for different time intervals was performed. The microstructure of high-temperature heat affected zone (HTHAZ) developed with the thermal experience was characterized via optical metallography and transmission electron microscopy (TEM). The duplex structure with equivalent phase components was drastically destroyed by the rapid thermal cycle. In the simulated HTHAZ structure, three different morphologies of newly formed austenite were observed in the coarse-grained δ-ferrite matrix; i.e. allotriomorphic austenite, Widmanstaten autenite and intragranularly nucleated autenite. During the exposure at 700 °C, the intragranularly nucleated austenite got coarse and the Widmanstaten austenite grew progressively. TEM revealed that several variants of rod-like Cr2N were precipitated selectively at intragranular and intergranular sites. From the analyses of diffraction patterns of TEM, Kurdjumov–Sachs orientation relationship was found to describe the interface between intragranularly nucleated autenite and δ-ferrite, while Pitch–Schrader orientation relationship to describe the disposition between hexagonal Cr2N precipitates and δ-ferrite matrix.

High strength steel sheet having excellent formability and method for production thereof

Abstract: A high strength steel sheet having (2-1) a base phase structure, the base phase structure being tempered martensite or tempered bainite and accounting for 50% or more in terms of a space factor relative to the whole structure, or the base phase structure comprising tempered martensite or tempered bainite which accounts for 15% or more in terms of a space factor relative to the whole structure and further comprising ferrite, the tempered martensite or the tempered bainite having a hardness which satisfies the relation of Vickers hardness (Hv)≧500[C]+30[Si]+3[Mn]+50 where [ ] represents the content (mass %) of each element, and (2-2) a second phase structure comprising retained austenite which accounts for 3 to 30% in terms of a space factor relative to the whole structure and optionally further comprising bainite and/or martensite, the retained austenite having a C concentration (CγR) of 0.8% or more.

Ductility and Formability of Newly Developed High Strength Low Alloy TRIP-aided Sheet Steels with Annealed Martensite Matrix

Abstract: Formable high-strength low-alloy TRIP-aided sheet steels with annealed martensite matrix or "TRIP-aided annealed martensitic steel" were developed for automotive applications. The steels possessed a large amount of plate-like retained austenite along annealed martensite lath boundary, whose stability against the strain-induced transformation was higher than that of the conventional "TRIP-aided dual-phase steel" with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the TRIP-aided annealed martensite steels exhibited a superior large elongation and reduction of area. In addition, they possessed the same excellent stretch-flangeability and bendability as "TRIP-aided bainitic steel" with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress and so on.

Development of ultra fine grain structure by martensitic reversion in stainless steel

Abstract: Austenitic stainless steels have good corrosion resistance and good formability but they have also relative low yield strength. It is well known that the mechanical properties of austenitic stainless steels are very sensible to the chemical composition (which can induce hardening by both substitutional and interstitial solid solution) and to microstructural features (such as grain size and δ-ferrite content). Recently there have been commercial developments to exploit the effect of these variables in stainless steel taking advantage of changes in the chemical composition induced by nitrogen addition [1, 2]. Another effective way to increase yield strength without impairing good ductility is grain refining. Although this approach has induced the development of ultrafine grain carbon steels (e.g. [3]), no attempts have been still reported on this approach for austenitic stainless steels. In fact, austenitic stainless steels do not undergo phase transformation at typical annealing temperatures and then the only way to refine the grain is recrystallization after cold rolling. However, the strengthening by grain refining is limited, due to the high recrystallization temperature of this stainless steel grade. For instance, the recrystallization temperature of the AISI 301 steel is above 900 ◦C and the minimum grain size obtained is in the range 10–30 μm [4]. In austenitic stainless steels, plastic deformation of austenite creates the proper defect structure which acts as embryo for martensite deformation: the successive reversion of deformation-induced martensite (α′) enables a marked grain refining [5, 6]. In this letter the production of an ultra fine microstructure in an AISI 301 stainless steel by martensitic reversion is reported. The chemical composition of the steel used is shown in Table I. The procedure used to refine the grain is the following (see Fig. 1): • Metastable γ is almost entirely transformed to α′ by heavy cold rolling: in fact the retained γ cannot be refined during the subsequent annealing. • α′ reverts to recrystallized austenite γR during annealing at low temperature.

Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels

Abstract: Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of low- and ultralow-carbon microalloyed steels containing additions of boron and/or molybdenum to enhance hardenability. Each alloy was subjected to simulated recrystallization and nonrecrystallization rolling schedules, followed by controlled cooling at rates from 0.1 °C/s to about 100 °C/s, and the corresponding continuous-cooling-transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element contents, through to bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. Combined additions of boron and molybdenum were found to be most effective in increasing steel hardenability, while boron was significantly more effective than molybdenum as a single addition, especially at the ultralow carbon content. Severe plastic deformation of the parent austenite (>0.45) markedly enhanced PF formation in those steels in which this microstructural constituent was formed, indicating a significant effective decrease in their hardenability. In contrast, in those steels in which only nonequilibrium ferrite microstructures were formed, the decreases in hardenability were relatively small, reflecting the lack of sensitivity to strain in the austenite of those microstructural constituents forming in the absence of PF.